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Big Brain Theory: Have Cosmologists Lost Theirs?

By DENNIS OVERBYE
Published: January 15, 2008 - New York Times

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 there 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, 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 Buosso 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.