[tt] NYT: Big Brain Theory: Have Cosmologists Lost Theirs?

[Premise Checker]

Big Brain Theory: Have Cosmologists Lost Theirs?
http://www.nytimes.com/2008/01/15/science/15brain.html

By DENNIS OVERBYE

It could be the weirdest and most embarrassing prediction in the
history of cosmology, if not science.

If true, it would mean that you yourself reading this article are
more likely to be some momentary fluctuation in a field of matter
and energy out in space than a person with a real past born through
billions of years of evolution in an orderly star-spangled cosmos.
Your memories and the world you think you see around you are
illusions.

This bizarre picture is the outcome of a recent series of
calculations that take some of the bedrock theories and discoveries
of modern cosmology to the limit. Nobody in the field believes that
this is the way things really work, however. And so in the last
couple of years there has been a growing stream of debate and
dueling papers, replete with references to such esoteric subjects as
reincarnation, multiple universes and even the death of spacetime,
as cosmologists try to square the predictions of their cherished
theories with their convictions that we and the universe are real.
The basic problem is that across the eons of time, the standard
theories suggest, the universe can recur over and over again in an
endless cycle of big bangs, but it's hard for nature to make a whole
universe. It's much easier to make fragments of one, like planets,
yourself maybe in a spacesuit or even -- in the most absurd and
troubling example -- a naked brain floating in space. Nature tends
to do what is easiest, from the standpoint of energy and
probability. And so these fragments -- in particular the brains --
would appear far more frequently than real full-fledged universes,
or than us. Or they might be us.

Alan Guth, a cosmologist at the Massachusetts Institute of
Technology who agrees this overabundance is absurd, pointed out that
some calculations result in an infinite number of free-floating
brains for every normal brain, making it "infinitely unlikely for us
to be normal brains." Welcome to what physicists call the Boltzmann
brain problem, named after the 19th-century Austrian physicist
Ludwig Boltzmann, who suggested the mechanism by which such
fluctuations could happen in a gas or in the universe. Cosmologists
also refer to them as "freaky observers," in contrast to regular or
"ordered" observers of the cosmos like ourselves. Cosmologists are
desperate to eliminate these freaks from their theories, but so far
they can't even agree on how or even on whether they are making any
progress.

If you are inclined to skepticism this debate might seem like
further evidence that cosmologists, who gave us dark matter, dark
energy and speak with apparent aplomb about gazillions of parallel
universes, have finally lost their minds. But the cosmologists say
the brain problem serves as a valuable reality check as they
contemplate the far, far future and zillions of bubble universes
popping off from one another in an ever-increasing rush through
eternity. What, for example is a "typical" observer in such a setup?
If some atoms in another universe stick together briefly to look,
talk and think exactly like you, is it really you?

"It is part of a much bigger set of questions about how to think
about probabilities in an infinite universe in which everything that
can occur, does occur, infinitely many times," said Leonard Susskind
of Stanford, a co-author of a paper in 2002 that helped set off the
debate. Or as Andrei Linde, another Stanford theorist given to
colorful language, loosely characterized the possibility of a
replica of your own brain forming out in space sometime, "How do you
compute the probability to be reincarnated to the probability of
being born?"

The Boltzmann brain problem arises from a string of logical
conclusions that all spring from another deep and old question,
namely why time seems to go in only one direction. Why can't you
unscramble an egg? The fundamental laws governing the atoms bouncing
off one another in the egg look the same whether time goes forward
or backward. In this universe, at least, the future and the past are
different and you can't remember who is going to win the Super Bowl
next week.

"When you break an egg and scramble it you are doing cosmology,"
said Sean Carroll, a cosmologist at the California Institute of
Technology.

Boltzmann ascribed this so-called arrow of time to the tendency of
any collection of particles to spread out into the most random and
useless configuration, in accordance with the second law of
thermodynamics (sometimes paraphrased as "things get worse"), which
says that entropy, which is a measure of disorder or wasted energy,
can never decrease in a closed system like the universe.

If the universe was running down and entropy was increasing now,
that was because the universe must have been highly ordered in the
past.

In Boltzmann's time the universe was presumed to have been around
forever, in which case it would long ago have stabilized at a
lukewarm temperature and died a "heat death." It would already have
maximum entropy, and so with no way to become more disorderly there
would be no arrow of time. No life would be possible but that would
be all right because life would be excruciatingly boring. Boltzmann
said that entropy was all about odds, however, and if we waited long
enough the random bumping of atoms would occasionally produce the
cosmic equivalent of an egg unscrambling. A rare fluctuation would
decrease the entropy in some place and start the arrow of time
pointing and history flowing again. That is not what happened.
Astronomers now know the universe has not lasted forever. It was
born in the Big Bang, which somehow set the arrow of time, 14
billion years ago. The linchpin of the Big Bang is thought to be an
explosive moment known as inflation, during which space became
suffused with energy that had an antigravitational effect and
ballooned violently outward, ironing the kinks and irregularities
out of what is now the observable universe and endowing primordial
chaos with order.

Inflation is a veritable cosmological fertility principle.
Fluctuations in the field driving inflation also would have seeded
the universe with the lumps that eventually grew to be galaxies,
stars and people. According to the more extended version, called
eternal inflation, an endless array of bubble or "pocket" universes
are branching off from one another at a dizzying and exponentially
increasing rate. They could have different properties and perhaps
even different laws of physics, so the story goes.

A different, but perhaps related, form of antigravity, glibly dubbed
dark energy, seems to be running the universe now, and that is the
culprit responsible for the Boltzmann brains.

The expansion of the universe seems to be accelerating, making
galaxies fly away from one another faster and faster. If the leading
dark-energy suspect, a universal repulsion Einstein called the
cosmological constant, is true, this runaway process will last
forever, and distant galaxies will eventually be moving apart so
quickly that they cannot communicate with one another. Being in such
a space would be like being surrounded by a black hole.

Rather than simply going to black like "The Sopranos" conclusion,
however, the cosmic horizon would glow, emitting a feeble spray of
elementary particles and radiation, with a temperature of a fraction
of a billionth of a degree, courtesy of quantum uncertainty. That
radiation bath will be subject to random fluctuations just like
Boltzmann's eternal universe, however, and every once in a very
long, long time, one of those fluctuations would be big enough to
recreate the Big Bang. In the fullness of time this process could
lead to the endless series of recurring universes. Our present
universe could be part of that chain.

In such a recurrent setup, however, Dr. Susskind of Stanford, Lisa
Dyson, now of the University of California, Berkeley, and Matthew
Kleban, now at New York University, pointed out in 2002 that
Boltzmann's idea might work too well, filling the megaverse with
more Boltzmann brains than universes or real people.

In the same way the odds of a real word showing up when you shake a
box of Scrabble letters are greater than a whole sentence or
paragraph forming, these "regular" universes would be vastly
outnumbered by weird ones, including flawed variations on our own
all the way down to naked brains, a result foreshadowed by Martin
Rees, a cosmologist at the University of Cambridge, in his 1997
book, "Before the Beginning."

The conclusions of Dr. Dyson and her colleagues were quickly
challenged by Andreas Albrecht and Lorenzo Sorbo of the University
of California, Davis, who used an alternate approach. They found
that the Big Bang was actually more likely than Boltzmann's brain.

"In the end, inflation saves us from Boltzmann's brain," Dr.
Albrecht said, while admitting that the calculations were
contentious. Indeed, the "invasion of Boltzmann brains," as Dr.
Linde once referred to it, was just beginning.

In an interview Dr. Linde described these brains as a form of
reincarnation. Over the course of eternity, he said, anything is
possible. After some Big Bang in the far future, he said, "it's
possible that you yourself will re-emerge. Eventually you will
appear with your table and your computer."

But it's more likely, he went on, that you will be reincarnated as
an isolated brain, without the baggage of stars and galaxies. In
terms of probability, he said, "It's cheaper."

You might wonder what's wrong with a few brains -- or even a
preponderance of them -- floating around in space. For one thing, as
observers these brains would see a freaky chaotic universe, unlike
our own, which seems to persist in its promise and disappointment.

Another is that one of the central orthodoxies of cosmology is that
humans don't occupy a special place in the cosmos, that we and our
experiences are typical of cosmic beings. If the odds of us being
real instead of Boltzmann brains are one in a million, say, waking
up every day would be like walking out on the street and finding
everyone in the city standing on their heads. You would expect there
to be some reason why you were the only one left right side up.

Some cosmologists, James Hartle and Mark Srednicki, of the
University of California, Santa Barbara, have questioned that
assumption. "For example," Dr. Hartle wrote in an e-mail message,
"on Earth humans are not typical animals; insects are far more
numerous. No one is surprised by this."

In an e-mail response to Dr. Hartle's view, Don Page of the
University of Alberta, who has been a prominent voice in the
Boltzmann debate, argued that what counted cosmologically was not
sheer numbers, but consciousness, which we have in abundance over
the insects. "I would say that we have no strong evidence against
the working hypothesis that we are typical and that our observations
are typical," he explained, "which is very fruitful in science for
helping us believe that our observations are not just flukes but do
tell us something about the universe."

Dr. Dyson and her colleagues suggested that the solution to the
Boltzmann paradox was in denying the presumption that the universe
would accelerate eternally. In other words, they said, that the
cosmological constant was perhaps not really constant. If the
cosmological constant eventually faded away, the universe would
revert to normal expansion and what was left would eventually fade
to black. With no more acceleration there would be no horizon with
its snap, crackle and pop, and thus no material for fluctuations and
Boltzmann brains.

String theory calculations have suggested that dark energy is indeed
metastable and will decay, Dr. Susskind pointed out. "The success of
ordinary cosmology," Dr. Susskind said, "speaks against the idea
that the universe was created in a random fluctuation."

But nobody knows whether dark energy -- if it dies -- will die soon
enough to save the universe from a surplus of Boltzmann brains. In
2006, Dr. Page calculated that the dark energy would have to decay
in about 20 billion years in order to prevent it from being overrun
by Boltzmann brains.

The decay, if and when it comes, would rejigger the laws of physics
and so would be fatal and total, spreading at almost the speed of
light and destroying all matter without warning. There would be no
time for pain, Dr. Page wrote: "And no grieving survivors will be
left behind. So in this way it would be the most humanely possible
execution." But the object of his work, he said, was not to predict
the end of the universe but to draw attention to the fact that the
Boltzmann brain problem remains.

People have their own favorite measures of probability in the
multiverse, said Raphael Bousso of the University of California,
Berkeley. "So Boltzmann brains are just one example of how measures
can predict nonsense; anytime your measure predicts that something
we see has extremely small probability, you can throw it out," he
wrote in an e-mail message.

Another contentious issue is whether the cosmologists in their
calculations could consider only the observable universe, which is
all we can ever see or be influenced by, or whether they should take
into account the vast and ever-growing assemblage of other bubbles
forever out of our view predicted by eternal inflation. In the
latter case, as Alex Vilenkin of Tufts University pointed out, "The
numbers of regular and freak observers are both infinite." Which
kind predominate depends on how you do the counting, he said..

In eternal inflation, the number of new bubbles being hatched at any
given moment is always growing, Dr. Linde said, explaining one such
counting scheme he likes. So the evolution of people in new bubbles
far outstrips the creation of Boltzmann brains in old ones. The main
way life emerges, he said, is not by reincarnation but by the
creation of new parts of the universe. "So maybe we don't need to
care too much" about the Boltzmann brains," he said.

"If you are reincarnated, why do you care about where you are
reincarnated?" he asked. "It sounds crazy because here we are
touching issues we are not supposed to be touching in ordinary
science. Can we be reincarnated?"

"People are not prepared for this discussion," Dr. Linde said.