Dark Matter vs. Aether

This is an easier one than dark matter vs. modified gravity. As mentioned, I’m going to be on Science Friday today, and they asked me to contribute a guest blog post, which I’m cross-posting below. Old news, I’m sure, for longtime CV readers, but here you go.

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Probably the biggest single misconception I come across in popular discussions of dark matter and dark energy is the accusation that these concepts are a return to the discredited idea of the aether. They are not — in fact, they are precisely the opposite.

Back in the later years of the 19th century, physicists had put together an incredibly successful synthesis of electricity and magnetism, topped by the work of James Clerk Maxwell. They had managed to show that these two apparently distinct phenomena were different manifestations of a single underlying “electromagnetism.” One of Maxwell’s personal triumphs was to show that this new theory implied the existence of waves traveling at the speed of light — indeed, these waves are light, not to mention radio waves and X-rays and the rest of the electromagnetic radiation spectrum.

The puzzle was that waves were supposed to represent oscillations in some underlying substance, like water waves on an ocean. If light was an electromagnetic wave, what was “waving”? The proposed answer was the aether, sometimes called the “luminiferous aether” to distinguish it from the classical element. This idea had a direct implication: that Maxwell’s description of electromagnetism would be appropriate as long as we were at rest with respect to the aether, but that its predictions (for the speed of light, for example) would change as we moved through the aether. The hunt was to find experimental evidence for this idea, but attempts came up short. The Michelson-Morley experiment, in particular, implied that the speed of light did not change as the Earth moved through space, in apparent contradiction with the aether idea.

So the aether was a theoretical idea that never found experimental support. In 1905 Einstein pointed out how to preserve the symmetries of Maxwell’s equations without referring to aether at all, in the special theory of relativity, and the idea was relegated to the trash bin of scientific history.

Aether was a concept introduced by physicists for theoretical reasons, which died because its experimental predictions were ruled out by observation. Dark matter and dark energy are the opposite: they are concepts that theoretical physicists never wanted, but which are forced on us by the observations.

Dark matter, in particular, is nothing at all like the aether. It’s something that seems to behave exactly like an ordinary particle of matter, just one with no electric charge or strong interaction with known matter particles. Those aren’t hard to invent; particle physicists have approximately a billion different candidate ideas, and experiments are making great progress in trying to detect them directly. But the idea didn’t come along because theorists had all sorts of irresistible ideas; we were dragged kicking and screaming into accepting dark matter after decades of observations of galaxies and clusters convinced people that regular matter simply wasn’t enough. And once that idea is accepted, you can go out and make new predictions based on the dark matter model, and they keep coming true — for example in studies of gravitational lensing and the cosmic microwave background. If the aether had this much experimental support, it would have been enshrined in textbooks years ago.

Dark energy is conceptually closer to the aether idea — like the aether, it’s not a particle, it’s a smooth component that fills space. Unlike the aether, it does not have a “frame of rest” (as far as we can tell); the dark energy looks the same no matter how you move through it. (Not to mention that it has nothing to do with electromagnetic radiation — it’s dark!) And of course, it was forced on us by observations, especially the 1998 discovery that the universe is accelerating, which ended up winning the Nobel Prize in 2011. That discovery took theoretical physicists around the world by surprise — we knew it was possible in principle, but almost nobody actually believed it was true. But when the data speak, a smart scientist listens. Subsequent to that amazing finding, cosmologists have made other predictions based on the dark energy idea, which (as with dark matter) keep coming true: for the cosmic microwave background again, as well as for the distribution of large-scale structure in the universe.

There is still much we don’t know about dark matter and dark energy; in particular, we certainly haven’t nailed down what exactly they are (although we have many plausible ideas), and the only way we’ve detected them is indirectly, through their effects on gravitational fields in the universe. But they are not arbitrary; both ideas make very specific predictions for what those gravitational effects should be, which astronomers have tested and verified. Unlike the aether, which shrunk and eventually disappeared under experimental scrutiny, the case for dark matter and dark energy continues to grow stronger.

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82 Responses to Dark Matter vs. Aether

  1. AI says:

    “1998 discovery that the universe is accelerating”

    It’s probably better to clarify that it’s expansion of the universe that is accelerating.

  2. david says:

    It seems that another specific prediction of dark matter (as we currently hypothesize it) is that there should be no strong correlation between the distribution of visible matter in a galaxy and that galaxy’s rotation curve. However, there seems to be one. This appears to be a glaring failure of our current understanding of DM, and a strong hint that there’s something fundamental that we’re missing.

  3. Meh says:

    You have to admit, the mistake is forgivable, they do have a lot in common. Aether is like the beautiful daughter who you really have to go out of your way to impress, while Dark Matter and Dark Energy are the butt ugly obese twins who have the hots for you; ugh, leave me alone and go tell your hot sister why she should hook up with me.

    I’m kind of a fan of the idea that Dark Energy, Dark Matter, and Gravity are conceptualized as different harmonic octaves of a string.

  4. Roger says:

    The aether was not necessarily thought to have a “frame of rest” in the 1800s. See Maxwell’s 1878 essay for a survey of aether theories. It was Lorentz who said in 1895, “It is not my intention to … express assumptions about the nature of the aether.” Something like the aether later became essential for quantum electrodynamics.

  5. Meh says:

    Roger #5,

    What ‘something like the aether’ later became essential for quantum electrodynamics?

  6. Roger says:

    In quantum electrodynamics, the vacuum is not empty, but has something in it for the propagation of light. It is not usually called the aether, but it is a similar concept.

  7. Hemo_jr says:

    There is still the possibility that both dark matter and dark energy are artifacts of observational deficiencies.

  8. Charles says:

    david:
    Since dark mater has mass just like regular mater and is affected by gravity the quantum fluctuations in the very early universe that caused regular mater to come together would do the same for dark mater. The only difference is that dark mater doesn’t clump like regular mater does.

  9. david says:

    OK, so if the observed correlation today is due to correlated initial conditions, then it should be possible to demonstrate it mathematically via numerical simulations or whatnot. While I don’t recall such a demonstration, maybe I’m ignorant or maybe groups are working now to show it?

  10. Ben says:

    Charles, what david refers to is the observed *anti-correlation* (not correlation) between the surface density of baryons and that of dark matter in galaxies. Observations show that there exists a universal baryonic surface density of the order of 800 solar masses per parsec square, above which virtually no dark matter is ever present in galaxies, and below which it appears in exactly the right proportion as to give the baryonic Tully-Fisher relation between the visible mass and the fourth power of the asymptotic circular velocity. See e.g. Figure 4 of http://arxiv.org/pdf/astro-ph/0509305 . There is currently no understanding at all of this observation. It is really a kind of exclusion principle: if the baryonic surface density is large enough, there is apparently no room for dark matter. With our current understanding of dark matter as being the plain simple cold dark matter particle, one would not expect that at all: if the cold dark matter potential well is deep enough, nothing should prevent baryons to clump into that deep potential, with as high a baryonic surface density as they want, while still being subdominant to the even larger density of dark matter. But this is not what happens: wherever the baryonic surface density is high enough, observations say bye bye to the dark matter effect. So, where this anti-correlation comes from is still a complete mystery, except that it is predicted by MOND. Anyway, whatever the explanation for this phenomenology, we should always listen to Nature, and not pick and choose the observations. There are actually many other observations defying explanation in the cold dark matter context on galaxy scales. But of course, the same can be said about MOND: there clearly are observations fully disfavouring it on the largest scales. But that doesnt necessarily mean it is wrong on all scales.

  11. Bob says:

    Sean, I don’t think it it correct that “dark matter and dark energy are concepts that theoretical physicists never wanted.” I think this is a common misunderstanding that astrophysicists have. They think that dark matter and dark energy are wild, crazy concepts that are completely disconnected from all our other ideas in physics. But its not true. For instance, when theorists hypothesized the axion to solve the strong CP problem, they wanted dark matter to found. When theorists hypothesized supersymmetry to obtain unification of the forces of nature, and to maximally unify relativity with quantum mechanics, they wanted dark matter to be found. And when theorists tried to compute zero point energy and always found it non-zero in any reasonable framework, such as QFT or string theory, etc, they really wanted dark energy to be found. On the other hand, astrophysicists were mainly unaware of progress in fundamental physics, so for them the discoveries of dark matter and dark energy were “unwanted”, if you like, because they had to start rewriting the astrophysics text books.

  12. Bill says:

    If dark matter feels gravity, then why doesn’t it clump?

    And if it doesn’t clump, then shouldn’t it be distributed uniformly? (In that red-and-blue picture that I’ve seen numerous times on various science-related Web sites (IIRC, of two galaxies colliding), it looks to me like there’s more of it some places than other places.)

    This non-scientist is very confused.

  13. Pingback: Stretch the Intellect Saturday… « blueollie

  14. Ben says:

    @Bob: indeed, there could be axions out there, LSSP too of course, as well as sterile neutrinos, and many other things… But in what exact proportion is the real question. Possible (or probable) existence doesnt mean being out there in exactly the right amount, both globally and locally. Globally, the WIMP miracle for the relic density of dark matter from the typical weak scale cross-section seemed to be an excellent argument, but is this argument still really alive with the latest results from direct detection XENON-CDMS constraints, as well as complementary LHC constraints?? If not, we could well have a mix of all these things out there, and an infrared modification of gravity on top of all this is still a real possibility too. By the way, my understanding was that the axion mass is varying as the temperature changes, due to the running of its couplings to matter during the process of symmetry breaking. And the effective consequence of the presence of such mass-varying DM could be an effective modification of gravity indeed, through the effective breaking of the weak equivalence principle due to the varying mass of DM itself. So both aspects are not necessarily mutually exclusive. Anyway, astrophysical observations of the relations between the gravitational field and the observed distribution of baryons in various different systems is still something very useful for giving us hints about the answer(s): these observations currently strongly point towards collisionless dark matter on cluster scales, but do not exclude (or even slightly favor) an infrared modification of gravity on galaxy scales, where coincidences of scales between the transition surface density I mentioned above (square root of the cosmological constant in natural units) and dark energy might still point towards something really interesting in linking the two supposedly separate dark sectors together. This might well all end up as “garbage in the sky” in the end, but let me perhaps be a bit biased in saying that I strongly doubt that it will actually end up as garbage.

  15. Meh says:

    Roger #7,

    That doesn’t really answer my question. What is the concept you’re talking about? I’m just starting my study of electrodynamics, so you’ve got my mind hooked on this. If you can’t answer it, then I have to assume you have no idea what you’re talking about. Please don’t make me assume that.

  16. Aiya-Oba says:

    Aether by any name, is absolute space; the oneness, resultant and equator of the relative pair of spaces. -Aiya-Oba (Philosopher)

  17. Brad says:

    13. Bill Says:

    If dark matter feels gravity, then why doesn’t it clump?

    To clump into a more condensed form it has to lose energy somehow, that requires some sort of interaction. Interstellar gas clouds condense by becoming hotter and radiating electromagnetic energy up into the x-ray region. Dark matter can’t because it doesn’t interact with anything electromagnetically, and doesn’t collide with normal matter to lose momentum directly.

  18. Gaston Moens says:

    Aether, dark matter, dark energy: different ‘things’, same function: save a theory. No direct empirical evidence.

  19. s johnson says:

    #18 Brad says that dark matter must interact to lose energy to clump. By definition EM interactions are ruled out, as is cancellation of momentum by collision with baryonic matter. This leaves gravitational waves for one. Although they are weak, on cosmic scales, both spatial and temporal, wouldn’t this have visible effects?

    Also, wouldn’t there be a certain amount of random cancellation of momentum by collisions between dark matter particles? Or are dark matter particles imagined as not even interacting with each other? Also, wouldn’t there be the effects from the dark matter equivalent of decay? Why would we expect dark matter to be composed solely of stable particles, unlike baryonic matter?

    In view of these and similar considerations, does the concept of thermal equilibrium play any physicial role in cosmology? Shouldn’t all models of cosmic expansion default to dark energy and dark matter as the dominant factors?

  20. martenvandijk says:

    Dark Matter vs. Aether: modern myth vs. antique myth.

  21. Valatan says:

    @sjohnson #20: gravitational waves are not just weak, they are EXTREMELY weak. It would take the Earth several ages of the universe to have its orbit decay into the sun, for example. You’re not going to get significant clumping of galaxy-sized objects via gravitational radiation.

    @Hemo_jr #8: if DM is an artifact of observational deficiency, then it must be a defect across several different sets of observations, that all point toward the same DM abundance. You’re going to have to have some combination of DM and modified gravity in any model.

    @martenvandijk: if you hate them so much, come up with a better model.

  22. Bill says:

    Hmm…trying to process…

    I’m remembering that red-and-blue photo that appeared on various Web sites not too long ago (IIRC, of two galaxies colliding). I can’t remember whether the red or the blue is the dark matter; but whichever it is, it certainly looks like there’s more of it some places than other places. Is that just normal clustering rather than clumping due to gravitational attraction?

    And if dark matter can’t lose energy, does that mean that it’s in a state of maximum entropy?

    Or do I misunderstand so thoroughly that explaining it to me is hopeless (entirely possible)? 😎

  23. Christian Takacs says:

    Dark Matter and Energy are what you get when you have equations that can’t explain observation.
    Try working with three spatial dimensions (confirmed and measurable), and no point particles, since anything that is something enough to bump into something else has extension (NO VIRTUAL POINTS, PARTICLES, ANGELS DANCING ON VIRTUAL PINS ALLOWED!!)
    Some really big basic observations:
    1. Something with spin, like a thrown american football for example, or a particle, will have a wave like action if viewed two dimensionally.

    2. After all the “We wanna a unified field theory!! Waaaaaah! ” I’ve heard over the last several decades, I find it hilarious that physicists when confronted with the observational evidence that gravity alone ‘ain’t all that’s making things go round other things’ are surprised by this, and start making up stuff to save their ‘gravity only’ models. You want a damn unified field theory, start noticing gravity ain’t the only thing in it making stuff move a certain way.

    3. Instead of ‘pretend/virtual’ photons which are required to make current theory work, what would happen if “REAL” photons made it work? Last I checked, real photons are non baryonic, and do actually have mass for goodness sake, small as it may be, but there are lots and lots of them.

    4. Ummmm, about the whole accelerating universe thingy… Please consider for a moment, you are in an orbit, which is ellipitcial. As you are on certain locations along your orbit you notice other objects seem to speed up and slow down.. like say other planets. Would you think it likely that the other planets are actually speeding up or slowing down (due to dark matter/energy, entrophy, big bang, kitchen sink, etc. of course!)…. or perhaps… just perhaps… that other planets would APPEAR to speed up or slow down because of where you were in relation to them in your orbit? Whew, thank goodness our solar system doesn’t orbit anything ellipically… or our galaxy, or our galaxy cluster… I mean that would just suck if someone came up with an idea that WE were accelerating due to our location within an orbit inside of various stellar entities, not the universe.

    5. If first you don’t succeed, try not to make stuff up and complicate things further with magic, miracles, virtual anything, unicorns, endless dimensions, endless math designed to hide flaws and avoid criticism, or infinite anything.

    6. Read, rinse, repeat.

    from the collection “Space
    Child’s Mother Goose” by Frederick Winsor and Marian
    Parry (Simon and Schuster, 1956)

    This is the Theory Jack built.
    This is the Flaw
    That lay in the Theory Jack built.
    This is the Mummery
    Hiding the Flaw
    That lay in the Theory that Jack built.
    This is the Summary
    Based on the Mummery
    Hiding the Flaw
    That lay in the Theory that Jack built.
    This is the Constant K
    That saved the Summary
    Based on the Mummery
    Hiding the Flaw
    That lay in the Theory that Jack built.
    This is the Erudite Verbal Haze
    Cloaking Constant K
    That saved the Summary
    Based on the Mummery
    Hiding the Flaw
    That lay in the Theory that Jack built.
    This is the Turn of a Plausible Phrase
    That thickened the Erudite Verbal Haze
    Cloaking Constant K
    That saved the Summary
    Based on the Mummery
    Hiding the Flaw
    That lay in the Theory that Jack built.
    This is the Chaotic Confusion and Bluff
    That hung on the Turn of a Plausible Phrase
    That thickened the Erudite Verbal Haze
    Cloaking Constant K
    That saved the Summary
    Based on the Mummery
    Hiding the Flaw
    That lay in the Theory that Jack built.
    This is the Cybernetics and Stuff
    That covered Chaotic Confusion and Bluff
    That hung on the Turn of a Plausible Phrase
    That thickened the Erudite Verbal Haze
    Cloaking Constant K
    That saved the Summary
    Based on the Mummery
    Hiding the Flaw
    That lay in the Theory that Jack built.
    This is the button to Start the Machine
    To make with the Cybernetics and Stuff
    To cover Chaotic Confusion and Bluff
    That hung on the Turn of a Plausible Phrase
    That thickened the Erudite Verbal Haze
    Cloaking Constant K
    That saved the Summary
    Based on the Mummery
    Hiding the Flaw
    That lay in the Theory that Jack built.
    This is the Space Child with Brow Serene
    Who Pushed the Button to Start the Machine
    That made with the Cybernetics and Stuff
    Without Confusion, exposing the Bluff
    That hung on the Turn of a Plausible Phrase
    And, shredding the Erudite Verbal Haze
    Cloaking Constant K
    Wrecked the Summary
    Based on Mummery
    Hiding the Flaw
    And Demolished the Theory that Jack built.

  24. Entropy says:

    Dark matter can lose energy, it just has to wait for something as weak as or weaker than the weak interactions in order to do it. It can also gain energy, through the same weak mechanisms. Basically, how would you add or subtract energy to/from a neutrino? It’d be very, very difficult, but it can be done in principle. Dark matter is in the same situation.

    The image you’re recalling is the bullet cluster.

    http://apod.nasa.gov/apod/ap060824.html

    The dark matter is the blue on the outside (detected indirectly via gravitational lensing by apparently empty space). What’s happened in that picture is the collision of two galaxies. Most of the visible mass of the galaxies has slowed down via friction and sits in the red areas. The dark matter halos, roughly spherical, have sailed through since dark matter basically does not interact. If one rejects dark matter, it must be explained how a typical galaxy warps light ~7 times as much as it should (ok, fine, modify GR), and how two galaxies that collide warp light exactly as much as they should in the collision region, but “empty space” on either side of them warps light many times more than the visible galaxies (uh…I’m pretty sure Occam’s Razor likes dark matter here).