*From Eternity to Here*

The author is Sean Carroll and the subtitle is The Quest for the Ultimate Theory of Time.  This book-to-appear offers a very good summary of the paradoxes of time.  The new contribution (new to me, at least) is to offer an integrated discussion of the multiverse, the law of entropy, de Sitter space, and the foundations of the so-called "arrow of time."

Carroll argues that the invocation of baby universes clears up a lot of apparent puzzles:

The prospect of baby universes makes all the difference in the world to the question of the arrow of time.  Remember the basic dilemma: The most natural universe to live in is de Sitter space, empty space with a positive vacuum energy…most observers will find themselves alone in the universe, having arisen as random arrangements of molecules out of the surrounding high-entropy gas of particles…

Baby universes change this picture in a crucial way.  Now it's no longer true that the only thing that can happen is a thermal fluctuation away from equilibrium and then back again.  A baby universe is a kind of fluctuation, but it's one that never comes back — it grows and cools off, but it doesn't rejoin the original spacetime.

What we've done is given the universe a way that it can increase its entropy without limit.

…[pages later]  In this scenario, the multiverse on ultra-large scales is symmetric about the middle moment, statistically, at least, the far future and the far past are indistinguishable…[yet] The moment of "lowest" entropy is not actually a moment of "low" entropy.  That middle moment was not finely tuned to some special very-low-entropy initial condition, as in typical bouncing models.  It was as high as we could get, for a single connected universe in the presence of a positive vacuum energy.  That's the trick: allowing entropy to continue to rise in both directions of time, even though it started out large to begin with.  There isn't any state we could possibly have chosen that would have prevented this kind of evolution from happening.  An arrow of time is inevitable.

Is it all true?  Beats me.  But if you read this book you will come away more hopeful about the prospects of a relatively simple "theory of everything."

Here is the author's home page; he teaches at Cal Tech.  Here is his personal page.  Best of all, here are his talks.  His Twitter feed is here.

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He's also a contributor to the physics blog Cosmic Variance: http://blogs.discovermagazine.com/cosmicvariance/

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I'm writting up a summary of Gary Drescher's solution to the time paradoxes for LessWrong.com. It should be up in a few days.

Long story short: entropy increases in *both* the positive and negative direction away from the initial state of the universe, so you should really be thinking in terms of "futureward" (away from t=0) and "pastward" (toward t=0). The increase in entropy *is* futureward direction of time.

As global entropy increases, it can also decrease in localized regions, and it is these localized regions that can, in effect, store information about which direction is futureward, and about pastward states, which is why beings can remember the past.

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book-to-appear? Sounds like you might have gotten a review copy for FREE, Tyler. How could you rape my trust in this way, saying nice things about a book you got for free and not explicitly disclosing it? I feel vulnerable, shocked and horrified.

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Bob Murphy: Sean Carroll gave a talk last week which was very accessible.

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How would one test this?

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How would one test this? One couldn't, so the idea is forever safe. If one did test it, and it was falsified, this would provide a wonderful impetus for physicists and mathematicians to provide a new theory. And so the cycle repeats. While physics is fabulous, if you want certainty you won't find it in science. But you could check out my blog.

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