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All disk galaxies rotate once every billion years: study (astronomy.com)
490 points by clayt6 10 months ago | hide | past | web | favorite | 185 comments

This is extremely suspicious for various theories of dark matter, right?

How is it that all disk galaxies have just enough dark matter in just the right distribution to make up for the various radii and distributions of visible matter and end up rotating at the same rate?

I have no answers, but as they say, the most interesting words in science are, "Huh, that's funny..."

The constant rotational speed is the observation that requires dark matter to exist. Otherwise, small galaxies would rotate faster just like close planets do.

Dark matter theory tells why galaxies rotate faster than they should. The fact that this speed is constant requires some other explanation.

It's not "constant" though, it's "cosmologically constant". Cosmologists would routinely assert that 1 == 1000 holds true.

Couldn't a large galaxy rotate as fast as a small galaxy, without the need for dark matter, if the large galaxy had a sufficiently more massive black hole at its center than the small galaxy.

The formula for the orbital period around a central mass is:

               3  1/2
           /  R  \
  T = 2 Pi |-----|
           \ G M /
where T is the period, R is radius of the orbit, G is the gravitational constant, and M is the mass of the central body. That's Newtonian, but I think that is good enough for this.

This says that if Big Galaxy has a radius k times that of Small Galaxy, it won't rotate slower than Small Galaxy as long as Big Galaxy's mass is at least k^3 times Small Galaxy's mass.

You are right that this could explain the velocity at a particular radius, but it's the radial velocity profile as you go out that hints at dark matter. The velocity as a function of radius is strange.

I don't believe it is just one thing. I believe the observed shape of galaxies is another problem that is dark matter is a proposed solution.


I was just reading more and there is also the fact that galaxies stay together at all. Apparently without including dark matter, they wouldn't "hold together." Sounds like the problem with the atom back before quantum mechanics. :)

Dark matter also has a measurable gravitational lensing effect, as I understand it.

I don't think so. Dark matter still behaves like any other massive object, as far as I'm aware, beyond the obvious issues with detecting it.

It may be more an artifact of the size of each disk galaxy being related to the amount of mass in that galaxy, and perhaps nearly all the angular momentum each galaxy still has is from one source: the big bang. In other words, a galaxy with X amount of mass will have Y amount of angular momentum from the origin of the universe, which will naturally disperse to a size Z as a function of X, Y and the time passed since the origin of the universe. So maybe it's all one big exercise in conservation of angular momentum.

Dark Matter was first discovered because the galaxies are not rotating at the correct amount, if you only consider the 'light' matter we can see. That all the dark matter halos should rotate at this rate, despite the decoupling from light matter, is strange given what we know about dark matter [0]. Obviously, more data needs to be taken, but the result that all the galaxies are rotating at the same rate is not something we should suppose from first principals.

AFAIK, dark matter is kinda thought of as a sparsely non-interacting gas. Over the eons, the dark matter particle with enough momentum to have escape velocity have mostly evaporated from the galactic halos, leaving the light matter galaxies and large clouds/halos of dark matter. As we have seen in the Bullet Galaxy collision, the halos really don't interact, thus dark matter is non interacting.

What then makes the similar spinning rates of galaxies interesting is that the halos of dark matter should not be the same size, given that they don't interact. The halos should not all be the same size, they should be different sizes. If they are different, then the rotation rates should be different too.

Just like a figure skater pulling in her arms to spin faster, the smaller galaxies should be spinning faster. This was the original problem that made us look for dark matter, the galaxies are spinning faster than we think they should. Big galaxies should fling themselves apart. We now think that all the galaxies are just embedded in dark matter halos, solving the angular momentum issue. Essentially, the light matter is like an ant on a spinning Frisbee.

But if the galaxies are now all spinning at the same rate, and that the amount of light matter in a galaxy is independent from spin rate, that must mean that all the dark matter halos are of roughly the same size. Which sounds crazy, thus the news article.

If anyone with a better understanding is out there, PLEASE let me know where I am making a mistake. Thank you!

[0] Essentially all we know about dark matter is that 'it falls down', in that it interacts gravitationally and not in really any other way. This is opposed to so-called dark energy (the stuff driving the acceleration of the cosmos) in that the dark energy 'makes things fall up.'

I'll try to help:

The paper itself is at http://s3-ap-southeast-2.amazonaws.com/icrar.org/wp-content/...

From the very first paragraph it takes cold dark matter as a given, and refers back to its reference MMW98 many times.

Two key sentences in the Conclusions: "While R_max appears to mark a sharp truncation in the [luminous] disc of galaxies, it does not enclose all baryons. Stars in the halo are distributed to much larger radii, and their kinematics indicate the dark matter also extends further, likely to the virial radius."

So this is the wrong paper to be looking for departures from the standard structure formation (a good overview of which is http://www.esa.int/Our_Activities/Space_Science/Planck/Histo... )

> that must mean that all the dark matter halos are of roughly the same size

No, the issue here is that the luminous "surface" of the carefully selected sample of galaxies is nearer the cores than a number of previous numerical simulations of galaxies firmly rooted in the standard structure formation. The paper discusses a number of possible reasons for this, including (very bluntly) "Baryonic physics is messy" [introduction, second paragraph] and "Theory and observations indicate that feedback from star formation ... or active galactic nuclei ... can rearrange the distribution of baryons, and in the process drag along dark matter ... into an altered distribution, affecting all scaling relations" [ibid].

As to the coreward surface, "... our results are best explained by a true physical truncation of discs. Whlie the formalism presented this far implies continual accretion limits the extent of discs, section 5.4 considers other scenarios for limiting the extent of discs [.......] [including] the limitations in the angular momentum in an initial proto-galactic collapse ... truncation in star formation due to disc stabilization ... ionization by the UV background ... and spreading of the disc due to angular momentum transfer."

So baryonic gas and dust falling (back) onto galaxies is their favoured model for squashing the luminous matter inwards, and they look for evidence of old (and likely to explode) stars beyond the luminous edge (but well inside the edge of the CDM halo) as a source for some of that gas and dust. They have a number of ideas about why such stars may be in the halo in the first place, consistent with the standard structure formation model (but not precluding at least one or two other models).

> Essentially all we know about dark matter is that 'it falls down', in that it interacts gravitationally and not in really any other way.

It doesn't fall down much because it can't radiate away its angular momentum in a scattering interaction with other DM or baryons. So particle DM in the halo tends to stay in the halo, rather than migrating inwards. Assuming the CDM is collisionless, inward migration is solely because of gravitational interactions, which are extremely weak and thus a very slow way to ditch enough momentum to descend to a lower orbit. The lingering DM constrains the lower orbits available to baryons when they collide and ditch momentum radiatively, which is why there's still so much luminous matter outside the core.

> opposed to so-called dark energy (the stuff driving the acceleration of the cosmos)

In the standard cosmological gauge (which takes a specific slicing of 4-spacetime into 1+3 time+space, and treats all the stress-energy in the bulk as strictly inertial in that slicing, represented as compressible homogeneous fluids (each with a particular density/pressure relationship, remembering that pressure is the inverse of tension) under constant spatially isotropic tension, and imposes a set of coordinates that fix on Eulerian objects in the baryonic matter fluid) the "comoving" coordinates from one spatial slice to the next are related by the cosmological constant. In this representation, all the matter fluids (baryons, dark matter, radiation) dilute away -- their density decreases -- with the metric expansion. However, one of the fluids has constant density and tension (i.e., negative pressure): it does not dilute away.

If we consider vacuum de Sitter space in the cosmological gauge, i.e., if we take the above and remove all the distracting matter and radiation fluids and focus on an expanding spacetime which is empty except for this constant (positive) rest density and (negative) pressure, then we can work out that the pressure must be -1/3 of the energy density. The strictly timelike worldlines of Eulerian obsevers in this setup diverge with the expansion of the universe. There is no acceleration felt by any of the observers, but they calculate mutual recession distances that rise extremely high at large spatial distances (and large spatial distances are more common in the future).

> dark energy 'makes things fall up.'

The critical point is in the previous paragraph: if our galaxy clusters are practically always Eulerian observers, they don't feel the effects of the expansion as they do their own internal gravitational interactions. If we have a galaxy cluster of non-radiating dark matter, nothing falls up and away from it during its history from the beginning of the dark energy dominated epoch. (Real clusters in the DE-dominated epoch will radiate photons at the very least, but would do so even with no expansion; any ejected matter is thrown out into inter-cluster space by internal processes, not by dark energy.)

Note that we are not required by nature to use the standard cosmological gauge, but we do have to preserve the central observables of the spacetime geometry and the observables of galaxy clusters: stitched into the expanding spacetime (well-modelled by a Robertson-Walker metric) are concentrations of matter that source real metrics that asymptotically decay to (near-enough-to-be-practically-indistinguishable-from) Schwarzschild at reasonably short spatial distances. The crucial thing in that is that these Schwarzschild spacetime patches DO NOT EXPAND, but the (effectively vacuum) RW spacetime they're stitched into does. If you start playing around with the expanding part you can choose a bunch of different ways to "explain" the features observed by an astronomer in one Schwarzschild patch examining the radiation originating at other Schwarzschild patches, but the standard cosmological gauge is hard to beat in terms of simplicity.

When I learned about dark matter I remember it being discussed about how the spin more like a solid rigid disk (ie: homogeneous mass distribution) vs a non-rigid (non-homogeneous) disk. That the distribution of visible matter did not predict the observed rotation behavior (moment of inertia problem, not center of mass. Though angular momentum is key).

An analogy for a non-rigid disk would be like swinging a flimsy plastic pipe. You'll notice that it is not straight when spinning and the far end lags the hand holding onto the rod. Vs if you spin with a metal pipe, the whole thing is rigid and your hand and the tip of the rod are in the same place.

The figure skater analogy is usually used in discussions about center of mass and moment of inertia (figure skater pulls arms in and the center of mass changes, but the energy is transferred into spin energy). Which rigid disks have a different moment of inertia than a non-rigid disk (eg: hoops spin different than disks).

There is some evidence that dark matter is self-interacting: https://arstechnica.com/science/2015/04/new-evidence-that-da...

Love the ant on frisby analogy ;)

For clarity: the finding is about the rotation of the galatic structure (e.g. the spiral arms), not the orbital speeds of the individual bodies, which is different. Google the "density wave" theory of galactic structure for details.

Mass distribution is obviously a factor in that analysis, but it's not as critically determined as you think.

No, the measurements are of the redshift of hydrogen in the galaxy so it's talking about the orbital speeds. Also this quote from the press release: "if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round.”[1]

[1]: https://www.icrar.org/cosmic-clocks/

It's a bit more abstract than that. The finding is actually that there's an approximately linear relationship between the measured distance of the outermost edge of a galaxy from its centre, and the average linear velocity of objects at that distance.

Define a rotating circle based on those two measurements, for each galaxy. These hypothetical circles will all have similar angular velocities no matter how big they are, hence they all take roughly the same amount of time to make one rotation.

Perhaps we are all in a simulation and the galaxy rotation is tied to a single clock source. Developers are sometimes a little lazy.

I'd love to see that datasheet.

"To use the PLL (Planck-Locked Loop) as an oscillator source, use the 1-billion year galaxy tick as a source and set the PLLMUL register to divide by its maximum scaling factor of 1.855 * 10^52."

  function getRotationSpeed() {
    //just make it a billion for now, we'll randomize in the next version
    return 1000000000;

Probably a bug in using that function, though: 1 billion years is the rotation period, not the rotation speed. The rotation speed is 2 * pi * r / (1e9 * 365.25 * 86400), where r is the distance from the center of rotation (in meters).

Maybe that's the problem with dark matter: objects get heavier when speed increases, so since the speed is 18 orders of magnitude too large, everything is heavier than it ought to be. All because of an incorrectly named function.

> Maybe that's the problem with dark matter: objects get heavier when speed increases, so since the speed is 18 orders of magnitude too large, everything is heavier than it ought to be. All because of an incorrectly named function.

Reminded me of "Tlön, Uqbar, Orbis Tertius" by Borges:

> By 1942, Tlönian objects began to inexplicably appear in the real world. One of the first instances in which this occurs is when Princess Faucigny Lucinge received via mail a vibrating compass with a Tlönian scripture. Another instance is witnessed by Borges himself: a drunk man, shortly after dying, dropped coins among which a small but extremely heavy shining metal cone appeared. It is suggested that these occurrences may be forgeries, but yet products of a secret science and technology.


Spec said "make things spin" and so they did. After all, this isn't a rogue-like game with random procedurally generated content and permadeath,

or computation and/or memory space is limited! so trade-offs are needed

Isn’t it just as suspicious absent dark matter? Seems just plain weird regardless.

If you accept the notion that all disc galaxies are oriented on exactly the same plane (which is current scientific consensus I believe), then it doesn't seem that far-fetched to think that they all rotate the same way as well.

They're not all aligned on the same plane, they're somewhat aligned along the matter filaments and sheets (including the suspected dark matter) that make up the large scale structure of the universe.

"Galaxies are not distributed randomly in the cosmic web but are instead arranged in filaments and sheets surrounding cosmic voids. [...snip...] We found evidence that the spin axes of bright spiral galaxies have a weak tendency to be aligned parallel to filaments. For elliptical/S0 galaxies, we have a statistically significant result that their spin axes are aligned preferentially perpendicular to the host filaments;..."


"Galaxy shapes are not randomly oriented, rather they are statistically aligned in a way that can depend on formation environment, history and galaxy type."



"If you accept the notion that all disc galaxies are oriented on exactly the same plane (which is current scientific consensus I believe)"

That is easily disprovable, no? We can see many many different angles of disc galaxies. A simple google search will show tons and tons.

I agree. It looks pretty random. Its probably even more random since we can't reliably eyeball the chirality in most of photographed galaxies so half the angles in one of the axis are practically mirrored at first glance.

That observation in itself is not a disproof.

(Imagine standing in a room with a bunch of CDs suspended horizontally from the ceiling. From any vantage point you can see some CDs edge-on, some from the top, some from the bottom, etc. But they're all oriented in the same direction.)

This situation is trivially detectable though. Simply plot the inclination angle of disc galaxies as a function of azimuth and inclination, if it's like you say, there will be a very clear banded structure.

That analogy does not hold up when the CD you are observing from is in the same plane as all the others.

I suspect that in your example you would observe a number of different apparent orientations, but that there would be some types of apparent orientation that could never occur.

That boggles my mind, do you have any sources I can read to start digging into on this?

Not sure why a question like this would be voted down but my own searches have only turned up articles about galaxies spinning in the same direction.

Someone else commented with some clarification. https://news.ycombinator.com/item?id=16586901

Ok, my mind is boggled too... Anything we can read on the topic ?

What topic? Basic geometry?

I feel like they might have noticed if it was that simple.

Galaxies are distributed throughout a volume of space, they don't even exist on a single plane.

Edit: here is a video which clearly shows the orientations of rotation are not aligned.


Maybe their projections? Like all their axis of rotation are parallel?

We see some galaxies from top-down so their axis of rotation cannot be parallel to any galaxy we see side-on (like the milky way)

Did you maybe mean: small, satellite galaxies show up in the same orbital plane as the main galaxy's disk.

"which led them to conclude that the outer rims of all disk galaxies take roughly a billion years to complete one rotation"

I was curious about "outer rims". I know little about astrophysics, but is the "outer rim" a definable thing? Because my sense is that, like a whirlpool (and planetary physics), material within a galaxy have different rotational rates depending upon their distance from the center of mass. It seems odd to describe "the spin" of a galaxy at all — but since they specify "outer rim" I expect that to have an agreed-upon definition.

Actually a galaxy's spin curve significantly diverges from a what we'd expect, like a "whirlpool" or say a solar system. It turns out stars on the rim do not complete orbits significantly slower than ones nearer to the center. It acts more like a frisbee. They're moving faster than they should but not getting "flung" out.

It is one of the reasons why dark matter was originally postulated; basically the galaxy system is much larger/massive than what we can see.

Adding a "Halo" of massive particles that are weakly interacting explains the curves fairly well--hence Dark Matter.

In case anyone gets the wrong idea, when dark matter is referred to as weakly interacting in the literature, as in WIMPs, they are specifically discussing particles that interact via the Weak force, not merely particles that don't interact very often. It's a good idea to keep the usage of that phrasing limited to avoid confusion.

Well... Sort of. The 'weakly interacting' means it interacts via gravity and another force no stronger than the Weak force. As per Wikipedia [0]: "There exists no clear definition of a WIMP, but broadly a WIMP is a new elementary particle which interacts via gravity and any other force (or forces), potentially not part of the standard model itself, which is as weak as or weaker than the weak nuclear force, but also non-vanishing in its strength."

[0] https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...

I think the radial scales on which galaxies are frisbee-like (i.e., the tangential velocity is roughly proportional to radius so that rotation period is constant) are much smaller than the scales on which we notice the discrepancy from naive expectations that suggests dark matter. The discrepancy in the velocity curve is at radiuses where both dark-matter and non-dark-matter models predict that velocity increases sub-linearly with radius. (It's true the dark-matter models are closer to linear than the non-dark-matter models, both are clearly distinct for it, and hence show whirlpool-like motion rather than frisbee-like motion). See the third figure here:


Dark matter is considered a likely candidate for catalysing galaxy creation. This could lead itself to keeping the ratios of regular matter to dark matter densities being largely the same across most galaxies.


Theory: If #gravity were modeled as a constant egress flow of spaceelevator harpoons coming out of each atomic planet, the scale invariance of the rotational frequency of #galaxies would not be surprising. No need for #DarkMatter, gravity isn't a field, but it can come in waves.

I just hope when the next simulation starts my mind gets carried forward.

I hope there's a reroll option. My build is interesting but I can't figure out how to play it.

it won't, these are stateless containers

It could also be that dark matter is distributed proportionately with observed regular matter.

This is extremely suspicious for various theories of dark matter, right?

Whoever wrote the procedural generation for this universe forgot to update the debugging scaffold code in the galaxyRotationRate() function.


Thanks for reporting this bug, it'll take just a few moments to fix it and restart the simulation ...

It'll be a day, though, it's Wednesday and the CI only runs on Thursdays.

Nah. It should definitely be a Monday!

So, now it has restarted perhaps the Dev can tell us what the fix was?

I'm only a very small passively interested layperson here, but does this seem much faster than anyone else thought?

I looked it up and a "galactic year" for our sun is only 250 million years. Meaning our planet has been around the galaxy 18 times since its formation! That seems like so much faster than I would have thought!

This is indeed fast. In fact, it's much faster than they should be spinning, going by the visible mass; a galaxy spinning at the speed ours is ought to tear itself apart.

It doesn't, which is a lot of the reason why we think dark matter exists.

Here's a video on the subject: https://www.youtube.com/watch?v=Dx1Wf84bC2M

As an alternative, could we also be wrong about the gravitational constant at a large scale, or perhaps it's not really exactly an inverse square? Or maybe there's another relatively weak force that's only apparent at a larger scale that maybe only drops off linearly so it's predominant at large distances?

The Google term for that if you want to read more about it is "MOND", MOdified Newtonian Dynamics. It's not the theory favored by the consensus, but it's not out of the fight yet.

There's also TEVES which is MOND with relativity, since MOND doesn't predict any of the relativistic effects we observe constantly.

The problem is, they still end up needing some dark matter before they work very well. I think every bright person assumes dark matter must be fake when they hear about it, but nobody has succeeded in getting rid of it. Dark matter plus the physics we know about is actually a really great fit to what astronomers see, and it has been hard to improve on.


Yes, I should say that I'm not necessarily advocating for it, just giving a hint as to where to look the ideas up.

Personally I'm just fine with either. Particle physics makes it pretty clear IMHO that the idea of a weakly interacting massive particle is perfectly reasonable, because the particle zoo that we have is already rich far beyond "normal matter" and adding a few more isn't conceptually too difficult. (It'll break our theories, of course, but we kinda already know there must be something wrong with them, so that's not a huge shocker.) And the idea that physical laws may not be exactly perfectly the math we thought they were is also not that shocking to me, for all kinds of reasons, again not least of which is that we know something is wrong with our current theories.

Awesome. Thanks for the starting off point!

Density wave theory [1] posits that the "arms" of a spiral galaxy are more like stellar gravitational traffic jams than persistent objects-- stars pass through and clump up in arms, but the stars in each arm change through the eons.

If you plot mass extinction events against when the solar system transits through the arms, eleven extinction events in the last 500 million years occurred while going passing through one of these high density regions. [2]

[1]: https://en.wikipedia.org/wiki/Density_wave_theory [2]: https://arxiv.org/abs/1309.4838

I find this concerning. When is the next time the sun will do this? are spiral arms a great filter, or effective semi-regular eliminator of advanced galactic life?

Fast or slow compared to what? If you think of it as 18 times in a third of the age of the whole universe it doesn't seem particularly peppy.

... Also, don't the disks spin at the same speed/uniformly? Meaning closer to the axis things go slower than on the edges?

That's the apparent paradox in the galaxy rotation curve: https://en.wikipedia.org/wiki/Galaxy_rotation_curve

Mright. Looks like someone left the default value in the universe's config file.

Almost as bad as the bug where they made light the same speed in all reference frames. I heard they didn't even fix the bug, they just put in some wonky fixes that mess things up when you go really fast or get close to the world boundary, since they figured nobody would ever do that!

I think that's a feature rather than a bug -- or, rather, a requirement for preventing buffer overflows. A hard wall at the limits of the addressable memory would be too obviously artificial, so that was a no-go. But time-dilation allows them to push faster particles into lower and lower priority threads. A lot of handy optimisations there.

No, where I think they really screwed up was in level of detail -- you know, where you get to smaller scales and start generating details procedurally rather than pulling geometry out of memory. I mean, I understand why it's necessary to do that -- who wants to store the position and momentum of every particle in the observable universe? -- but they could have at least faked some kind of continuity between observations, rather than calling Math.random() every time!

And that damned issue with shared memory of entangled particles. That will not end up well.

It's not really shared memory. It's kind of a compression hack that coalesces a particular state matrix of a whole set of particles into a single vector and when you read out the state of a particular particle it unfolds this coalescion by partial orthogonalization, starting with a random state vector from the spanning vector space.

You're guaranteed that for any other particle of the remaining set, the state vectors are orthogonal to the state you just read out. If you do the experiment with two entangled particles, by reading one, you'll immediately know the state you're going to read for the other one.

If you do it for more than one particles, for each state you read you reduce the size of the remaining set of vectors that my come out.

From a compression/encoding point of view it's kind of neat. If you do it for lots and lots of systems of many particles in a certain microstate, on average you're going to end up with nearly identical results for each total readout process, although the precise values and the order in which they appear will vary wildly.

Now because all this iterative state unfolding more or less comes down to be a kind of hash function you want to make sure, that users of the middleware don't rely on hidden internal state, or assume some kind of hidden seed. The downside to this is, that this particular implementation detail destroys locality, which kind of goes against the whole idea of the fixed-event-propagation-differential system, that aims to isolate high energy processes from neighboring parts of the simulation by easing their timespace metric.

There are a few corner cases (which actually came to happen in a lot of instances in the simulation), where out-of-bounds stress-energy densities are (successfully) isolated from the rest of the simulation, leaving visible to the rest of the simulation only a meta-description of the contents inside the region, that boils down to mass, charge and spin (where due to some interesting interaction charge and spin happen to have the same kind of visible effects on the outside timespace metric) and the surface area of the boundary region. However right at the boundary region, the cursor iterating over the aforementioned state vector unfolding may cross into the isolated region. At first it looked as if this could break the simulation. But it allowed for a wonderful hack for an incremental garbage collection inside the isolated regions, by treating the whole isolated region as a single meta particle, holding N instances of the state vector, where N is proportional to the surface area of the boundary region. Randomly selecting one of the quasi frozen states from inside the isolated area, we can call its destructor, be unfolding its complement an entangled particle that happens to by just outside of the boundary region.

This goes nicely with another hack, introduced early in development: The on-demand spawning of entangled particle/antiparticle pairs, which can be used to transmit forces between the actual particles you want to simulate.

By applying these on-demand spawns on the isolated regions, it turns out, these regions can be garbage collected, by kind of "evaporating" their contents through a entropy maximizing process, thereby avoiding the need to faithfully reconstruct the original information; instead the remaining hash value is uniformly distributed over the simulation and used to seed the entropy pool from which random numbers for the unfolding process are taken.

Kind of neat, don't you think?

Oh you're good.

Watch them quietly deploy a fix now that they know we've noticed.

Hmm...the simulators have the power to rewrite our memories...they might erase memory of the glitch from out minds after they deploy the fix...

I wonder...how many times have they deleted our memories already?

If you guys have not watched HBO's westworld, you should. It goes so much further on this situation.

In a simulation, the beings running it have so much power that there is probably nothing we could do to stop that. (Although if whatever the simulation is running on has connections to outside, it is theoretically possible we could exploit some bug from within the simulation to make changes outside that make their memory wipe not work...but this is probably extremely unlikely. It requires us to discover a bug and understand its implications and exploit it before the operators figure out they need to do a reset and wipe).

But this reminds me of another memory wipe situation I've wondered about. One of the common things in many tales of alien encounters is the aliens messing with the memories of those who interact with them.

Unlike with simulation operators, who are essentially gods to us with omniscience and omnipotence and completely unconstrained by any laws of science or logic that we know, aliens would presumably be constrained. They would not be omniscient and omnipotent--just more advanced than us.

So with aliens we would have hope of combating their memory wiping, or at least detecting it.

For example, if you often drive at night in isolated areas where you might be particularly vulnerable to alien abduction, you could keep some innocuous physical item in your car, such as a book with a bookmark in it, or a Rubik's cube, or a cassette tape of a band you hate. Pick an item that you can put in a certain state that is alterable, and have a standard state you keep it in. The bookmark is always on page 100, the Rubik's cube is always solved, the tape is always rewound.

If you ever see anything strange that even suggests "UFO", you alter the item. Move the bookmark to page 110. Add a couple twists to the cube. Start playing the cassette.

If you remain aware of the possible UFO until it goes away or you figure out what mundane thing it is you are seeing, you fix the item.

If, however, you either remember that you saw something but don't remember what happened, or you don't remember anything weird but find you have lost time, you can check the item and if it is out of its normal state you know that you thought you saw a UFO and now you don't remember it.

The idea here is that the stories of aliens usually include something making much of our technology fail so that we can't record them, and they also probably know enough about to us to recognize when someone who sees them writes a note on paper and so deal with that. But unless they have a way to read minds or extract and interpret memories, they probably won't recognize that, say, starting to listen to music is actually a form of note taking, and so won't know they need to rewind the cassette before they let you go with your memories wiped.

So...anyone of you actually do anything like that?

This is the theme of a few episodes of Doctor who. Characters marked there skin with a pen/sharpy, every time they saw an alien. The aliens automagically wiped the memory of anyone who saw them.

Honestly, I specifically suggested that we have a separate config file for each galaxy. Now look, they're all spinning at the "same speed."

So, we are living in a simulation?

I think it's very likely: https://en.wikipedia.org/wiki/Simulation_hypothesis#Ancestor...

It's not a testable claim right now, but it might be in the future: https://en.wikipedia.org/wiki/Simulation_hypothesis#Testing_...

Your link only provides a way to falsify a theory, not provide evidence for simulation.... which is impossible by definition.

Not to mention the idea of a simulated universe is, you know, philosophically boring and implies mostly false things in most peoples' minds.... like an anthropomorphic scientist god. In reality, it would change virtually nothing about how we view our universe.

> not provide evidence for simulation.... which is impossible by definition.

I disagree.

Science cannot prove that a theory is 100% true, only that it is not wrong by repeatedly testing the predictions made by the theory. The best we can do is say: this theory (e.g. general theory of relativity) is the best explanation for data we have so far and every experimental prediction it has made has come true.

The paper (https://arxiv.org/pdf/1703.00058.pdf) linked on the wiki page proposes four experiments that seeks to test the simulation theory. None of these experiments will single-handedly prove the simulation theory, but if they all pass we can only keep testing and seeking alternate explanations.

If the experiments keep passing and no alternative theories can be found, then we're either in a simulation, or the universe just happens to behave exactly in the way that a simulated universe would, but it isn't. I agree that we can never truly know which is true, but at that point the difference is reduced to semantics. It's like saying electrons don't really exist, instead they're just wave/particles that are exactly like electrons in every way except for some immeasurable quality.

This assumes the entity running the sim will never directly influence it, of course if that ever happens (requiring a _very_ high burden of proof) it'll be proof of the sim.

I’m saying the semantics of a simulation are boring. I think the actual tests are pretty cool. :)

Dunno about you but if I were pentesting a machine and found out it was a VM, then maybe I'd try to get a foothold into the hypervisor, host, or sibling VMs. Not boring.

That metaphor is ill fitting unless there is an anthopomorphic entity designing flawed software. It smells a hell of a lot lite Pascal’s wager.

Maybe, or we just don’t understand all of the rules.

Could be an artifact of the preconditions required for the formation of disk galaxies.

A spiral/disk galaxy's mass is proportional to the square of the radius:

Mg = C1 * Rg * Rg

where C1 depends only on shape and is basically the same across all the spiral/disk galaxies.

Orbital speed of stars in the disk is proportional to the square root of the galaxy's mass (the observed absence of the other otherwise must-have orbital velocity equation component - ie. 1/SQRT(r) - is what causes crack pot theories like dark matter) :

Vdisk = C2 * SQRT(Mg) = C2 * SQRT(C1) * Rg

where C2 also dependents only on shape and is basically the same across all the spiral/disk galaxies.

Thus Vdisk = C * Rg where the C is pretty close to being the same across all the spiral/disk galaxies.

Angular speed of a star in the disk at the distance "r" from center is

Vdisk / (2pi * r) = (C * Rg) / (2pi * r)

which at the edge of the galaxy, ie. where r = Rg, becomes

(C * Rg) / (2pi * Rg) = C / 2pi

ie. the same constant across all the spiral/disk galaxies. The end.

> "...crack pot theories like dark matter"

Out of genuine interest, do you have any citations from reputable physicists who agree dark matter is a "crack pot theory"?

I think he was being sarcastic. The point is the basic law of gravitation is being violated here, and it's like, zero point energy or something.

not really. All the works postulating dark matter from the flat speed curve that i've seen so far are using basic orbital speed formula (sqrt(M/R)) which is only applicable to either spherically-simmetric masses and/or large (compare to the mass containing radius) distances between the interacting masses. Such simplification is just not valid for disk star orbital speed calculation because 1. disk galaxies are far from being spherically symmetric and 2. distance between a star and the galaxy center is of the same scale as the galaxy's radius. Very detailed Newton based calculation of those orbital speeds taking into account the shape of the galaxy mass distribution produces much flatter curve and, for example, for Milky Way to get a flat curve matching observations i needed to speculate only additional 20%-30% of galaxy mass to be present in the galaxy halo (like gas and dark dwarfs of all kinds which some works suggest there are and even that may be unnecessary - need to find and check some additional observational data for that) instead of additional 900% of dark matter that typical "dark matter" works speculate for.

Of course you are completely wrong:


There is a great deal of evidence for dark matter that does not depend on Kepler's laws, please stop spreading misinformation.

Source: assistant editor of a physics journal, know several astrophysicists personally, currently dealing with a MOND paper this very week...

Finally! Someone else made the same observation I did. All that "expected" rotation curve stuff is bullshit since it's based on Keplers laws, which don't apply to a flat disk.

They do that based on a misapplication of the divergence theorem. They assume that all the mass inside an orbital radius can be treated as a point-mass which is true for uniform spherical distributions. They also assume the mass outside the radius has no net infuence - which is true for uniform spherical shells. Neither of these is valid for non-uniform distributions - a flat disk in not symmetric in 3-space. If they'd just do some electrostatics problems they'd know the different formulas for disks and rings. I was going to google a sample page but there's too many to sift through and I gotta run an errand.

Welcome to the club - it's incredibly small...

I really encourage you if you are actually interested in the arguments to progress beyond the entry level explanations. Binney & Tremaine (https://press.princeton.edu/titles/8697.html) is the standard work - I highly recommend it. At the very least, it will put to rest your notion that astrophysicists only consider spherically symmetric potentials or fail to consider the effects of a disks etc., which is just patently untrue.

And here I thought Hacker News was going to get the Nobel Prize! I can't believe quickly deriving something from first principles also occurred to the people devoting their lives to this field.

Thanks for the link. They have a .pdf of chapter one which I gave a look. Right there on page 14 when estimating the mass of our galaxy inside our suns radius we have:

"The approximation that the mass distribution is spherical is reasonable for the dark halo, but not for the flat stellar disk. Better models suggest that this estimate is probably high by about 30%, since a disk requires less mass to produce a given centripetal acceleration"

And there you have it. While he notes that a spherical distribution is different than a flat disk, he estimates the error at about 30%. The problem is that the error is not one of a constant factor, it's a different function. This unspecified better model also doesn't seem to care about the matter outside the solar radius - which also affects our velocity in a disk, but not in a sphere.

There is a lot of math in chapter one, but also a lot of hand waving.

Yet DM is so sexy and mysterious ... Even Sheldon couldn't resist the temptation.

What are your credentials, if you don't mind my asking?

He can do the math himself.

Whereas professional physicists can't and haven't taken those maths into account?

That would be correct. I wondered if the problem was me reading things like Wikipedia which says the expected galactic rotation curve should follow something based on Kepler's laws. Then someone got me a subscription to Nature... You know the prestigious "peer reviewed" journal where the serious stuff is. Don't get me wrong Nature had some really awesome content. But there was a paper about a galaxy found in the early formative stages and the authors measured a rough rotation curve and felt the need to compare it to something Kepler would predict. Ugh.

If anyone can tell me why Kepler applies galaxies (vs 2-body systems with certain constraints), I'd be happy to show where the error in that analysis is. Kepler doesn't apply to galaxies. It's really that simple.

All the math?

Credentials are irrelevant here as what I'm talking about is high school level integration of Newtonian forces.

And you, personally, correctly calculated the actual expected rotation?

I can't imagine the amount of superiority you must feel to assume that physicists don't know high school level integration of Newtonian forces.

I used to work in a well-regarded Astronomy and Astrophysics department at a research university. Every few months, I would go through the department "crackpot folder" when I was especially bored.

Every couple days the department chair or some well-known professor would get a letter (this was before most of the crackpots had email accounts, I guess) claiming that some theory--particularly general relativity--was wrong and all it took was a simple application of high school arithmetic to see it.

One of our faculty happened to coin the phrase "dark energy" while I was there, and man did the letters start coming in after that. Of course, the only absolutely universal commonality among all of the letters was that they were all nonsense, like the above.

In general, I felt sort of bad because you could tell that a lot of these people were really not well, but it's still hard for me to see things from someone's perspective who genuinely believes that the entire astrophysics community (save a tiny handful of contrarian actual physicists) has somehow engaged in a massive conspiracy to ignore introductory physics in favor of some other vastly more complicated theory for absolutely no possible benefit. (Especially when these professors are literally teaching introductory physics to students while they're pursuing their research.)

There was a time when the entire scientific community believed the entire universe was filled with a light-bearing medium (luminiferous aether) because they couldn't explain propagation of light in empty space; or that there was another planet between Mercury and the Sun (Vulcan) because they couldn't explain Mercury's orbit.

I'm not claiming it is the case now (though it does smack of luminiferous aether a bit) but the scientific community can and has been wrong sometimes in the face of as yet undiscovered principles.

I think you might have missed my point. I'm not saying that dark matter skeptics are crackpots because they don't believe in dark matter. I'm saying that if people think they can disprove the existence of dark matter because you can apply some high school arithmetic and claim that Keplerian law is violated, then you are a crackpot.

Someone else unwittingly suggested a parallel to Fermat, which I think hits the nail on the head exactly. If someone thinks they can prove Fermat's last theorem with some high school geometry, odds are pretty good they're a crackpot. If they're a well-regarded mathematician and they can build off new mathematics and make a few very complicated leaps by discovering new mathematics of their own, then people take them more seriously.

Scott Aaronson has a really great take on this here: https://www.scottaaronson.com/blog/?p=304

As an aside that's probably only interesting to me, I went to a Great Books college. It's one of those hippy dippy liberal arts schools where there are no electives, no textbooks, no lectures, just primary sources. You learn mathematics by reading Euclid, Ptolemy, Newton, etc.

At any rate, for first-year "Labratory" class, you trace the atomic theory from the Greeks to roughly Avogadro. Along the way you read papers by and replicate the experiments of many influencial scientists along the way. When I mention that the above crackpots are "wrong", I don't mean it in the sense of the aether or analogous theories along the way to the atomic theory, namely phlogiston or caloric, two previous theories of heat. Those theories did not explain new observations and were rightly discarded along the way. Priestly and Lavoisier were brilliant and tried to make theory match observation, so now even though their theories are discarded, their work still stands on its own for the time. I wouldn't call them wrong. There's a definite difference between them and theories that are just wrong.

Nobody's going to get a Nobel Prize when they just make the world believe that they have obviated the need for dark matter by pulling out some high school math and some Kepler proofs.

Someone will win it if they ever make an observation that proves a MOND theory in a way that shows conclusively that dark matter is not needed to explain all current observations and in fact cannot remain compatible with this new observation.

There's a big difference between the two, I think.

>"Someone will win it if they ever make an observation that... shows conclusively that dark matter ... cannot remain compatible with this new observation."

This requirement is impossible to meet for sufficiently flexible theories. Eg no one will ever "win" over "God did it" according to your metric.

Also, if you see how many people in high academic positions are obviously wrong about simple stuff they use every day like p-values, nothing will surprise you any more.

Regarding your first point, that's not what I meant and you know it. Dark matter is the current theory explaining many different astronomical observations. It explains quite a lot of observations, so something pretty conclusive will need to happen for people to abandon it. Either new observations will disprove it or a new theory will come about that makes some prediction that can be observed that cannot for with current theories. I hope something like that happens! New science would be so much cooler than dark matter, but at the moment it doesn't seem likely.

I'm not sure I see how your second point is relevant. You seem to imply that I think that academics are infallible in all areas not even related to their expertise. Do you truly see no difference between non-statistics researchers incorrectly calculating statistical values and dozens of unrelated experiments all leading to the same result?

Flexibility is a main feature of the dark matter explanation, so it is relevant. It amounts to putting otherwise undetectable halos of mass wherever is needed to explain deviations from the model. There are further constraints people put on it for now, but none are crucial and will be quickly discarded (or more dark matter added) if required.

>"I'm not sure I see how your second point is relevant. You seem to imply that I think that academics are infallible in all areas not even related to their expertise. Do you truly see no difference between non-statistics researchers incorrectly calculating statistical values and dozens of unrelated experiments all leading to the same result?"

This is series of strawmen... I'm saying widespread confusion can, and currently does, exist on a topic even amongst the experts. Therefore it is not unbelievable that it does on other topics as well.

Also, dark matter is an interpretation of a result, not a result itself, so an experiment cannot lead to it directly. Finally, I don't think there are any actual experiments that have supported dark matter, only astronomical observations. Correct me if wrong but I found this after a quick search: https://www.sciencenews.org/blog/science-ticker/results-slew...

>"non-statistics researchers incorrectly calculating statistical values"

Actually another point is that this is not the issue. The calculations are fine, they are just calculating something other than they think: p(Data | Hypothesis) when they want p(Hypothesis | Data).

That is very similar to what the OP claimed about calculating a model that assumes a sphere to describe a disc (no idea if they are correct on that).

I feel your pain :) Back then the most such fun we had were Fermat theorem proofs coming to the algebra department.

Newtonian forces are a generalization anyway. Do they even apply to objects as large as galaxies?

I don't mean to just be a pedant when I say this, but the use of the word crackpot is missplaced here.

Crackpot theories are eccentric and not commonly believed.

I am interested in crackpot theories because sometimes they end up turning out right, and I think within that dynamic there are important lessons for humanity that we end up learning in very hard ways.

For example, the Great Depression paved the way for the acceptance of Keynesian economics--ideas previously written-off as crackpot theories. Another: NASA engineers warned of the exact dangers of a foam strike, under the exact conditions in which it took place, were at the time written-off by senior management. Social scientists that studied this described part of the problem as Groupthink.

Possibly naive question from a non-physicist... does this not follow from relativity as a natural a consequence of the effect gravity/mass has on spacetime?

By that I mean that the edges of galaxies experience a "reference" spacetime (e.g. constant period), while the interiors experience a stretched out version spacetime (i.e. inverse time dilation and length constriction)?

no, the point is that it follows from basic algebra, and high-school physics defitions of velocity.

Highschool definitions of velocity don't really apply once the distances involved grow beyond a couple light years. Atleast not without corrections for stuff like relativity (which has been measured and confirmed to exist by simply observing curvature of light over the sun.

Or did the entire community of astrophysics simply ignore high school physics in favor or something vastly more complicated because reasons?

Even at galactic scales the correction is not that great. I don't thing the OP's claim is that they are exact to say 3 sig figs but more like exact to within an order of magnitude

But how does the proposed model compare to the accuracy of the dark matter model?

In physics, how well your model can predict past and future states from the current state is rather important. Though future states more than past state but any model should be able to also somewhat agree with it's the past it generates.

dark matter is irrelevant. The speed law is empirical (whether or not you explain it with dark matter) and the first poster points out that the constant edge rotational rate falls out from that, and geometry.

It is relevant when you compare models. All I asked is how accurate both models are. If Dark Matter is more accurate in predicting how galaxies move and shape then it is obviously the better model even if a simpler one also exists but it's predictions are worse.

You can't simply say "this model also predicts that" without giving on how accurate it does predict and comparing to existing models. Otherwise we'd be using flat earth models for building bridges and planning ship routes.

The original poster's original comment didn't make claims about the validity of dark matter, it was only in a different subthread that he talked about dark matter directly, challenging it. The original comment's content is invariant on how you derive the distribution of stellar velocities.

Also not a physicist. At the galactic scale gravity forces are too weak to take into account any non-Newtonian considerations. At least for myself, most things observed so far at galactic scale, like flat disk star orbital speed for example, can be easily explained by straight 6th grade Newtonian physics.

sigh I wouldn't have replied to this if you hadn't posted half a dozen comments. When I was 16 I thought special relativity was a hoax and I tried to demonstrate this with triangular diagrams scribbled in pencil in my notebooks. I ended up just rediscovering the Doppler effect. I never showed it to anyone, but when I think back, I still cringe a little. Relativity and time dilation are real things with real theory and real experiments.

You'd do well to do a little more research on the topic before posting so many comments on HN.

There is a ton of evidence for dark matter. We still don't know exactly what it is, but we do know lots about it (e.g. it is not baryonic matter because observation differs from theoretical models of how it would interact with ordinary baryonic matter). As for all that other evidence, the first hit on Google is for laymen: https://medium.com/starts-with-a-bang/five-reasons-we-think-...

TLDR: gravitational lensing, missing mass, and three other really good reasons that dark matter is a serious subject of study.

Disclaimer: I am not a physicist. And I'm trying not to represent myself as one on HN.

Talking about this stuff is fun, sure. It's great to learn! But please don't spread ignorance with such high confidence. As for "just" Newtonian physics, there's a whole research field called "Modified Newtonian Dynamics" (MOND). https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

TLDR; there are lots of problems with MOND, and it doesn't explain gravitational lensing and other effects of dark matter that we see.

Another fun experiment is to write script to cover a disk with points, then do a full calculation of all N gravitational forces at a point along the radius. Calculate the acceleration of a mass at that point and back-calculate the velocity that would orbit with that inward force. Repeat this for many points along the radius and plot the rotation curve without doing any dynamic simulation. There can be some interesting numerical issues with this but a random set of points to "measure" the curve will still show it.

For anyone interested in why there's a lot more to it than this argument, the first equation:

Mg = C1 * Rg * Rg

Is not a good assumption for the discs we observe in the universe, because they do not have a constant surface density. The picture is further complicated by the fact the many disc galaxies also contain a central bulge, large enough to effect the potential (to say nothing of Dark Matter).

> However, the researchers note that further research is required to confirm the clock-like spin rate is a universal trait of disk galaxies and not just a result of selection bias

Exactly, and that's why the article's title is misleading and clickbaity. The article also basically copies the press release [0] line for line.

Still, if this is true that's a pretty groundbreaking discovery.

Perhaps we are witnessing a previously undetected force at work, one that operates on much larger scales with much larger masses.

[0] https://www.icrar.org/cosmic-clocks/

This is strikingly scale invariant. I know it seems insane to ask it of something that big, but could it possibly be an effect that is bootstrapping up from the smallest scales?

What do you mean?

These are galaxies of very different sizes but yet sharing an oddly similar time signature which have another similarity, that is that they have developed without a major collision since the big bang, given they are disk galaxies.

I am asking whether there could be a scale invariant function acting here that makes the rotation of the galaxy more dependent on age rather than size, that got started when the vortices that ended up as galaxies were much closer together and at much smaller scales.

That's 3.17 * 10^-17 Hz

Most accounts claim that thanks to dark matter, stars orbit at the same speed regardless of distance from the galactic center. They don't say whether this is linear or angular velocity, but even if we assume it's linear velocity, how can it be that the sun (which is half way out) orbits every 0.25 Gyr and stars at the rim orbit every 1 Gyr?

Sun at 26 Kly and the outer edge stars at 80 Kly have the same 200km/s orbital speed, and as result have 3 times difference in the orbital period.

Seriously, can't the sheer speed of the inner components of a galaxy cause the gravity variance we attribute to dark matter? Wouldn't the speed increase the mass of the objects and therefore also their gravity?

The sheer speed you're talking about isn't nearly high enough for mass to change significantly. You need to get very close to the speed of light before this is the case.

Not that I have looked at the numbers myself, but I would think physicists would make those calculations using General Relativity, in which the effect you mention is already considered, not Newtonian Mechanics.

It's a bit like this: https://xkcd.com/1758/

Perhaps, due to folding in a higher dimension, all galaxies are one galaxy in different 3D 'cross section'

is this not normal? Like how planets will orbit a mass of star or black hole at a certain distance depending on their speed?

Curiously, the turning effect we observe is not coming from stars moving in an orbit around the centre of the galaxy. When we plot the movement of the light coming from the spiral arms (or bar) of a galaxy, given their size and speed of movement it would require the stars to be moving faster than the speed of light. Also if the spiral arms were made up of orbiting stars, the ones closer to the center would move faster than the outer stars causing a winding effect, which we do no observe.

One theory suggests that we are observing a density wave sweep through material, which causes compression and heat, increasing the likelihood of new stars to be born, these stars burn bright and white. As this wave passes through, it leaves behind those stars in its wake, which over time burn less brightly than before.

One analogy that comes to mind an electronic circuit, although electricity travels at the speed of light, the actually charge carriers are moving relatively slowly, know as the drift velocity.

Homopolar motor, maybe?

I was just going to say: all 33-1/3 rpm records - regardless of their thickness or chemistry - are observed to rotate at about the same rate, regardless of record size.

And (I'll note that) the drive belts are usually hidden from view.

I’d actually love to get an explanation for this. Do galaxies work similar to solar systems in that all objects are in orbit around a central mass or is it a different phenomenon that binds matter together into a galaxy?

The mass of the central black hole is a tiny fraction of the mass of the galaxy, it might be more correct to say that the objects in a galaxy orbit the common mass (including dark matter) of the galaxy.

For example, the black hole at the center of the Milky way is estimated at 4.1×10^6 M (solar mass) and the total Milky Way mass is estimated at 0.8–1.5×10^12 M (figures from Wikipedia).

Some interesting articles found while digging for these numbers:




Planets orbit around the center of mass. It's coincidental that the center of mass is (at least in our case) at the sun's location, because the sun has most of the mass.

I believe the current theory is that most (all?) galaxies have a supermassive black hole at their heart.

Yeah, or so I heard. What's the cause and effect order for galaxies and their corresponding supermassive black holes?

constant angular velocity is not the same as constant linear velocity. One of them, things move faster or slower against a fixed reference point depending on where you are on the disc.

So I'm a bit confused about what 'all' and 'rotate once every billion years' means here. If they all have the same angular velocity, thats pretty bizarre. If the net effect of scale is that on average, for a given frame of reference, stars in the disc move at much the same rate no matter what size the disc is, depending on how close to the core, thats different, and possibly just about ytivarg, the thing which makes centipede force work

Is it strange that it’s a round human number like one billion? Why not 883,259,631 years?

To be clear it's not a human number, because a year is not arbitrary, like an hour or a minute. It is still weird that it would be a clean decimal (arbitrary) figure based off of our solar system's year.

Well a year is still a human-centric number, because it is related to the planet humans live on.

However, it is not weird and most likely it means nothing but a simple coincidence. When you look at the number of time units that it could be a power of n multiple of, then such a coincidence becomes less surprising.

The universe breathes at a set rate

the article say "about one billion years"

humans probably rounded it to one billion so they can talk about it more easily


> "It’s not Swiss watch precision," said Gerhardt Meurer, an astronomer from the International Centre for Radio Astronomy Research (ICRAR), in a press release. "But regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round."

Benford's law shows that leading digits of measured values are not evenly distributed, and tend to be smaller.

How is that relevant? A bias in a distribution does nothing to prohibit the existence of outliers.

Milky Way is every 200 million years, as every school child knows:

"The sun, and you and me, and all the stars that we can see Are moving at a million miles a day In an outer spiral arm at forty thousand miles an hour Of the galaxy we call the Milky Way

Our galaxy itself, contains a hundred billion stars It's a hundred thousand light years side-to-side It bulges in the middle, sixteen thousand light years thick But out by us its just three thousand light years wide

We're thirty thousand light years from galactic central point We go round every two hundred million years And our galaxy is only one of millions of billions In this amazing and expanding universe"

- The Galaxy Song.

The observation is for stars at the _edge_ of the galaxy. Sol is not on the edge.

the rotation of a galaxy isnt the same thing as the rotation of the stars contained within in. The rotation of the galaxy is measured by the rotation of arms/other features, which are due to a traveling density wave (think about how traffic speeds up/slows down)

If stars orbit faster than arms, doesn’t this mean that stars have to occasionally change which arm they’re in? How does that work - some sort of gradual re-clumping of stars?

Yes, our solar system travels from arm to arm! while the overall shape of the galaxy remains.

By the way, our transit between and into these galaxy arms is theorized to coincide with past mass-extinction events, across tens of millions of years.

Whaaat! I never heard about the galaxy arm transit coinciding with mass extinction events. I'm about to google, but do you have a favored source?

I commented here 10 months ago


in relation to "Mass Extinction and the Structure of the Milky Way (2013) (arxiv.org)"

But, the arms don't rotate either! There are some great animations here: https://en.wikipedia.org/wiki/Density_wave_theory

The article was about stars on the edge of a galaxy.

> “But regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round.”

As another commenter points in another thread, this article is explcitly about the red-shift-calculated velocity of stars at the outer edge of galaxies, and not about the spiral arms' apparent motion.

is this a thing? please make this a thing

It's a thing. Monty Python


EDIT: Stephen Hawking singing Galaxy song

Stephen :(

Can I set my watch by that?

> “It’s not Swiss watch precision,” said Gerhardt Meurer, an astronomer from the International Centre for Radio Astronomy Research (ICRAR), in a press release.

>> “It’s not Swiss watch precision,”

This is a weird case, like "20/20 vision", where the metaphor expresses the opposite of the reality. Swiss watches are so expensive because they're clunkier and less accurate than quartz crystal watches.

I think the idea is that the Swiss mechanical watches are more precise than other mechanical watches...

Score one for the deist "divine watchmaker" idea.

I'd like to see a time-lapse video of that :)

Referring to "All" in the title:

All of them? Even those outside the observable universe? Or only the observed ones?

How can we know anything about what cannot be observed?

How well-defined is the angular velocity of a galaxy? Since it's not a solid disk, there's no reason for it to be the same for different regions. Is there any uniformity at all, e.g. do stars at the same distance from the center move at similar speeds?

Somebody else in the thread points out today our own son makes 4 revolutions around the galaxy center in a billion years, not 1.

You may find this helpful: https://en.wikipedia.org/wiki/Galaxy_rotation_curve

If you look at a 33rpm spinning record, you'll see that any point on the edge of the label is moving a lot slower (linear speed) than the record's edge ... even though it has the same 'angular velocity' (33 rpm).

But galaxies are not solid like a record, and as the article explains, "most stars in spiral galaxies orbit at roughly the same speed" (that's linear speed). So the edges 'fall behind.' (That was not recognized until about 1975.)

How well defined is your son? Even if the group of atoms which he is composed of is not solid for 1 billion years, there is a way to approximate his average angular velocity around the center of the galaxy. They chose a bunch of stars on the outer rim of a galaxy, and they all had a similar angular velocity, despite large differences in their radii. Here they define it as the average angular velocity on the outermost edge of the galaxy.

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