How do you explaining gravitational lensing with no visible mass, without dark matter?
In other words situations where we see twin images of a very distant galaxy because light curves around intervening mass, without sufficient mass observable to explain the deflection?
I was under the impression that we could map dark matter localized to a precision that tended to rule out measurement error or a general divergence from Einstein's laws, unless it had a peculiar non-uniform distribution.
>How do you explaining gravitational lensing with no visible mass, without dark matter?
Black holes?
The physicist says:
>What struck me was some regularity in the anomaly. The rotational velocities were not just larger than expected, they became constant with radius. Why? Sure, if there was dark matter, the speed of stars would be greater, but the rotation curves, meaning the rotational speed drawn as a function of the radius, could still go up and down depending on its distribution. But they didn’t.
That seems pretty damning for any theory that relies on dark matter.
>>How do you explaining gravitational lensing with no visible mass, without dark matter?
>Black holes?
Well, black holes are not really black, you can see their accretion disk. Also, a centralized huge mass would create a different velocity profile from what we are seeing. So black holes are pretty much out.
Only if they have an acretion disk. If they've cleared their neighborhood of most infalling matter then they would be invisible except by gravitational effects.
Sorry, but I don't understand your argument. When asked 'how do you explain lensing without dark matter?', your answer is 'with this particular flavor of dark matter'?
Well we're seeing a bunch of stuff happening out there and calling it dark matter.
1. The big problem is not enough stuff. Galaxies don't have enough stuff in them to let gravity hold them together.
2. A second problem is that we have stuff out there that our instruments can't detect but seems to be putting out gravity and bending the light.
The orthodoxy of dark matter says 1 and 2 are connected. That dark matter is bending light and causing extra stuff in galaxies to allow them to form and keep them from flying apart.
This physicist says no, just gravity doesn't work quite the way we think it does, because the stars in galaxy are moving at a constant speed relative to their distance from the galaxies center - but if there were a whole bunch of extra stuff in there that we can't see creating extra gravity all over the place, it should mess with the speed and cause it to be different depending on what parts had more dark matter in it.
My parent was saying, how do you explain the invisible stuff putting out gravity and bending light, and suggested that it could not be explained by black holes. I was pointing out that one of the main explanations for it is black holes, but that still doesn't solve the first big problem of not enough stuff.
Well, right. You could have a distribution of black holes. But that would mean a lot of black holes - all of them in space so void of other matter that they have no accretion disks. Also, with black holes so common, we would see many collisions with plain stars - and those would be very visible.
Can't be black holes. For one thing they would start becoming visible through Hawking radiation if they were small enough to be homogenous like dark matter. Secondly baryon acoustic oscillations set a limit on how many black holes there can be.
For your second part, the constant velocity is very well explained. The dark matter is just very broadly distributed. That's a natural consequence of not interacting with anything. It clumps up much less.
Yes, black holes and other compact objects have pretty much been ruled out as candidates for dark matter. The issue is that they would produce gravitational microlensing signals which are not observed. In brief, when the black hole passes in front of a star, its gravitational field magnifies the light from the star in a characteristic way. By analyzing the shape of the light curve, you can figure out pretty much everything about the system like the masses and the distances. Systematic surveys for these microlensing events like OGLE have not observed black hole microlensing events at the rate that would be needed for black holes to constitute most of the dark matter out there. They do observe quite a few planetary microlensing events, though.
If you assume a black hole, only certain masses and distances work, and they lens other objects.
Picture I had was, if you had multiple objects affected, you could map the distribution of the gravitational anomaly, but it's over my head in terms of what the math and the assumptions are.
No. Dark matter is spread relatively homogeneously so you would need very small back holes, and those would be visible or wouldn't have existed this long. Baryon acoustic oscillations also limit how much normal matter we can have, and they say there has to be a great deal of non-baryonic matter.
I think you only grok half the story here. You're correct - black holes are not a popular candidate for dark matter, for reasons you described. But this article is not about candidates for dark matter, it's about removing dark matter from the equation entirely. If I understand correctly, black holes here are given as an explanation for isolated examples of strange gravitational lensing effects.
There aren't isolated examples of strange gravitational lensing. The lensing being discussed occurs at the scales of galaxies and clusters of galaxies, so you need an incredible number of tiny black holes, brown stars, or other MACHOs.
When we're confronted with something we can't explain using our existing equations and theories, the conclusion is that there must be some mysterious thing there that fixes our current equations and makes them work again?
A thing we can't detect or see or have any other evidence of ... other than our equations and theories need that thing there for them to work?
> In other words situations where we see twin images of a very distant galaxy
It's entirely possible our equations do not function well on those scales.
Dark matter is predicted at every level of astrophysics. It's not just "hey, galaxies look weird".
The amount of visible matter only accounts for 4% of the cosmic microwave background.
The CMB is extremely smooth, but visible matter is highly clumped up. This can only be explained by a huge amount of matter that doesn't clump up ie non-baryonic.
The ratio of deuterium to hydrogen rises based on the density, since there's more pressure from gravity. There's way more deuterium than can be explained by visible matter, so there must be some extra compression happening.
Galaxies have more kinetic energy than they can have given their visible matter and how fast it's moving. This implies a huge amount of invisible mass.
Gravitational lensing around the edges of galaxy clusters indicates a halo of invisible mass.
Colliding galactic clusters show their mass splits into two parts: one that clumps up into a bright, dense blob, and another that has much higher inertia and continues past the bright blob as if it didn't interact with it in any way besides gravity.
We can map variations in empty space where there is more mass than is expected, in a way that can't be explained by a self-consistent theory of gravity but can be explained by invisible mass.
Dark matter as a theory hasn't changed for 80 years and every single one of these discrepancies is solved exactly by using the measurements of dark matter that already existed. They all agree with each other perfectly. There is no theory that even competes with dark matter. Nobody has come up with another idea that explains all of those or even most of those problems.
>The CMB is extremely smooth, but visible matter is highly clumped up. This can only be explained by a huge amount of matter that doesn't clump up ie non-baryonic
This is not correct. Datk matter's presence is inferred from the CMB anisotropy not because it doesn't "clump up" but because it does not interact with photons (EM field). Inflation is responsible for excess smoothness. In fact dark matter drives regular matter to "clump". See eg:
>every single one of those discrepancies is solved exactly
This is quite an oversell as far as I know. The margins of error on these calculations are often large.
In particular the version with a slightly modified gravity field and smaller amounts of unobserved regular matter is not too outlandish. What would be needed to exclude this is a proof that the amount of unobserved matter cannot be reduced significantly by modifying G, which has not been written yet.
> The amount of visible matter only accounts for 4% of the cosmic microwave background.
Could you explain what do you mean by 'accounts for 4% of the cosmic microwave background'? How is one supposed to account for CMB by matter density, when its spectrum is almost equilibrium one (thus consistent with almost any value of matter density)?
The CMB has a certain spectrum and temperature. The amount of energy in the universe correlates to the amount of mass in the universe. The more energy and mass was present in the big bang, the hotter the CMB will be. The amount of visible matter corresponds to a CMB temperature that is far lower than what we see.
> The amount of visible matter corresponds to a CMB temperature that is far lower than what we see.
Why would there be a relation between CMB temperature and amount of (visible+invisible) matter? I suppose the CMB temperature decreases in time. Would`s the relation you talk about imply that the mass or density of mass of the Universe then has to decrease as well?
> solved exactly by using the measurements of dark matter that already existed...
Just be careful. Dark matter distributions are highly paramterized quasi-continuous functions, so getting measurements to match exactly is a function of their definition, not an emergent property. While we do feel confident that there could something reasonable there, a priori exactitude cannot be evidence because that's begging the question and also possibly over fitting parameters.
why? Until direct evidence and predictions of properties emerge, from an epistemological point of view it's still a very shaky model. If it all turned out to be epicycles it wouldn't be surprising. Or maybe a better analogy is Vulcan. https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
The closest we have come to direct evidence are the independent calculations of the properties if it's a self-annhilating particle that agree with a certain set of observations. Even so that seriously breaks a conservation rule, because if there's no proposed way to create dark matter, the universe makes no sense.
Firstly, they function well enough to predict that gravitational waves exist, the ones which we measured being produced by an extreme gravitational event.
Secondly, many other areas of physics have worked this out. The prime example of Quantum Field Theory and the Standard Model, which has been incredibly successful in predicting the existence of new types of particles from purely mathematical equations and theories (and trust me that stuff is pretty damn abstract, even compared to General Relativity)
It is of course a possibility, although you make it sound somewhat obvious, as if physicists just haven't bothered to question that the widely-regarded theories might be wrong. I assure you this is not the case; there are no politics involved in this kind of research which is stopping people presenting questions such as "are we sure our current models are correct?"
In cases like this, it is safe to assume that researchers have already explored any ideas you could come up with, especially the particularly simple ones...
"there are no politics involved in this kind of research"
While it might not be true in this case, you couldn't be further from the truth. there is an EXTREME amount of politics. people's careers are based on entire assumptions, thesis', papers, and work could be all for not if based on false assumptions.
Yeah there's a huge amount of politics in research IN GENERAL, but not so much in physics. Like there are no big questions covered up because they're inconvenient or anything.
Dismissing a theory that is (now known to be) right is not necessarily political; it can be just a mistake. There are certainly cliques that are invested in believing the orthodoxy, but there are ways in the hard sciences of absolutely disproving that orthodoxy; and, though they are subject to fierce scrutiny, as they should be, they can be, and have been, accepted after surviving that scrutiny.
it can be but you offered no evidence contrary. the data WAS there, but it conflicted with their well established views, call them political or not - it doesn't matter.
it would take a great deal of hubris to say 'yes, all other sciences and research suffer from the human condition, but not physics!'
> Firstly, they function well enough to predict that gravitational waves exist, the ones which we measured being produced by an extreme gravitational event.
I don't think that aspect of the theories is in dispute?
Newtonian physics well enough at certain scales, but breaks down at others. The question is if there's something better out there and if we adopt something better, would it remove the need for all this "dark matter".
> Secondly, many other areas of physics have worked this out. The prime example of Quantum Field Theory and the Standard Model
Wait, what? Quantum Field Theory suggests the existence of dark matter?
>The question is if there's something better out there and if we adopt something better, would it remove the need for all this "dark matter".
I'm not sure about this, personally I don't know enough about quantum gravity to comment on that. Dark energy is a tricky question though because it is somewhat explainable within our current theories, and it is unobservable.
> Secondly, many other areas of physics have worked this out.
I don't know why I put this. I meant to put that QFT is an example of a theory which predicts many more phenomena than it was originally invented to describe.
MOND might explain rotation curves of galaxies. But it doesn't explain all the other places in astrophysics, where one needs dark matter. Especially MOND (and any modification to gravity) cannot explain the Bullet Cluster: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...
The Bullet Cluster is especially significant, because it shows gravitational lensing by dark matter spatially separated from visible matter. No modification to gravity can explain that without giving up the idea that gravity is a https://en.wikipedia.org/wiki/Central_force .
It seems like the takeaway is not that galactic cluster anomalies support or break MOND, but that MOND is a way to reduce the general astroöbject mass anomaly problem to a modified gravity plus the cluster mass anomaly problem. This makes sense if the cluster anomaly is tractable (?) and there are no other problems created by MOND (the real question IMO).
The problem then is that we want to have enough examples where MOND "works" that we don't think we'll run into another cluster anomaly. However, there aren't too many types of objects in the sky. So we either need very good numerical evidence for MOND in all other systems (unlikely) or a solution to the cluster anomaly which might not be too hard, unless MOND is actually wrong of course, and that's a risk you often face as a scientist.
1) "But MOND (or other modified gravity theories) predict that the "DM" should be found around the visible matter not elsewhere"; so they say. However, this last statement is incorrect. For example, in galaxies the baryons are concentrated at small radii but the putative DM according to MOND is far beyond that. "
It is important to stress the word "around". In Milgroms example the DM is still around the baryons, just far out at a different radius. This is the modification you can get, as long as gravity is still a central force in your theory. However in case of the bullet cluster the DM simply doesn't follow the center of the baryons. It is completely separated at a different place. I think he doesn't get this important point.
2) "As I said we do know that there is some (still) dark matter in clusters and that's what they found."
So Milgrom is admitting that even with MOND one needs some dark matter to explain the results. But with only dark matter we don't need to use MOND. So according to Occams razor the Dark Matter theory wins, because it explains more with less.
Is that part of the Pocket acquisition? I was comparing Safari and Firefox a while ago and Safari's reader view was one of the deciding factors in my choice.
This, whereas there's every possibility that dark matter isn't what we think it is, and holds some serious surprises, the evidence that there's a whole extra physical phenomenon (dark matter) that we've never observed seems pretty incontrovertible.
Dark energy, though, currently appears to be a "stuff we really don't understand" factor.
What is puzzling about MOND and to lesser extent about Dark Matter is that both uses violation of Newton mechanics to justify themselves when we know that Newtonian mechanics is wrong. The problem is that we cannot yet model behavior of galaxies using just General Relativity. Typically models assumes Newtonian physics with minimal accounting for relativity. However there is no sound proof that it is a valid approximation. The General Relativity is extremely non-linear and even small contribution may lead to very big consequences.
We cannot yet rule out that there are no problems with dynamic of galaxies. It can be that we model them using wrong approximations.
There is something that always puzzles me and I would like if something could illuminate me. How are those kind of massive scale astrophysical calculations made? Do they take into account every visible mass aggregate in the galaxy and simulate them, or just apply cinematic at scale assuming the galaxy centre is a point of certain mass?
I have played a little with N body simulations and I can't but be amazed about how numerically unstable physical simulations are, and how much the result can change depending on trivial simplifications.
I'm not much of an expert on the topic, but have also played with writing N-body simulations. At some point, obviously N^2 is unfeasible, so most simulations seem to use some approximation, such as the Barnes-Hut approach [1]. That approach divides the simulation space into an octtree. One can then use said octtree to approximate the force of distant quadrants by simply using the center of mass of an entire region. Due to the rate at which gravity drops off over large distances, it's "accurate enough", and fortunately the majority of "N" is "distant".
A good (but pretty shallow) overview exists on Wikipedia about other approaches [2] (under "Calculation optimizations").
EDIT: It's occurred to me that you were referring more to stability of the simulation, rather than what optimisations are used. I have less information to offer on that part, and I agree that it would be a huge problem, if complete accuracy was desired. I suspect, however, that in the kinds of simulations we're talking about, it's more about a general result such as "the whole galaxy should be divergent", not whether a single star went left or right when interacting with a black hole.
Actually in a lot of new HPC codes for doing physical simulation we are now starting to incorporate Uncertainty Quantification (UQ)[1] directly into the code such that we can determine where the instabilities and bifurcation points exist in non-linear systems. UQ is really expensive to compute, but it at least allows us to know when our simulations are accurate or are sensitive to perturbations.
we are witnessing here the power of the nonlinearities inherent in general relativity in the context of weak gravity to effect very significant changes relative to the results expected on the basis of Newtonian theory. It suggests that hitherto unexplained astrophysical phenomena
be re-considered on the basis of the application of general relativity to weak-field gravity.
I can deny the Earth is flat, because there is an abundance of evidence that it is round.
I can't deny that there are 650 elements, because as far as I know, other elements may be discovered. But I can certainly doubt it, and supply alternative theories.
I can deny allegations of wrong doing, because they either happened, or didn't.
But Dark Matter isn't assured yet. Denying it means it either exists or doesn't. But the truth may be a mix of MOND and something different but similar to Dark Matter. It isn't "fixed" into a yes/no situation.
That's evidence, not proof. We still don't have enough evidence to say conclusively that Dark matter is as we currently imagine it. And of course the same can be said about MOND.
I am not saying Dark Matter is something that actually exists that cannot be ruled out by other explanations (a revision of GR would be ideal IMO), just that this phenomenon makes the approach much trickier.
He expressed it badly. The point I think he is trying to say is that dark matter can be neither proven nor disproven at this time with the information available. An intellectually honest person should remain open to both possibilities.
You can say "I am not convinced of the dark matter hypothesis."
But unless you have information the rest of us do not, you can't really say "I deny the dark matter hypothesis could be real."
I [too] don't think it's about bad wording, yet pure bad logic.
The article is positioned on the principle that was imagined based on wrong assumtions of other theory and which acceptance will dissmiss the other theory.
I'm not even being close to think of being positioned on a side there, but afaic one could easily take one in this subthread.
TL; DR yep, dark matter could be denied no matter of the state of its understanding - just bring the better explanation of those irregularities its existence explains!
Acceptance doesn't dismiss theories, evidence does. A physicist shouldn't truly deny anything, just look at the evidence and say it is not the best theory.
Even with a better explanation, we can't be sure more information won't see the return of dark matter. Milgrom should understand this well, he's suggesting the return of the cosmological constant.
Forget dark matter, use the analogy from the article: afaik nobody has ever proven ether is not existing.
I know nothing about dark matter, but I could play a zero-sum game based on this article content - just the way anybody else could play - and outcome of that game is according to this article proven theory of this guy denies dark matter and dark energy theory.
And that's all, we could prove bad logic here without waiting for the third season of Dark Matter to be broadcasted.
This subthread deals with the completely different issue, it's about bad logic. No matter how well Milgrom and/or his theory could be discredited, that doesn't change the fact we can dismiss something prior to fully understand it.
It doesn't matter if that - what we dismissed -
shapeshifts into something completely different (or similar...) afterward, on the contrary, regarding resources spent it would be sad if not.
>This subthread deals with the completely different issue, it's about bad logic. No matter how well Milgrom and/or his theory could be discredited, that doesn't change the fact we can dismiss something prior to fully understand it
It's not bad logic, it only seems like it is because you're approaching it from an Aristotlean view of logic. These things can't be proven, so dismissing them is never an option in science.
No, as the entire concept of "fully understand" is ridiculous. We can't fully understand anything, so if you assume the answer is yes then we could dismiss everything.
The actual quote agrees with me here. If we understood it better the best you could do is doubt its existence.
>> could something be dismissed if we don't fully understand it?
> No, as the entire concept of "fully understand" is ridiculous. We can't fully understand anything, so if you assume the answer is yes then we could dismiss everything.
It's not a concept, it's just my lousy English and awkward abbreviation of "to the point where" with "fully".
I'm here just to play a game of logic.
Did we dismiss the ether at the point when we managed to understand it better/enough or we managed to dismiss it after we started to play with other cards?
It's not your English, I understand what you are saying, you're game of logic is using an old copy of the rules.
Science doesn't dismiss things. Theories about ether were put aside when we started getting experimental results that it could not support and developed other models that explained it better. The theory still exists, and there is a possibility that someone will use it to better explain things in the future.
Your logic boils down to "if we understand something, it can not be dismissed. If we do not understand something, it can be dismissed..." This doesn't work in science though, the first assumption can not be true, so the second ends up saying you can dismiss everything. Leibniz and other's noticed and addressed this flaw in logic in the 1700's, and expanded logic to deal with it.
I am not great at this logic either, but a laymen's version goes "if `theory` supports experimental evidence better than anything else, you should act as if it were true until a better theory is available."
Thank you very much for your effort here in explaining things.
There's nothing here in the thread powered by "my logic". I'm just certain that dark matter, ether, the Holy Grail, or anything, could be abandoned in our minds once we find the prettier chick to cope with, no matter how well we understood the previous one.
Yes, we could abandon it. The point of science is that we never should.
Before the scientific method was adopted, we would take that pretty woman, ignore any flaws, and make them essentially god. A new one had almost no chance to compete, even of they were much prettier.
With the scientific method, we say that she's the best around, but there's probably a better one out there, and with a little work it might be the old one. So it's easier to adopt new ideas, and the old ones still provide some value.
It appears to be a very common trend. Modern mass culture creates more interest in people's personalities than in anything else. I feel it every day myself, when I can't help noticing how easy it is for any serious discussion to quickly decay into talking about someone's personal traits, habits or preferences.
Humans have evolved for thousands of years to be social creatures, and to judge the merits of things that they may not understand by the characteristics of the person championing them. That's not modern mass culture, it's the way humans operate. I personally think it's also why many scientists fail to convince people versus (comparatively) charismatic novices.
The use of the word "deny" is deliberate, it's used to invoke the phrase "holocaust denier" to smear the person you're accusing of it. An underhanded tactic from the author of this hit-piece.
Actually what's disturbing (and I am a paid subscriber to nautil.us) is that nautil.us is duplicating content. They took that old interview and passed it off as new content in their latest edition.
You really ought to check the search funtion for a url before you post it. But the dupe detector/merger ought to be smarter, in a world where medium and feedburner, to take two examples, tack all manner of querystring garbage onto a url.
As per the HN FAQ (link at bottom of page), you can repost something if it hasn't gotten significant attention in the last year. The previous discussion of this story is only three months old.
Whether or not inflation theory holds long-term, I highly doubt that "most of current research will prove itself completely useless." Even if Dark Energy process to be done yet-undiscovered property of ordinary matter, the research on its effects and properties will still be very valuable.
We didn't throw out Galileo's research when Kepler discovered new principles, or Newton's when Planck and Einstein performed their research.
A.Meessen suggest a simpler explanation. Dark matter is made of electrically neutral elementary particles subject to scattering and a pressure. These particles may combine and dissociate which accounts for drak energy and the cosmological constant. The properties of all elementary particles (except mass) are predicted by his space time quantization (STQ) theory. It provides a blue print of all elementary particles. It predicts the neutral elementary particles that may constitute dark matter. There is no need to call back in question Newton's theory.
A.Meessen does pure science and studies whatever he consider of interest. Regarding UFOs he did inductive research. A priori we don't know if the reported phenomenon is real or not. To test the reality of the phenomenon you have to pose the hypothesis that they exist and see if this leads us to a logical contradiction. Even then, all we could say is we don't know if it's real or not.
As a physicist he studied their propulsion system. There was two main reason to do so. First, we know the physics law and their constrains well. Second, the witness reported the side effects of a totally unknown propulsion system. If witness were making up the data, logical contradictions would be found quickly because most witnesses are not physicists.
For instance, a witness reported seeing a UFO moving at very low altitude above trees. The strange thing was that the top of the trees were attracted by the UFO. Any conventional propulsion system would result in pushing the tree top away. If the UFO was real, how could we explain this observation ? What underlying physic mecanism could produce such effect ? That is how he started studying the UFO phenomenon. As a physicist he focussed in trying to identify a possible propulsion mechanism that could produce the reported effects. The only hypothesis he used was that UFO are real and that they may use the known physics law in a way yet unknnown to us, an original way.
You have to initially suppose the UFO data valid to do this reserach. It can't work by supposing UFO don't exist.
This research led him to two significant results. First the PEMP, a pulsed EM propulsion system [1]. Second, a new type of oscillator using a supraconducting surface that can generate very intense EM fields required for the propulsion [2]. It may not be a coincidence that a supraconducting shell would also protect occupants from the very intense oscillating EM field. Since humans can't produce supraconductive materials at room temperature, there isn't much room left to suppose a human origin of UFOs. The logical and scientific consistency, as well as the sound theory does also point to the reality of the UFO phenomenon. A.Meessen has data that you and scientists don't have to make their opinion, although it is published and in open access.
The idea that UFOs are not real is just a belief supported by a few bogus cases and, I guess, by the fear of ET with far supperior technical skills than us. The idea that UFOs are real is now supported by scientific results. It's just theories that are waiting experimental validation. But at this stage we don't need to make an hypothesis about the reality or not of UFOs. These theories can be tested today in a lab with our technology.
This is pure science, not crack potery. Sorry for your beliefs.
Beside, the validity (or invalidity) of his STQ theory can't be inferred from his work on ufology because they are totally independent. If his theory on STQ is bogus, then say in what it is bogus. Otherwise, you are in the gossip domain, not science domain.
This is really great. It has always bothered me (as a non-physicist) for precisely this point:
"Sure, if there was dark matter, the speed of stars would be greater, but the rotation curves, meaning the rotational speed drawn as a function of the radius, could still go up and down depending on its distribution. But they didn’t. "
It is not. Dark matter is not some smooth distribution that could be explained by adjusting the formula for gravity. There are places where it is, and places where it isn't. Not only that, but it has its own mass and inertia.
Not only that but dark matter is missing in a host of other ways that can't be explained by the laws of gravity being more complex. Microwave background radiation is 30x more powerful than can be explained by visible matter, but matches exactly if you add in dark matter and dark energy. The ratio of hydrogen to deuterium is inconsistent with visible matter, but matches perfectly if you add dark matter.
Galaxy clusters collide and we can literally see dark matter pass through the collision since it doesn't interact.
Dark matter is as well proven as the atom was when we made the bombs. We just don't know what it's made of yet. You simply don't know enough about it.
Dark matter isn't just dark, it doesn't interact with normal matter. When galaxies collide when can verify via gravitational lensing that the normal matter smashes together while the dark matter continues unimpeded in straight lines through each other.
I think you got caught up by the misnaming of 'dark matter'. It's better to think of it as 'transparent matter'. It's non-interacting, not like a Dyson sphere which would obviously be interacting e.g. from it blocking what's behind it even if it itself makes no emissions.
A dyson sphere would block the light of stars sitting behind it on straight line of sight from us. That doesn't happen with dark matters. It's transparent. Unlike dyson sphere.
We're compelled to choose the option which requires the fewest assumptions. Dyson spheres involve a level of assumptions far beyond dark matter and dark energy. There would have to be Dyson spheres everywhere, that do not radiate heat, and that support some civilization that leaves very few traces.
Dyson spheres emit lots of light, just at lower temperatures than that of the star they encompass. You're basically running a Carnot engine with the heat reservoir at the temperature of the star, some number of thousand degrees, and the cold reservoir at a few degrees kelvin.
Clouds of gas and dust don't emit light, yet we know they exist and can estimate their mass. Couldn't we apply the same methods to Dyson spheres if this was the case?
The fact that someone with more expertise is warning that a discussion is going off the rails is a useful, constructive fact. Demanding that that person explain in detail why in order to contribute is a good way to make sure they don't bother.
The fact that someone is knowledgeable about a subject doesn't exempt them from being a constructive contributor to a discussion.
Additionally, no where was it "demanded that you explain in detail". I don't mean to be too blunt here, but there is quite a huge gap of comments between a detailed explanation and what you posted.
As an example: "There were a number of things incorrect in this post. The largest one of them was stating X when in fact Y. For more reading on it, look at Z."
And notably this response required fewer words than those spent defending your initial comment.
1. The proper question is "When a commenter is very wrong on a technical topic, is it net-positive for someone with expertise to point this out even if they don't contribute further?". I think the answer is obviously "yes". You seem to be more concerned with whether people are pulling their fair share, vs. being exempt/special, which I claim is the wrong question.
2. Effort scales with detail. The example comment you give is 3 times the effort, and I wouldn't have made the original comment if it required that much detail. My comment took me a few seconds to write, and merely tracking down a URL link for explanation would have made it not worth it.
3. Rebutting the N-th confused post on HN about dark matter doesn't interest me, but the meta point of what contributes to a discussion does interest me.
As the official self appointed Em-Drive denier here, my recommendation is to provide more information in the comments. Sometimes it's difficult. Most commenters here are not experts in the particular field, but are curious. Sometimes it's better to center in the worst part of the comment if it has too many errors.
Sadly, I don't know enough cosmology and general relativity, so I use this approach to this problem https://xkcd.com/793/
I think that general relativity give minimal corrections in the scale of a galaxy, only some local corrections that are important near black holes and some spots. So it's enough [citation needed] to use Newtonian physics, perhaps with some correction near the galaxy center and perhaps a small effective field with the average distribution of matter.
To track an individual star during many galaxy revolutions it may be necessary to be more careful, specially if the stars pass near another star. But to have a good estimation of the rotation speed it is possible to almost ignore general relativity.
Question time: Do the current simulations in the area use special/general relativity corrections? Do you have a link to a recent paper? A link to a simulation program? [bonus point if the link points to the option page to enable/disable relativity or select the approximation method] Is there a Wikipedia page about this? [Most of the times the Wikipedia page is easier to understand than the paper.]
I reviewed the credentials of the person who made this parent post I am now responding to. I found him to be an exceptionally qualified expert and published scientist. He not only has high levels of credibility to make his statement, but is very likely to be the most qualified person in the entire thread to comment on the subject.
Thus it is very interesting and fascinating that his comments in his own field of expertise were downvoted by people who are almost certainly ignorant about the details of topics in his field. Also astonishing in its hubris was the ultimate detachment of the entire thread and the pronouncement by an unqualified individual that it is not on topic despite its clearly being on topic and exceptionally relevant.
Such are the dangers of fascist systems where unqualified individuals rule with an iron fist. The result no doubt will be similar to that of the Soviet farming system when ruled by small minded unqualified bureaucrats rather than experts in the field.
Dark matter appears, to a layman with some historical knowledge, as nothing more than modern day epicycles. When we can't explain something with the models we have, and have to invent magic terms to balance the equations, that indicates that we really don't know what the fuck is going on.
>One of the recent most convincing things was the bullet cluster as described here.[1] We saw two galaxies collide where the "observed" matter actually underwent a collision but the gravitational lensing kept moving un-impeded which matches the belief that the majority of mass in a galaxy is collisionless dark matter that felt no colliding interaction and passed right on through bringing the bulk of the gravitational lensing with it.
How can you possibly even begin to explain that without dark matter?
We don't understand how matter acrually works on a large scale. Inventing imaginary terms that we can't observe to satisfy the equations in the models to match the observed reality seems a poor practice.
We understand that when two galaxies hit each other, the visible mass gets all tangled up and suns and nebula smash into each other. Meanwhile the invisible mass continues onwards, oblivious to everything else. We CAN observe it and we have.
Seriously this is basic object permanence. We can see hidden things moving around by themselves. Dark matter is still there even when there's normal matter on top of it.
Right it's like when people say, I don't believe in Creation, that stuff is just nonsense. And you say, so what's your alternative theory then? And they reply, well there was a "big bang" and everything just sort of appeared, for no reason. Really, that's the alternative explanation?
Well yes, if you ignore all the evidence of something then you can confidently say there is no evidence.
Christian genesis: There was a garden of Eden, and a god.
Evidence: No garden of Eden, no evidence of angels or god, no reason those things would suddenly stop being apparent. Direct contradictory evidence that YHWH[1] was a minor god in the original jewish pantheon rather than an actual creator god.
Big Bang: The universe started for an unknown reason and rapidly expanded outward.
Evidence: The universe is visibly expanding outwards and formed in a way consistent with an explosion.
Did you think that the big bang theory was just something someone thought of, and then they looked for evidence? Evidence came first, then the big bang was proposed.
> Christian genesis: There was a garden of Eden, and a god.
That's a really bad example to go to.
The tale of Eden follows a poem, and if you take both literally, then you end up with contradictions.
For why? Genre.
Neither were intended to be literal. They were designed to convey meaning.
> YHWH[1] was a minor god in the original jewish pantheon rather than an actual creator god.
Origins are highly disputed, but El seems to be the most likely title, who is the head of the Caananite pantheon, and influential enough to be remembered in Babylon. Not a minor deity.
I understand you were responding to a statement which sort of ignores the basic premises of science, but that doesn't mean your narrative should ignore a scholarly approach to documents whose origins predate writing.
>Neither were intended to be literal. They were designed to convey meaning.
Even Genesis has direct geographical references to it. It has directions. Calling it a metaphor is moving the goalposts. Ad reductio Abrahamic religion is simply that god made whatever we can't understand.
It's not like I'm saying the world wasn't made in seven days. I'm saying that god said we were kicked out of where we came from and would never be allowed back, and that place doesn't exist. I'm saying god says he talks to us and appears to us, and those things don't happen. The basic, central tenants of the bible aren't real. These are important things, not minutia.
>Origins are highly disputed, but El seems to be the most likely title, who is the head of the Caananite pantheon, and influential enough to be remembered in Babylon. Not a minor deity.
That's not true. The introduction of Yahweh to the Semetic religion was very visibly followed by a shift in personal names referencing El (eg Michael) to names referencing Yahweh. Yahweh was an individual, entirely separate god that aggressively replaced El, and El was relegated to an increasingly minor role. The thing called YHWH in 1200 BC is a 4th tier divine warrior, and the thing called YHWH now is a sole god.
Bare minimum, the Semetic religion was demonstrably polytheistic before it was monotheistic, and that alone massively undermines the bible.
> Even Genesis has direct geographical references to it. It has directions.
Treating Genesis as a single book is insane. Its a collection of oral traditions. Multiple authours, genres, and hundreds of years separate each tale.
> geographical references to it. It has directions. Calling it a metaphor is moving the goalposts. Ad reductio Abrahamic religion is simply that god made whatever we can't understand.
Eden isn't a metaphor, its an allegory, using as its basis a common Mesopotamian myth, as was common at the time. Its origin lies in the story of Lilitu.
The directions to the Euphrates are in Babylonian and Egyptian myths too. It was an area considered symbolic of the spiritual world. You get similar directions to forests that never existed in Japanese myths.
And as a people, the Caananites, and their descendants, were incredibly symbolic. See Kabbalah's number system, and references to it throughout the Bible.
> The introduction of Yahweh to the Semetic religion was very visibly followed by a shift in personal names referencing El (eg Michael) to names referencing Yahweh.
Firstly, "Yahweh" was a best guess about an unspoken name that wasn't allowed to be written in complete form. We are fairly certain that guess was wrong, and the actual name was "Elohim". They're the same name, and yes, the same spelling. (We transliterate to English using sounds in this case).
El wasn't replaced, nor were his attributes. The Caananites set out to seize their promised land, backed by their warrior god.
> the thing called YHWH is a sole god.
Not to begin with, in Biblical record. Or commands such as:
> I am a jealous god, you will have no gods before me
Wouldn't make sense. Nor would the Caananites hospitality to those of the Ba'al religions.
The Biblical record shows this, and it doesn't undermine the record. The Psalms frequently reference belief in other gods.
The first reference to monotheism comes in the Davidic era, after Israel has been established, but it isn't entirely fixed, and instead is something that slowly took hold over a great deal of time.
Different scholars and theologians have different theories on the why of that.
> that alone massively undermines the bible.
I wasn't arguing for or against it. I was simply explaining a couple of documents in a large collection have different intended meaning than that which you gave it.
We don't simply lift out parts of Herodotus and think that we should take his mentions of Achilles literally.
The Bible is a collection of documents. The meaning ascribed to them doesn't exempt them from treating them as ancient documents.
I'm sure they seem insane to you, but no moreso than our ancient Greek records, or the Roman ones either.
There is no evidence that locations such as that were meant as allegories. They grew from real sacred places, as appear in other religions. As cultures grew the religions simply grew with them and was re-interpreted. The metaphor is not the original meaning.
>Firstly, "Yahweh" was a best guess about an unspoken name that wasn't allowed to be written in complete form. We are fairly certain that guess was wrong, and the actual name was "Elohim". They're the same name, and yes, the same spelling. (We transliterate to English using sounds in this case).
I have no idea where you're getting this from. They are quite different. Aleph lamed he yud mem vs. yud he waw he. There was heavy redaction in the 5th century BC in favor of "Elohim" but YHWH definitely existed first. Elohim in ugaritic originally meant "children of god" ie the pantheon under El, which Yahweh was a part of. Later, with the revisionist history, Elohim became "God of gods". What's more, YHWH is forbidden to speak aloud whereas elohim is not.
There was an outright movement as YHWH became more important and El became less important. It wasn't just a name change. They literally switched, and even after Yahweh became the most important god he still didn't fully absorb El's traits until later.
>Wouldn't make sense. Nor would the Caananites hospitality to those of the Ba'al religions. The Biblical record shows this, and it doesn't undermine the record. The Psalms frequently reference belief in other gods. The first reference to monotheism comes in the Davidic era, after Israel has been established, but it isn't entirely fixed, and instead is something that slowly took hold over a great deal of time. Different scholars and theologians have different theories on the why of that.
I dropped a "now", that was meant to say "the thing now called YHWH is a sole god". But even in Kings, Baal and others are no more than false idols eg Ahab, Jezebel and Elijah.
>The Bible is a collection of documents. The meaning ascribed to them doesn't exempt them from treating them as ancient documents.
I did not mean to imply that. Certainly it is not nothing. However it is not realistic evidence for a god any more than the Greek pantheon. We know that there are no gods on top of Olympus. The hyperbolic and contradicted aspects of the bible are likewise not real.
>>Christian genesis: There was a garden of Eden, and a god.
We'll, technically, Genesis starts with a universe filled with water, and science stars with a universe filled with hydrogen (that can be detected) and potentially other stuff that can't.
On the outside, both stories are the same and have gaping holes in their logic.
Creationism is nonsense and has no place in a scientific discussion because it's not a falsifiable scientific theory. Thanks for derailing this discussion.
It's still a nontrivial observation that the equations can be corrected to account for reality most simply by adding a term for unseen matter (rather than some factor or some other mathematical operation).
Maybe the term doesn't have a straightforward physical explanation but there's a long history of introducing math without any corresponding mechanistic explanation---this is what gravity itself was until general relativity, nothing more than an inexplicable inverse-square attractive force.
If a Newtonian model of gravity doesn't "explain" gravity, then neither does general relativity. These are all different models of reality with different levels of model complexity and predictive accuracy, but there's no fundamentally binary difference between them in their power of "explanation" etc.
So new models like "dark matter" or "MOND" are all just different models that model observations better than prior theories but currently have the problem of being difficult to unify with the rest of theoretical physics.
There was/is nothing magic about epicycles. Epicycles was part of a model which described the movement of the planets. Now we have better and more precise model, but that does not mean epicycles was "magic".
There is nothing magic about the the word "dark matter" either. Yes indeed it describe a phenomen we don't understand, this does not mean it is magic.
Using epicycles just means to decompose the motion into its Fourier series. It may not be the most elegant description, but its not wrong per se, either.
> Using epicycles just means to decompose the motion into its Fourier series. It may not be the most elegant description, but its not wrong per se, either.
One issue here is that (as I understand the history) epicycles were not originally proposed in the context of "just add them until you've got something that looks right"; it was proposed that, occasionally, one needed to add one epicycle. After a while it was found that even this didn't work, and we needed sometimes to add another. I forget whether the theory collapsed before instances were found where we'd need a third.
This, I think, is 'wrong' in the sense of physics: if your model is constantly scrambling to catch up with newly acquired data, then it has no predictive, and hence little scientific, value. Further, the very ability of epicycles constantly to scramble to catch up—the ability of trigonometric polynomials to approximate continuous functions arbitrarily well over arbitrary bounded intervals, as you mention—is also a mark against them: a scientific theory that is so general that it could be tuned to predict anything can, in fact, for that reason actually predict nothing.
Epicycles were added to make the model better fit the observations. There is nothing wrong or unscientific about this. It is just like new planets are added to the model of the solar system when new planets are observed.
> There is nothing wrong or unscientific about this.
To be sure, there is nothing wrong with changing your model in response to data—what would be wrong would be the opposite. What I was calling 'wrong' was two things: (1) a model that can only be tuned post hoc to observations (that is, it's always being changed to fit them, and never predicting new ones); and (2) a model that is so flexible that it can be tuned to fit any observations (again, because it will never have the ability to predict future observations).
The predictions by the system is the movements of the planets in the sky. The Ptolemaic system with epicycles actually did this pretty well, considering the available data. It is just that as better observations and better calculations became available, the model have to be more precise which required more epicycles.
I don't really get the thing about "too flexible". According to Wikipedia there is apparently some myths floating around regarding the science history of epicycles, but I assure there is nothing magical about them - at least they are not any more magical than Newtonian mechanics.
> I don't really get the thing about "too flexible". According to Wikipedia there is apparently some myths floating around regarding the science history of epicycles, but I assure there is nothing magical about them - at least they are not any more magical than Newtonian mechanics.
I think we may be talking at cross purposes here, but I wanted to give it another shot. I do not think that there is anything magical about epicycles, and my objection to them is scientific; it has nothing to do with their history. My claim is that a scientific tool is valid science only to the extent that it has predictive power. If your model has so many parameters that, by setting them to appropriate values, you can fit any outcome, then (absent some other data telling you which values of the parameters to use) you'll never actually predict anything, only scramble setting them to 'post-dict' existing observations—which is, effectively, useless.
(For example, any finite collection of observations can be 'post-dicted' by a high-degree polynomial, and we can accommodate more observations simply by increasing the degree of the interpolating polynomial. This is not magical, and mathematically totally sound; but its very flexibility means that it is scientifically useless, because there's no reason to believe that our perfect fit to the `n` values we've seen so far says anything about the (`n + 1`)st value we'll see next.)
OK how is it you think epicycles differ in this respect from say the keplerian model of the solar system? I struggle to understand what you think is "special" about epicycles. No model of the solar system will allow you to predict the movement of the planets if you don't enter the correct parameters, which you have to find through observation. Given the correct initial parameters, the ptolmaic system with epicycles will yield predictions about the movements of the planet, predictions you can verify or falsify through observation.
> OK how is it you think epicycles differ in this respect from say the keplerian model of the solar system?
Just to be clear, I'm not in any way insulting the intelligence of those who proposed them. It was a clever mechanism for dealing with what seemed at the time to be a blemish on an otherwise beautiful theory, and the fact that it turned out to be (factually) wrong does not mean that it was wrong to propose and test it.
With that said, here's my stab at what I see as the difference. The Keplerian model predicts elliptical orbits. To be sure, one must do some tuning of parameters to find the semimajor and semiminor axes of these ellipses; there is no way for someone sitting in a windowless room, with access only to Kepler's laws, to predict exactly, say, the length of a Jovian year, or even the existence of Jupiter.
However, what is crucial is that that person sitting in a windowless room can say, definitively, that, if Jupiter has been observed in these five positions at these five times, then it will be in this position at this sixth time. If it's not there, then the Keplerian model is wrong; it has been falsified.
Now suppose instead that another person in another windowless room knows about, not just Kepler's laws, but also epicycles. Given the same five measurements, he or she can make a prediction (which won't require epicycles). This prediction might or might not agree with the sixth measurement, but, if it doesn't, then that's no problem; just add on an epicycle to account for that measurement. If the result of that correction doesn't agree with the seventh measurement, then just add another epicycle.
To me, these two systems are meaningfully different. It is not the reliance on parameters to which I object, but the extreme flexibility; a model that can literally never be falsified, because it can always be adjusted to match new observations, is worthless as a scientific model.
That would be a valid criticism if epicycles were added to the model post-hoc to explain previous observations, and adding those epicycles to the model did not improve future predictions.
> Is that really how you think epicycles were used?
Yes. So, if that's not true, then you've been right all along, and my objection was mostly founded on a misunderstanding of the history. Do you have a reference I could read?
(I should say that, even if it's not actually how they were used, it's still how they could have been used, and I think that the spirit of my objection applies; but the actual practicalities of it would certainly be undermined by my historical misunderstanding.)
In short: Before the Ptolemaic system, it was assumed the planets moved in circular orbits around the earth. The problem is, this did not explain the phenomenon of "retrograde motion", which is when a planet apparently slows down and moves backwards in its path over the sky, as observed from the earth. Ptolemaus added the epicycles which describes the planets as additionally moving in smaller orbits along the path - just as the moon moves in a smaller circle around the earth, which would result in retrograde motion if observed from the sun. In other words, the epicycles explained a phenomenon and allowed the model to better predict the future movements of the planets.
The thing is, this model is not wrong and makes reasonable correct predictions about the motion of the planets as observed from the earth. The Copernican model is just simpler because by describing the planets as orbiting the sun rather than the earth, you don't need as many epicycles in the model. The Tychonian model, by the way, solves the same issue by having the planets orbit the sun, but the sun oribiting the earth, thereby additionally solving the parallax problem which the Copernican model could not explain given the knowledge at the time. All three models needed additional epicycles, which Kepler simplified by describing the orbit as a elliptical.
You will note these models are actually equivalent in modern understanding, where neither the sun or the earth is at the center of the universe.
The current model of the solar system still have epicycles (eg. for moons), so if the presence of epicycles somehow turns a model into pseudoscience, then what?
Journalism misleads people very hard about physics research. Physicists dont run around claiming they've observed dark matter. That's the wrong direction. Physics has a problem with gravity because some things don't behave in such a way that makes theory agree with observation. Since we are talking about gravity here, we call the thing we are observing "matter". Simply because things that interact with gravity have historically been called matter. Because its "dark" in the sense that we can't see (observe) it, its called dark matter. As a placeholder for something that has not yet been explained.
Then journalists come around and spin this tale about how physicists have found this super magical stuff called "dark" "matter" and its so exotic if you dropped it onto des moines it'd turn the place into honolulu.
Dark Matter is just the placeholder name for an observation that disagrees with conventional theory.
But dark matter is theorized to he a particle that we haven't observed yet, which sounds like "matter" to me. The specific idea to fix up the discrepancies is, there are huge clouds of invisible particles. There are competing theories on what sort of particle they might be, but dark matter theories predict that these particles are out there.
again, thats what theoretical physicists do. Someone makes a weird observation and your theory is left with a hole. You try to plug it.
They don't preach these things. Physicists don't run around preaching unprovable holy grail theories. They just make something up. Out of nothing. Because its their job. They need to come up with falsifiable theories that can be verified in lab experiments.
In order to do so, they HAVE TO hypothesize something. They can't just say "err dark matter right?". They have to say "there could be a particle with these properties, a mass of roughly X, producing a cross-section of Y when bombarded with tau-leptons etc.". That's just the scientific method.
Any physicist who postulates particles does that from a position of "it probably doesn't exist". The same way when you measure something to be faster than the speed of light, you assume that a cable is broken.
Then some journalist comes around saying "lol paper says weird particle exists, scientists are whack". That's annoying. But it happens.
If journalists wrote about CS the way they write about physics, the next time someone loses 2 BTC from their wallet, they'd claim that thats hard evidence of quantum computers being invented by north korea. Hypothetically plausible but as outlandish as permissible by reality.
Dark matter isn't theorized to necessassrilly be a particle either (finding new symmetries in the standard model would be extremely surprising)- it's literally just a placeholder name for a massive (as in, exhibits mass) unknown.
Yes, one way to restate my post is that dark matter could or could not be a new particle. The important part is that dark matter proponents assert there is something(s) whose mass we can detect, but whose other properties we can't measure as it doesn't fit into our current physical models.
Even as someone who would rather MOND be true than dark matter, I have to admit that dark matter is internally consistent in ways that MOND is not.
By comparison to the theories of planetary movement (since you seem to love comparing dark matter to them so much), MOND is rather like the Copernican model and dark matter rather like Kepler's model. It's well-known that the traditional equations are off. MOND provides a fix that starts off looking better, but the more precise you do the measurements, the more you need to add the fudge factors in to make the numbers come out right, and it ends up being just as bad as the original--which is a reasonable description of the evolution of the Copernican model. By contrast, both Kepler's model and dark matter provide a much more accurate picture with a somewhat more complex addendum to the original non-fudged equations. But they also both lack mechanistic explanations for their origin: Kepler's model ruined the traditional explanation of how planets moved and inherited the stellar parallax death-knell from Copernicus's theory.
In the case of planetary motion, it took the development of Newton's Law of Gravitation and the derivation of Kepler's laws of planetary motion from said theory to provide a sound mechanistic basis. We lack that mechanistic basis for dark matter, but that doesn't really mean that dark matter is inherently wrong.
In other words situations where we see twin images of a very distant galaxy because light curves around intervening mass, without sufficient mass observable to explain the deflection?
I was under the impression that we could map dark matter localized to a precision that tended to rule out measurement error or a general divergence from Einstein's laws, unless it had a peculiar non-uniform distribution.
Edit: maybe somebody can explain these maps - http://www.sciencemag.org/news/2017/03/scientists-unveil-mos...
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