Dark matter doesn’t emit light – otherwise it wouldn’t be dark! But Wikipedia insists it doesn’t absorb radiation either; how is that known?
If it were composed of large opaque objects it would block light from beyond, but what if it’s made of small opaque objects? If it did absorb radiation, thermodynamic principles might imply that it would re-radiate that energy; is that the only reason we “know” it doesn’t absorb radiation, or is there more to it?
The laws of thermodynamics absolutely require that if something be able to absorb radiation, it also be able to re-emit it. We can never be absolutely certain, of course, that everything in the Universe actually follows the laws of thermodynamics, but then, the same is true of literally everything else we know.
That’s it? The statement “Dark matter neither emits nor absorbs radiation” is just a phrasing of “Dark matter doesn’t emit radiation; physicists therefore infer from thermodynamic principles that it doesn’t absorb radiation either”?
Is it impossible that some objects have a thermodynamic arrow reversed relative to ours? Would such objects appear as dark matter? If so, the thermodynamic argument becomes circular: We think the thermodynamic arrow points the same way everywhere because radiation absorbers have not been observed. But when we do detect a possible such object we know it’s not a radiation absorber because we think the arrow points the same way everywhere.
I realize this is not a Board where such things can be discussed fruitfully …
… but I am curious what possibilities for dark matter have been ruled out.
Here ya go :
From here.
If it absorbed radiation, we would be able to see it. Not directly, but it would stop us from seeing light-emitting things behind it. Since it doesn’t, we know it doesn’t absorb radiation.
True if the dark object is large, but what if it is small – say the size of a star?
ETA: I plead extreme ignorance, and am willing to believe that experts may have a good reason why they know dark matter doesn’t absorb radiation. But I’ve not seen it in this thread.
“Thermodynamic principles” does not mean the same thing as “the arrow of time”. In this case, the principle in play is the principle of detailed balance, which actually arises from the idea that microscopic processes don’t have an arrow of time. In other words, if I film a microscopic process and run the film backwards, the film should show me another allowed microscopic process. If I “film” a photon of a particular wavelength being emitted by a particle of dark matter, and I run the “film” backwards, I get a photon of that same wavelength being absorbed by a particle of dark matter.
Now, if you’re asking whether the microscopic laws of dark matter behave differently under time reversal: maybe they do; as far as I know, it’s an open question, like most questions concerning dark matter. After all, quarks are believed to behave very slightly differently under time reversal (though the evidence is indirect, and the time-reversal effects have never been directly measured.) But for the most part, the microscopic laws that govern conventional matter are the same when time is flowing forwards or backwards, so at least on this basis it seems plausible that dark matter behaves the same way. What’s more, one of the more compelling models of dark matter that we have (supersymmetry) requires that the dark matter behave much the same way regular matter does. In the absence of any evidence that dark matter has such exotic behavior compared to conventional matter, and in the presence of interesting (if yet unproven) models that require it to behave the same way, our best guess is that it behaves the same way under time-reversal.
First Law of Thermodynamics -
“The total amount of energy in an isolated system is conserved.”
Take the case of dark matter. If it absorbs radiation then you have this scenario : Energy flowing in, energy flowing in … wait a minute, where’d it go? Did it change into something else? Don’t look like it. Was it (eventually or otherwise) re-emitted? Duh, naming rights already claimed so, no.
This is my man-on-the-street, definitely not a physicist understanding of the situation. So, please don’t yell at me, Chronos, et al.
So it’s transparent?
“Thermodynamic principles” refers to the increase of entropy between the past and the future. Reversing the “arrow of time” refers to the possibility that, for some objects, their low entropy state is in what we call the future, rather than the past.
For example, imagine a star which is evolving backwards, from a low-entropy state in our future (perhaps a Counter Big Bang?) toward a high entropy state in its “future” (but our past).
Even if we stipulate that no such objects exist, can we imagine how such an object would appear to us? AFAICT, it would appear as dark matter, at least from a distance. No?
Yes! In the far-fetched scenario, it changed the internal state of the dark-matter object, converting helium into hydrogen (according to our time sense).
I read interesting books by Huw Price and Julian Barbour. (Barbour, BTW, has an interesting new paper claiming that one past with two futures is the natural development of Newtonian gravitation!) I don’t begin to understand much of what I read, but think I perceive the “blind spot” Price identifies.)
In trying to understand these books and papers, I have basic questions of physics for which I don’t know the answers. I think I got one answered in this thread: The claim that dark matter doesn’t absorb radiation is just an assumptioon derived from the assumption that it shares our “thermodynamic arrow.”
Sooner or later, one would cross between us and a (visibly shining) star, and we’d see the shining star temporarily disappear.
I’ll defer to real astronomers, but I assume someone has done the (I imagine fairly simple) calculations to show that if there were enough normal matter dark stars in the Milky Way to account for the required gravitational effects of dark matter, then we would see them regularly blocking stars (at least, regularly enough that we would have noticed it by now).
Re : The last part of Septimus’s last post :
Yikes! I feel a little violated here.
Isn’t it also true that dark matter is a placeholder? If physicists’ theories are right, there needs to be a lot more matter. If that matter exists, it has to have these properties (one of which is it doesn’t absorb radiation)?
Dark matter is kind of the accommodator in your family who takes all the blame and smooths everything out and agrees with everyone so we can get through Thanksgiving dinner. Anything that doesn’t work, posit something about dark matter to make all the numbers come out.
I think (correct me if wrong) the chief argument for dark matter is that galaxies shouldn’t spin like they do, with the edges rotating as fast as the centers. Gravity says that things far away should orbit much slower than things close by.
I think it’s a mistake to try to talk about what Dark Matter is. We only call it dark matter because we don’t know what it is.
What we do know are certain observations that cannot be explained through our current knowledge of the universe.
So, we know that dark matter must interact with gravity. The structure and behavior of galaxies requires a certain amount of mass that we do not see.
The fact that we can’t already see the mass means, by definition, that dark matter must NOT interact with light. If it did, then we’d see it, if only by blocking light.
The size of dark matter doesn’t avoid this issue. Even a nebula can block light, and a nebula is nothing but gas and dust so thinly spread that we’d call it a vacuum on Earth. An individual atom flying through deep space is sufficient to block some light. The more atoms, the more light is blocked.
Scientists have been coming up with possible ideas for dark matter for a long time. Every time they do, they come up with some prediction that should be observable. Then they go try to find it. Whatever dark matter is, it hasn’t fit any theory or observation yet.
… but I am curious what possibilities for dark matter have been ruled out.
[/QUOTE]
What exactly was a the point of quoting a comment totally out of context from a five year old thread?
Stranger
You’ve got the right idea here, though the galactic rotation was just the first piece of evidence—we have more evidence for dark matter (and dark energy) nowadays. Basically, the evidence comes from three main sources: the anisotropies on the cosmic microwave background; the level of clustering of galaxies; and the relationship between the brightness of distant supernovae and their redshift. All told, this leads to a Universe that has a certain amount of matter (dark and conventional) and a certain amount of “dark energy”. The “matter” in this case is shorthand for “anything that doesn’t travel near the speed of light and tends to cluster together”. You can then look at galactic rotation curves, or at gravitational lensing effects, or at primordial nucleosynthesis, to conclude that there’s a lot of “matter” out there that we can’t see via electromagnetic radiation.
I got it.
It was exactly the same underlying question, asked the other way around. My quoting was to indicate I didn’t hope for useful discussion of that underlying question; that’s why I asked the much more limited question in the thread title.
That question has been answered, AFAICT. Some posters appear to take offense at my mentions of alternate thermodynamic assumptions.
So, @ Mods: Please feel free to close the thread.
The idea of “dark matter” being a “placeholder” – that is, an artificially imagined construct to account for observational gaps in the evidence – sounds to me like the classic “finagle factor”: The quantity which, when added to, subtracted from, multiplied by, or divided into, the answer you got, gives you the answer you should have got.
Sort or reminds me of Einstein’s hypothesized “Cosmological constant”.
Historically speaking, that perception isn’t entirely unfair. As I noted above, though, cosmologists have built up a model over the last 20 years, based on several different lines of evidence, that has converged on a picture in which there really is some amount of matter in the Universe that only interacts gravitationally with everything else. Moreover, attempts to “fix” the laws of gravity to get rid of the need for dark matter have not been terribly successful to date. And physics has a history of postulating entities solely to “make the sums come out”, with direct confirmation of their existence coming much later. If I had to bet, I would say that 20 years from now we will have detected dark matter directly as well; but I wouldn’t bet more than about, say, $50 on it.