Thread: Boltzman brains
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Jul31-09, 03:32 AM
Sci Advisor
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Quote Quote by Haelfix View Post
The obvious problem with that idea is the following.

If there was some special property of the early universe/cosmology that allowed us to evolve from a low entropy state to a large entropy state, then there should be another universe that has the opposite behaviour, at least in principle. So they would by hypothesis go from a high entropy state to a low entropy state. Or in other words, an observer in that universe would see humpty dumpty spontaneously coming back together again. He would see large scale structures flying apart, rather than being clumped by gravity.

But thats crazy!
It's not crazy at all. It just means that the arrow of time is dependent upon the initial conditions. It's counterintuitive, to be sure. But the physics work out identically for an arrow of time like the one we experience, or one in the opposite direction.

Quote Quote by Haelfix View Post
The large scale behaviour of spacetime absolutely cannot change the local laws of physics (say solar system physics). Ergo the perpetual rise in entropy and the irreversability that it entails (even on human scales) cannot possibly be explained by cosmological initial conditions.

The only thing that cosmology can tell us, is in principle why is the initial condition so low relative to what it could be (eg not why z1 < z2 for any universe, but why z1 was not ~ z avg).
That would require that the arrow of time be different in different regions of the universe, or different on different size scales. And that does appear to be disallowed.

And besides, I think you're forgetting that all of the stuff around us originally came from a very small expanding patch.