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by Dennis Overbye
July 12, 2010
from
NewYorkTines Website
It’s hard to imagine a more fundamental and ubiquitous aspect of
life on the Earth than gravity, from the moment you first took a
step and fell on your diapered bottom to the slow terminal sagging
of flesh and dreams.
But what if it’s all an illusion, a sort of cosmic frill, or a side
effect of something else going on at deeper levels of reality?
So says Erik Verlinde, 48, a respected string theorist and
professor of physics at the University of Amsterdam, whose
contention that gravity is indeed an illusion has caused a
continuing ruckus among physicists, or at least among those who
profess to understand it.
Reversing the logic of 300 years of
science, he argued in a recent paper, titled “On
The Origin of Gravity and The Laws of Newton,”
that gravity is a consequence of the venerable laws of
thermodynamics, which describe the behavior of heat and gases.
“For me gravity doesn’t exist,” said
Dr. Verlinde, who was recently in the United States to explain
himself.
Not that he can’t fall down, but Dr.
Verlinde is among a number of physicists who say that science has
been looking at gravity the wrong way and that there is something
more basic, from which gravity “emerges,” the way stock markets
emerge from the collective behavior of individual investors or that
elasticity emerges from the mechanics of atoms.
Looking at gravity from this angle, they say, could shed light on
some of the vexing cosmic issues of the day, like the
dark energy, a kind of anti-gravity
that seems to be speeding up the expansion of the universe, or the
dark matter that is supposedly
needed to hold galaxies together.
Dr. Verlinde’s argument turns on something you could call the “bad
hair day” theory of gravity.
It goes something like this: your hair frizzles in the heat and
humidity, because there are more ways for your hair to be curled
than to be straight, and nature likes options. So it takes a force
to pull hair straight and eliminate nature’s options. Forget curved
space or the spooky attraction at a distance described by Isaac
Newton’s equations well enough to let us navigate the rings of
Saturn, the force we call gravity is simply a byproduct of nature’s
propensity to maximize disorder.
Some of the best physicists in the world say they don’t understand
Dr. Verlinde’s paper, and many are outright skeptical.
But some of those very same physicists
say he has provided a fresh perspective on some of the deepest
questions in science, namely why space, time and gravity exist at
all - even if he has not yet answered them.
“Some people have said it can’t be
right, others that it’s right and we already knew it - that it’s
right and profound, right and trivial,” Andrew Strominger, a
string theorist at Harvard said.
“What you have to say,” he went on, “is that it has inspired a
lot of interesting discussions. It’s just a very interesting
collection of ideas that touch on things we most profoundly do
not understand about our universe. That’s why I liked it.”
Dr. Verlinde is not an obvious candidate
to go off the deep end.
He and his brother Herman, a Princeton
professor, are celebrated twins known more for their mastery of the
mathematics of hard-core string theory than for philosophic flights.
Born in Woudenberg, in the Netherlands, in 1962, the brothers got
early inspiration from a pair of 1970s television shows about
particle physics and
black holes.
“I was completely captured,” Dr.
Verlinde recalled.
He and his brother obtained Ph.D’s from
the University of Utrecht together in 1988 and then went to
Princeton, Erik to the Institute for Advanced Study and Herman to
the university.
After bouncing back and forth across the
ocean, they got tenure at Princeton. And, they married and divorced
sisters. Erik left Princeton for Amsterdam to be near his children.
He made his first big splash as a graduate student when he invented
Verlinde Algebra and the Verlinde formula, which are important in
string theory, the so-called theory of everything, which posits that
the world is made of tiny wriggling strings.
You might wonder why a string theorist is interested in Newton’s
equations. After all Newton was overturned a century ago by
Einstein, who explained gravity as warps in the geometry of
space-time, and who some theorists think could be overturned in turn
by string theorists.
Over the last 30 years gravity has been “undressed,” in Dr.
Verlinde’s words, as a fundamental force.
This disrobing began in the 1970s with the discovery by Jacob
Bekenstein of the Hebrew University of Jerusalem and Stephen
Hawking of Cambridge University, among others, of a mysterious
connection between black holes and thermodynamics, culminating in
Dr. Hawking’s discovery in 1974 that when quantum effects are taken
into account black holes would glow and eventually explode.
In a provocative calculation in 1995, Ted Jacobson, a theorist from
the University of Maryland, showed that given a few of these
holographic ideas, Einstein’s equations of general relativity are
just a another way of stating the laws of thermodynamics.
Those exploding black holes (at least in theory - none has ever been
observed) lit up a new strangeness of nature.
Black holes, in effect, are
holograms - like the 3-D images you see on bank cards. All the
information about what has been lost inside them is encoded on their
surfaces. Physicists have been wondering ever since how this
“holographic principle” - that we are all maybe just shadows on a
distant wall - applies to the universe and where it came from.
In one striking example of a holographic universe, Juan Maldacena of
the Institute for Advanced Study constructed a mathematical model of
a “soup can” universe, where what happened inside the can, including
gravity, is encoded in the label on the outside of the can, where
there was no gravity, as well as one less spatial dimension.
If dimensions don’t matter and gravity
doesn’t matter, how real can they be?
Lee Smolin, a quantum gravity theorist at the Perimeter
Institute for Theoretical Physics, called Dr. Jacobson’s paper “one
of the most important papers of the last 20 years.”
But it received little attention at first, said Thanu Padmanabhan
of the Inter-University Center for Astronomy and Astrophysics in
Pune, India, who has taken up the subject of “emergent gravity” in
several papers over the last few years.
Dr. Padmanabhan said that the connection
to thermodynamics went deeper that just Einstein’s equations to
other theories of gravity.
“Gravity,” he said recently in a
talk at the Perimeter Institute, “is the thermodynamic limit of
the statistical mechanics of “atoms of space-time.”
Dr. Verlinde said he had read Dr.
Jacobson’s paper many times over the years but that nobody seemed to
have gotten the message. People were still talking about gravity as
a fundamental force.
“Clearly we have to take these
analogies seriously, but somehow no one does,” he complained.
His paper, posted to the physics archive
in January, resembles Dr. Jacobson’s in many ways, but Dr. Verlinde
bristles when people say he has added nothing new to Dr. Jacobson’s
analysis. What is new, he said, is the idea that differences in
entropy can be the driving mechanism behind gravity, that gravity
is, as he puts it an “entropic force.”
That inspiration came to him courtesy of a thief.
As he was about to go home from a vacation in the south of France
last summer, a thief broke into his room and stole his laptop, his
keys, his passport, everything.
“I had to stay a week longer,” he
said, “I got this idea.”
Up the beach, his brother got a series
of e-mail messages first saying that he had to stay longer, then
that he had a new idea and finally, on the third day, that he knew
how to derive Newton’s laws from first principles, at which point
Herman recalled thinking,
“What’s going on here? What has he
been drinking?”
When they talked the next day it all
made more sense, at least to Herman.
“It’s interesting,” Herman said,
“how having to change plans can lead to different thoughts.”
Think of the universe as a box of
scrabble letters.
There is only one way to have the
letters arranged to spell out the Gettysburg Address, but an
astronomical number of ways to have them spell nonsense. Shake the
box and it will tend toward nonsense, disorder will increase and
information will be lost as the letters shuffle toward their most
probable configurations. Could this be gravity?
As a metaphor for how this would work, Dr. Verlinde used the example
of a polymer - a strand of DNA, say, a noodle or a hair - curling
up.
“It took me two months to understand
polymers,” he said.
The resulting paper, as Dr. Verlinde
himself admits, is a little vague.
“This is not the basis of a theory,”
Dr. Verlinde explained. “I don’t pretend this to be a theory.
People should read the words I am saying opposed to the details
of equations.”
Dr. Padmanabhan said that he could see
little difference between Dr. Verlinde’s and Dr. Jacobson’s papers
and that the new element of an entropic force lacked mathematical
rigor.
“I doubt whether these ideas will
stand the test of time,” he wrote in an e-mail message from
India.
Dr. Jacobson said he couldn’t make sense of it.
John Schwarz of the California
Institute of Technology, one of the fathers of string theory, said
the paper was “very provocative.” Dr. Smolin called it,
“very interesting and also very
incomplete.”
At a workshop in Texas in the spring,
Raphael Bousso of the University of California, Berkeley, was
asked to lead a discussion on the paper.
“The end result was that everyone
else didn’t understand it either, including people who initially
thought that did make some sense to them,” he said in an e-mail
message.
“In any case, Erik’s paper has drawn attention to what is
genuinely a deep and important question, and that’s a good
thing,” Dr. Bousso went on, “I just don’t think we know any
better how this actually works after Erik’s paper. There are a
lot of follow-up papers, but unlike Erik, they don’t even
understand the problem.”
The Verlinde brothers are now trying to
recast these ideas in more technical terms of string theory, and
Erik has been on the road a bit, traveling in May to the Perimeter
Institute and Stony Brook University on Long Island, stumping for
the end of gravity.
Michael Douglas, a professor at
Stony Brook, described Dr. Verlinde’s work as,
“a set of ideas that resonates with
the community," adding, “everyone is waiting to see if this can
be made more precise.”
Until then the jury of Dr. Verlinde’s
peers will still be out.
Over lunch in New York, Dr. Verlinde ruminated over his experiences
of the last six months. He said he had simply surrendered to his
intuition.
“When this idea came to me, I was
really excited and euphoric even,” Dr. Verlinde said. “It’s not
often you get a chance to say something new about Newton’s laws.
I don’t see immediately that I am wrong. That’s enough to go
ahead.”
He said friends had encouraged him to
stick his neck out and that he had no regrets.
“If I am proven wrong, something has
been learned anyway. Ignoring it would have been the worst
thing.”
The next day Dr. Verlinde gave a more
technical talk to a bunch of physicists in the city. He recalled
that someone had told him the other day that the unfolding story of
gravity was like the emperor’s new clothes.
“We’ve known for a long time gravity
doesn’t exist,” Dr. Verlinde said, “It’s time to yell it.”
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