Comments on: From Eternity to Book Club: Chapter Three

By: sonasi

sonasi — Fri, 12 Feb 2010 02:44:34 +0000

Why is Sean doing his book club on the cosmic variance blog? I vote for way less Sean and more of everyone else.

By: Sean

Sean — Wed, 10 Feb 2010 21:06:13 +0000

Lumpy means *high* entropy, if the density is high enough. Once the universe expands (quite far in the future), high entropy will be smooth again.

By: RantingNerd

RantingNerd — Wed, 10 Feb 2010 20:47:05 +0000

Sorry to be late catching up here (I just got the book a few days ago ).

If lumpy = low entropy for the universe, then how does a cloud of intersteallar hydrogen (which is fairly smooth) collapse into a higher-entropy star, which is much lumpier? (I am clearly missing something here, but Stat Mech was always my weakest subject.)

By: Fill

Fill — Sat, 06 Feb 2010 21:51:18 +0000

Yeah that actually makes perfect sense.

By: Sean

Sean — Sat, 06 Feb 2010 20:46:39 +0000

Fill, we’ll talk about this more in Part Four. But the increase in entropy is not due to the expansion of the universe; the universe can be high-entropy at any size, although the configurations will look different. At high density, when gravity is important, a high-entropy configuration will look very non-smooth. Think of a collapsing universe; there’s no reason at all to expect structure to smooth out.

By: Fill

Fill — Sat, 06 Feb 2010 19:12:00 +0000

I tried to skim the comments to see if my question had already been asked, but I didn’t see it. So apologies in advance if this is a retread.

Why is it that the early universe, which was essentially a homogeneous(to 1 part in 100,000) mixture of elementary particles and radiation was extraordinarily low in entropy? Shouldn’t that be a high entropy configuration? Is the low entropy due to the unification of the forces or the amount or the free energy in the system?

How much of the increase in entropy is simply due to the increase in size of the universe?

By: Sean

Sean — Fri, 05 Feb 2010 02:17:01 +0000

Lorianna, this probably isn’t the best place to get into this topic. But roughly: general relativity predicts gravitational waves, and quantum mechanics is correct, and quantum mechanics says that when your classical theory has waves your quantum theory has particles. So essentially every physicist who has thought carefully about the question believes that gravitons exist. It doesn’t mean anything at all for general relativity, which is only a classical theory. Whether or not you call gravity “a force” or not is largely a matter of taste; the GR perspective suggests that you don’t have to think of it as a force, but it doesn’t really matter.

By: Lorianna

Lorianna — Thu, 04 Feb 2010 23:54:41 +0000

I am a complete novice so pardon me if this question seems silly. I’m going to keep the question shorter than I want to since I don’t really know what I’m talking about. You can thank me later for that. Why do physicists think the graviton exists and if it does what does that mean for general relativity? Is gravity a force or not, or both?

By: Sean

Sean — Thu, 04 Feb 2010 17:05:09 +0000

Philoponus– Black holes are important, but they’re not the whole story; they’re simply the end-state of gravitational collapse whose entropy is easy to calculate (and extremely large). The evolution of black holes is part of a more comprehensive story, in which high-entropy states at high density are very lumpy, but the real early universe is very smooth. (Note the question is certainly not “why we evolved so many black holes,” which is easy to answer — it’s “why weren’t all sorts of black [or white] holes to begin with?”)

By: From Eternity to Book Club: Chapter Three « JackTales

From Eternity to Book Club: Chapter Three « JackTales — Thu, 04 Feb 2010 16:58:26 +0000

[...] to this week’s installment of the From Eternity to Here book club. Next up is Chapter Three: “The Beginning and End of Time.” Remember that next week [...]