Entropy Reversal?

Factual Questions
1

Stephen Hawkings proposes, based on mathematical arguments, that there would be a negative entropy arrow if the universe were to begin contracting, i.e. if there was enough matter in the universe to cause its contraction. This leads to four pertinent questions.

  1. What is black matter?

  2. How is black matter measured and how much is there?

  3. Is there enough black matter to cause the contraction of the universe at some point in time (since, as I understand it, there doesn’t seem to be enough visible matter for a contraction of the universe)?

  4. The third law of thermodynamics (entropy concept) simply stated, says that all matter will attempt to reach its maximum state of disorder. Does Hawkings propose that this law will be reversed in a contracting universe? Mathematically maybe? However, even today, on a localized basis, the third law can be reversed with the application of energy, but the total universal entropy must increase. From a physical basis is the concept of a reverse entropy arrow realistic or just a “mathematical” concept (e.g. would a match still burn if the universe was contracting)?

2

Well, I haven’t read Hawking in 10 years or so, but I’ll tackle the first three questions:

  1. Any matter in the universe that we can’t see. There are two major proposed groups: MACHOs (Massive Compact (?) Halo Objects) and WIMPS (Weakly Interacting Massive Particles). I think that’s what they stand for. MACHOs are things like black holes, planets, brown dwarfs, etc - big stuff that doesn’t glow so we can’t see them. WIMPs are particles that are tiny and hard to detect but may make up much of the mass of the universe. Neutrinos would be the major component if we can prove that they have rest mass.

  2. Well, by definition, we can’t see dark matter directly, at least with current technology. As our telescopes get better, we can see more MACHOs - planets and brown dwarfs have been discovered fairly recently, for instance. There are also big sensitive experiments going on to try and measure neutrino rest mass.
    As to how much there is, like I said, they’re still doing experiments. This is a matter of much argument. The reason we suspect dark matter’s existence at all is because visible matter only accounts for ~10% of the mass of the universe as inferred from gravitational interactions between galaxies, though I seem to remember a report that the 10% figure may be too low. Anyway, the short answer is that no one knows.

  3. This is why people are so interested in dark matter. This is one of the Big Questions in astronomy today. No one knows. They’re working on it, though.

Hope that helps. I bet this is a simulpost…

3

To start with, Hawking does mention the possibility that entropy would reverse direction if the Universe were to start collapsing, but he immediately starts poking holes in the idea, and concludes that they’re probably independent, so entropy would still work the same in a contracting Universe.

As for the rest: There’s currently strong evidence that most of the “matter” in the Universe (approximately 70%) is in the form of the vacuum energy, or Cosmological Constant. There seems to be just exactly enough to cause the Universe to be geometrically flat, which would in the simplest models result in a speed of expansion asymtotically approaching, but never reaching, zero. However, because some of that is in the form of Cosmological Constant, the simplest models do not apply, and the end result is that the Universe currently appears to be expanding, and will probably continue to do so forever, at an exponentially-increasing rate.

By the way, thanks for all the great questions, DeutschFox. I like talking about physics :slight_smile:

4

Thanks Chronos … I appreciate you putting up with me :-). The “black matter” subject, I believe, was best being addressed by the research at Harvard which measured the red shift of various double(?) stars rotation based on their sizes. The results seem to indicate the presence of black matter affecting their motion and a measure was inferred. Haven’t kept up on the research though. Also, I guess I will have to read Hawkings again … just his discussing the concept of a reverse entropy arrow based on mathematical concepts, without relating it to physical phenomena, seems like book filler to me …