From: Hal Finney (hal@finney.org)
Date: Tue May 06 2003 - 13:25:22 MDT
Anders points to http://arxiv.org/abs/hep-th/0208013 and
http://www.nature.com/nsu/020812/020812-2.html, and Damien B. comments:
> What? What? Why should something have `set the peculiar initial conditions'
> of *our* Universe? On this Poincare recurrence argument (well, it looks
> awfully familiar to me, anyway), ours might well be the remixed result of
> many earlier *non*-us-like universes, surely? It's *not* special pleading
> for me to note that I hold the views I do and speak English because by
> happenstance I was born in Australia last century, rather than two thousand
> years ago. The Copernican default assumption can be taken too far,
> especially when we're juggling universes.
We discussed this paper a few months ago on Wei Dai's "everything" list.
Unfortunately the physics was over the heads of all of us so it was hard
to understand exactly what is going on. I sent email to the authors
asking for clarification, but they didn't respond, no doubt assessing
my level of ignorance as being too deep for a quick answer.
Damien is right that this is a variation of a very old argument, going
back to Boltzmann in the 19th century, relating to Poincare recurrence.
Suppose that the universe goes on forever in a steady, disordered state,
say as a uniform gas. Then eventually, at random, some of that gas will
collect together in a locally low-entropy state, and it could collect
to form solid matter or even stars. This would be extremely rare, but
it would happen eventually, and we're assuming that the steady state
lasts forever. Then, with even enormously greater rarity, you could
get a much larger random fluctuation which could form whole galaxies, or
even billions of galaxies as we see. Hence in a universe of this kind,
there would occasionally be observers like ourselves.
The problem and paradox is that this is not a plausible explanation of
what we see (even ignoring Big Bang considerations), because it is so
much enormously less likely that you will get zillions of galaxies than
that you would get just a single star. It's far more likely that the
bare minimum would happen that was sufficient to create life; something
like a single star and a single planet, or maybe even just a large brown
dwarf whose surface was cool enough for life. And "far more likely" is
really an understatement here; the degree of difference in probability is
beyond astronomical. Hence almost all observers would see a nearly empty
universe, and observers like us would be so rare among the collection
of all observers who ever exist as to be practically nonexistent.
All this is generally considered a historical footnote in the conceptual
foundations of thermodynamics, but this recent paper claims that it
could apply to our own universe. Here is where my physics understanding
breaks down. Despite the fact that the universe is predicted (under
current models) to expand at an ever-increasing rate, thinning the
density of matter and energy down to approximately zero, the authors
boldly re-scale their coordinates, using an expanding coordinate frame in
which the universe is constant in size. Presto, we have a more or less
steady-state universe. The problem I couldn't get past is that it would
have a zero density, hence no opportunity for random collections of matter
to form, but perhaps there is some esoteric effect that overcomes this.
So if this is true, we are back to the paradox above, that almost
all observers that will ever exist will see a very empty universe.
They will live potentially happy, healthy lives, just as we do; but
they won't have any stars to look at. The fact that we see uncounted
trillions of stars is fundamentally inconsistent with any application of
the principle that we are typical observers, if the authors are correct.
(The authors also point out that even those few observers who see a big
universe will not see one that plausibly came from a recent Big Bang,
since it did not in fact do so. However I think that's icing on the cake,
the size issue is more fundamental.)
The conclusion that their work points to is not that the anthropic
principle is wrong, but rather that physics is wrong. Assuming they
are right about the properties of this expanding de Sitter mode, then
this work suggests that the current picture of the eventual future of
the universe must be mistaken. The universe will not get into a state
of rapid, exponential expansion that goes on forever. Our physicists
have made a mistake.
Well, that wouldn't really be that big a surprise; the current model is
only a few years old, and it is certainly possible that ten years from
now we'll have different ideas about how the universe will turn out.
The interesting thing about this paper is that it uses anthropic-style
reasoning to argue that current physical theories must have this sort
of error.
Hal
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