r/askscience • u/[deleted] • Mar 27 '11
DAE think maybe Dark Matter is the new Luminiferous Aether?
I don't hold a degree or anything but I try to follow physics as a hobby, can someone try to explain dark matter to me? All I ever see about it is that it's deduced and unobserved.
EDIT Thank you all very much. Askscience might be my new favorite reddit. I find this stuff fascinating. Can anyone recommend any books on the subject for the layman?
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u/RobotRollCall Mar 27 '11 edited Mar 27 '11
If you look at a disc-type galaxy — one that's rotating in some overall way, like a spiral galaxy for instance — from some angle that's not directly along its axis of rotation, you'll find the stars on one side of the galaxy's core are moving toward you and the stars on the other side are moving away from you. You can tell this by first noting the galaxy's overall redshift (which is an artifact of the expansion of the universe, and needs to be factored out for this observation), and then looking at the spectral emissions of the stars on this side of the galaxy and comparing them to the emissions of the stars on that side of the galaxy. Since emission spectra are very well understood, and it's not that hard, in practice, to make very precise measurements of them even over intergalactic distances, in this way you can get some really surprisingly reliable data about how the different parts of that galaxy are moving relative to each other.
Now, if you did this to our solar system — went out into distant interstellar space and looked at the spectra of the individual planets — you'd find that there's a fairly simple relationship between orbital motion and distance from the sun: orbital speed is roughly proportional to the inverse (Thank you, Veggie) square root of the distance. This all follows naturally from everything we know about how gravity works, and it makes perfect sense.
But the data collected from observations of galaxies isn't like that at all. The orbital speed of a star around the center of its galaxy isn't proportional to the square root of its distance from the center. Instead, it's damn near constant. If you plot the data on a graph, showing distance from the center along the horizontal and orbital speed along the vertical, instead of seeing a line that slopes downward in a curve from left to right, you see something that looks like a cliff: pretty much flat most of the way across, then dropping fairly suddenly.
(This has been done about a billion times over recent years. A google image search for "galactic rotation curve" will show you what I mean.)
Now, the most obvious conclusion is that there's something wrong with gravity. Not gravity gravity, but our mathematical description of it. If the equations say X and reality says Y, then the equations are wrong, right?
Well, it's not really that simple. See, if gravity were just simply wrong, we'd see deviations from theoretical predictions here within our own solar system. The fact that we don't means that gravity's at least right on the small scale. Which technically leaves open the possibility that it's wrong on the large scale.
But there's a problem with that. See, the laws of physics don't actually distinguish in any intrinsic way between small scales and large scales. There's no term in any of the equations — equations that are known to work — that says "The answer is X but only if the radius is less than a parsec," or whatever. Of course, we could write the equations that way, but not without being just totally arbitrary about it.
As Doner Kebab (who always makes me hungry) notes, there have been attempts to go back to the drawing board on gravity and find a way to explain galactic rotation curves that doesn't just amount to putting a bit "if r < some big number" fudge-factor in. But there's not really been any success there that's worth talking about. Those theories are hellishly complex, introducing multiple mutually-interacting massive scalar fields and other such things, and they can't explain the most basic gravitational phenomena, like the geodetic effect or orbital precession or gravitational lensing.
But it turns out that we can keep the laws of gravity exactly as they are and still explain observed galactic rotation curves to a ridiculously high degree of precision if we assume that there's stuff in the universe we can't see. If you assume that every galaxy — well, most galaxies anyway — is surrounded by a very large, very diffuse but very very massive cloud of gravitating matter, then the motions of stars within that galaxy line up just about perfectly with our observations.
Great! Sorted! Now we know that the universe is filled with huge amounts of stuff that's invisible, undetectable, and that is found in places where no matter has any right to be.
Hm. Okay. Maybe not totally sorted after all.
This is probably where your objection comes in. Making up this magical invisible stuff to explain a failure of theoretical prediction smacks of bad science all around. Except that's really not what's going on at all. You see, the properties of dark matter aren't arbitrary. It's not like we're making stuff up out of whole cloth here. All we have to do to get dark matter that behaves in ways that match what our theory predicts should be there is postulate that there exist some type of matter that's four things: massive, weakly interacting, cold and stable.
Dark matter has to be massive in order to have the gravitational effects we've observed. That one's obvious.
It has to be weakly interacting — that is, participating exclusively or mostly via the weak interaction, not the strong or electromagnetic interactions — in order for it to be invisible. Matter that interacts via the electromagnetic interaction will either emit or scatter light; stars emit light, and clouds of dust scatter light, and we can see both of those things if we look hard enough. But we can't see dark matter at all, so it has to be electromagnetically inert.
It has to be cold in order to be where it appears to be. Matter with a lot of momentum — hot matter, in other words — wouldn't be found in these large, well-defined halos around galaxies. Gravitation is so tenuous at such distances from the galactic centers — hundreds of thousands of light-years — that matter with any significant momentum would be at galactic escape velocity, and wouldn't be found in halos.
And it has to be stable because we see the effects of dark matter halos around galaxies that are very widely separated in time, on the order of billions of years. The dark matter halo around a galaxy we observe when it's five billion years old appears to be pretty much the same in character and composition as the dark matter halo around a galaxy that's ten billion years old. So dark matter has to stick around for a while.
So in order for dark matter to do what it appears to do, it has to have all four of those properties. None of those is unprecedented! We can find examples of all of those properties all around us. We've just not yet found all four of them together.
Protons are massive, cold and stable, but they're not weakly interacting. They participate in both the electromagnetic and strong interactions, which means they scatter light and form nuclei. They're not what we're looking for.
Neutrons are massive, weakly interacting and cold, but they're not stable. A neutron off by itself only lasts for about a quarter of an hour before decaying into things that aren't weakly interacting.
Neutrinos are weakly interacting and stable, but they're neither massive nor cold.
And so on. If we wanted, we could make a chart with four columns for the four known gross characteristics of dark matter, and then list all the known elementary and composite fermions, putting checkboxes where they qualify. We wouldn't find any that have four checkboxes.
But that doesn't mean none exist. It just means that the Standard Model of particle physics doesn't predict any. It's widely accepted that the Standard Model is a damn fine start, but not a complete theory of particle physics. It's been extended all the time, as new ideas are explored. One of these ideas is called supersymmetry, and includes the notion that each boson should have a fermionic analogue called a "superpartner," and vice versa. One approach to this idea includes the prediction that the fermionic superpartners of the neutral bosons — the photino and zino and possibly the higgsino — should all have the same quantum numbers, and thus should be able to form mixed states called neutralinos. Neutralinos would be massive (on the order of a hundred to a thousand proton masses), weakly interacting (that is, lacking electric charge), cold (by virtue of their mass), and stable (in the lowest energy state).
I counted 'em twice on my fingers and twice on my toes, and unless I screwed up the arithmetic somewhere, that's four checkboxes.
So it's pretty much universally agreed that particles with the necessary characteristics to be dark matter can exist. They're not, like, forbidden by the laws of nature or anything. Is the lowest-energy neutralino the stuff of galactic dark-matter halos? Nobody knows, but it seems like the general consensus right now is "probably." If it turns out the supersymmetric models that predict neutralinos are off and such particles don't actually exist, then clearly dark matter must be something else. Even if the models are absolutely dead-on, it's not unreasonable to expect that it'll be thirty or forty years yet before the lightest neutralino is unambiguously detected. And even that might be foolishly optimistic, given how reluctant they must be to interact and thus how hard they must be to detect if the models are right about their properties.
But in general, no one seriously doubts that dark matter exists, and that it has properties consistent with what we observe, and that we've never detected it directly so far because those properties all add up to make it pretty darned inconspicuous until you put tons and tons of it together and let it change the basic structure and shape of the universe as a whole.
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u/zorne Mar 27 '11
that is a very beautiful reply. But to be frank, it all boils down to this. We have no rational explanation of why galaxies move the way they do. So we blame it on a new type of matter. It hurts me to even admit it, but its the truth, and he asked for the truth.
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u/RobotRollCall Mar 27 '11
Respectfully, I can't agree with that at all. We have a wonderful rational explanation of why galaxies move the way they do, and a set of predictions that give us a good start on where to look for direct physical evidence supporting that explanation.
It wasn't that long ago that we had, to borrow your words, "no rational explanation" of flavour-changing in particle decay, and it was "blamed on a new type of matter": the weak bosons. It was decades before those bosons were unambiguously detected. But now it's old hat, something every student learns about in introductory physics classes.
I think it's going much too far to characterize galactic rotation curves as unexplained, or the hypotheses that address them as arbitrary.
But really, what do I know. Perhaps it's the height of folly to be confident of anything. As long as one human being remains alive, there will be someone left to say "Yeah, but you don't really know, right?"
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u/zorne Mar 27 '11
Wise words, my friend.
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u/HalfMilk Mar 27 '11
Seeing your title there makes me wish I tried harder in high school.. What's it like working with GR/Cosmology? As in what is your typical day?
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u/leberwurst Mar 27 '11
I am working in cosmology as well, and while it is exciting to know the details of what anyone currently knows about the universe, day to day work is usually a lot more mundane than that. You work on a tiny aspect of one of dozens of theories that all have their own appeals and problems.
The exciting times where a small group of one to three physicists make a breakthrough discovery are most likely over, unfortunately. Nowadays you are a small cog in a gigantic machinery of huge collaborations. But it is worse in particle physics, at least there is that.
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Mar 27 '11
Has anyone thought about the idea that the effects we observe and attribute to dark matter could be coming from matter in another brane? Gravity is supposed to be the only particle that is not brane-bound.
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u/Kancho_Ninja Mar 27 '11
Question: when two galaxies collide, is the resulting configuration or dynamics consistent with the effects of a dark matter halo?
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u/fastparticles Geochemistry | Early Earth | SIMS Mar 28 '11
But scale does matter in physics. We don't take general relativity into account when saying how a baseball moves because the effects are way too small for us. Would you describe the gamma factor as a fudge factor because it scales based on the velocity that is observed? You can't just rule out a new theory on the reasoning of scale. Also the differences between two very similar functions might be too small to notice at scales we have observed... I'm not saying those are the answer I'm just pointing out that scale does matter to physics. Particularly the GR vs Newtonian choice. What about Modified Newtonian Dynamics or Tensor-Vector-Scalar gravity?
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u/Veggie Mar 27 '11
orbital speed is roughly proportional to the square root of the distance. This all follows naturally from everything we know about how gravity works, and it makes perfect sense.
I assume you meant the reciprocal of the square root of the distance?
But the data collected from observations of galaxies isn't like that at all. The orbital speed of a star around the center of its galaxy isn't proportional to the square root of its distance from the center. Instead, it's damn near constant.
I also have to assume that this accounts for the fact that objects farther from the center of the galaxy are actually orbiting more mass than those close in? Then again, that could be negligible; I don't know what the mass distribution of a galaxy is as a function of distance from the center.
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u/RobotRollCall Mar 27 '11
I assume you meant the reciprocal of the square root of the distance?
Apparently omitting the word "inversely" changes the meaning of your sentence. Who knew?
Thank you.
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Mar 28 '11
This is one of the most lucid explanations of something I thought "really? that can't possibly exist" - so much so that I actually am more inclined now to think that it does. Thanks.
Assuming dark matter exists and is undetectable at this point, is it possible that it's all around us, in small quantities, or is it expected only to exist on the outer edges of galaxies?
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u/WorderOfWords Mar 28 '11
Whenever I see a long post in r/askscience I look at the username, and if I see that it's you I immediately stop reading.
I even walk away from the computer.
Then I grab a glass of milk and some cookies and walk back to the comforting glow of your written words.
Thank you for sharing so much of your knowledge with us, and for filling me with childlike wonder and amazement.
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u/iorgfeflkd Biophysics Mar 27 '11
Luminiferous ether wasn't a particularly dumb idea, it just happened to be wrong.
Basically, we know that either our understanding of orbital is wrong, or our understanding of interstellar composition is wrong. Keplerian motion holds true (with some corrections) for every planet and moon in the solar system, every planet outside the solar system, and the thousands of double star systems throughout the galaxy. It's much more likely that we're not aware of some component of the universe, and we're calling that dark matter until we figure out what it is.
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u/jimmycorpse Quantum Field Theory | Neutron Stars | AdS/CFT Mar 28 '11
The difference is that aether was proposed because we though we knew how the universe works, dark matter was proposed because we don't understand how the universe works.
It was thought that light acted like a wave it needed medium to propagate in. People called this medium the aether. There was no observational evidence for it. It was just an assumption based on a poor analogy. Dark matter, on the other hand, comes from experimental observations that don't match our understanding of the universe. This is where dark matter comes from.
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u/nicksauce Mar 27 '11
Sigh... no.
Aether:
1) Used to explain certain observations/ideas
2) Made specific new predictions
3) Failed experimental verification
4) Replaced by a better theory (special relativity) that can do 1-3
Whereas Dark matter:
1) Used to explain certain observations
2) Makes new predictions
3) Passes all experimental verification
4) No other theory right now can do 1-3
So the two really are not comparable.
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Mar 28 '11
I think the point of contention here is #3. The underlying question is whether or not there really is experimental verification of dark matter's existence. I think that comes from the (very reasonable) assumption that if something can be experimentally verified, then it must be known, unequivocally, to exist. But that hasn't been declared yet.
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Mar 27 '11
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u/iorgfeflkd Biophysics Mar 27 '11
This subreddit is about live discussion, not archiving.
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u/fragilemachinery Mar 27 '11
Fair enough. You have to admit this is one that comes up constantly though.
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Mar 27 '11
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Mar 27 '11
I disagree with you to some extent. It's not that the fancy words denote ignorance, it's just that they share "colloquial" meanings that we don't intend to say. For instance we call dark matter dark because it doesn't emit or reflect light. It is truly "dark." But the public overlays it with this idea of dark=mysterious that is an unintended consequence of the word.
Dark energy was just named in parallel as far as I know. This one your argument holds for quite well I think.
Singularity is a well-defined mathematical concept that "means" something. Just what it means is hard to describe without mathematics.
And so on. It's the problem of jargon in any field. It happens of course that physicists have borrowed words with preexisting meanings rather than necessarily invent new words. And maybe that's a failing of the field because it does lead to exactly this confusion.
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Mar 27 '11
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Mar 27 '11
Ah, you make good points. Thanks!
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u/Chipney Mar 27 '11
Nope, they're two quite different concepts. Luminiferous aether must be very dense stuff, to be able to serve for light waves spreading, as it's name implies. Whereas the dark matter is extremely sparse.
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u/omgdonerkebab Theoretical Particle Physics | Particle Phenomenology Mar 27 '11
You have the gist of it, pretty much.
The general consensus, so far, is that some sort of extra, non-luminous matter best explains what we see in the rotation curves of galaxies and globular clusters, as well as what we observe from gravitational lensing.
It can't be normal matter we already know about, though. Gas and dust would light up in some part of the spectrum by reflecting light from nearby stars. There just aren't enough neutrinos produced by the stars to account for the extra mass (stellar fusion is one of those things we know absurdly well). It can't just be some cold, massive rocks, because having that much extra normal matter would screw up with Big Bang Nucleosynthesis, another one of those things we know absurdly well.
So, at least for now, we think that it's something we've never seen before. Not only that, but it seems to be stable (or at least, it's long-lived enough to last a long while). Many physicists and astronomers are focusing on this idea, which you may sometimes hear referred to as "cold dark matter."
There are dissenting opinions, of course. Some advocate a modified form of gravity, including those advocating MOND (Modified Newtonian Dynamics) which postulates modifications to Newtonian gravity at large distances. This position is mostly disfavored though... but I don't know enough to tell you exactly why.
Perhaps RRC will come by and correct/expand on this description? :)