How do Physicists know that Dark Matter doesn't absorb radiation?

Factual Questions
21

So you elect to immediately poison the well by quoting from another thread which has no link to this one and immediately suggest that no useful discussion will be forthcoming? And far from taking offense, I only see posters patiently attempting to correct your misapprehensions of the principles of thermodynamics, and your original question was addressed authoritatively and completely by Chronos without reference to entropy or asymmetry in time, and MikeS followed up with an expanded explanation.

If you posit a universe in which the laws of thermodynamics are other than what we observe them to be, i.e. that a body can absorb radiation and not radiate it back to maintain thermodynamic balance, then no one can say what would happen; it’s as if trying to predict the outcome of a baseball game if each runs scores a random number between -5 and 5. We can only talk sensibly about this universe, or a universe with slightly altered properties that still fundamentally adhere to deterministic principles, which is why quantum mechanics, as currently formulated, is such a problematic field.

Yes, it is. That’s not a flaw in the hypothesis, it’s a feature. That is, astronomers, astrophysicists, and cosmologists are well aware that “dark matter” is not only not strongly observationally verified but even lacks a fundamental physical basis beyond having to satisfy some essential properties (have mass, be weakly interacting via electromagnetic force, sufficiently diffuse throughout the universe to not create observable gravitational foci but be concentrated enough to cause galaxy-sized structures to rotate as a highly viscous mass). With these properties, we can at least identify the scale of what to look for and falsify hypothesis which rely upon matter that can’t behave in this fashion. No one is claiming that having a label is the end of the question of what “dark matter” consists of, whether it is even a previously unidentified type of matter, or even some other fundamental interaction entirely.

To restate the answer, we know that “dark matter” doesn’t absorb radiation in any significant quantity because we can see through it. If it does interact with some part of the spectrum, it must be weak enough or sufficiently far away from the normal spectral wavelengths that astronomers typically look for that it doesn’t show significant absorption on either radio or visible frequency spectral lines coming from known sources with well-predicted intensity.

Stranger

22

On the other hand, an impartial model of physics would start with the assumption that there is dark matter, and then set out to determine how much of it there is, leaving open the possibility that the amount is zero. We know that some particles are subject to some of the fundamental forces but not others; why should electromagnetism be any different? It’s perfectly reasonable to suppose that there might be some, or even many, particles which don’t interact with electromagnetism. The proposal is in principle refutable, of course, if we looked for such particles but failed to find them, except a lack of electromagnetic interaction makes them very difficult to even look for.

23

If “dark matter” is some species of WIMP, based on how much dark matter there should be in our galaxy how much would we expect to find in our solar system, or traveling through the Earth? The sun emits neutrinos constantly which zip through the Earth without interacting electromagnetically, but we think neutrinos are massless, correct? If there were particles something like massive neutrinos passing through the Earth constantly what method could we use to detect them? I know there are neutrino detectors, but given the low interactivity of neutrinos it takes gigantic detectors. Or do neutrinos have mass after all? Why do we believe neutrinos couldn’t be dark matter?

24

Neutrinos do have mass, but it’s very small. And the main objection to neutrinos as dark matter is that they tend to be “hot” (i.e., fast-moving), but the evidence is that the majority of dark matter is “cold” (slow).

And yes, people have built experiments to try to directly detect dark matter particles zipping through them. None has succeeded yet, but that’s not too surprising. Most candidate particles are of comparable difficulty to detect as neutrinos. Some are in principle easy to detect, but require things like detectors that are precisely tuned to the particles’ properties, like a resonant chamber that’s some exact length, and we don’t know those properties well enough to do better than guess.

25

In the WIMP scenario, the Earth should be sitting in a “WIMP wind” with a speed centered on that of the solar system through the galaxy and with a local density of roughly 0.4 GeV/cm[sup]3[/sup]. In different terms, this is the same density as 60 hydrogen atoms in the volume of a coffee cup. Folks have looked for this local density within the solar system through observations of planetary motion, but the density limits from this technique are five orders of magnitude above the target implied by cosmological and galactic observations.

As a side note, one signature for WIMP dark matter thanks to the “wind” is a modulation of the dark matter’s velocity and its interaction rate. Throughout the course of a day and a year, the velocity of the WIMP wind should change direction and magnitude. Some of the current WIMP detector R&D is aimed at making a directional detector to look for the direction modulation. A rate modulation signal has been seen by the DAMA/LIBRA experiment, but its interpretation as a dark matter signal (as opposed to something more mundane) is highly controversial.

Dark matter detectors aren’t nearly as big as neutrino detectors yet. To be sure, if they could be made that large, that would be great, but dark matter detectors are rather precise (and usually cryogenic) devices that are not trivial – or cheap – to scale up. For some detector approaches, this is countered by the fact that they are “background free”, meaning if you see even a single interaction, you can be sure (barring a goof up) that it came from a dark-matter-like particle.

A passing subtlety that maybe no one will care about: the resonant chambers that Chronos mentioned aren’t used for WIMP searches but rather axion searches, with axions being a different class of hypothetical (but well-motivated) particle that could be a part of the dark matter story.

26

One can challenge existing assumptions about time, without overthrowing the “thermodynamic arrow.” I remain surprised that the simple explanations for the EPR or GHZ paradox, hinted at by Einstein and Bell themselves, aren’t generally pursued. You guys know this stuff much much better than I do, but obviously I’m very poor at explanation. For starters

And beyond that, physicists have proposed universes in which two phases operate with thermodynamic arrows opposite to each other. Thomas Gold proposed such a model based on a “Big Crunch”, which Hawking endorsed, I think, for at least a short while. And, as I mentioned upthread, Barbour has modeled Newtonian gravity to show two opposite-directioned futures evolving from one low-entropy state.

I acknowledge that all you physicsts know far far more about all of this than I do. Perhaps it’s very clearly proven that all versions of Time’s Arrow are marching in one direction with no deviation ever. But it would be nice to read something more convincing than “Entropy increases because entropy increases.” Whatever the deficiencies of Price’s book may be, he does give many examples of how easy it is for circular reasoning about time’s arrow to stifle open debate.

If physical laws are time-reversible, Huw Price’s suggestion seems worthwhile – imagine the universe time-reversed. Now entropy doesn’t increase; it decreases! We’d never notice because

[QUOTE=John Baez]
As with the time arrow of radiation, in the last analysis it appears to be nothing but a raw experimental fact that the entropy of our universe is increasing. In a sense this is not surprising, because pondering chemistry and biology a bit it becomes apparent that life as we know it requires the entropy to be changing monotonically, rather than staying about the same.

Processes like remembering and planning, which define the psychological notions of future and past, are only able to occur in the direction of increasing entropy.
[/QUOTE]

My suggestion that some dark matter objects might be reverse-stars is somewhat whimsical. But if anyone has refuted the possibility here without relying on the circularity “We know entropy increases because we know entropy increases” then I missed it.

I am sorry to have offended you; you are one of the posters I most admire here. OTOH, if you could somehow bring yourself to believe my questions are legitimate, I think you’ll see that I’d be irritated that you “poisoned the well” for my five-years ago question.

Doesn’t the arrow of time derive from the happenstance that we have a past of very low entropy? If we postulate instead a region with a future of very low entropy, then its thermodynamic arrow and causality arrow will reverse, no?

Doesn’t this apply to large objects but not small objects? As jimbuff314 pointed out, “Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements.” We wouldn’t be able to “see through” such dwarfs, but (unless we were very close to them) we wouldn’t know we couldn’t because they’re so small right?

I’m certainly here to learn, not to try to teach! I started with an easy question “How do Physicists know that Dark Matter doesn’t absorb radiation?”, but am not sure anyone here answered it from an arrow-agnostic viewpoint. Good news is: I’ll spare you any follow-up questions! :smiley:

27

There is such a thing as baryonic dark matter, and we know definitively that at least some of it exists. We ourselves are one example, as would be planet or planet-like objects free in the void. But there are some pretty solid limits on the total amount of baryonic matter in the Universe, and they fall significantly short of the total dark matter.

28

To be honest, I couldn’t figure out what you wanted the reverse-matter to accomplish, so I sorta skipped that part of the thread. If I have a chunk of this special matter out in space, what is it doing that helps it be dark matter better than, say, rocks?

29

Adding to my earlier reply to this question:

There is a long list of these WIMP direct detection experiments, past, present, and future. Each looks for evidence of a WIMP bumping into a nucleus. Issues:

  • WIMPs don’t bump into nuclei very often since they’re weakly interacting (with a lower-case “w”, i.e. it’s not necessarily the weak force that’s involved). So you need as much detector as you can muster.
  • When they do bump, they leave very little energy behind. To notice it, you need a very specialized setup.
  • Other things can mimic the signal from a WIMP-on-nucleus collision. Neutrons are always a concern, but depending on the detection method employed other things in the environment can be an issue. These false signals are collectively termed “backgrounds”.

Even the most radio-pure (low radioactivity) materials available usually have orders of magnitude more background radioactivity than the WIMP signal rate. To get around this, many of the techniques below are designed to pick apart the subtle differences in a WIMP interaction and the relevant backgrounds. Some experiments use a single technique, but many combine techniques to add to the discriminating power. Identifying (and then throwing out) background events works pretty well for backgrounds from alpha, beta, and gamma radioactivity. Neutron backgrounds are more pernicious because they interact in the exact same way that WIMPs would – by bumping into a nucleus. To battle neutrons, you need shielding to keep outside neutrons out. Since cosmic rays kick neutrons around, you also need to go deep underground to get away from the main source of neutrons.

Some cues that get used…

Scintillation light: A bumped nucleus gives up some of its energy to the excitation of atoms or molecules which re-emit that light detectably. Background events due to bumped electrons in the material (rather than nuclei) give a subtly different scintillation signature, both in amplitude and time profile. For instance, a background electron moves farther before running out of steam than does a bumped nucleus, so fast-enough electronics can look for the extra travel time. The scintillator material can be a special crystal, organic compound, or liquified noble gas.

Dual-phase nobles: In the noble liquid case, you can put an electric field across the detector to cause any ionized atoms or free electrons to drift out of the liquid. When they reach the boundary, they can be accelerated through a gas phase to make a second burst of scintillation light or to get collected on wires that record their spatial pattern. Again, all this looks a little different for WIMP signals versus many background types.

Phonons: A collision in a very pure crystal will induce phonons (quantized vibrations) that can be recorded. These devices are always cold and usually made from a semiconductor (silicon, germanium) since the readout electronics are tightly coupled with the active detector elements themselves. For sake of example, superconducting transition edge sensors are one technology in use.

Ionization: Ionization of the material is detected electronically instead of through any secondary production of light. Semiconductor crystals are again appropriate. WIMP and background signatures are different in terms of amplitude and timing.

Threshold detectors: A localized energy deposition (like from a WIMP-on-nucleus collision) can induce bubble nucleation in a super-heated liquid. If you tune the pressure and temperature just right, only nuclear recoils can do this and not electron recoils, so “all” you need to do is get your internal alpha backgrounds down and shield from neutrons.

Directional detection: If you can determine the direction the bumped nucleus is traveling in, you can see if it correctly aligns with the ever-changing (but known) trajectory of the Earth through the galaxy. Seeing this direction is very tough in anything other than a gaseous detection medium, and a gaseous medium has a lot few nuclei present than a solid or liquid for useful WIMP detection (unless you make the thing enormous). Thus, raw interaction rate is a challenge here.

Rate modulation: A detector that isn’t so great at distinguishing signal from noise might be easier to make large, in which case you can look for fluctuations of the total rate to see if they go up and down with the expected phase of the WIMP wind.

Spin-dependent searches: If WIMPs interact in a manner than scales with the spin of the particles involved, you can choose interaction methods that allow high-spin nuclei to be used. WIMPs may not interact in such a manner, though.

30

Wow, Pasta, I wasn’t aware of any of that WIMP detection technology. That’s very exciting to me, that people have come up with so many insanely precise methods to detect such elusive effects. (and that we can actually build these things.)