Welcome 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 we’re doing two chapters at once, Four and Five.
For those who missed them, here’s the Science Friday discussion, and here’s the Firedoglake book salon with Chad. I should also point to some substantive review/discussions: Wall Street Journal, New Scientist, USA Today, and Overcoming Bias.
Excerpt:
For the most part, people interested in statistical mechanics care about experimental situations in laboratories or kitchens here on Earth. In an experiment, we can control the conditions before us; in particular, we can arrange systems so that the entropy is much lower than it could be, and watch what happens. You don’t need to know anything about cosmology and the wider universe to understand how that works.
But our aims are more grandiose. The arrow of time is much more than a feature of some particular laboratory experiments; it’s a feature of the entire world around us. Conventional statistical mechanics can account for why it’s easy to turn an egg into an omelet, but hard to turn an omelet into an egg. What it can’t account for is why, when we open our refrigerator, we are able to find an egg in the first place. Why are we surrounded by exquisitely ordered objects such as eggs and pianos and science books, rather than by featureless chaos?
This chapter is a fairly straightforward review of the modern understanding of cosmology, with a particular eye on those issues that will become important later in the book. We zip through the expansion, structure formation, and dark energy. There I got to tell a fun personal story of my wager with Brian Schmidt. At least I think it’s fun — including personal stories is not my natural tendency, but at the right moments it can help to humanize all the forbidding science. Hopefully this was one such moment.
A few topics go beyond the standard cosmology summary. I discussed the Steady State theory a bit, because it’s a relevant historical example when we will much later turn to the question of what the universe should look like. I also dwell a bit on vacuum fluctuations and dark energy, because those will pay a crucial role in my personal favorite explanation for the arrow of time. And we close the chapter with a very brief overview of the evolution of entropy. It has to be brief, because we haven’t laid nearly enough groundwork to do the job right. This is a conscious choice, which may or may not work: rather than simply progressing on an absolutely logical path from foundations to conclusions, I felt free to mention points that would be important later, on the theory that they would come as less of a shock if we had established some familiarity. Again, hope that worked.
Tom Levenson, who is an actual writer, advised me to omit “smoking a pipe” from the caption to Figure 7, on the theory that what is shown should not also be told. I left it in anyway. It’s my book!
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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?”)
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?
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.
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?
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.
Yeah that actually makes perfect sense.
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.)
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.
Why is Sean doing his book club on the cosmic variance blog? I vote for way less Sean and more of everyone else.