[tt] Sean M. Carroll: Does Time Run Backward in Other Universes?

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Sean M. Carroll: Does Time Run Backward in Other Universes?
http://www.sciam.com/article.cfm?id=3Dthe-cosmic-origins-of-times-arrow&print=3Dtrue
[Linked by Arts and Letters Daily.]
May 21, 2008

One of the most basic facts of life is that the future looks different
from the past. But on a grand cosmological scale, they may look the same.


The universe does not look right. That may seem like a strange thing
to say, given that cosmologists have very little standard for
comparison. How do we know what the universe is supposed to look
like? Nevertheless, over the years we have developed a strong
intuition for what counts as "natural"--and the universe we see does
not qualify.

Make no mistake: cosmologists have put together an incredibly
successful picture of what the universe is made of and how it has
evolved. Some 14 billion years ago the cosmos was hotter and denser
than the interior of a star, and since then it has been cooling off
and thinning out as the fabric of space expands. This picture
accounts for just about every observation we have made, but a number
of unusual features, especially in the early universe, suggest that
there is more to the story than we understand.

Among the unnatural aspects of the universe, one stands out: time
asymmetry. The microscopic laws of physics that underlie the
behavior of the universe do not distinguish between past and future,
yet the early universe--hot, dense, homogeneous--is completely
different from today's--cool, dilute, lumpy. The universe started
off orderly and has been getting increasingly disorderly ever since.
The asymmetry of time, the arrow that points from past to future,
plays an unmistakable role in our everyday lives: it accounts for
why we cannot turn an omelet into an egg, why ice cubes never
spontaneously unmelt in a glass of water, and why we remember the
past but not the future. And the origin of the asymmetry we
experience can be traced all the way back to the orderliness of the
universe near the big bang. Every time you break an egg, you are
doing observational cosmology.

The arrow of time is arguably the most blatant feature of the
universe that cosmologists are currently at an utter loss to
explain. Increasingly, however, this puzzle about the universe we
observe hints at the existence of a much larger spacetime we do not
observe. It adds support to the notion that we are part of a
multiverse whose dynamics help to explain the seemingly unnatural
features of our local vicinity.

The Puzzle of Entropy

Physicists encapsulate the concept of time asymmetry in the
celebrated second law of thermodynamics: entropy in a closed system
never decreases. Roughly, entropy is a measure of the disorder of a
system. In the 19th century, Austrian physicist Ludwig Boltzmann
explained entropy in terms of the distinction between the microstate
of an object and its macrostate. If you were asked to describe a cup
of coffee, you would most likely refer to its macrostate--its
temperature, pressure and other overall features. The microstate, on
the other hand, specifies the precise position and velocity of every
single atom in the liquid. Many different microstates correspond to
any one particular macrostate: we could move an atom here and there,
and nobody looking at macroscopic scales would notice.

Entropy is the number of different microstates that correspond to
the same macrostate. (Technically, it is the number of digits, or
logarithm, of that number.) Thus, there are more ways to arrange a
given number of atoms into a high-entropy configuration than into a
low-entropy one. Imagine that you pour milk into your coffee. There
are a great many ways to distribute the molecules so that the milk
and coffee are completely mixed together but relatively few ways to
arrange them so that the milk is segregated from the surrounding
coffee. So the mixture has a higher entropy.