On Gravity, Oreos and a Theory of Everything

On Gravity, Oreos and a Theory of Everything
By DENNIS OVERBYE
The portal to the fifth dimension, sadly, is closed.

There used to be an ice cream parlor in the student center at the
Massachusetts Institute of Technology. And it was there, in the summer of
1998, that Lisa Randall, now a professor of physics at Harvard and a bit of
a chocoholic, and Raman Sundrum, a professor at Johns Hopkins, took an
imaginary trip right out of this earthly plane into a science fiction realm
of parallel universes, warped space and otherworldly laws of physics.

They came back with a possible answer to a question that has tormented
scientists for decades, namely why gravity is so weak compared with the
other forces of nature: in effect, we are borrowing it from another
universe. In so doing, Dr. Randall and Dr. Sundrum helped foment a
revolution in the way scientists think about string theory - the vaunted
"theory of everything" - raising a glimmer of hope that coming experiments
may actually test some of its ineffable sounding concepts.

Their work undermined well-worn concepts like the idea that we can even know
how many dimensions of space we live in, or the reality of gravity, space
and time.

The work has also made a star and an icon of Dr. Randall. The attention has
been increased by the recent publication to laudatory reviews of her new
book, "Warped Passages, Unraveling the Mysteries of the Universe's Hidden
Dimensions," A debate broke out on the physics blog Cosmic Variance a few
weeks ago about whether it was appropriate, as a commentator on NPR had
said, to say she looked like Jodie Foster.

"How do we know we live in a four-dimensional universe?" she asked a crowd
who filled the Hayden Planetarium on a stormy night last week.

"You think gravity is what you see. We're always just looking at the tail of
things."

Although it is the unanswerable questions that most appeal to her now, it
was the answerable ones that drew her to science, especially math, as a
child, the middle of three daughters of a salesman for an engineering firm,
and a teacher, in Fresh Meadows, Queens. "I really liked the fact that it
had definite answers," Dr. Randall said.

At Stuyvesant High School, where she was in the same class as Brian Greene,
the future Columbia string theorist and best-selling author, she was the
first girl to serve as captain of the school's math team, and she won the
famous Westinghouse Science Talent Search competition with a project about
complex numbers. She went on to Harvard where she stayed until 1987 when she
emerged with a Ph.D. in physics.

Those were heady times in physics. Fired by the dream of a unified theory of
everything, theorists flocked to string theory, which envisioned the
fundamental elements of nature as tiny wriggling strings.

Dr. Randall, however, resisted this siren call, at least for a while. For
one thing, physicists thought it would take a particle accelerator 10
million billion times as powerful as anything on earth to produce an actual
string and test the theory.

String theory also stubbornly requires space-time to have 10 dimensions, not
the 4 (3 of space and 1 of time) that we experience. Preferring to stay
closer to testable reality, Dr. Randall was drawn to a bottom-up approach to
theoretical physics, trying to build models that explain observed phenomena
and hoping to discover principles with wider application. But Dr. Randall
and string theory had their own kismet.

In the mid-90's, theorists discovered that the theory was even richer than
its founders had thought, describing not just strings but so-called branes,
as in membranes, of all dimensions. Our own universe could be such a brane,
an island of three dimensions floating in a sea of higher dimension, like a
bubble in the sea. But there could be membranes with five, six, seven or
more dimensions coexisting and mingling like weird cosmic soap bubbles in
what theorists sometimes call the multiverse.

"The stuff we're really famous for was really lucky in a way," Dr. Randall
said.

In the summer of 1998, after postdoctoral stints at Harvard and the
University of California, Berkeley, she was a tenured M.I.T. professor ready
to move to Princeton. She wondered then whether parallel universes could
help solve a vexing problem with a favorite theories of particle physicists.

That theory, known as supersymmetry, was invented in turn to solve another
problem - the enormous gulf known as the hierarchy problem between gravity
and the other forces. Naïve calculations from first principles suggest, Dr.
Randall said, that gravity should be 10 million billion times as strong as
it is. You might find it hard to imagine gravity as a weak force, but
consider, says Dr. Randall, that a small magnet can hold up a paper clip,
even though the entire earth is pulling down on it.

But there was a hitch with the way the theory worked out in our universe. It
predicted reactions that are not observed.

Dr. Randall wondered if the missing reactions could be explained by positing
that some aspects of the theory were quarantined in a separate universe.

She called up Dr. Sundrum, who was then a fellow at Boston University and
happy to collaborate, having worked with her before. A lot of physics is
taste, he explained, discerning, for example, what is an important and a
potentially soluble problem. Dr. Randall's biggest strength, he said, is a
kind of "unworldly" instinct. "She has a great nose," Dr. Sundrum said.

"It's a mystery to those of us - hard to understand, almost to the point of
amusement - how she does it without any clear sign of what led her to that
path," he continued. "She gives no sign of why she thinks what she thinks."

They began by drawing pictures and making crude estimates over ice cream and
coffee in that ice cream parlor, which is now a taqueria. What they drew
pictures of was a kind of Oreo cookie multiverse, an architecture similar to
one first discovered as a solution of the string equations by Edward Witten
of the Institute for Advanced Study and Petr Horava, now at Berkeley. Dr.
Randall and Dr. Sundrum's model consisted of a pair of universes,
four-dimensional branes, thinly separated by a five-dimensional space
poetically called the bulk.

When they solved the equations for this setup, they discovered that the
space between the branes would be warped. Objects, for example, would appear
to grow larger or smaller and get less massive or more massive as they moved
back and forth between the branes.

Such a situation, they realized to their surprise, could provide a natural
explanation for the hierarchy problem without invoking supersymmetry.
Suppose, they said, that gravity is actually inherently as strong as the
other forces, but because of the warping gravity is much much stronger on
one of the branes than on the other one, where we happen to live. So we
experience gravity as extremely weak.

"You can be only a modest distance away from the gravity brane," Dr. Randall
said, "and gravity will be incredibly weak." A result was a natural
explanation for why atomic forces outgun gravity by 10 million billion to 1.
Could this miracle be true? Crazy as it sounded, they soon discovered an
even more bizarre possibility. The fifth dimension could actually be
infinite and we would not have noticed it.

In this case, there would be only one brane, ours, containing both gravity
as we know it and the rest of nature. But it would warp space in the same
way as in the first model, trapping gravity nearby so that we would
experience space-time as four-dimensional. This new single brane model did
not solve the weak gravity problem, Dr. Randall admitted, but it was a
revelation, that an infinite ocean of space could be sitting next to us
undetected.

"So when we wrote this paper, what we were concentrating on was this amazing
fact that really had been overlooked for 100 years - well, years, whatever -
that you can have this infinite extra dimension," she said. "I mean it was
quite wild."

This was not the first time that theorists had tinkered with the extra
dimensions of string theory, dimensions that had been presumed to be coiled
out of sight of experiment, into tight loops so small that not even an
electron could enter. In 1998, three theorists - Nima Arkani-Hamed of
Harvard, Gia Divali of New York University and Savas Dimopoulos of Stanford
(a group known in physics as A.D.D.) - had surprised everybody by suggesting
that if one or two of the curled-up extra dimensions had sizes as big as a
tenth of millimeter or so (gigantic on particle physics scales), gravity
would be similarly diluted and weakened.

When Dr. Randall and Dr. Sundrum published their first paper, describing the
two-brane scheme, in 1999, she said that many physicists did not recognize
it as a new idea and not just an elaboration on the large extra dimensions
of the A.D.D. group. In fact, she said, the extra dimensions don't have to
be very large in the two-brane theory, less than a millionth of a trillionth
of a trillionth of an inch.

When they published their second paper, about the infinite dimension, she
said, even some of their best friends, reserved judgment.

But by the time a long-planned workshop on strings and particle physics at
the Kavli Institute for Theoretical Physics in Santa Barbara rolled around
that fall, string theorists were excited about the Randall-Sundrum work and
the earlier A.D.D. proposal.

The reason was simple: If they were very lucky and one of these versions of
string theory was the one that nature had adopted, it could actually be
tested in the Large Hadron Collider, the giant particle accelerator due to
go into operation at CERN near Geneva in 2007. Colliding beams of protons
with a combined energy of 14 trillion electron volts, the collider could
produce particles like gravitons going off into the fifth dimension like
billiard balls hopping off the table, black holes or even the illusive
strings themselves.

"If this is the way gravity works in high-energy physics, we'll know about
it," Dr. Randall said.

Although physicists agree that these theories are a long shot, the new work
has captured their imaginations and encouraged them to take a fresh look at
the possibilities for the universe and their new accelerator.

Dr. Greene of Columbia said, "Sometimes it takes an outsider to come into a
field and see what is being missed, or taken for granted." At first the idea
that extra dimensions could be bigger than any of us had thought was
shocking, he said.

Andrew Strominger, a Harvard string theorist, said: "Before A.D.D. we
believed there was no hope of finding evidence for string theory at the
Large Hadron Collider, an assumption that was wrong. It shows how
unimaginative and narrow-minded we are. I see that as cause for optimism.
Science and nature are full of surprises, we never see what's going to
happen next."

It was shortly before a conference that Dr. Randall had organized during the
Kavli workshop that she had her own experience with gravity: she fell while
rock climbing in Yosemite, breaking several bones. Only a day before, she
said, she had completed a climb of Half Dome and was feeling cocky.

Another symptom of gravity's weakness is that a rope is sufficient to hold a
human body up against earth's pull, but Dr. Randall was still on the first
leg of her climb and hadn't yet attached it to the rock.. She woke up in a
helicopter. For a long time, she said, new parts kept hurting as old ones
healed. "I was very much not myself. I didn't even like chocolate and
coffee."

Since she was the conference organizer, her ordeal was more public than she
would have liked. "In some ways you sort of want to do this in private," Dr.
Randall said. "On the other hand people were really nice."

After two years at Princeton, Dr. Randall returned to M.I.T. in 2000, but
then a year later moved to Harvard, by then a powerhouse in string theory.
She was the third woman to get tenure in physics there.

Dr. Randall, 43 and single, prefers not to talk about "the women in science
thing," as she calls it. That subject that gained notoriety earlier this
year when Harvard's President Larry Summers famously ventured that a
relative lack of women in the upper ranks of science might reflect innate
deficiencies, but Dr. Randall said it had been beaten to death.

Asked if she would rather be a woman in science than talk about women in
science, Dr. Randall said, "I'd rather be a scientist."

She did say that part of the reason she had written her book was to
demonstrate that that there were women out there doing this kind of science.
"I did feel extra pressure to write a good book," she said, adding that the
response in reviews and emails from readers had been much greater than she
had expected.

She was particularly pleased that some of her readers were attentive and
studious enough to catch on to various puns and games she had inserted in
the book, like the frequent references to Alice in Wonderland, which, she
said, is a pun on "one-d-land."

Dr. Randall is intrigued by that fact that her results, as well as other
results from string theory seem to paint a picture of the universe in which
theories with different numbers of dimensions in them all give the same
physics? She and Andreas Karch of the University of Washington have found,
for example, that the fifth dimension could be so warped that the number of
dimensions you see would depend on where you were. Our own universe might
just be a three-dimensional "sinkhole," she says.

"It's not completely obvious what gravity is, fundamentally, or what
dimensions are, fundamentally," she said over lunch. "One of these days
we'll understand better what we mean, what is the fundamental thing that's
given us space in the first place and dimensions of space in particular."

She held out less hope for time, saying, "I just don't understand it.

"Space we can make progress with."

Is time an illusion?

"I wish time were an illusion," she said as she carved up the last of her
chocolate bread pudding, "but unfortunately it seems all too real."

Peace,

Sean
--
"Every now and then when your life gets complicated and the weasels start
closing in, the only real cure is to load up on heinous chemicals and then
drive like a *** from Hollywood to Las Vegas." Hunter S. Thompson

My music list for trades: http://db.etree.org/FionaRCB


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