Is it subject to the same physical laws as ordinary matter?
For example, does its mass have the potential to produce energy equivalent to mc[sup]2[/sup]? Is it composed of particles? I assume it cannot be composed of atoms given that photons are produced when electron orbits collapse, but maybe my understanding of that is very naive. Are there stable atoms whose orbits never (or metaphysically can’t) collapse? Does it come into contact with ordinary matter in the same sense that ordinary matter does? In other words, could I hold some dark matter in a coffee cup? Or would it just pass through? Finally, is the existence of dark matter the only possible conclusion from observation of light bending around the gravity wells of galaxies? Or is it possible that dark matter stays to itself, surrounding galaxies in a halo without moving about anywhere or being affected by gravity?
Taking on any or all of the above questions would be greatly appreciated. Thanks.
We can calculate how much of the dark matter out there could be made up of baryons (protons, neutrons, and other ordinary matter particles). The answer is “not all of it”. We know that some dark matter is made up of atoms- it’s called planets. We couldn’t detect the planets in our solar system from a distant galaxy, so they’re dark matter in a sense- they don’t emit light the way stars do.
Neutrinos, which are the only form of non-baryonic dark matter we know much about, can’t be held in a coffee cup. They’d whiz right through it. Neutrinos don’t interact with the electromagnetic force, which is what holds stuff inside a coffee cup. If other kinds of dark matter also don’t interact electromagnetically, you couldn’t hold them in a coffee cup either.
My understanding (from a scientist speaking on The Universe series) is that dark matter doesn’t reflect light either, but planets do. Was he incorrect, or did I misunderstand him?
Here’s a brief article on candidates for dark matter. The answers to your questions will vary greatly depending on which of the candidates are looked at.
A leading competing theory is MOND (Modified Newtonian Dynamics). My understanding is that was invented to explain why the rotation of galaxies does not correspond to Newtonian physics - the reason that dark matter was first invoked - and works only so-so for other areas in which dark matter is now used as an explanation. I’ll let our physics gurus discuss that in more detail, though.
For most of the period of time when the number crunching which predicts dark matter was possible, the only characteristics that we were sure it had was, gravitation, and being dark.
Now, the reason for that rather spare definition is that what we needed to make all the number crunching come out even was more gravitational force. And nothing that could provide it was emitting light that we could see. So, what we needed was dark matter.
Since that time, there have been other observations that imply other characteristics. If something with mass is in some particular places, which seems to be the case given our number crunching, then it is an anomaly that the amount of radiant energy from ordinary matter which seems probable for the same general location is not greater because of interactions with the dark matter which should be there. So, perhaps some of the Dark matter is not simply matter that doesn’t radiate, but rather some more exotic particle.
Given certain number crunches, it also seems that light must be shining directly through areas which really ought to have a fairly large amount of dark matter, and, well, golly gee, why doesn’t it show us some interaction with the light? Every type of matter we can describe would.
Recently, some volumes of dark matter which ought to be associated with galactic clusters of ordinary matter appear to be in the wrong place. How did that happen? When did that happen?
So far, as near as I can understand, Dark Matter is a description of anything that has mass, but lacks some other characteristics we generally associate with mass. It also seems more and more likely that it is not one thing, but a set of things related only by our observations.
I’ve certainly heard MACHOs (massive compact halo objects) listed as candidates for baryonic dark matter, and planets cited as examples of MACHOs.
He may have been talking about non-baryonic dark matter. That gets more study these days, since we have a pretty good idea what the baryonic dark matter is (the same kind of stuff we see in the solar system and nearby in our galaxy). We’ve also managed to rule out MACHOs as the only kind of dark matter, so we know there’s some other kind out there.
The problem is, we don’t know what most of the dark matter is. Current theory says that most dark matter is cold dark matter, or something that is not moving at a high percentage of the speed of light. Current theory also says that most of the dark matter is not baryonic. The two known forms of dark matter, MACHOs and neutrinos, don’t fit that description- MACHOs are baryonic, and neutrinos move at relativistic speeds. We think there’s something else out there, but all we really know is that it is non-baryonic and not moving at relativistic speeds.
We do know that dark matter is affected by gravity. That’s how we know it’s there. It can’t be sitting still in a cloud around the center of a galaxy, because if it were, it would fall into the center of the galaxy.
I would hesitate to call it either “leading” or “competing”, actually. The basic idea behind MOND is to say that Newton’s laws aren’t quite accurate: the force law falls off as r[sup]-2[/sup] until you get to a certain acceleration scale, and then it falls off more slowly (proportionally to r[sup]-3/2[/sup], I believe.) Using this, you can explain the fact that the outer stars in galaxies are moving faster than we would otherwise expect: since gravity doesn’t fall off as quickly, then you don’t need as much mass in the center of galaxies to keep the outermost stars in their orbits. Tune things a little and hey presto, you’ve explained away dark matter.
The problem with this picture is that this isn’t how most physicists think about gravity any more; Newtonian dynamics is valid in an awful lot of regimes, but Einstein’s General Relativity is valid in a whole hell of a lot more. And despite the best efforts of MOND’s adherents, they haven’t managed, in the twenty-odd years since MOND was invented, to figure out how to fit it into a theory that has the same nice properties as GR does. The attempts to do so have become increasingly baroque with the successive “patch jobs” that have been done, and although dark matter seems kind of philosophically iffy at this point, relativistic MOND seems equally so (in my opinion, at least).
Are you referring to recent measurements of the Bullet Cluster? These, in particular, seem to present real problems for MOND. Some people have called it “the final nail in MOND’s coffin”; but then, that’s been said before, and MOND is still kicking.
One is that non-Newtonian theories were mentioned even in that brief article I linked to.
The other is that if Lib were to search on “dark matter” on a website of a science magazine like Scientific American or New Scientist - which he really ought to do since they seem to do a dark matter article every five minutes - you’ll find a million references to MOND. From a layperson’s pov, therefore, MOND is a leading competitor, if only because it gets mentioned so often.
Astronomers and physicists of the inclination to mull over dark matter need to amuse themselves somehow, you’d go loony looking out into the great abyss for too long with only breaks to crunch numbers.
Same object, different report, but this one and several other articles about similar mapping observations involving lensing seem to show dark matter “objects” not following the expected behavior that gravity only interaction would involve.
My own personal suspicion is that we are trying to imagine either too few, or two many phenomena to account for what we see.
Planetary astronomers are MACHO, particle astrophysicists are WIMPs!
Maybe so- if it is, one of my professors at UC Santa Cruz is, by my conservative estimate, responsible for about half the dark matter out there.
Another theory I have is that it is made up of the egos of astronomers and physicists.
Oh- and socks. When socks disappear from the dryer, they tunnel out to the outer reaches of the galaxy. They then clump together with other lost socks to form sock stars.
To get back to the coffee cup question, most of the dark matter out there seems not to interact via the electromagnetic force, or only to a very slight extent. In addition to making the stuff dark (since light is electromagnetic), it also means that it wouldn’t sit in a coffee cup, since the interactions which cause “normal” stuff to stay in a coffee cup are all electromagnetic. Really, almost all of the interactions you’re familiar with are electromagnetic, so non-electromagnetic dark matter wouldn’t be subject to any of them.
