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The Reason We Haven’t Directly Detected Dark Matter (medium.com)
334 points by alex_young 6 months ago | hide | past | web | favorite | 295 comments

My attempt to summarize the reasons would be:

We haven't directly detected dark matter because we don't really know what we are looking for, and there are only a few things we can search for. Dark matter might not be structured in a way we can investigate with current technology.

Also, dark matter doesn't interact much with regular matter, which makes the search even harder.

> Also, dark matter doesn't interact much with regular matter, which makes the search even harder.

This is (one of the many places) where I get lost. It has mass, so it by definition interacts with anything with mass?

Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? (How much dark matter would be in the solar system? What would be the average density of the dark matter? what would be the mass of single dark matter particle?) Or what do I miss here?

> It has mass, so it by definition interacts with anything with mass?

Yeah, but the only interaction is gravity. And nothing that clumps these particles together (such as the electro-magnetic force for ordinary matter).

> Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system?

The additional mass can be measured, this is why we suspect the particles are there in the first place :)

> The additional mass can be measured, this is why we suspect the particles are there in the first place :)

Yes, in interstellar scale. How much there should be dark matter within solar system? 1 gram? 1 kilogram? I mean, if there was supposed to be 5/6 parts of mass of dark matter within solar system, it would quote obviously be somehow observable?

The thing you are missing is that the "missing mass" isn't observable on the scale of the solar system. It's missing on the scale of large scale galactic structures -- i.e. lots and lots of galaxies. When we look at those structures, we can't figure out how they got that way unless there is a whole bunch more mass than there looks to be. Apart from that, we know nothing.

People speculate that maybe there is some weird particle that we can't see in any other way than through looking at the mass of these large (really, really, really ridiculously large) structures. But is there any of it in the solar system? In our galaxy? Nobody knows. Does it hang out between galaxies? Between galaxy clusters? Does it even really exist? Nobody knows.

You're just jumping way, way, way too far down the way ;-) Literally, dark matter could be anything -- even a misunderstanding about how the universe works. That's what's interesting about it.

Okay, let's try to reformulate. What is the average density of dark matter in places we know there is dark matter? Then, assuming here were dark matter with that density in earth, how much that would be?

I am just trying trying to get my head around how sparse the dark matter actually is, wherever it actually exists. Would we have one kilogram of dark matter to observe?

Damn. Should have googled in the first place. Here is an estimate of dark matter density in solar system:


6x10^-28 kg/cm3

Volume of earth is around 10^27 cm3 so that makes the dark matter mass within earth around 600 grams. Admittably that is a bit difficult to measure...

But even then, there's no reason to assume that there is any in that space. For example, imagine that all the dark matter was in a disk around the solar system, way out farther than the asteroid belt. If you could see through it, you would never know. Or maybe it rings galaxies. Or maybe it hangs out in clumps between galaxies. Or... It literally could be anything because we can only measure it on the scale of mind boggingly massive structures.

I believe it’s generally believed that dark matter clumps near galaxies because it interacts through gravity. So would it be reasonable to say that it should clump together near massive objects in the solar system?

DM usually forms a halo as it orbits the center of mass of whatever it orbits (galaxy, globular cluster), and because it doesn't interact otherwise, it cannot slow down, cannot shed momentum, so it is likely mostly not a disk, but a big sphere, and probably a shell around galaxies.

That sounds a little like an additional dimension or alternative something truly “out there”. Ie maybe gravity’s force relies on some process that reverses at the macro scale.

I mean, If there were appreciable amounts of it (i.e. not on the scale of kilograms in or around the earth), so that it could have a measurable impact on planet orbits, we could gain information on its distribution within (or interaction with) the solar system. That's why this density estimate is interesting - it sort of determines what the smallest scale is at which we'd have a chance of observing interactions/structure with dark matter.

It's also observable on the scale of galaxies. Evidence for dark matter includes observed inconsistency between galactic rotation curver and total mass estimates. But yes, some of the earliest evidence (decades of it) was unexpected behavior of galactic clusters.

Anyway, what we expect to see here is what's typical at our distance from Sagittarius A[star], and distance from the galactic plane.

Also, I gotta say that the galactic dark matter distribution reminds me a lot of Vinge's "Slow Zone" ;) A Fire Upon the Deep came out in 1992. I wonder whether he had dark matter in mind. He never used the term, as I recall.

Could it be antimatter? Ie are we sure through direct observation that antimatter reacts with light the same way matter does? I know antimatter has been made in laboratories, but plenty of phenomena act differently outside of the lab.

(I’m pretty sure we’re sure dark matter isn’t antimatter, but just the same, worth the thought when 5/6th of the universe is unexplainable.)

I think that the properties of antimatter are pretty well established to be identical to that of matter. For instance, the light spectrum of antihydrogen is known to be exactly the same as ordinary hydrogen. I don't think many (any?) researchers are looking at antimatter as an option for dark matter.

(Also, I always thought of antimatter as an abstract thing that gets made in the lab and that was cool ... But we use it for practical things every day. For instance, PET scanners in medicine produce positrons and watch the annihilations inside your body. Astoundingly cool to me.)

Good point! I forgot about PET scans.

The problem with antimatter is that it very much interacts with normal matter, quite violently in fact. So if dark matter was antimatter we’d be bathed in a sea of radiation as it continually annihilated with ordinary matter, which would make it quite easy to detect ;)


Short answer: 10^-18 as much as the mass of the sun, 1 proton-mass per 3 cubic centimeters. Not enough to be detected gravitationally - it just gets swamped.

Moreover, locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible.

Nevertheless, our gravitational experiments can say things about the properties of dark matter (it generally obeys the Equivalence Principle): https://arxiv.org/abs/1207.2442

Furthermore, for certain classes of ultra-light dark matter, gravitational and spin-coupled searches can have something to say, e.g.: https://arxiv.org/abs/1512.06165

We're trying.

> locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible.

This seems like a weird thing to expect. What else has approximately uniform local density of distribution? Why would dark matter be different?

Dark matter doesn't physically interact with itself or regular matter, so it doesn't "clump" the way regular matter does. A particle of dark matter will fall towards the Sun or Earth, but it doesn't stop when it gets there - it just carries right on through, with just as much energy as it had before. We expect there to be a dark matter "wind" passing through the solar system at galactic speeds, so it doesn't stick around.

But! There may be seasonal variations in the amount of dark matter reaching Earth due to a solar "lensing" effect. Attempts have been made to find this signal, and an annual modulation has been found, but the debate as to its cause is ongoing.




In general, though, shouldn’t we have dark matter “orbiting” massive bodies bound by their gravitational field?

The galaxy's dark matter is largely rotating with the bulk of the visible galaxy. However, the solar system's peculiar motion through the dark matter (DM[a]) does perturb the DM, and some DM will entrain to solar system objects (mostly the sun) leading to small overdensities.

However, remember that within the orbit of Neptune there is only about ten Phobos-masses worth of dark matter, or barely more than Jupiter's small moons Lysithea (disc. 1938) or Sinope (disc. 1914, until 2000 the outermost known moon of Jupiter).

Moreover, Jupiter can't really gravitationally entrain anything beyond 0.35 astronomical units away from it (otherwise the sun dominates), and gas (whether dark matter or electrically neutral atoms or light molecules[b]) is too low-mass to be drawn into a orbit around Jupiter with such a small radius.

There is likely a small overdensity of DM within the sun, but that's really a focusing of dark matter gas through gravitational lensing rather than dark matter gas staying trapped within the sun. It may help understanding if you hold the sun stationary and blow a wind of dark matter gas past (and through) it -- the gravitation of the sun pinches some of the gas inwards. Since on timescales of small numbers of years the sun has roughly constant velocity against the gas (or the wind blows with constant strength from a constant direction), the pinched wind is at a constant location and constant density deep within the sun.

Whether any of the less-massive bodies of the solar system have overdensities within them (or possibly tails[1]) depends on the mass of dark matter particles, and right now that's not well-enough constrained to answer with any confidence.

- --

[a] here I mean specifically cold dark matter (from the standard cosmology) rather than neutrinos. Solar neutrinos and (relativistic) neutrinos from far away sources are "hot" and so run away from the galaxy too quickly to account for much of its non-visible mass; cosmic neutrinos (the neutrino analogue of the cosmic microwave background) are cold and dark, but individually they're too low-mass to form galaxy or even galaxy-cluster size halos. The total mass of the cosmic neutrino background is also small.

[b] of course, ionization of neutral atoms and gas molecules is pretty likely in the solar system, and Jupiter has an enormous magnetotail. Dark matter doesn't feel magnetism (and isn't ionized by UV or X-rays), otherwise it wouldn't be dark. So while gases can be drawn around Jupiter electromagnetically, dark matter cannot.

[1] https://www.nasa.gov/feature/jpl/earth-might-have-hairy-dark...

What do you think of the possibility of 'dark sector' interactions?[1]

The idea that dark matter might consist of a class of self-interacting particles, and that we might be embedded in a universe full of hidden phenomena as rich as the ordinary-matter phenomena that are visible to us (e.g. dark 'planets', dark 'stars', dark 'galaxies', or something very different) was always intriguing to me, but it seems that a consensus is emerging, based on observations of large-scale distribution, that dark matter is dominated by a single type of particle incapable of self-interaction.

Is it still possible that some fraction of the dark matter in the universe is self-interacting, capable of 'clumping' and exhibiting physics similar to ordinary condensed matter, or are all the indications now pointing strongly towards a single non self-interacting particle?

[1] https://en.wikipedia.org/wiki/Dark_photon

> What do you think of the possibility of 'dark sector' interactions?

It remains a possibility. It does not seem to be required by observation, though. Worse, if you move away from parsimonious non-interacting quantum field theories to more complex models, you have to suppress a lot of symmetries that inevitably produce observables which are not seen. Most people working with general relativity just use non-interacting scalar fields, but specific ideas about dark matter have to consider the ins and outs of gauge theory (e.g. does DM only feel gravitons and Higgs or does it also feel one or more of the other non-photon gauge bosons? if it feels the weak force, what goes on at electroweak scales? and so on...). The microscopic details of the microscopic alternatives within the broad family of QFT dark matter get hairy quickly, and there's very little astrophysical evidence to prefer one over the other (people favour axions or sterile neutrinos for reasons from within particle physics, and are looking for such things to complete their extensions to the standard model, they have to be very weakly interacting for particle-physics-in-laboratories reasons, but oh by the way as a side effect dark matter could be wholly or at least partially these proposed standard-model-problem-slaying particles).

> [what if we propose dark photons, dark atoms, etc.?]

One problem you run into is that if you can form composite dark particles analogous to atoms, or dark molecules, what prevents them from forming larger structures that collapse gravitationally? Likewise, if you can emit dark photons, you're removing momentum-energy from a particle in an orbit, and you would then expect the particle to fall into a closer orbit. Again, how do you prevent gravitational collapse? You might fix that by feeding back (squash DM together in galactic cores, release enormous "dark shine" dark-photon-analogues which then kick the massive DM particles into wider orbits, but it's like balancing a pencil on its tip; this is called DMAF (dark matter annihilation feedback), and is speculative. On the other hand baryon-flow feedback is a thing in solving e.g. the core/cusp density problem in particle dark matter, and that's a lot less speculative, because we know things like galactic jets are practically mandatory.

You're generally stuck with appealing to rareness, which is in conflict with Copernican principles which work really remarkably well in cosmology (and astrophysics too), or slowing down dark chemistry so much that it basically doesn't have to enter into equations anyway.

Carroll blogged about this a decade (!) ago (how to feel old: remember reading his cosmic variance blogpostings and making the discovery, pardon the pun, of how many years it's been since he stopped blogging there...) here : http://www.preposterousuniverse.com/blog/2008/10/29/dark-pho...

In astrophysics instead of using base-ten for enumerating interesting things in the sky, the counting system goes roughly: forbidden-everywhere, unique, mandatory-everywhere. If you introduce dark matter stars, you would expect there to be so many of them that you could not miss the Einstein lenses they generate. (Similar to MACHO hunting). Dark matter galaxies, being much more massive, would be even harder to miss. You will struggle to find a deep-field image that isn't filled with background galaxies (or clusters) lensing even more background ones. If there are dark galaxies, surely they would be in the foreground of some of the visible galaxies -- otherwise what prevents that?

Finally, we do have some gravitational structuring of dark matter; the standard description of structure formation requires it, and it's hard to get the late-time structures we see without dark matter filaments.

There's undoubtedly some meat on the Structure Formation wikipedia page and the things it links to, but a briefer and simpler overview is in the last three paragraphs at http://sci.esa.int/planck/51560-the-history-of-structure-for...

This is exactly the comprehensive reply I was hoping for, thanks. I dug up a Carroll post on the arrow of time and the big bang for this thread which turned out to be from 2004, so I know the feeling.

If there are any other non-experts like me this far down this reply chain who are interested in dark sector speculation, in addition to raattgift's excellent links I'd recommend the Wikipedia page on the Lightest Supersymmetric Particle [1] and Rob Reid's recent podcast with dark matter researcher Priya Natarajan [2].

[1] https://en.wikipedia.org/wiki/Lightest_supersymmetric_partic... [2] https://after-on.com/episodes-31-60/036

Dark matter is a supersolid that fills 'empty' space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.

The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.

Displaced supersolid dark matter is curved spacetime

Yeah, we get it. You believe in the ether. Perhaps you would prefer to call it "the firmament." The only people who believe this are certain fringe (read that pseudoscience) speculators who will never be taken seriously because they ignore the actual data, and have only a rudimentary understanding of the rigorously verified physics involved.

Robert B. Laughlin, Nobel Laureate in Physics, endowed chair in physics, Stanford University, had this to say: https://en.wikipedia.org/wiki/Aether_theories#Quantum_vacuum

> "the empty vacuum of space … is filled with 'stuff' ... The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether."

Laughlin’s ‘stuff’ is the smoothly distributed, strongly interacting, supersolid dark matter that fills ‘empty’ space and is displaced by ordinary matter.

Einstein: Ether and Relativity http://www-history.mcs.st-and.ac.uk/Extras/Einstein_ether.ht...

> "According to the general theory of relativity space without ether is unthinkable"

Einstein’s ether is the supersolid dark matter that fills ‘empty’ space and is displaced by ordinary matter.

And… you make my point for me. Thanks. Tell me, does your “supersolid dark mater” also rotate once every 24 hours?

Nobel laureates are "fringe (read that pseudoscience) speculators"?

No. You proved my point by demonstrating that you have no idea what they are talking about.

You could educate yourself. Go study quantum mechanics and the Heisenberg uncertainty principle. Learn about particle fields, vacuum fluctuations, virtual particle pairs, and the Casimir effect. Find out what this "boiling sea of vacuum energy" actually is, and why one might call it ether, and in what sense that would be true.

It is the chaotic nature of the supersolid dark matter which causes the Casimir effect.


> What else has approximately uniform local density


Cool-phase neutral atomic gas in interstellar settings.

Cool-phase neutral molecular gas ditto.

What breaks the uniformity of the latter two is mainly electromagnetic interactions. UV or X-rays will ionize them, and the freed electrons will cause secondary ionizations. This is the main pathway for heating neutral interstellar gases. Subsequent cooling is by photon emission. This drives dust-grain-forming chemistry; these grains are more dense than gas, and so have different gravitational observables as well as different emission/absorption characteristics. Very roughly, the dust grains can collide and stick to one another chemically, leading to further density non-uniformities.

Cold dark matter doesn't feel UV or X-Rays or it wouldn't be dark, and if it is collisionless (as in the standard model of cosmology) then there is no dark chemistry that can locally densify dark matter.

> What else has approximately uniform local density of distribution?

Really, anything that approaches an ideal classical gas. "Local" is an important qualifier.

A cubic centimetre of taken from near the middle of a small jar of water inside your household refrigerator or a cm^3 of gas taken from near the middle of a helium-filled balloon.

The middle, because the density differs at the boundary of the material in the container. Likewise, the density changes sharply at the edges dark matter clouds, but is fairly uniform in large volumes far from the boundary.

The overdensity in the jar's contents compared to the contents of the fridge overall and the underdensity of helium in a balloon in a room at sea level compared to the whole room are very roughly analogous to overensities and underdensities of dark matter at scales much larger than solar systems.

Great questions !

> How much [dark matter] [is] in the solar system ?

A lot, because the solar system is a huge volume and dark-matter fills it fairly uniformly (there is a small overdensity inside the sun, and the planets will also cause small departures from essential uniformity).

However, it's extremely sparse, so there isn't much in any small fraction of the solar system.

Compare that with the planets: they are extremely dense, but do not fill more than the tiniest fraction of the whole volume of the solar system. However, even so, even Phobos and Deimos have much more mass than all the dark matter inside Mars's orbit (see a couple paragraphs below).

~ one third of a million proton-masses for every cubic metre

~ 6 * 10^-22 kg for every cubic metre (Earth's density is 524 kg/m^3)

~ 0.65 kg / earth volume


Of course the total mass scales with volume.

Inside Neptune's orbit there is about 10^17 kg of dark matter; that's about ten Phobos-masses, or about the mass of 253 Mathilde (a carbonaceous intermediate-belt asteroid).

The volume of the galaxy is enormous, and with dark mater filling all of it roughly uniformly, the mass of all the visible matter is dwarfed -- there is an awful lot of space between star systems.

> somehow observable

It's not moving anywhere close to relativistically compared to the Earth's surface, and it doesn't feel electromagnetism. If we compare two other neutral particles, we have no practical ability to detect non-relativistic neutrinos (we can only spot a microscopic fraction of relativistic neutrinos from known sources) and have trouble spotting thermal neutrons (again, we generally need a known source that is "loud" with them, and additionally the collision momenta will still be larger than most collisions with solar system dark matter -- neutrons spit out of nuclear reactions are much faster than Earth's orbital motion through the extremely sparse dark matter the inner solar system sweeps through, and neutron beams used experimentally are generally a lot denser than ~ 3 neutron-masses per cubic centimetre).

Not within the solar system, but within the stellar neighborhood. If you construct a voronoi volume around the sun, then very roughly you'd expect 5 solar masses of dark matter in that volume. But that is a HUGE volume, on the order of tens of thousands of AU on a side. In the area we can directly observe (via orbits of bodies we can see), the fraction of dark matter comes out to like one part in a trillion or thereabouts.

Dark matter’s interaction with “regular” matter via gravity is covered in the article. It also interacts with light via gravity.

Measuring the interactions between galaxies, and things like gravitational lensing are providing evidence of dark matter’s existence.

We've already detected on type of dark matter: neutrinos. A single neutrino can travel through a light-year of solid lead (if such a thing existed) and only have a 50% chance of interacting. Every day something like 10^20 neutrinos pass through your body (most from the Sun) without doing a thing. So weakly interacting particles are not completely unknown, we already have examples of them. Dark matter is just some particle we haven't discovered yet (again unsurprising because we know our theory of particle physics is incomplete) which is even less interacting and possibly more massive than neutrinos.

We can detect dark matter via its mass, that's why we know it exists. We can see it speeding up galactic rotations and acting to gravitationally lens more distant galaxies and so forth. But we have yet to detect it on the small scale, especially at the individual particle level.

Keep in mind that one of the key differences between dark matter and atomic matter within a galaxy, for example, is that dark matter has a nearly uniform density over very large volumes (many light years across) whereas atomic matter has enormous density variations, with huge expanses of near vacuum punctuated by ultra dense stars, planets, neutron stars, etc. Within our own Solar System the amount of dark matter inside a spherical volume that would extend out to Neptune's orbit is only as much as a comparatively small asteroid. As you scale out to larger and larger scales the fact that the density of dark matter is relentless causes the mass enclosed inside a volume to start to match (and ultimately exceed at the largest scales) the mass of stars, nebulae, planets, etc.

You need to think about what types of interactions we normally perceive-they're all dependant on the electromagnetic force. DM does not appear to absorb or emit light, so it stands to reason that it also won't repel electromagnetically (ie touch things.)

As I understand it, gravity is the interaction between things with mass, but it's a weak interaction, thus hard to detect on a particle by particle basis. You might be able to detect if there's an extra potato in a bag because a potato is a whole bunch of particles. But identifying that just a few of those particles are not the regular stuff -- protons, electrons, etc. -- by weighing the bag, would be difficult.

"Think of the amount of mass required to generate a gravitational pressure needed to overcome the electromagnetic binding force between molecules inside the mass--the equilibrium occurs, basically, when an object in space becomes spherical. This happens at about 10^20 - 10^21kg. Divided by the mass of a proton implies you need about 10^47 atoms to generate the amount of gravitational pressure to break the electromagnetic strength between atoms." (1)

Richard Feynman would tell you that this immense difference is "what's keeping you from not falling through the floor down to the Earth center." Or why the apple hanging on the "few atoms" of its stalk is not falling from the tree, when the summary gravitation of all atoms of the whole Earth are pulling it down.

Or, again to compare such big numbers, there are "only" 10^86 atoms in the whole Universe observable to us! (2)

1) https://www.physicsforums.com/threads/how-are-the-gravitatio...

2) https://www.universetoday.com/36302/atoms-in-the-universe/

But you do not need to measure it particle by particle. Just because those have mass, there should be a bunch of dark matter particles hanging around with earth. And given that we have quite good idea what earth consists of, there should be a discrepancy in some of the measurements that use earth's mass against the mass we have from our understanding of earth's composition. Unless, of course, the extra mass of earth due to dark matter is calculated in e.g. kilograms. That's why I would like to know the expected density of the dark matter.

Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all! No "clumping" even around e.g. Sun can be observed. The "hanging around" is on the level of the whole galaxies, and sometimes the dark matter even remains outside of the whole galaxies, being too slow to follow their gravitational interaction(!) That's the famous example of the "bullet cluster":


That's how weakly the dark matter interacts with anything else. And it obviously doesn't even interact strong enough to "fall" to the center of the galaxy. Otherwise it would be there, but it remains to the outside of even where the "normal" matter is (mostly the stars and black holes, the "supermassive black hole" in the center is only at most 1e-5 of the estimated total mass of our Galaxy).

Or more precise:


There’s “just a little more” of the dark matter in our Solar system but way below the levels it could affect the measurements we can perform.

> Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all!

If it interacts so weakly I don't understand why clumps around anything. In fact if it only interacted via gravity I would expect it to start slowly accelerating toward normal matter, spend a short time near it, then speed away as it slowed down again. If the path is an eclipse this would mean it spends most of its time away from the matter rather than near. It would be almost as if gravity caused normal mater to repel dark matter.

> Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all!

Would I be right in thinking that also puts severe limits on how much it interacts with itself? Because my intuition would be, if you loose normal matter into a gravity well, it will clump, even if it doesn't interact with the source of the gravity.

Am I inferring correctly?

> Because my intuition would be, if you loose normal matter into a gravity well, it will clump, even if it doesn't interact with the source of the gravity. Am I inferring correctly?

Now I have a little of Newton for you: look at our Solar system: you see the planets, and even more interesting, all the small asteroids circling around the Sun? Can you answer why don't they all fall to the Sun, but move in the orbits?

The way the gravitation works was not "intuitive" before Newton, 300 years ago, and now it's obviously still so for many non-professional readers.

What's actually happening, according to the dark matter model, and the dark matter actually more easily fits much more of our cosmological observations than anything else, is that there is a lot of dark matter but it is simply much more "spread" around the volume of the galaxies. And just like all the visible stuff of the whole galaxy doesn't fall to the galaxy center (like the planets don't fall to the Sun!), the dark matter remains "around" the galaxies, where more of dark matter is "outside" (as in "in the outer regions of it") than in the "inside" of the galaxy (and in the case of the "Bullet Cluster", that I've mentioned in some other comment, dark matter is obviously lagging all the movement of non-dark matter! (1)). That dark matter that is in the inside of the galaxies actually initially "clumped" somewhat, but that "somewhat" is, according to our estimates, significantly below what we are able to measure, when we're interested in the gravitational effect on the Solar system.

1) https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bul... and https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0...

> Now I have a little of Newton for you

Sweet of you to bring me Newton. Always a welcome gift.

But I think you misunderstand where my question is pitched.

The solar system is rather clumped, you see. A little Aristotle for you. :)

In all seriousness, the more dark matter is around, the less it can have mutual interactions, before it would clump, surely? Assuming such forces exist, there is a nonzero probability that two particles of dark matter will approach close enough that non-gravity forces will be significant, And they will no longer act under an ideal Newtonian gravity. Dust clouds coalesce into suns, given time. Can dark matter have its own dark-only version of the electromagnetic force? Or is that ruled out by the lack of clumping? Or is there just too little of it to make a conclusion on that? My question was entirely consistent with ol' Issac.

> Can dark matter have its own dark-only version of the electromagnetic force? Or is that ruled out by the lack of clumping?

My "feeling" is, it's not "intuitive enough" to "guess" any answer without a lot of computing:


We think we're quite sure in our observations, so that helps, but to be able to claim how the simple laws can exactly produce what we see, we have to do a lot of work. Just like what Newton figured out was not provable before without all the computations:


Indeed the mass of dark matter is why we think there is dark matter -- discrepancies such as you mention, but evident from astronomical observations. The problem for characterizing the particles is: Mass and what else? The what else is the thing people are trying to detect.

We don't have that good idea of what the earth consists of. I think much of those theories are based on counting backwards from knowing the mass, and if we have 5% dark mass orbiting Earth, it wouldn't make any material difference.

They are not distributed evenly throughout space. So if you measure more gravity in a region of space that you cannot see (because dark matter doesn’t interact with em), is it dark matter, or an error in your theory of gravity?

As I understood it (I'm not a physicist so probably wrongly) dark matter is the proposed answer to reconcile General Relativity Theory (which is the current gravity theory) with actual observations of the universe.

So there is two possibilities:

- There is no dark matter and the general relativity is wrong.

- There is dark matter and the general relativity is correct.

And maybe a combination of both: there is something we don't detect but the theory is also wrong.

It's not just General Relativity -- if there is no dark matter, the very Newtonian gravitational force having an 1/distance² relationship would need to be wrong on large scales.

However, no attempt at formulating a theory that propose a different long-range gravity than 1/distance² (and still reduces to the known behavior in the short range) has held up to observations.

(And with "short" I mean "on the scales inside our solar system", not meters).

Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on the inside of this sphere, and then orient the sphere so that we are at the bottom (like a penny inside of an inflated balloon). Now imagine (don’t believe, just imagine) that our universe is expanding at a constant rate. As we approach the equator of this sphere, the area that we must cover grows larger, but once we pass the equator it begins to grow smaller. And so what might appear to us as slowing down and speeding up could just be the shape of space changing, and not the speed of expansion. Again, just food for thought.

Minor correction: The accelerating expansion of the universe implies the existence of dark energy, not dark matter.

This is not a minor correction simply because they both contain the word "dark". It would have been slightly more correct to confuse the terms dark matter and dark chocolate. Dark energy is an entirely different kettle of fish.

Or maybe it's nothing at all ;)

Correction noted and accepted. Thanks. In this particular thought experiment, dark energy (not dark matter as originally stated) has been replaced by the shape of space, and due to our inability to directly detect such extra-dimensional curvature we have defined this energy as a placeholder (which would not exist if this experiment proved to be true).

I used to be a bit interested in astrophysics, so I can't vouch 100% for this being accurate, but it's my understanding:

The universe isn't the penny, it's the balloon. Physicists believe we are living on the surface of a hypersphere. One important consequence of this idea is that the big bang didn't occur at a specific point in our 3D space, but at the center of the sphere.

Furthermore, the concept of a balloon expanding vs. deflating is a bit of a misconception. The argument used to be that whether or not the balloon is expanding, depending on the rate of the expansion, gravity could eventually win out and cause the matter to collapse back together (big crunch scenario). The problem with that theory is that we now know that galaxies are speeding away from us at a growing speed that (not sure the exact details, probably based on red shift in light from nearby galaxies). So the idea of gravity winning out was not based on evidence, just one of a number of possibilities, but the evidence proved it wrong beyond a doubt.

The analogy of an inflating balloon is a useful one because, as in the universe, an observer at any given position on the surface of the balloon sees all other points receding from them. This leads to the illusion that any observer's position is the 'center' of the expansion, but there is actually no center on the surface of the balloon, just as there is no unique origin point for the expansion of space in the universe.

The analogy isn't perfect though. I don't think it's quite right that cosmologists believe we are living on the surface of an expanding hypersphere; that would imply that the expansion of the universe had a real spatial center somewhere in a large extra dimension, just as the inflating surface of a balloon has a real spatial center in the balloon's three dimensional interior, inaccessible to observers that can only probe the surface.

That the universe has a real, albeit extra-dimensional spatial center isn't a mainstream idea, but there are theorists exploring the possibility that the universe exists on the surface of a brane in a higher dimensional 'bulk', and that the big bang resulted from a collision between branes in that higher dimensional space [1].

There might be some utility in thinking of the universe as an inflating hypersphere whose radius corresponds to time, rather than to an additional spatial dimension. In that analogy, the center of the hypersphere (or balloon) would represent a point in time, rather than a point in higher-dimensional space. The surface of the hypersphere (corresponding to the space of our universe) would appear to expand the further an observer was from the temporal 'center', which would be equivalent to the big bang. There is a consensus among cosmologists that the big bang appears to be a special point in time, if not in space.

[1] https://en.wikipedia.org/wiki/Brane_cosmology

Wouldn’t that be not the best fit you can imagine with the holographic principle?

Think of 4 dimensional spacetime as an oddly-shaped rectangle, which is much wider at one end than another. Maybe more like a pyramid with the top bit missing. All of matter and energy started as an explosion at the narrow end, and is traveling across towards the much broader end. The expansion of the universe is merely the expansion of the barrel, and the dark energy spreading apart the universe is really the slight trajectory differences caused by the shape of the charge that started the process.

To me, it's like 3d canon shot flying down a 4d barrel.

Or trace the worldlines of these particles. Any given point in time represents a 3d slice of the 4d pyramid-rectangle.

That’s a really awesome thought experiment! I got a related idea during undergrad. What if the apparent symmetry between the three special and one temporal dimensions was at one point complete? Ie time could at one point be swapped out with any of the other three dimensions. But somehow that symmetry collapsed- and maybe it’s collapse was the result or creation of mass-energy. I imagine a hypercube being “pulled down” into a confined region. But the confinement “pushed” the two directions of time into the one direction and mass-energy.

Interesting ideas.

Wouldn't this result in relative acceleration which correlates to the angle of the body being observed relative to the viewer? Anything aligbed directly between the viewer and a pole would move at a constant speed while speeds would appear to increase as you approach perpendicular? I can't imagine how this would play out in a 3d projection of a 4d surface however.

Observations of "cosmic inflation" are not uniformly distributed, so this could very well be true.

Is that the current consensus? Do you happen to have sources for that?

This strikes me a lot like “flatland”, ie. there’s something there but we’re not equipped to perceive it. Our experiments are mostly like extreme extensions of our own senses - seeing and touching things. But could something exist entirely within a dimensional space that we can’t touch or see? And if so how would we ever figure out how to detect it ... and even more meta, what if it was impossible for us to perceive the detection?

It’s all really fascinating.

The problem is that gravity, like EM, follows an inverse-square rule. If gravity had a 4th dimensional component, we would expect to see an inverse-cube situation. The equation is: 1/(1-dimensions)

Youtube video that explains this in depth: https://www.youtube.com/watch?v=3HYw6vPR9qU&t=726s

Re: EM, the Biot-Savart force law can result in long-range attractive forces proportional to d^-1 instead of d^-2. So if it’s operative at galactic and intergalactic scales (between plasma filaments) and we’re not fully taking it into account, then we may be discounting the most powerful long-range force in the universe.

Well, it has a gravitational effect, so it definitely exists inside of our normal spacetime.

Scientists have explored alternative models of gravity or spacetime with additional spatial dimensions and things like that. As far as I know, none of them are very promising. There was an interesting PBS space time video about one example recently: https://youtu.be/3HYw6vPR9qU

Of course, it is definitely interesting to think about :)

Interesting, I didn’t know that!

I guess for me the analogy is to the sphere in flatland being perceived as a circle - we can see evidence of dark matter but so far we can’t really detect or measure it directly. I wonder if the gravitational effect is the only clue that we’ll find inside “normal” spacetime.

The most exciting thing about that is the idea that the hunt for dark matter might eventually lead to an understanding of the “rest of the universe” where we would otherwise never have a hint of its existence.

Nice article, especially showing how much experimental data about dark exists. Dark matter isn't just a crazy concept to "fix" gravity. As nicely described in the article, there are several experiments clearly showing some invisible matter. We know a lot about it, except which unknown particle causes it.

Really well written article, especially as someone who is interested in the topic but not an expert.

As I was walking the dog last night I was thinking about SEO and how even the "experts" don't really know Google's secret sauce or how the recipe will change. There are best-practices that seem to work but with Google's algo hidden and changing it seems funny to be an expert. But certainly they know more than their clients. That led my thoughts to dark matter.

Expert: I am an expert on Dark Matter. Me: What is it? Expert: I have no idea but I think it exists. Me: Ok, you must be really smart.

My glib little dialog contains no sarcasm. No doubt they are very smart. Expertise is measured differently in different fields.

Expert on Oak Island knows all the rumors and theories but not where the treasure is. Expert flat-earther knows all the wrong facts. Expert politician might know .0001% due to the vastness of government. Experts on religion know the experts in other religions are wrong. Expert MLB hitters fail more than succeed.

I guess it gives me hope I may one day be an expert at something.

I think to qualify as an expert on dark matter you should at least be familiar with General Relativity, the standard model of cosmology, gauge theory (and in particular extensions of the standard model of particle physics), and have a good overview of the literature on dark matter experiments. Add in a novel contribution to the literature, and you have done the bulk of the work required to gain a Ph.D.

It is very difficult for a poser to fool actual experts. It is a lot easier for a poser to fool non-experts. Also it's often more lucrative.

Experts may disagree with one another without either party risking be considered non-expert by her or his peers.

Experts, also, having a decent overview of the literature (which includes observational and experimental results) will generally have a decent understanding of what they don't know. It is very rare for a poser to admit what he or she does not know. It's also fairly common for non-experts, who are not experts in some other discipline, to have no idea the amount of knowledge experts have been exposed to during the course of developing their expertise.

> No doubt they are very smart

Really, it's more that actual experts are excpetionally well-read in their narrow little disciplines, and along the way have lots of practice writing as well. The key to expertise in theoretical physics is actually reading theory (and the results of tests thereof). A lot. Writing enough will probably result in a dissertation. And, perhaps, getting a paper published, writing a book or a chapter of a textbook, and so on.

The other thing you have right is that experts can be wrong. But being wrong usually offers up an opportunity for more writing that other experts are likely to get around to reading. So being wrong can increase expertise, as one learns from others' mistakes and how they were caught, and as one thinks about how to avoid similar mistakes in the future (which offers up another opportunity for writing... and so on and so forth).

> I guess it gives me hope I may one day be an expert at something.

Noone becomes an expert without effort.

I think the definition of "expert" can be pretty ambiguous. We have one type of "expert", the MLB hitter who is an expert because he is more skilled than others at hitting. And we have another type of "expert" who is an expert because she knows more about something than others. These are two very different types of "experts" but what makes them experts is their skill/knowledge relative to the general populance rather than skill/knowledge relative to some absolute.

Even the best batter in the league goes to batting practice, studies what other good batters do, takes advice from coaches, some will likely study sports science literature themselves (rather than relying on coaches doing that for them). Batters will always want to think of ways to improve their mechanics, and will always want an "edge" over pitchers, and pitchers vary in their techniques.

The great hitters from decades ago did not have access to video footage of themselves, their rivals, and opposing pitchers, for example. So while some of them were exceptionally skilled at hitting baseballs (and not just home runs) (and also running bases and being competent in field positions usually) they were not really experts for want of a body of rigorous literature.

Conversely, one can of course have sub-major-league skills but enormous expertise -- there are batting coaches and sports science academics after all, and even popular analysts. And sports skills decay with age.

Nobel-prize-winning scientists can get senile dementia too; sadly that not just wrecks their skills, it also wrecks their expertise as they forget much of what they've read and studied.

I'm guessing that "guessing in the dark" is the way experiments for particles in the standard model have been done for a while now. I would assume that a single success would set off a chain reaction as the author said.

I'd say it's not completely in the dark. The experiments try to test theories that suggest where to look. For instance if a theory predicts a previously un-discovered particle, with some rules for how it interacts, then you can refine your search based on those rules -- what range of masses you're looking for, what decay patterns, and so forth.

The simplest “proof” of dark matter - Kepler’s Laws dictate that objects with smaller orbits (closer to center) move faster (in terms o fangular velocity). Our solar system works like that. Our galaxy doesn’t - its spiral shape indicates that the angular velocity varies minimally with radius.

The spiral structure isn't fixed to specific matter, it's a wave moving "through" the galaxy. But broadly yes: you can look at orbital velocities of galactic matter directly (via doppler shifts of their spectra) and make a map of the mass distribution in the galaxy. And then you can add up all the mass we can see as stars, and they don't remotely add up.

This carries the implicit assumption that all forces fall off monotonically with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor impact crater: raised in the middle, a deep valley, and raised at the outer edge?

Of course, with more evidence, such as the observations of the Bullet cluster, these simple explanations fall apart. But at any rate, it's not enough to take one observation and assume it holds at all scales.

Look at tiny water droplets. They don't behave at all like large bodies of water.

Inverse square makes sense because you can model it as a finite number of particles with decreasing density as they move away from the source, like shotgun pellets. MND is like if more pellets just sort of appeared out of thin air at a certain distance... It would open up all sorts of different questions, starting with: how do those particles know when and where to spawn more particles? Where does the energy come from (or where does it hide until it decays out in the form of more gravitons?)

Gravity behaving like that would be a lot weirder than there simply being some types of mass we aren't able to detect yet. Physics has had a long history of predicting particles that hadn't been detected yet, and then actually going out and finding them. See neutrinos, the Higgs boson, etc.

General relativity being wrong would be waaaay weirder.

>>General relativity being wrong would be waaaay weirder.

If I'm not wrong, Newton's laws can still be held to be true in some cases even inside GR?

Some where in the past, some one could come up with a new model which could show us new truths, and yet could contain GR to be true in some cases.

In GEB, Hofstadter show us part of the ways so see intelligence emerging from noise and garbage, is to understand when you design a system, you start with axioms, truths and rules that are true in a system, could be total garbage in another.

Models are what we use to understand things, That's not how things are.

Yes, you are correct. This is actually expected to happen at some point, as GR and quantun field theory are incredibly well tested and yet haven't successfully been combined into one theory.

However, when this happens, it will be one of the greatest physics breakthroughs of all time.

Because of this, any scientist needs very strong, compelling reasons for trying understand their results in a way that holds GR as somehow broken. The vast majority of of experiments don't have good justification for not treating GR as a given truth.

> General relativity being wrong would be waaaay weirder.

It only got empirical confirmation recently, it should still be open to disconfirmation.

General relativity is incredibly well tested. Showing that general relativity is wrong would be the physics discovery of the century.


That's a common urban legend. General relativity has been tested and confirmed many, many, many times, dating back to soon after it was first proposed.

> This carries the implicit assumption that all forces fall off monotonically with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor impact crater: raised in the middle, a deep valley, and raised at the outer edge?

I always wonder about the interplay between a gravity well and dark energy specifically at the galactic boundary. If dark energy is in a sense "anti-gravity", would it not be stronger outside of a gravity well and weaker within? Would there be an interference pattern ( wave or valleys ) between dark-energy and gravity? Isn't a rotated wave just a spiral? Could the difference in rotational velocity at the edge be dark energy "pushing" the matter away to make up for gravity's weakness at the boundary?

In the standard cosmology, dark energy is \Lambda, the cosmological constant.

The cosmological constant is taken to be a geometrical phenomenon rather than some dynamical field.

We can within the limits of observational accuracy use a "swiss-cheese" cosmology model. We start with a Friedmann-Lemaître-Walker-Robertson (FLRW) background, which is an expanding spacetime with a uniform dust that dilutes away uniformly with expansion. The expanding spacetime's metric is Robertson-Walker, which is an exact solution of the Einstein Field Equations (EFEs) of General Relativity. The dust particles are galaxy clusters, which are gravitationally bound, and are manifestly not expanding in the same way, so they cannot have the same metric. Around each particle, we cut out a "hole" in the background and replace it with a collapsing spacetime metric, like Schwarzschild or Lemaître-Tolman, which are two other exact solutions of the EFEs.

We can use Israel junctions to stitch together a pair of metrics like Robertson-Walker and Lemaître-Tolman, and while it's annoying procedurally, it produces good results. Indeed, a simpler case is the Einstein-Strauss swiss cheese, which served as a practical cosmological model until the late 1980s, when the evidence began piling up for the presence of a small positive cosmological constant.

In swiss cheese models, the cosmological constant vanishes in the inner metric (the "holes") and is only non-zero in the outer metric in which the holes are embedded. Since dark energy is simply the representation of the cosmological constant given a particular slicing of the universe into things-in-space+time rather than spacetime-filling fields, this means that mathematically there is no dark energy in galaxy clusters and other gravitationally collapsing "holes" in the otherwise expanding universe.

This seems shocking ("why isn't there dark energy everywhere?" seems to demand a mechanism rather than just a statement of geometry) but it's testable, and so far there is no evidence for the metric expansion of space within the solar system, or within galaxy clusters.

If we find out that space does expand near and in galaxies, then we have a variety of ways to go beyond the swiss-cheese approach, and we might explore a couple of them anyway since we now have powerful computers and do not have to lean on exact or analytical solutions of the Einstein Field Equations. This is the research field of "inhomogeneous cosmology".

Alternatively, we can stop making holes and instead treat the cosmological constant as a (location-dependent) dynamical field that is weaker near matter except in the early universe (when matter is all squashed close together). Various proposals along those lines like "quintessence" ("quint" because such a field automatically produces a fifth force) have been written about. Evidence strongly constrains a fifth fundamental force of nature, however, so this approach seems much more speculative than either simply accepting that the cosmological constant is geometry and the universe is swiss-chese-like geometrically, or pursuing a much more complicated "real" metric rather than starting with a simple, exact, analytical metric and perturbing it where that's important (e.g. because of how the matter in large structures might be laid out in ways that are hard to be considered pointlike or axisymmetric when viewed from large distances).

> gravity well

Not a useful concept and definitely not an object in General Relativity.

> anti-gravity

Kinda, but it's important to understand what that means. When gravitation squashes matter into a denser shape you can think of it as creating pressure on and in the matter. The metric expansion of space is not preventing the gravitational collapse of galaxy clusters: they're still shrinking and the pressure inside them is still increasing. Galaxy clusters are essentially bubbles floating in a sea that is getting larger around them. By contrast, the pressure in the "sea" is reducing over time, proportional to the value of the cosmological constant. Since we can treat pressure as a component of the stress-energy tensor, the "matter" or "sources" side of the EFEs, we can treat increasing pressure inside collapsing stars as a source of gravitation (an IMPORTANT source when stars collapse into white dwars, neutron stars or black holes -- pressure dominates the other masses and energies as a gravitational source in those cases). Likewise the increasingly negative pressure in the regions outside galaxies source can be treated as a source of gravitation, but really this is just a special way of looking at the fact that galaxy clusters are separating from one another without any motion-distortion (shear, for example) being evident in our images of the galaxy clusters at increasing distances. There is obvious shear within collapsing galaxy clusters and their internal components -- galaxies and objects near the centres of clusters are more stretched radially than those further from the centres of clusters, and the shearing strength depends on the overall mass of the cluster. There is no mass-dependency on cosmological redshift from receding galaxy clusters; individual galaxy clusters are not stretched towards us at different redshifts.

> interference pattern between dark-energy and gravity

Well, between dark-energy and collapsing matter, yes. The closest concept to your idea of an interference pattern is the presence of holes in the swiss cheese. If superclusters are gravitationally bound and form long filament structures that collapse collectively (rather than there being a line-up of individually roughly-spherically-collapsing galaxy clusters, with the individual clusters not moving towards each other over time) then the geometry would not be quite so swiss-cheese like, or at least not everywhere.

You are free to do handstands and other contortions to describe this in terms of waves-and-interference. You'd probably use perturbation theory, where you throw away the holes and complicate the background metric or the matter fields. When you do that you're engaging in the study of inhomogeneous cosmology or quintessence-like dynamical dark energy. Those are decent search-engine terms if you want to do a quick survey of those fields of research. They aren't popular because the standard cosmology with swiss cheese matches observations to extremely high precision while being much easier to work with than the other two approaches.

> ... at the galactic boundary ...

The Israel junction is described in Chapter 21 of one of the standard textbooks, _Gravitation_ by Misner, Thorne & Wheeler ("MTW"). In brief, there is an infinitesimally thin shell drawn as a boundary around the collapsing spacetime arranged carefully so that an internal time coordinate matches the external time coordinate, which is the scale factor (or lookback time) in the standard model of cosmology. There's a mathematical matching of values of the electromagnetic fields and other fields of the standard model of particle physics on either side of that thin shell.

In reality, the boundary around real galaxy clusters is not that sharp; the edge is a fuzzy end to the sparse gas and dust one finds at the outer limits of galaxy clusters' gravitational influence, so it ends kinda like Earth's atmosphere. It's mostly gone at 100km up, but not enough that satellites and spacecraft much higher don't have to deal with tiny drag from stray molecules and atoms. But in practice, above 100km the residue of atmosphere doesn't enter into equations, and in practice far from the centres of galaxy clusters the residue of gas doesn't enter into equations either.

But if you had much much much more powerful computers and software than we have today, you could in principle do numerical relativity that accounts for all that, and would be "fuzzing out" the Israel junction procedure most likely. (Or, again, you could go right in and study and account for inhomogeneities right down to photons travelling between galaxy clusters, wheeeee! But where do you cut it off? A stroke of lightning on Earth around eight million years ago flashed some light in a direction that caused a tiny fraction of the flash to exit our galaxy cluster. Should we subtract that out from our galaxy cluster's position in the expanding universe? We're also near edge than the centre of our cluster, so the direction of the flash is relevant to how much of it exited, and when it exited. And so on and so on and so on. At some point, the light in question is still contributing to the gravitational collapse of our galaxy cluster; at another point, it's removed some of the galaxy cluster's stress energy from the region in which everything in it is gravitationally bound. We just choose an arbitrary point and say "there's the crossing-over". (We also would ignore the flash because it is such a tiny fraction of the total stress-energy of the galaxy cluster).

As with almost all physical models at some point you have to say "I can only be so precise in modelling and in matching the model to nature" and hope that precision keeps improving over time.

It's not just galaxies it's also the interactions between galaxies and while you can tweak gravity to handle galxey rotations it doesn't work for those bigger structures, though dark matter does work for both.

Since nobody has linked it yet - there is an xkcd about literally this argument: https://xkcd.com/1758/

Why are there so many people who trot out their theories that they've put about 30 seconds worth of thinking into and are based on the assumption that professional scientists fundamentally don't know what they are doing and are completely clueless?

You need more than this. The alternative is modified gravity, meaning gravity rules are different at different scales. Most people arguing against dark matter use that one example as their battle ground, aha, I can get the same result with just a tweak to Newton's laws.

It's when you add in all the other problems that a little tweak to Newton's won't be enough.

This may be a dumb question, but could dark matter just be regular matter, like planetoids? What if interstellar space had a light sprinkling of things like Oumuamua?

On an interstellar scale, maybe this could create the lensing effects and explain some other phenomena. Is it unreasonable that 5/6 of the mass of the universe is like the stuff that makes up planets, but isn't lit up as brightly as a star?

Not a dumb question, but obviously they've thought of that:


This is the "MACHO" (massive compact halo objects) theory, which has been tested and disproven. The missing mass can't be gas or dust clouds because that would have certainly observational traces which we have looked for and don't exist. It can't be planetoids because that would cause a frequency of gravitational micro-lensing of stars in other galaxies that we do not observe.

This got me thinking about the difference between "direct" and "indirect" observation. What is the difference?

For example: in these experiments, what would "direct" observation be? We have instruments that detect changes in certain variables, and we look for changes that align with our expectations of how a particle affects these variables. But we're not directly observing the particle... we're observing the particle's effects on these variables. So there's always an intermediary between us and the phenomena we're attempting to explore.

It seems to me that this intermediary must exist for all phenomena that cannot be perceived by our 5 senses. So how do we determine when an intermediary is "direct" vs "indirect"?

Looks like it's time for me to revisit Philosophy of Science...

Indeed, modern science is based on lots of prior knowledge. I like to remember that we could still be very wrong about how everything works. That’s what the missing 5/6ths of unexplainable behavior tells us, for one. For two, I hope (personally) that we are wrong about many things. Our current theories lead to some very bleak realities.

But, ultimately, it’s important to not become skeptical to the point of tearing down progress. It’s one thing to keep an open mind and consider other options. It’s all too easy for bad actors to sell snake oil as a product of the unknown or unexplainable - or even just attack and destroy progress.

"The theory. This tells us that around every galaxy and cluster of galaxies, there should be an extremely large, diffuse halo of dark matter. This dark matter should have practically no “collisions” with normal matter — upper limits indicate that it would take light-years of solid lead for a dark matter particle to have a 50/50 shot of interacting just once — there should be plenty of dark matter particles passing undetected through Earth, me and you every second, and dark matter should also not collide or interact with itself, the way normal matter does."

I've always heard that the lack of interactions was an observation, not a deduction: we can't see it, therefore it doesn't interact.

When matter particles interact they release energy that we can detect in form of x-ray or light. When dark matter particles interacts with matter or dark matter, they don’t seem to release any energy. Their only giving away clue is gravitational effect. If dark matter does exist and can be directly detected then we are looking at very new physics.

Anyone trying to use machine learning to make a "black box" mathematical framework? Feeding known measurements as "learning" (including known anomalies and discrepancies)?

Using techniques such as deep learning for program synthesis. https://www.microsoft.com/en-us/research/blog/deep-learning-...

Yep, it's been done before by Cornell researchers, in 2009. You provide a bunch of data to a program and it hums for awhile and derives some equations from the observation. In the story written around the time, it took positions of a pendulum swing and derived laws of motion from it. See https://www.wired.com/2009/04/newtonai/ (and lots of other articles around the time.)

You could feed a spreadsheet into and then it would iterate for awhile, converging toward most accurate equations describing the system. Or in my case, diverging from what I was sure was the solution. It was a hit and miss with most data (honestly, an eyeball and a few braincells did better with some data sets), but all the same I used it to lazily approximate position equations for some programs I was writing.

The desktop program was called Eureqa which then got ported to the cloud for obvious capacity increases, and relabeled into a company/product called Nutonian. Most recently bought by "DataRobot" and it's now being sold for sales optimizations. Because science.

Can someone that knows a bit more about this explain why we believe that this dark matter substance must exist and not simply that we have an incorrect model of gravitation?

Because such a minimal change to the model ("there's just some more stuff we can't see") fixes a number of problems in one fell swoop (off the top of my head, galactic rotation curves, observations of gravitational lensing, a cosmological model compatible with obervations of CMB fluctuations, structure formation, cosmic evolution).

Theories of modified gravity can fix those individually, but as far as I'm aware, no alternative has been shown to be viable once taken in combination.

No one has been able to come up with a compelling alternative model of gravity. All the attempts so far have largely failed (their have been many), as the proposed theories break in one or more critical ways.

I think their may be a small number of languishing models that aren't totally disproven but don't look very promising. But I'm not sure.

In short: "because science", quite literally.

Astronomers are scientists, they don't make shit up just out of boredom, they only believe theories when they've withstood rigorous testing through observational evidence.

It's important to understand the history of dark matter / "missing matter". It's a multi-decade history that originally started out with a small amount of intriguing but seemingly persistent observational data that couldn't be explained easily (namely that when you measure how much galaxies weigh by looking at how fast the stars are orbiting that figure differs a lot from the weight you get by calculating up the contribution from all the stars and gas and dust and whatnot that we can see (the "light" matter)). In response a huge number of different "theories" (more properly hypothesis) were brought up to try to explain the mystery, while new ways of studying the problem were thought up as well.

And over the many years after the initial evidence came to light (in the '70s) considerably more evidence came to light from a wide diversity of observations. I won't list them here but I'll point out that the wikipedia page on dark matter has a good run down. Anyway, the fascinating twist to the story here is that as this evidence came to light it started eliminating various hypotheses about this missing matter until finally only one was left: the current theory of dark matter. It's not just a crazy idea, it's a crazy idea that fits all of the evidence when nothing else did.

So don't look at the WIMP dark matter theory as though it's just some half-baked idea, it's a hard fought veteran of numerous campaigns to kill it, but it just keeps going because by all the evidence it seems to be the only theory that explains reality.

As to specifically the "modified gravity" theories that compete with dark matter, they have a hard time explaining several observed phenomena, especially things like the famed "Bullet Cluster". There are a few examples where we can observe collisions between galaxy clusters and through different techniques we can map out the distribution of stars and of gas and of mass in the collision. What we observe in the Bullet Cluster as well as some others is that the gas is in a completely different place than the stars (because stars in galaxies mostly pass through one another whereas gas clouds squish together) and the center of mass of the stars of the galaxies is separated from the center of the non-visible mass of the galaxies. This absolutely cannot be explained by any of the modified gravity theories.

For a very simple reason that, you have to start explaining things from the concepts you already know are true.

The article describes the rationale, as does the "observational evidence" section of the Wikipedia article on dark matter[0].

tl;dr no alternative models for gravity can sufficiently explain all of the data, but general relativity and "dark matter being particles" does, so that's what we're going with.


« On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse galaxy (UDG) with the mass of the Milky Way galaxy, but with nearly no discernable stars or galactic structure, may be made almost entirely of dark matter.[6][7][8] »

Meta-source: https://en.m.wikipedia.org/wiki/Dark_galaxy

Isn't this a proof of the invisibility of dark matter ?

It's an observation that fits our theoretical models when dark matter is assumed.

It's indirect proof. The indirect proof is so overwhelming strong that any rational person will concluded with reasonable certainty that dark matter is real.

Directly detecting a dark matter particle will give us more data than "dark matter is real".

I learned a lot from this post. I didn't know that so many observations resulted in the same 5/6 ratio.

So my takeaway from this article is: where there is a concentration of mass, there is dark matter. (Quote: "According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers.")

Wiiild speculation: perhaps mass is actually a complex number with real and imaginary parts (analogous to how quantum mechanics describes fields), and what we're able to measure directly (by weighing) is the real part. The imaginary part ("dark matter") is some yet unknown interaction with gravitational force and what we measure indirectly with gravitational lensing is the complex magnitude.

> So my takeaway from this article is: where there is a concentration of mass, there is dark matter.

Not always: https://www.nature.com/articles/nature25767

I have no chops whatever in this field, but it's fun to think of experiments we might do:

If I understand current theory, dark matter only interacts with itself and with ordinary matter through gravity. Hence the importance of Vera Rubin's observations [1] that some galaxies were rotating too darned fast to hold together based on the ordinary matter we could see. Gotta be something invisible generating more gravitational force.

If so, couldn't we expect larger masses to attract more dark matter than lesser masses? And mightn't very high resolution measurements of those masses' gravitational forces disclose a discrepancy attributable to more dark matter clustering around a larger mass?

I think of constructing two spheres, one of lithium (density = 0.534 g/cm^3), the other of platinum (density = 21.45 g/cm^3). Both have equal diameters, and very different masses. Park them out in space - maybe in an orbit inclined 90° to the ecliptic so there's some time when they're far away from the complicating effects of planetary masses.

Then release test objects with accurately known masses (think the silicon spheres made for Gravity B Probe's gyroscopes [2]) near each of the two spheres. Minimum approach speeds would be given by the assumption of only ordinary matter in the spheres, no dark matter present. If there is dark matter, and if it accumulates according to gravitational interactions, the test mass approach speeds should be greater than calculated from ordinary matter gravitational force. Also, the larger mass should attract more dark matter and exhibit a greater deviation from ordinary matter force.

Depending on the local density of dark matter, one might see the results change over time with differential accumulation of dark matter around the two spheres.

Do we have the measurement capabilities to do something like this? I recall LIGO measures distances four orders of magnitude less than the width of a proton.

[1] https://en.wikipedia.org/wiki/Vera_Rubin [2] https://einstein.stanford.edu/TECH/technology1.html

Your experiment has several fundamental flaws. Firstly, dark matter is weakly interacting (even with itself) which is why it doesn't "clump" the way atomic matter does. The only "clumping" of dark matter happens on galactic scales, because the typical speeds of dark matter particles are in the range of orbital speeds around galaxies. Our measurements of the dark matter mass in galaxies don't have nearly enough precision to be able to determine the relationship you posit with much certainty.

As to your own experimental design, using large spheres of matter, it's completely non-workable. As mentioned above, dark matter particles have velocities in the range of orbital speeds around the galaxy, which is 100s of km/s. So most of the dark matter particles in the vicinity of Earth are going to simply pass through your spheres without stopping, and certainly without increasing their mass.

Additionally, you seem to be confused about the scales of densities here. Dark matter is distributed in a somewhat uniform density on interstellar scales (there are density gradients across the galaxy however). Near the Sun the density of dark matter is about 0.0025 solar masses per cubic light-year, or about 5e-20 kg per cubic meter. So, there really is not a lot of dark matter passing through objects around Earth, mass wise. There's only maybe 6 kg within the entire volume of the Earth at any given time, for example.

Thanks for your reply, it's informative and interesting. Helps me understand why detection of dark matter is a continuing challenge.

Despite recent some initial evidence to the contrary,[1] I'm still a fan of the idea that dark mater is actually concentrations of matter/energy outside of our universe.

If string theory is true, gravity does leak into the multiverse, and we survive the Great Filter, then someday we may be able to use gravity to communicate universes outside of our own.


Then why would dark matter be detected in galaxies but not in void space ? Why would those external world matter/energy be spatially syncrhonised at the place of galaxies ? Ockham Razor to the rescue.

I'm just a layman, but the parent said "if string theory is true, gravity does leak into the multiverse". Given that I'd imagine the clumpiness of our universe could influence the clumpiness of nearby universes (and vice versa).

Exactly. Not all galaxies have the same % of dark matter.In fact some have none.[1] Maybe those with little to none, formed by random fluctuations, while others formed because of the attraction to dark matter in their vicinity.


> And that’s okay! Unless dark matter happens to be of a certain mass with a certain interaction cross-section, none of the designed experiments are going to see it. That doesn’t mean dark matter isn’t real, it just means that dark matter is something else than what our experiments are optimized to find.

That's the rather underwhelming conclusion. And it's wrong insofar the size argument is not certain, but depends trivially on the distance of observation. Otherwise I'd like to know what magical number we are talking about.

Slightly off topic, but is there a resource for slightly less scientific minds to understand the universe as seen by those who do. I get an impression reading here and there that are theories to describe start and end of the universe (Or the limitations of human mind to grasp the concept of it), but I give up the moment I see too much of mathematics.

Hawking's "A Brief History of Time" is really good, and only has one equation. My mom read it, even.

I highly recommend reading the book: we know no idea.

You mean We Have No Idea: A Guide to the Unknown Universe?


> But that’s indirect; we know there’s supposed to be a particle associated with it, and that’s what the hunt is all about.

I think is the is the most important line in the article. Why does it have to be a particle? That's a large assumption. We've conceptualized everything in our models so far as waves/particles. Maybe we need a different concept.

You might want to check out a very interesting non-linear spinor theory[1] which contains a chapter on dark matter and dark energy analytically explaining the formation and inner structure, as well as the ratio of dark matter and dark energy throughout the development phase of the universe. Page 29 lists all components of DM/DE and normal matter.

[1] http://norbert-winter.com/wp-content/uploads/2018/07/2018_02...

Full theory can be found here: http://norbert-winter.com/wp-content/uploads/2018/02/2017-03...

As an uneducated layman I have 2 questions I have failed to understand that continue to confuse me. 1. How can relatavistic time resolve to a instant for which the mass in the universe applies? 2. Why does spacetime not have wave like tensional disortions?

Not following your first question, but isn't 2 the recently-observed gravitational waves?

We detect most matter through its interaction with electromagnetism-we touch things and see things, essentially. I think the "aha" moment is when you realise that something invisible to light would also be incorporeal.

I see a lot of comments on mass, but I thought not too long ago there wasn’t even consensus this mass existed.

For example the guy who tried to modify or extend gravitational theories to account for it.

Maybe this was a fringe idea or has lost momentum.

There still trying to hold up with such a silly idea, which is not supported by the standard model, measurements and not common sense. Time to admit that MOND is the better theory.

Excuse a layman's observation here.

The dark matter problem rings as yet another mass-related anomaly we stumbled upon previously but on a nuclear scale.

"Mass defect" [1], as appropriately named, is observed on a nuclear scale (whole nucleus mass is less than mass of nucleons that the nucleus is composed of). It is explained with binding energy required to keep the individual nucleons of a nucleus together.

That is an 'invisible' mass that gets subtracted from the 'free' mass of nuclear particles as measured.

Any possibility to project such analogy onto a galactic scale?

[1]: https://en.m.wikipedia.org/wiki/Mass_defect

In the Mass defect case, it's not an invisible mass what it is substracted. When some nuclei react to produce another nuclei, the energy of the mass defect is emitted as a gamma ray or as the energy of a fast electron/positron or something else. But all the mass can be found somewhere. It is only missing because you can't keep the gamma ray inside the balance.

There should be a similar effect in a galaxy, because when the stars/gas/whatever get closer they have a small gravitational binding energy. I didn't do the calculations, but I'm sure it is so tiny (relatively) that you can safely ignore it.

For comparison, in the Helium nuclei the difference is only 0.8%, in a galaxy it is (I think) abysmally smaller because the gravity is smaller than the strong force and the stars are much far away than the nucleons. But the dark matter is much bigger than the normal mater, something like x5 or x6. So it's not possible to explain the dark matter with a something that has a tiny effect.

Can dark matter be reminiscent of past, a time travelling effects of matter..

I like how we have an article explaining the rationale behind dark matter and the scientific principles that lead to the conclusion that it exists, but most of the threads here are simple declarations that no, they must all simply be wrong or not considering some simple alternative.

Can people maybe, just maybe not assume that scientists are idiots or, as once dead commenter puts it, charlatans?

If you disagree with the consensus, then offer some alternative more convincing (and more useful for civil and interesting discussion) or dispute the article rather than than just writing it all off as "fake science."

Agreed 100%. Indeed, this is one of the best articles I've come across which covers several of the reasons we believe dark matter exists, as opposed to us just needing to modify the theory of gravity.

For whatever reason, dark matter seems to repeatedly rub people the wrong way today, more so than any other scientific concept I can think of. I actually wonder if it's something psychological -- if the universe is only 1/6 normal matter, it makes us feel even more insignificant than we already do, in the vast, seemingly infinite universe? Or maybe it's just the name, sounding too much out of fictional Star Trek.

It's really difficult to have something useful or interesting to say as a layperson about dark matter (or most science fields) so people rely on tropes.

Special and General Relativity - "it's all relative man" Quantum Mechanics- "spooky action at a distance" Dark Matter - "if you can't see it, maybe it's just not there" Flat Earth - "you can't prove it's round"

There are more complex versions of this like the interesting, but debunked Tao of Physics and various cranks with new theories of everything (or perpetual motion), but they aren't accessible or interesting to the masses because they don't fit a convenient narrative.

Most big science and engineering takes a lot of work, understanding, and time to be accepted or have a big impact, but we're always looking for the new News.

I think one reason may be that people are used to the "completeness" of physics; it's the science that people seem historically most ready to declare "done", and it does explain a lot very well. This may make it particularly unsettling to learn about this vast frontier of ignorance.

Really? Who are these people that consider physics to be complete? I'm kind of skeptical that there's a large body of folks that know enough about physics to even ponder its completeness, but who also are unaware of the discrepancies between general relativity and quantum mechanics.

>For whatever reason, dark matter seems to repeatedly rub people the wrong way today, more so than any other scientific concept I can think of.

I think that it is because it is so hard to understand. You can't see it or touch it but it is supposed to make up the majority of mater in the universe. So what is it then? For a lot of people that's a hard thing to get and it would be easier if the answer was something we understood like the scientists being wrong.

In that way it isn't any different than creationists or flat-earthers. Of course dark matter "deniers" don't have the same religious convictions of creationists.

> Of course dark matter "deniers" don't have the same religious convictions of creationists.

Can you support that "of course"? I'm not so sure that there is no correlation. E.g. it is known that the same persons who tried to convince us that the "smoking is not dangerous" try to convince us that the "global warming doesn't exist, or at best it's beneficial." There are definitely the circles that immediately welcome and use any way to raise the doubt in the relevance of the majority of the scientific claims. It is intentional, it is supported by a lot of money, and it comes not only from one political direction. It is complex, it's not only a single ideology or a single religious group, but there are multiple correlations.

See the books: "The War on Science" by Shawn Otto and "Merchants of Doubt" by Oreskes and Conway.

Consider something. What is it that makes god did [x] an improbable hypothesis? The fundamental reason is that there is no direct evidence of said god. There is indirect evidence and logical arguments that can favor a god, but that means nothing when you cannot observe a god, you cannot measure a god, and there is no direct evidence for that god.

Dark matter still holds more in common with the divine than the practical, for now. We've developed and carried out a slew of extremely clever experiments to try to affirm its existence, yet each and every experiment has returned a resounding negative. This is one of the biggest problems with the gulf between experimental and theoretical physics that's been rapidly expanding over the past several decades.

I get what you’re saying. I think you could be way more clear and articulate (took me about 3 reads to understand that you’re just agreeing with the well received parent, I think). I hope that is the reason for the downvotes and not simply people not wanting to hear that rationalism is just as dogmatic as anything it has displaced. Sure it’s better (in the more pragmatic sense), but it still depends on sets of axioms regarded as truth and fails to present explanations for plenty of observed phenomena. It also is strictly not philosophic so it can’t answer “Why?” nor can it yield any sort of ethical/moral framework(s) for understanding reality.

For me, I have always had a problem with how dark matter was portrayed in the media, even more scientific media. It is only fairly recently that scientists have been communicating dark matter as a problem with several possible solutions to laypeople. Before it went kinda like this: "we are unable to detect 80% (or whatever number) of the mass in the universe and we think it is non-baryonic matter". As someone trained in science, but not physics, that sounds like a huge discrepancy suggesting an incorrect theory. But I all hear until recently is that "our theories are right, our observations are wrong". But usually it is the opposite. I'm oversimplifying but that was the general impression. More recently the media and scientists have been doing a better job admitting that something may be wrong with general relativity but the leading hypothesis is non-baryonic matter.

Maybe it's just me but I remember back in the 1980's the first simulations of galaxies didn't work unless you added a lot more matter than was known even then to exist. That said to me that there is an unknown... of some sort. Because we're not even talking relativistic effects really. I have no dog in all of this because I'm supremely confident I would understand it even if the problem proves tractable. (And it might not be, for instance maybe dark matter just does not interact with normal matter at all)

Getting po'd at the media for not being able to explain stuff that scientists don't understand, might as well yell at a jellyfish.

Even scientists communicating directly to the public is what I'm talking about rather a journalism major not understanding physics.

The discrepancy though is that a lot of the "observation" as I'd put it, too, is rather theoretic. It's a false dilemma and that's the problem. It's not just better measurements that are needed, but knowing where to look. If one is so far removed from the topic that there is no difference, it doesn't really make sense to speculate, except to show how much one doesn't understand.

Not sure I'd want to play the psychologizing game here. It could just as easily be used to explain the success of Dark Matter as a topic of discourse in the public imagination.

"Wow, look how insignificant it makes us feel, how insightful", or, "Just imagine how much is hidden from us every day!"

Yes, that must be it. Everything else about astronomy I'm perfectly fine with, but dark matter makes me feel insignificant. It's no problem however that many ... equate dark matter with some tangible notion of one theory or another and that their (and my) ignorance is staggering; Or that a negative result, like we can't bring two calculations into agreement, is treated like a result that found a real thing, while it's quite the opposite.

The simple problem is that DM is often defined as "a form of matter", when it could be multiple different forms. It's just the semantics that irks me. And then, if they will find maybe a new particle to account for at least some of the motion we couldn't explain, I will still be on the fence, because its not clear at all how to distinguish such from a virtual particle. That's nevertheless just a matter of semantics. The actual theory is probably way over my head. Therefore, doubt about the prediction for dark matter is just as much provoked by curiosity as for the proposed explanations. Ultimately, most explanations would be, in a mundane sense, turn out rather boring. So the question is, what's the deeper insight, as it goes for cosmology, what's the significance for life in general? There's a lot we don't know? I didn't even know that!

For me its the similarity to another pre-relativistic concept- the invisible ether between the worlds- that rubs me the wrong way.

And its tough to imagine the consequences of something you are absolutely not able to detect, you could be flying towards with 250 km/s. Its kind of scary- imagine you hit something like that and it causes earth quakes or a shake up of the solar system. Just think if that missing gas giant was actually a Blob of DM cycling the solar system.

And, well - the situation seems similar to this over focused on the problem situations you sometimes have in coding. Everyone has committed to a very narrowed down problem solution, that is just not working out, but instead of stepping back, a thousand angles are tried to solve the problem in the narrow scope.

Many here, just want to help. Which, given the Enlightenment as a project of everyone capable, against a Elite fighting for dogmatic ignorance - is a good thing in my book.

The article is missing in my opinion a confidence rating for every observation- how often this has been tested in experiments or observed in space. Otherwise it was good.

> And its tough to imagine the consequences of something you are absolutely not able to detect, you could be flying towards with 250 km/s. Its kind of scary- imagine you hit something like that and it causes earth quakes or a shake up of the solar system.

If it was capable of doing this, we would observe it doing the same to light on its way toward us - gravitational lensing in other words. If your scenario was possible, we would've definitely made observations to that extent, but we haven't.

For what it's worth - neutrinos fit your description right here. There are a trillion of them passing through your hand every second, essentially without interacting at all (from https://what-if.xkcd.com/73/).

> but instead of stepping back, a thousand angles are tried to solve the problem in the narrow scope.

What makes you think nobody has stepped back? Tons of people have stepped back and proposed lots and lots of alternatives. Dark matter is the only one that can explain all the phenomena mentioned in the article. It is the result of a lot of stepping back and failing even more in the other directions.

>> dark matter seems to repeatedly rub people the wrong way today, more so than any other scientific concept I can think of.

Because, while interesting to a great many scientists, the practical implications of this knowledge are centuries away. Defining dark matter, understanding the backbone of our universe, isn't going to cure cancer. It isn't going to fix global warming. It isn't going to get us to Mars. So when people read of massive experiments throwing unending brainpower and money into the DM hunt, it is natural for them to react as they do. Astronomers give us pictures of far away places that satisfy our natural need to explore. DM hunters stare at numbers and statistics, generating papers and messy diagrams. They aren't fighting an uphill PR battle.

There are also a not-small number of people for whom the DM hunt represents a challenge to their fundamental beliefs. Talk of colliding galaxies billions of light-years away conflicts with the young-earth model that is part of their daily lives. Rather than criticize on that basis and appear ignorant, they lash out on other grounds.

The technology spinoffs from DM-hunting are notable for the broader community.

The high-power dilution refrigerator that ADMX uses is of the same sort that the quantum-computing industry needs more and more of. Indeed, the students and staff being trained by ADMX are finding homes in both academia and across industry.

The high-sensitivity detector technology developed for WIMP searches have alternative use in nuclear non-proliferation monitoring. Improved detector ideas may continue to rattle down into medical imaging in the long run, improving some combination of sensitivity and dose.

The real prize, however, is what happens when the nature of dark matter is understood. It is a long-game play, but the technological implications might be on par with subjects like electricity, nuclear physics, quantum mechanics, etc. We won't know until we get there.

I know this. We know this. The public does not. DM hunters never talk about such things. All the public gets is "we used a massive detector to look something in this narrow spectrum, didn't find it, and will try again next year with an even bigger detector."

Any new detector should be described in terms of the new technologies it will require and how those new technologies will be used elsewhere. That gives it value regardless of whether it detects anything or not.

How do you know it won't cure cancer? Marie Curie checked out some invisible physics and discovered radioactivity. Now we use it to treat cancer.

I suspect that practical outcomes and technologies arising from understanding dark matter will be huge.

Also, I think you're missing the point of astronomy. "astronomers" today are generally astrophysicists or planetary scientists, and they are studying fundamental, mostly invisible, processes and substances.

> I suspect that practical outcomes and technologies arising from understanding dark matter will be huge.

I doubt it. More than 50 years after discovery of neutrinos, we are yet to find any practical outcomes or technologies.

So you're saying a better understanding of nuclear reactions has not had practical implications? Just because the particle itself is 'useless' from a technological perspective doesn't imply the same is true for the accompanying theory...

Maybe not neutrinos specifically, but muon detectors are now being used for archaeology.

The neutrino is part of the standard model. The dark matter may easily be entirely new physics, outside of the standard model. The implications may be enormous.

Moreover, even if the dark matter itself doesn't itself lead directly to to new tech, it is very likely that subsequent discoveries will.

Neutrinos theoretically can be used for nonproliferation at least :).

Time from discovery of nuclear physics to nuclear devices was less than 50 years

Same for experimental planes to first commercial flights.

So it might take centuries, but it also might not.

I think you misunderstand the threads.

Dark Matter isn’t a thing it’s a place holder it has many candidates some more probable than others it might be that there is only one class of dark matter it might be that there are multiple classes of dark matter.

It might be that all the dark matter will be new forms of matter it might be that some or all of it will end up being baryonic matter.

It also might be that we won’t find it at all in which case we might need to say that our understanding of gravity and the geometry of space time isn’t nearly as representative of reality as we thought maybe dark matter is gravity leaking in from other universes maybe it’s the affect of additional dimensions of space time maybe quantum gravity causes some wired emerging phenomenon that affects the geometry of space time that we attribute to missing mass.

There isn’t a consensus amongst scientists on anything related to dark matter other than based on observations we are missing a truckload of mass in the universe.

And currently people are trying to explain it by looking for new forms of matter, normal matter that is very cold and so we can’t see it, and even attempting at modifying the theories of gravity to account for the observations we see with new relativistic and classical modified gravity models.

It can't be baryonic matter, because baryons are made of quarks, which carry electric charge, and therefore interact with light, xrays etc. But we don't see dark matter scattering or absorbing the x-rays & whatnot emitted by stars, nebulae etc. Hence the 'dark' in dark matter.

So it has to be made of particles which have no electric charge. The only such longlived particle we know of is the neutrino, but these guys travel at nearly the speed of light, so can't clump together to form the concentrated blobs of dark matter we observe from gravitational effects.

So dark matter cannot be explained by the particle we know about.

Cold baryonic matter, MACHOs, primordial black holes etc were all candidates for dark matter at some point, yes they aren’t likely candidates at this point because we have much better telescopes and more complete surveys but the search for DM isn’t a process of elimination you can’t prove what is it by eliminating possible candidates you can only show where we might more likely to find a good candidate.

Neutrons are uncharged baryons. The problems with baryonic dark matter have little to do with the charge of quarks.

That quarks have charge, means they participate in electromagnetic interactions. So they can absorb, emit, & scatter photons.

It doesn't matter that a neutron is overall charge neutral, because it's the elementary particles that make up the neutron, that participate in interactions.

This is why, you don't fall through the earth, despite being made of charge natural atoms, because the surrounding electrons of our atoms repel those of the atoms making up the ground, counterbalancing the gravity pulling us down.

I think you are mistaken here.

Photons couple with charged particles due to their spin, this interaction can be seen through Compton scattering. https://en.m.wikipedia.org/wiki/Compton_scattering

However we have for example Neutrinos which are weakly interactive even more so than the photon and are not baryons (the real reason why neutrinos are not a baryonic DM candidate ;)).

A neutrino will not interact with a photon at all since photons do not have an interaction through W and Z bosons (both W, Z and Photons are electroweeak gauge bosons) and neutrinos are not charged.

There are a lot of reasons why you wouldn’t fall through the earth electromagnetism is just one of them, however even non charged baryonic matter can be “solid” and resist gravity for example neutrons will resist gravity through degeneracy it’s all a question of which type of interactions are possible.

There is much more to particle interactions than charge.

As far as photons interacting with neutrons, neutrons while being neutral in charge have a magnetic diepole which is why the photon can couple with them.

You can make baryonic matter that is massive and does not interact well with photons, the LHC has been making some new baryonic matter but none of it is a good candidate for dark matter so far.

But overall yes currently it looks like baryonic matter at least the one of the standard model without any extensions produces isn’t a good candidate for DM.



If you drop some neutrons on the ground, they will go through the ground some short distance until they interact with something, most likely via the weak interaction.

Photons do scatter off neutrons due to various effects, but the cross-section is really very low. I haven't checked the numbers, but I suspect that it's more than low enough that this type of scattering does not rule out neutrons as dark matter. (Plenty of other things do rule out neutrons as dark matter.)

The reason you don't fall through the ground even though you have almost no net charge has nothing to do with quarks.

Fair point r.e. low cross section.

Think the main reason neutrons are ruled out as DM because free neutrons decay, they have a half-life of ~10 minutes. So if DM was neutron, nearly all of it would have decayed into protons, electrons & neutrinos, the former two of which we would definitely be able to see.

Baryonic matter doesn’t have to be free neutrons, cold baryonic matter can be stable we have found it already just as we have found brown dwarfs there just isn’t enough of it to make up for the missing mass hence it’s not a likely candidate at this point.

PS Protons are also baryons and are stable.

> longlived

Why couldn't it be short-lived?

Because the decay process will likely emit particles we can detect. Also if they are short lived we now need a mechanism which constantly generates which ever matter it is which is going to be complex since we don’t really have mechanisms that create matter (at least in any substantial amounts) in the universe other than the Big Bang.

Isn't some weakly interactive massive subatomic particle pretty much the consensus?

WIMPs are just a whole class of placeholder particles, there is no consensus on anything currently everyone has their own pet theory or theories depending on their field.

Ultimately this is because we can't yet prove by experiment, which theory is the correct one.

We need to observe these guys being produced in a particle collider to definitively know what dark matter is made of.

It’s not just that say we find wimps in the lab we need to have some sort of a model that would explain how they are or were produced in the universe and more importantly that they were produced in the correct ratio to be the single explanation for dark matter.

It’s quite possible that we will find WIMPs in a particle accelerator but then be stuck with the fact that at best they’ll only account for 3% of the dark matter we observe and then it’s again back to the drawing board.

> it might be that some or all of it will end up being baryonic matter

It can’t be. It’s already known that 5/6 ths of all the matter in the observable Universe can’t be baryonic.

> There isn’t a consensus amongst scientists

Only for completely useless definitions of the “concensus.”

>It can’t be. It’s already known that 5/6 ths of all the matter in the observable Universe can’t be baryonic.

No it's not known, it's highly unlikely that the "final" dark matter candidate will be completely made up of baryonic matter but it's not been exclusively proven.

Only Siths deal in absolutes.

>Only for completely useless definitions of the “concensus.”

Do you have a more useful definition of consensus for this matter? if so please share it.

> it's highly unlikely that the "final" dark matter candidate will be completely made up of baryonic matter but it's not been exclusively proven.

That "highly" in "highly unlikely" is the key here, that highly is "immensely huge" e.g.:


"While future observations will determine the strength of the constraints from Eri II, existing data from Eri II and from the sample of compact ultra-faint dwarfs appear sufficient to rule out dark matter composed exclusively of MACHOs for all masses above ~10e−7 mass of the Sun."

And if your definition of the "consensus" is 100.0000% then I do consider it useless.


If you use Merriam-Webster's: "the judgment arrived at by most of those concerned" then yes, there is a consensus because "most" begins for me at "more than 60%." And I accept only those that are actually "in the field" and not those that studied something completely different but "have a strong opinion." Sorry, that's automatically useless. I know personally the Ph.Ds who believe nonsense as soon as they are out of the area they have researched personally. It's simply a https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect as in "I was successful while studying X my whole life and therefore I believe I can 'understand' Y better than those who studied Y their whole life." And I'm talking here only about the natural sciences.

By definition it's not baryonic, because we aren't detecting it and we would be detecting it if it were. "Dark" in this context means, in and of itself, non-baryonic.

And please don't bring silly Star Wars references here, they don't help and this particular one is basically offensive. HN isn't reddit.

I was about to post something similar. Before you act on the urge to go pontificating about how the scientists have clearly got it wrong for this reason or that reason, ask yourself - do you really think they haven't thought of that? Don't you think that, if there is mileage in your idea, there would certainly be scientists pushing it?

Addendum: Science (and more generally, specialization) requires that we trust that that the experts in the areas outside of our own chosen fields do in fact know what they are doing. It's become less fashionable to do so lately (one politician in the UK famously said a few years back that "the people of this country have had enough of experts"), but that is to our cost - the extreme end of this is flat-earth conspiracy.

I think the backlash against "experts" comes from the fact that way too many things get mixed into "science". When you look at fields like social sciences, nutrition and economics there are a lot of "scientists" that get publicity for results of very undisciplined work. They don't crosscheck their result for contradictions and often just seem to look for confirmation of their own biases.

This contrasts to most physics I read about. Physicists seem to make a big effort to reconcile all their measurements and if they contradict they admit that and say "we don't know yet".

So physics and other hard sciences suffer from being put together with a lot of other bad science in the public.

There are also people - usually rich and/or corporate - deliberately muddying the water in fields such as climate change, pharma testing, and so on.

The public doesn't understand science, but it understands corporate PR even less.

The corporate PR industry has no plans to change this.

One of the biggest and most destructive wins for corporate PR has been the steady erosion of scientific credibility with FUD and character assassination, combined with science-for-hire commercial shilling. This has done huge damage to the credibility of tentative research, and made it very hard to get away with saying "We just don't know yet. So we need to keep looking, because looking where we don't have answers yet is our job."

But... not only does the public not understand how corporate PR operates. The scientific community often doesn't either.

There's a presumption of good faith in scientific debate. No matter how personal the to-and-fro gets, there's an assumption that everyone is doing the same job for recognisably similar reasons.

That presumption is wholly wrong in any situation with significant political and/or financial consequences outside of science.

Contrary to your argument, there is also the Gell-Mann Amnesia effect: https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect

Note that I am not saying it is the case here, but simply pointing out that the "experts" authority can also be challenged.

Furthermore, this happens rarely, but notably that someone with no serious "expert" qualifications, they produce revolutionary work (Ramanujan, etc.)

The article invites that kind of response by presenting arguments for the existence of dark matter as if they are undeniable. In reality the road to consensus was anything but smooth.

Science works best when you are skeptical of claims, test alternatives, and find evidence that contradict the alternative hypothesis. If you want to discuss controversial science you need to anticipate objections, show the work has been done to test the alternatives, and then leave the reader with the sense that your hypothesis is undeniable (because they can't think up any more objections.) This absurdly high burden is what makes science both hard and effective.

For dark matter a large number of the initial objections have been tested thoroughly, but since they weren't covered people unfamiliar with the subject (and I don't claim to be an expert) will naturally dredge up their own objections.

Almost all of our current scientific knowledge has come from challenging the consensus of the times. Scientific skepticism is always healthy and should be welcomed. Even if the original theory is correct, that doubt will simply help science better explain and prove the current understanding. I'd actually be more worried if everyone agreed about something. Even gravity we thought was fully solved until Einstein doubted that we understood it fully. Let's be careful of putting anything in the category of "beyond doubt", especially theories that haven't been tested or confirmed directly.

>Scientific skepticism is always healthy and should be welcomed.

I strongly disagree.

Principled scientific skepticism from practicing scientist and expert is welcome and can contribute.

Layman scientific skepticism in hard sciences like physics is worthless and even harmful. No matter how skeptical a layman who reads popular science magazines is, they don't contribute. Only thing we can do is to learn. Our judgment and opinion is worthless.

People want to understand, contribute and discuss, but even if we have the education to understand the issues, we don't have the time or interest really think it trough. Everything valuable flows top down from experts to the layman.

Generic examples taken from the history like "Even gravity we thought was fully solved until Einstein doubted that we understood it fully" are just using different words to express tautologies. They don't provide information to the issues at hand.

Science is about repeatable experimental outcomes that are predicted In advance by theory.

Demanding that people believe in a theory that so far has not been supported by the experimental data is more like religion than science.

Which theory is not supported by experimental data? GR? There is hardly any theory with more experimental support.

I'm not asking anyone to put the consensus on dark matter beyond doubt, only for people to realize that the consensus exists for a reason. Even the Wikipedia page for dark matter mentions alternatives, so it's not even an absolute consensus, and scientists have considered possibilities like "gravity is just different at large scales." The problem is, none of those alternatives can yet explain the data as well as the dark matter hypothesis.

But unless we apply the principle of charity to fields that may lie outside our areas of expertise, then discussion won't move beyond the typical middlebrow dismissals HN seems to be fond of.

I think this is true, but I would add that the challenge usually came from someone who was thoroughly familiar with the consensus of the time. I think it's important to distinguish between healthy scientific scepticism from well informed observers/participants and a general out of hand dismissiveness, an extreme example being climate change. It's very difficult to critically evaluate the claims in many fields without having a good handle on the literature. (To be clear I'm not saying this requires having a PhD in the subject.)

That's precisely why we should be open to new theories and novel experiments... something which happens all the time in the scientific community. It's not a case for wild and uninformed speculation (as is happening all over this thread).

Dark Matter IS the scientific knowledge that came from challenging the consensus.

Scientific progress has no end as far as we can tell. Just because we've found a theory that explains more than the previous one doesn't mean that we're done.

Actually, most scientific progress has come from building on earlier scientific work..that is incremental science.

People somehow feel uniquely qualified to speculate wildly about theoretical physics. It's bizarre in a forum full of technical-minded people.

I don't know what it is about us technically minded people that makes us so confident in our knowledge in so many fields like that (note that I'm including myself in that generalization). It seems to go beyond your garden variety Dunning–Kruger effect.

My personal supposition is that most people don't know very much if anything about most subjects, and tech people tend to know a bit more than that quite low average (though still not anywhere near expert level) in an above average number of subjects, and that "novice polymath" status amplifies the Dunning-Kruger effect. Of course it's entirely possible that demonstrating the Dunning-Kruger effect by even thinking we tend to have an above average exposure to an above average number of fields.

Well said, that captures exactly the effect I was thinking about.

If it's any consolation, forums full of theoretical physicists can have discussions on topics like law, economics, or computer programming, and those tend to fill up with shocking displays of ignorance. Of course, one can find that in discussions touching on law and economics here too. :-)

ETA: you also get experts in one subdiscipline getting another subdiscipline terribly wrong, analogous to experts in programming language design making crazy statements about wide area networking.

We're tired of debating double-linked lists versus hashed sets.

its the only worthwhile thing left to speculate about. well that and i guess like number theory. aka life, universe, everything in it.

and even if someone figures it out i probably won't understand it :/

The problem is that some of the big questions in physics appear very simple if your knowledge of the subject lacks any depth. It is also very difficult for some people to gauge the depth of their knowledge. All of this is exacerbated by endless popular science shows which give people the sense of understanding, without the necessary substance. In short, it is not enough to understand by analogy, you need the math and the science. Most people who aren’t in the field lack that, but are culturally inclined to comment.

I’ve written about this here before: https://news.ycombinator.com/item?id=16703193

It feels unintuitive to think about “dark matter” that can’t be directly observed outside of its indirect gravitational effects, but that carries with it an implicit assumption that we should be able to directly observe all the matter in the universe, with the further implication that all the matter in the universe obeys the four fundamental forces that our matter does. Except we already know of other particles that don’t quite do that, like neutrinos, which we can just barely detect directly.

If you take at face value the observation that most observed mass in the universe (via the effects of gravity) is dark, then it appears that our fundamental understanding of reality only describes a minority of the universe and that what’s truly special isn’t the “dark matter”, but rather the “anthropic matter” that we can more easily observe. The dark matter may simply be multiple entirely alien classes of matter with their own fundamental forces, stuff that doesn’t interact with our matter at all outside of gravity and contains, perhaps, observers of its own trying to figure out what the hell we are and whether we exist, since we don’t respond to their fundamental forces.

> with the further implication that all the matter in the universe obeys the four fundamental forces that our matter does. Except we already know of other particles that don’t quite do that, like neutrinos, which we can just barely detect directly.

Left-handed neutrinos interact mainly by the weak force and also by gravity. We have a lot of knowledge about them. You can create a neutrino ray and a neutrino detector, and (if wired properly) measure their speed. We know that there are three types of neutrinos (or perhaps there is a fourth neutrino). We know that they can change midflight from a kind of neutrino to another kind of neutrino. The change takes some time, so you can create a neutrino ray of one kind and a neutrino detector of the other kind and measure how many of the neutrinos changed midflight. So the neutrinos are well understood and there are many labs that can measure them.

(Right-handed neutrinos are more difficult to measure because they interact only by gravity. Some people think that they don't even exist. Some people think that they may be part of the dark mater. I think this is still not settled. Anyway, all the neutrinos we know are left-handed neutrinos.)

This happens all the time here when it comes to physics. Other greatest hits are bickering endlessly about interpretations of QM deeming unacceptable the shut-up-and-calculate approach that 90% of physicists have no problem with and to a minor extent the failure of the LHC to make up new physics ex nihilo.

People should be able to recognize they haven't put the effort to learn the basics of what they're talking about and that maybe there are very good reasons for all this they can't be aware of because of it and should ask about... and start from there.

There is not scientific consensus on this issue. I would recommend reading another expert on this issue for comparison. https://tritonstation.wordpress.com

Dark matter and Lamda-CDM have some fundamental problems.

There is not scientific consensus on this issue. The article is written as if particle dark matter is the only viable option. I would recommend reading another expert on this issue for comparison. https://tritonstation.wordpress.com

Dark matter and Lambda-CDM have fundamental problems too.

That's why it's dangerous to pretend like really complex things actually are not that complex. Laymen think that they understand something about it and they are starting to have opinion which usually is wrong, because complex things are actually complex and opinion based on limited interpretation is wrong.

There is something else you should also consider here. As theoretical physics and experimental physics have started to grow further and further apart, there are people who have devoted their entire careers, in many cases their entire lives, to purely theoretical pursuits which may in the end simply be invalid. But at some point you reach a point of no return in your career - in that you would need to effectively reboot your entire life, which may be quite decorated by this point, in some entirely new direction leaving you going from leading expert to mid (or older) aged neophyte. There is a reason Max Planck famously said, "Science advances one funeral at a time." And that was back when experimental and theoretical were still very closely intertwined!

One problem with dark matter is that there have been an increasingly large number of quite clever experiments to try to detect various concepts of it, WIMPs in particular. And each and every one has been a negative. This should, in and of itself, begin to cast down on the hypothesis even lacking a better alternative.

This should, in and of itself, begin to cast down on the hypothesis even lacking a better alternative.

Only if your search was exhaustive: Scooping up a spoonful of ocean is not enough to conclude fish do not exist...

That's really the problem here though. You cannot falsify dark matter, and you cannot prove a negative. Imagine I say there is a 5 headed green deer that weighs 2000 pounds and can speak Gaelic, but it's really good at avoiding being spotted - so good in fact that you can even look at right at it and might not see it. And you can even touch it and not even know you're touching it! What would you could consider an exhaustive proof of its lack of existence?

You can leave carrots everywhere, but perhaps it doesn't like carrots. Or maybe you didn't spot it eating those carrots. Maybe you left them in the wrong spot, or at the wrong time. Maybe you need bigger carrots. Maybe you need smaller carrots. Maybe it likes cooked carrots instead of raw carrots. Maybe you need a better camera. Maybe the reason you camera didn't catch it is because you weren't using a flash, or maybe you need to setup an infrared camera, maybe you need to play a Gaelic greeting on loop, etc, etc. You can compile a literally infinite number of ideas to try to detect our dear deer.

Instead with non falsifiable theories, all you can do is approach them from an abundance of failures. You've tried carrots, you've tried infrared, you've tried ..., and it's still not coming out. Each failure makes it more and more likely that our deer simply does not exist. Of course this does not mean it definitively does not, but nothing will ever mean that.

> You cannot falsify dark matter, and you cannot prove a negative. Imagine I say there is a 5 headed green deer that weighs 2000 pounds and can speak Gaelic, but it's really good at avoiding being spotted - so good in fact that you can even look at right at it and might not see it. And you can even touch it and not even know you're touching it! What would you could consider an exhaustive proof of its lack of existence?

Dark matter isn't like this, though. It's more like an invisible deer that's been eating your grass and shitting on your lawn. At some point you have to say "I might not be able to see it, but the only thing that eats grass and leaves deer shit is ... a deer."

We have the problem that the observable mass of a galaxy is not sufficient to explain the motion of its constituents. As gravity is a pretty well validated theory, it can all be solved if we postulate some matter that we can't see which is still gravitationally affecting those things we can see. Dark matter, in this respect, is no more than the simplest explanation of what's happening via a single item that, if it were true, would explain things.

If you think you have a deer you haven't seen so far eating your lawn, you can put out infrared cameras and all sorts of equipment - and just maybe you might find that someone down the road has been messing with you and is mowing it at midnight. But equally, you might see grass disappear and see shit appear from mid air, and come to the conclusion whatever's doing it is invisible.

Ultimately, it's very conceivable that someone might disprove dark matter. All they have to do is posit some other candidate (eg a modification to gravity), and then predict a result that would follow from that. So far, all the other candidates haven't resulted in a hit, but the field is open for the next great idea to come up with one.

But dark matter no longer is particularly simple. At the time of its initial proposal it was little more than unobserved masses, which could have been something as simple as e.g. red dwarves. And so it made sense to reference dark matter as a place holder. But over time as we've discovered things are even more off, dark matter itself has started to morph into our 2000 pound Gaelic speaking deer. Now it comprises the vast majority of all matter in the world, it can't be baryonic, it mustn't interact with anything except itself and gravity, we need to have entire galaxies made of nothing but dark matter, and so forth, and so on. Add now add in the whole slew of experimental failures to detect it, as null hypotheses, and it features ever more exotic conditions as well.

And on the topic of those test failures, every single test we make to detect dark matter, which everything we hypothesize about it indicates we should be able to measure, says it's not there. This should not be a problem except for the fact that we've performed alot of experiments at this point to try to detect dark matter of all sorts, with increasingly high precision. And they've all returned a strong negative. With Bayesian logic it's almost surprising we haven't had a false positive yet, given all the failures.

I really do think the geocentric model has a lot in common here. Prior to Newton's discovery of strong mathematical indicators for gravitational mechanics, heliocentricism had long been proposed. And it was in many ways a much more simple explanation for many phenomena than geocentricism, which over the centuries became ever more convoluted and complex. The 'petal weaving' geocentric astronomic models are enough to boggle the mind even today. But one of the big problems was not even the church, it that there was an immense amount of academic work based around a geocentric model. As but one example astrology, which was considered a scholarly and academic pursuit in the past, was based heavily on the assumptions of a geocentric universe. For instance Mercury starting to 'go backwards' (which was required to glue together a geocentric astronomic view) is still a part of astrological 'theory' today.

Alternatives to dark matter look increasingly more likely with every failure of experiment designed to test it. And these alternatives need substantial refinement, but I think really the biggest issue with them is that if dark matter is not correct it would basically destroy more than a century of astronomical and cosmological work. Nearly everything we know about the cosmos is predicated on the assumption of dark matter. If it turns out dark matter is not the issue, we're looking at setting astronomical and cosmological work in many areas back a century. It would be, by fair, the biggest failure in modern science. So there is reason that alternatives to dark matter are held to a higher standard than dark matter itself, beyond simplicity - which dark matter no longer really is.

> It would be, by fair, the biggest failure in modern science. So there is reason that alternatives to dark matter are held to a higher standard than dark matter itself, beyond simplicity - which dark matter no longer really is.

I don't think this is anything to do with a higher standard. It's more like Occam's razor. If there's an alternative explanation that will upend our whole understanding of gravity and cosmology, have at it. But such an explanation must come with something testable that shows it's true.

Here's an example of an alternative explanation, "a wizard did it." But that's not science, because there's no way to prove it. As a result, in the absence of a testable hypothesis, you have to go along with the one that does the least violence to all currently existing knowledge, surely?

I.e. what I'm saying is that it isn't on the Dark matter theory to prove itself. We already have observable gravitational issues that need something weird to be true. Dark matter is just the simplest explanation anyone can come up with that fits the facts. It's on any other theory to either throw out less of already-established science by being an even simpler explanation, or to prove it's justified in throwing out more by showing experimental evidence.

Or, to put it another way. I agree that dark matter isn't a simple theory. But there isn't currently a simpler one on the market.

Let's go back to the late 19th century and give the physicists of the time a peek of what would come to be the future of dark matter -- that it would not only end up needing to be a rather exotic form of matter, but that it would need to account for nearly all matter in the universe, along with all the countless negative conditions we can also now apply to its existence as a result of various experimental failures. Of course we would also give them the pros of dark matter in that it also explains, after substantial 'refactoring', some other otherwise difficult to explain issues. The dilemma that they then face is pondering whether this material seems more probable, or whether it might be the case that e.g. Newtonian dynamics is somehow missing something? For brevity, I'm sidestepping relativity.

I think you'll see a lot of the issues with alternatives such as MOND are a 'lack of elegance', yet we've happily accepted that the universe on a very small scale is far from elegant, so it seems peculiar to insist that on the very large scale it must behave with elegance. Another issue is that MOND generally also has a requirement of some dark matter, but the critical difference is it's just that - matter (including baryonic) that cannot yet be observed, not the increasingly exotic material that dark matter has morphed into. And it also only requires a small fraction of the amount that current dark matter does.

I tend to agree that dark matter is the most probable explanation. But, at the same time I think the resource allocation for dark matter as opposed to everything else is probably not the one we would choose if we were being completely impartial.

The idea of non-visible matter as a fix for stellar motion dates back to the 19th century, when our knowledge about the cosmos was far more limited (remember, we only learned about other galaxies in the 1920s - it post-dates general relativity!). That the hypothesis is still viable today with our more elaborate understanding of the universe does validate the idea to some degree: Our cosmological models cut have killed it off easily.

So unless a better model that works equally well comes along, I see no reason to discard the dark matter hypothesis just yet: The universe has no obligation to be filled with a type of matter detectable with the means of today...

Yes, it dates to the XIX Century, and alternative types of it have been proven right or wrong at times.

Postulating unobservable things is still not a good way to do science. The one good thing of the dark matters theories was that physicists were clear in that it's not a done model, just a map of what's wrong with our understanding. This seems to have changed lately.

I'd generally agree with you, except for one thing. I'm not sure our models could have killed it off, excepting finding precisely whatever is causing the apparent mass abnormalities throughout the universe. Have you seen those ancient astronomic models used to illustrate a geocentric Earth? The one with the numerous spinning 'petals' surrounding the Earth? It's interesting imagining the amount of work that went into creating such models, constantly adjusting them over and over each time yet another little problem came up. Happily accepting planets just randomly changing direction, with no analog whatsoever anywhere else in nature, and so on.

When hypotheses are not falsifiable, they just tend to grow ever more elaborate rather than die. Dark matter started off as literally that - a place holder for bodies that did not significantly radiate. It's slowly changed into becoming the vast majority of all matter in the universe, existing practically everywhere, it can't be baryonic, it interacts with nothing except itself and gravity, there must exist near entirely dark matter galaxies, and so forth. So we're left with this ever evolving hypothesis that can't ever really be falsified and which every test, increasingly clever and precise, we create to detect it -- says it's not there.

And it seems that the zeitgeist is starting to increasingly treat dark matter as a fact whose proof is little more than an unscheduled formality. This seems illogical given that we've done more than ever to try to show it exists, and every result is saying - nope. And again this does not mean it does not exist, but each failure makes it more likely that this is the case, and there is an alternative explanation.

> Can people maybe, just maybe not assume that scientists are idiots or, as once dead commenter puts it, charlatans?

Have you met scientists? Or more accurately, PIs? Unfortunately, our system of being awarded grants and academic advancement does not necessarily reward the intelligent, those with the best scientific judgement, and rapidly burns out those who would in the ideal make the best scientists.

I like how the linked article is a laundry list of reasons to believe in dark matter but doesn't mention any problems with particle dark matter, MOND alternatives, or that the consensus on dark matter is far from settled, etc. And oh yeah anyone who questions this onesided cheerleading is strawmanned into an antiscience quack.

Because in one sense it sounds as bad as alchemist (who were once considered scientists).

We're riding on the back on a turtle

We're the center of the universe

All things are made from earth, air, water, fire

Space is filled with ether

Dark Matter "feels" like it's going to be yet another of those things that will be added to the list of things scientists used to believe.

I fully acknowledge they aren't stupid and that I am not remotely knowledgeable about any of it but go read the transcript from this Nova episode about actual scientists predictions about what we'd find on Venus


The scientists in 1995 we're not stupid either but their predictions certainly sound stupid in 2018.


How do you square the following notions?

a) dark matter does not interact with measurable matter

b) proof of dark matter's existence derived by the measurable, relative, interaction of mass and energy

Summarizing on the fly, so necessarily wrong in details:

Regular matter interacts via the four forces: strong interaction, weak interaction, electromagnetism and gravity.

Strong interaction: binds fundamental particles together to make atoms. In every day terms, it’s what makes mass, mass.

Weak interaction and electromagnetism: causes radioactive decay and EM radiation (photons). In every day terms, it’s how we get light, radio, cell phones, etc. Also nuclear power.

Gravity: attraction of mass across great distances. It’s how we stick to the earth and why the earth circles the sun.

There are particles that don’t act through forces but are affected by them:

Photons don’t have mass so don’t cause gravity but are affected by it.

Neutrinos don’t have an electromagnetism effect so they can’t be seen at all only measured in very rare weak interactions.

Dark matter: No strong interaction so not a part of atoms. No EM interaction so we can’t see it blocking light. No weak interaction that we have observed. However they do cause gravity, so we can see that. All of the proofs of existence are via gravity and relate only to mass — we can’t observe any other properties.

How is it even possible to detect dark matter, if it doesn't have EM interaction, weak interaction and strong interaction? Probably some ultra-sensitive gravity detector which could detect a single particle? Sounds like impossible for current technology. So we could have those dark matter particles orbiting with crazy speeds around our planet, sun and everything else without any chance to reliably detect them. But still there are experiments to detect them, does it mean that scientists expect some kind of interaction, may be rare?

If it interacts only gravitationally, the answer is that probably we will never detect a dark matter particle.

However, good news: several possible dark matter particles have been proposed, all of which interact very very slightly non-gravitationally as well. Practically all such proposals start with a particle physicist trying to repair some problems in the standard model of particle physics. When such proposed particles are decent candidates for cold dark matter, astrophysicists and physical cosmologists take note.

One family of candidates are the WIMPs, which feel the weak force, and so can produce a recoil reaction in atomic nuclei, and we can spot such recoils produced by neutrinos sourced by the sun or nuclear reactors. Galactic dark matter doesn't have a "bright spot" like the sun or the Super Kamiokande reactor, so distinguishing recoils from Brownian motion is tricky, since a WIMP may enter a recoil-detector from any direction. The density of WIMPs (if they exist) is much lower than the neutrinos streaming out of SK reactor or the sun, so there will be fewer recoils in the first place. WIMPs are generally found in various attempts to explain chirality in the standard model.

Another family of particle-physics-problem-solving dark matter candidates are the axions which feel both the strong and weak forces, and axions can be smashed up into photons (or formed from photons) in a very strong magnetic field.

There are several much less popular hypothesized particles that can be detected in principle because they feel one of the non-gravitational fundamental forces. This does not mean it is easy to detect them, though: whatever the microscopic makeup of dark matter, it is very sparse inside the solar system, and galactic dark matter reaches Earth with relatively low momentum, so even when it does interact with ordinary matter on Earth, it won't produce a large reaction.

> orbiting with crazy speeds around our planet

Galactic dark matter particles must move with the rotation of the galaxy, and for the most part so does our whole solar system, so the speeds will be slow, and in particular not at all relativistic. Also, because dark matter forms a dust where the individual bits of dust have extremely low mass, they will not be drawn into orbit around the Earth. The dark matter particles' orbits around the centre of the galaxy will be very slightly perturbed by the Earth, though.

That's exactly it. Nobody knows if it's impossible or not, but there's no way to find out unless experiments at least try, to whatever effort/cost we deem reasonable.

> Photons don’t have mass so don’t cause gravity but are affected by it.

They do have momentum though, and their momentum flux is encoded in the stress-energy tensor, which up to constant factors forms the right-hand-side (the "source" or "matter" side) of the Einstein Field Equations of General Relativity.

So photons are a source of curvature, and thus to say that they "don't cause gravity" is wrong.

This has been known since the late 1920s, and was made very clear in some correspondence between Einstein and Bohr on the topic of "Einstein's box" (box of light).

See e.g. the following subsection (and the figure it refers to) https://en.wikipedia.org/wiki/Bohr–Einstein_debates#Einstein...

A more extreme case is https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)

In flat spacetime, we can alternatively start by considering the special-relativistic dispersion equation, E^2 = (mc^2)^2 + (pc)^2, m being intrinsic mass and p being momentum. Usually you see this as E = mc^2, taking square roots and considering the centre of momentum to be fixed. When you let a beam of light (or even a photon) travel across a set of coordinates, rather than keeping it fixed at some coordinate (e.g. the origin), p is nonzero, even though m is always zero. Since (pc)^2 is positive, so is E^2, so even though light is massless, it has (frame-dependent) energy. Indeed, being more formal, one says that light has momentum-energy. A further relationship E = hf, h being Planck's constant and f being the frequency of the beam of light (or just a photon), also underlines this: E = hf = pc^2, so the momentum of light relates to it's frequency, or alternatively it's wavelength (as f = c / lambda, where lambda is the wavelength). Light's frequency is observer-dependent because of relativistic doppler effects or equivalently light's wavelength is observer-dependent because of relativistic length contraction. (And this should not be surprising as even in high school physics you will have learned that kinetic energy is a frame-dependent dependent quantity. Relativistic kinetic energy is E.)

When we add gentle curvature and use suitable coordinates, E is simply promoted into the time_time component of the stress-energy tensor. (Gory details if you look up "comma-goes-to-semicolon rule", which you can find discussed here https://ned.ipac.caltech.edu/level5/March01/Carroll3/Carroll... or in most decent textbooks on General Relativity. Carroll prefers to call it the energy-momentum tensor instead of the stress-energy tensor; they're the same thing.)

Since any nonzero component of the stress-energy tensor serves as a source of curvature, then light must generate curvature.

You were half-right though: photons do indeed respond to curvature.

I'm not an astrophysicist, but how do we know we're not measuring overlapping folds of relative spacetime, and erroneously counting the same observable matter more than once?

There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.

Just because we can't see it or measure it doesn't mean there's nothing there. Fascinating to get some insight into how scientists concluded such a thing exists.

We can measure it.

Because it doesn’t exist. Dark matter is a fudge factor to prop-up a theory that astrophysics is too afraid to put to rest.

What do you mean by "put to rest"?

Do you mean we should switch to using another theory, which reproduces all existing results, also correctly predicts some currently-unexplained observation or experiment, and has also correctly predicted previously-unobserved results (i.e. isn't just a "fudge factor")? If so, I would love to know what that theory is, since I certainly didn't encounter any such thing in the dark matter module of my masters degree (the options were basically MOND or (various possible forms of) dark matter, and the former is certainly much more of a "fudge factor").

If by "put to rest" you mean we just throw away our current theories without having a better replacement, then I don't even understand what that would mean. Would we ground all rocket launches since we don't completely understand gravity?

We know how stars work, and we know how gravity works...These two numbers don’t match, and they don’t match spectacularly. There had to be something more than just stars responsible for the vast majority of mass in the Universe.

I think the problem here is that maybe there is something else we don't understand. We also did not understand the propagation of light in a vacuum in the 19th century, and invented a hidden medium called the "aether" to explain it. I think there is some fundamental thing we are missing, and we're inventing dark matter to explain it.

Perhaps we could call that thing: "dark matter" for now.

In seriousness, we can observe the effect, and even map the distribution of the stuff.

"In seriousness, we can observe the effect, and even map the distribution of the stuff."

No, we can measure the dissonance between our best theories and reality. And make a conjecture about 'stuff'.

I'm not a physicist, but my basic understanding of the state of the science tells me you're simply incorrect. We can observe the effect of dark matter via gravitational lensing, and folks much smarter than me have devised studies to exclude differential behavior of gravity at classical and galactic scales.

Give me a recent and well supported study indicating that dark matter may be something other than matter, and I'll happily read it.

My understanding is that the existence of dark matter is uncontroversial among physicists, and that the best bet is some sort of WIMP.

As a non-physicist I assume the physicists have thought really hard about this.

But it feels very strange to accept something that:

- is a particle

- doesn't interact with other normal particles (except through gravitation)

- doesn't interact with particles of its own kind

Neutrinos are almost as strange. They interact a tiny bit via the weak force, and they interact via gravity like everything else, but that's it. We know they exist because of the weak interaction.

Maybe dark matter interacts with things via the weak force, but even less than neutrinos- not quite zero, but just small enough that we can't measure it, or can't measure it yet. Hopefully, they do interact a little bit by some other mechanism otherwise they'll be very hard to detect- I believe this is what current attempts to find dark matter are relying on.

This assumes that we are 100% certain that our understanding of gravitational lensing is accurate _AND_ that only mass affects gravitational lensing.

Nothing in science is assumed with 100% certainty. I would argue that working from our best available theories (while always looking for ways to falsify them) is perfectly reasonable.

That assumes that it is "matter". Maybe there is something we just fundamentally don't understand about gravity.

They've thought of that: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

It doesn't really work. Overwhelmingly the evidence points to mass, which is gravitationally affected by other mass, and nothing else.

I know the mind rebels against the notion of invisible matter, but is it really so much more implausible than the invisible spookiness of gravity to begin with? If the scientific evidence points that way, your monkey-brain intuitions do not provide a reliable veto.

MOND does work a lot better than Lambda Cold Dark Matter (L-CDM)... but only for explaining the rotation of galaxies. It can explain this rotation almost perfectly given only the mass distribution: no tuning is required. Dark matter, on the other hand, has issues with dwarf galaxies and has no predictive power: you just fit the dark matter to the results you observe (Which is why you end up with some dwarf galaxies that are almost entirely composed of dark matter, and ones that almost have none. With MOND it just works).

Of course MOND can't really explain the third peak of the cosmic microwave background radiation, so it isn't perfect either. It is also phenomenological, with no underlying physical theory at the moment. Still, it's surprising that it does work at all.

I should also mention that the bullet cluster, which is touted as proving dark matter, causes issues for dark matter as well as MOND. The velocities involved in the collision are higher than can be explained by the current dark matter models. MOND kind of sucks at dealing with clusters, as well.

TL;DR MOND is a lot better than LCDM at explaining galaxy dynamics, LCDM is a lot better than MOND at explaining the cosmic background radiation. Both aren't that great at dealing with clusters (but you can also make dark matter work with enough fiddling).


Of course, neutrinos are invisible, and no one seems to have much of a problem with them anymore.

This is another possibility, and would be exciting.

But it's very very hard to come up with plausible models for such modified gravity. There are lots of people trying, and their ideas tend to break all sorts of other things we know. For example (IIRC) lots of candidates gave a slightly different speed for gravitational waves, and were ruled out by recent detections where we also saw X-rays from the same event.

Sure, but all data we have rather points out to some undetected matter than problems with the theory of gravitation. The article shows several measurements which would be well explained with particles we did not detect yet.

That's not impossible, but I tend to believe the experts when they say it's not likely (and have explicitly ruled out several such explanations).

Who knows what future physics holds (I’d love to know!) but our best theory of gravity (Einstein’s general relativity) is shown to be amazingly accurate in every experiment ever conducted for it.

Some form of only-detectable-via-gravity matter fits the math. Coming up with provable alternative math would be the greatest scientific breakthrough of the past century...or possibly of all time.

I’m sure people are working on it (along with quantum gravity). So either it’s wrong or it’s dark matter.

Maybe our assumptions about how momentum, gravity and inertia work are incomplete. General and special relativity describe it as a warping of spacetime, but while I've read all sorts about the equations, I have not yet read any description of how that phenomenon physically manifests (I'm not a physicist). Maybe relativity works well enough, but doesn't describe things completely, just like Newton's laws weren't complete and were superseded by relativity.

I'm not sure which theory you refer to? Normal physics works pretty well for most stuff and no one claims it knows everything.

It worked for neutrinos.


Physics is wrong on many fundamental levels, but everytime physicists find fundamental errors, they propose another entirely theoretical layer of complexity, with the "benfit" that no one is able to practically refute it.

It's not adding a layer of complexity, it's peeling off a layer of abstraction. With the "benefit" that it describes better what we observe and that it allows to successfully make new predictions.

What's your alternative to the scientific method exactly?

100 or so years ago physics went into the wrong direction, ignoring people like Tesla who argued for the aether model.

> Physics is wrong on many fundamental levels

Physics, when practiced according to the principles of the scientific method, is never wrong (or right). Physical laws try to explain the world around us using mathematical models. Those models can be tested over a certain domain of experimental parameters D. A physicist, when speaking accurately, would never say that a physical law L is correct. They would say that L correctly describes our experiments on domain D. New or more accurate measurements can expand or shrink D, and in the worst case D can become empty. This constant refining is the essence of physics and the scientific method.

Arguing that physics is wrong implies a lack of understanding of what physics actually is.

No, it is fundamentally wrong in itself, because it went into the wrong direction 100 or so years ago, when physicists ignored the aether model.

It wasn’t ignored it was disproven. Come up with experimental evidence or gtfo.

It was "disproven" based upon a different philosophy.

You can't disprove a philosophy though, you can only try to understand different philosophical models next to each other.

The Einsteinian philosophy represents materialism and a world without meaning. It was problematic right from the start when mental gymnastics was needed to explain how energy can move through nothing, or how nothing (space) can have properties.

Funny that you mention experimental evidence, as most of physics is basically theoretical nowadays. (That's why it's called theoretical physics, dark matter included)

> It was "disproven" based upon a different philosophy.

It was disproven based on experimentation[0], no "philosophy" involved.

>The Einsteinian philosophy represents materialism and a world without meaning

There is no "Einsteinian philosophy," nor does anything in Einstein's theories relate to "meaning" or any lack thereof, in a philosophical sense. Whether you want to believe in aether, or God, or that the Machine Empire built the universe as a VR simulation, E=MC^2 remains true. It can be tested, has been tested has been proven true.

And its probably worth mentioning that the same is true for aether theory, because it also was not a philosophy, but a scientific theory (which was, as mentioned earlier, disproved by experimentation.) The universe is no more or less meaningful or materialistic either way.

>It was problematic right from the start when mental gymnastics was needed to explain how energy can move through nothing.

On the contrary, the mental gymnastics were needed to continue supporting aether theory after experiments and observations continually failed to produce any evidence of it, and the properties aether would need to have to conform to the current cutting edge of science started to become ludicrous.


Interesting. I'm not really into physics, so it's helpful to learn about the mainstream position. I know I use sloppy wording.

I still agree with Tesla that empty space can't have any properties though. Things that don't appear logical, probably aren't.

>Things that don't appear logical, probably aren't.

At the time that Galileo proved that objects fall at the same rate regardless of their mass[0], the prevailing and more intuitive theory was that heavier objects fell faster. Miasma theory[0] was far more intuitive and "logical" to people than "tiny invisible monsters."[1] Newton's theories of gravitation alone couldn't account for the orbit of Mercury... but the illogical theory of relativity could[2].

Tesla was an uncontested genius, but genius isn't omniscience. Empty space does have properties (notwithstanding that the aether would have been one of them) like warping under gravity and vacuum energy[0]. Relativity, quantum mechanics, dark matter and dark energy are counterintuitive, sometimes profoundly so, but the universe isn't obliged to conform to human intuition.

All that we can say is that, as far as we know, based on observation and experimentation, the universe is not only stranger than we suppose, but still stranger than we can suppose. And that the luminiferous aether isn't a thing (although the Higgs field is probably close enough...)





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