A New Slice on Physics
Is the world we see trapped on a thin membrane separating us from vast other
realms? Some scientists say that would explain a lot
By K.C. Cole
LA Times Staff Writer
May 17, 2003
Plato considered it first.
What if everything we hold dear is but a thin slice of some larger,
unreachable reality, like a flickering shadow cast on the craggy wall of a
cave? What if the moon and stars, your home, your thoughts, your cat, are
but projections on this wall - mere suggestions of unfathomable realms
In the last few years, a mathematically rigorous version of Plato's
2,000-year-old thought experiment has been refashioning the way physicists
think about everything from subatomic particles to the Big Bang. The
universe we see, according to this scenario, is stuck on a thin membrane of
space-time embedded in a much larger cosmos. And our membrane may be only
one of many, all of which may warp, wiggle, connect and collide with one
another in as many as 10 dimensions. Physicists call this new frontier the
The idea could help solve a long list of outstanding mysteries. Among them:
What is the "dark matter" that seems to make up 90% of the universe? And why
is gravity trillions of times weaker than electromagnetism?
The revolution was set off in the mid-1990s when UC Santa Barbara physicist
Joe Polchinski determined through mathematics that branes were a surface to
which things attach, like hair to skin - except the "things" in this case
were the minuscule "strings" that may well be the fundamental ingredients of
"I was just fiddling around with mathematics Within a week or two [other
physicists] had done things with it I hadn't envisioned. It was like taking
the stopper out of the dam. Things poured through."
Alan Guth of the Massachusetts Institute of Technology, creator of the
currently accepted version of the Big Bang, said recently he felt a little
like Rip Van Winkle - picking up his head from a long sleep only to notice
that the landscape of physics he thought he knew had suddenly, drastically,
Stephen Hawking of the University of Cambridge, among others, envisions
brane worlds bubbling up out of the void, giving rise to whole new
universes. He ends his latest book, "The Universe in a Nutshell," with a
call to explore this "brane new world."
One might well wonder why such a seemingly bizarre concept has attracted so
many well-established physicists. The short answer is: desperation.
The laws of nature that describe the large-scale universe to an astonishing
degree of precision (Einstein's general relativity) are incompatible with
the laws that describe the small-scale universe with the same astonishing
exactness (quantum theory). This means either that one of these well-tested
theories is wrong (all but inconceivable) or that there is some larger, more
encompassing theory that somehow accommodates both.
To date, the only theory that comes close to marrying the two is "string
theory" - a mathematically elegant set of ideas that has swept the world of
physics over the last few decades. According to string theory, the basic
ingredients of the universe are not point-like particles, but tiny strings
vibrating in 10-dimensional space. Although still untested, string theory
has scored a spectacular series of theoretical successes, earning it an
ever-widening circle of admirers.
And yet string theory remains a realm apart from day-to-day physics - lovely
to behold but innately aloof.
For one thing, the strings are so small that it would take a particle
accelerator larger than the solar system to create the energies needed to
"see" them. This means, in effect, that strings can never be detected.
For another, the complex mathematics required to deal with the tortured
10-dimensional landscape is beyond the reach of most physicists.
Brane models change all that. Unlike in string theory, the extra dimensions
in brane worlds can be big, infinitely big. "It led to a whole new bunch of
possibilities that could be experimentally tested," said physicist Jim Cline
of McGill University in Montreal.
What's more, branes don't require the full range of mathematical tools
required for string theory, opening the door to new groups of scientists.
"You can use methods that are part and parcel of more traditional physics,"
said Columbia University physicist Brian Greene. "So a person who's not a
string theorist can jump into the field and make contributions."
This sense of promise was palpable last summer at the Aspen Center for
Physics, where string theorists and cosmologists - the scientists who study
the origin and structure of the universe - gathered for a workshop to
explore links between the smallest scales in the universe and the largest.
Brane scenarios popped up everywhere, enveloped in the thick fog of
uncertainty that clouds the birth of new worlds.
The setting was strangely church-like. The faithful sat in rows under spires
of white-barked aspens, their round leaves fluttering in the wind.
In front, a maestro in sneakers tapped out symbols on a blackboard, chalk
flying like fairy dust, black jeans covered in white handprints. There was
lots of talk about the infinite; lots of recitation and response. Everyone
strained to channel some larger reality through equations.
"Your bulk could contain many 3-branes," one physicist said.
"The 9-branes could still annihilate."
This was not your grandmother's physics. There were no objects in the usual
sense. No matter, no particles. Not even numbers. Only "instantons," "alpha
vacua" and multidimensional membranes wrapping around one another, traveling
down throats of black holes and bouncing back, transformed.
Even to physicists, much of this seems unbearably strange. But in physics,
strangeness comes with the territory. "When I first learned about quantum
physics as an undergraduate, it just about destroyed my mind," said Stanford
post-doctoral fellow Stephon Alexander. "And now, 12 years later, it's just
There's actually nothing particularly new about the idea that space may
extend into unseen dimensions, or even that the world we know is somehow
trapped on a membrane.
Extra dimensions were such a hot topic in the 19th century that Victorian
schoolmaster Edwin Abbott wrote a famous science fiction novel, "Flatland,"
based on the notion that our limited perceptions prevented us from seeing
worlds existing right in front of our three-dimensional noses. Albert
Einstein made extra dimensions an integral part of physics when he used a
fourth dimension, time, in his theory of relativity in 1905. Ten years
later, he showed that this interwoven fabric of space-time could warp under
the influence of massive objects - "causing" the force we know as gravity.
Extra-dimensional membranes were kicking around in string theory since at
least the mid-1980s, but no one took them very seriously. One of the first
suggestions that the world we know might be stuck to such a membrane
appeared in a 1985 paper that was a parody of string theory titled "The
Super G-String" by V. Gates, et al., from the University of Cauliflower
(actually, physicist Warren Siegel of State University of New York, Stony
Brook). "It was based on a serious paper that was totally overlooked because
it was before its time," Polchinski said.
The branes playing such a large role in physics today are richer and more
mathematically rigorous than early versions.
Essentially, a brane is a discontinuity in space-time, a boundary where
things meet, like the surface of a pond where the water meets the sky.
"It's a defect in the quantum fabric," said Ruth Gregory of the University
of Durham in Britain. On one side of the defect would be the vacuum of empty
space. A vacuum with somewhat different properties might exist on the other
Imagine our brane as pond scum - a thin film that divides the air above from
a deep (perhaps infinitely deep) body of water below. Most of what we
experience is trapped in the scum. But beyond is a whole other world of
currents swirling beneath the surface. Their motion might tug on our scum.
We'd feel it as nothing but a gentle disturbance, never dreaming of what
A brane doesn't always divide one thing from another. It may just be a
condensation of stuff, "a localized lump of energy and curvature that likes
to hang together," Stanford University physicist Steve Shenker said.
Either way, it's a place where things get stuck - like the scum on the pond.
"That was the revolution," said Harvard University physicist Lisa Randall.
"To realize that branes were honest-to-goodness objects."
Randall played a pivotal role in the revolution when she and Johns Hopkins
University physicist Raman Sundrum realized that branes could be infinitely
large and yet remain invisible.
The reason: We can't see anything outside our brane, because light can't
escape or enter it. We can't hear anything outside, because sound travels
through matter, and matter is stuck to our brane. We can't use radioactivity
to sense what's beyond, or even break through with nuclear bombs, because
nuclear forces are also firmly nailed to our brane. There could be a big
blue elephant sitting not a millimeter away in another dimension, but we
wouldn't know it's there because everything we use to "see" is stuck to our
Only gravity can't be glued to a particular brane. Gravity, as Einstein
revealed, is the curving of space-time itself, so it wanders willy-nilly
where it will, leaking off our brane into what physicists call "the bulk" -
the rest of space-time.
Brane scenarios offer an elegant explanation for why gravity is such a
weakling: Maybe it's not any weaker than the other forces. Maybe it's just
concentrated somewhere else in the bulk, or on another brane.
Explaining the wimpiness of gravity is but a taste of what this Brane New
World might do.
Consider another embarrassing problem that has stumped astronomers for
decades. At least 90% of the matter in the universe is AWOL. Or more
precisely, it is known to exist because of its gravitational pull (without
it, galaxies wouldn't hold together) but can't be detected by any other
means. The standard approach has been to populate the universe with exotic
new forms of matter, too elusive to be readily seen.
If our brane is but a small slice of a much larger cosmos, however, the
"dark matter" might be nothing but ordinary matter trapped on another brane.
Such a shadow world, Hawking speculates, might contain "shadow human beings
wondering about the mass that seems to be missing from their world."
Or take the mystery of why elementary particles always appear in triplets,
each set heavier than the next.
One possibility is that each triplet is the same particle repeating itself
on three layers of branes. They would have different masses on our brane for
the same reason as shadows on a wall can be different sizes depending on the
distance of the object that casts them.
"One of the neat things about the whole extra-dimensional idea," Polchinski
said, "is that all the physics that we see - all the kinds of particles and
their detailed properties - are reflections of some inner geometry."
As in real estate, value depends on location, location, location.
The physicists most entranced with brane worlds are cosmologists. Over the
last decade, a new array of telescopes and satellites has provided them with
sophisticated tools for taking the measure of the universe. What was once
little more than navel gazing is fast becoming a data-drenched science.
But cosmologists need string theory to understand the origin of the
universe, because laws of physics break down at the tiny distances and
immense gravity at play in the Big Bang. For now, cosmologists can see back
in time only so far, and no farther.
Consider the Big Bang. According to current theory, the universe sprang from
an infinitely small speck of space-time known as a "singularity" - a paradox
in the accepted laws of physics, which hold that nothing can be infinitely
"A singularity is a euphemism for: 'Things have gone haywire Things make no
sense,' " said Greene, one of the coordinators of the Aspen workshop. "The
Big Bang singularity is an 'It doesn't make sense' on the most important
problem - namely, how did it all begin."
Branes can enclose the Big Bang singularity like a sheet of cellophane -
avoiding the problem of the infinitely small by giving the singularity some
Not surprisingly, the string-cosmology connection that brane worlds brought
about is also producing something of a culture clash. Until recently, string
theorists have remained skeptical of the grand theories of cosmologists.
String theory is mathematically rigorous. Cosmologists are a wilder bunch,
willing to try out almost any model of the universe and see where it leads.
"We know how branes work," said string theorist Nathan Seiberg of the
Institute for Advanced Study in Princeton, N.J. "We know what are properties
of branes, and what are not properties of branes. [Cosmologists] violate all
the rules. Is this good or bad? I'm not sure. Because if they come up with
something which violates the rules of string theory but does all sorts of
other wonderful things, then maybe we in string theory will have a
motivation to look into it."
Branes already have brought a whole new zoo of exotic species into the world
of physics. There are skinny branes and fat branes; empty branes and full;
active and still.
"A brane which is wiggling a lot would translate to a brane that has
excitations on it, particles on it," said McGill's Cline. That would be a
brane with atoms, forces, us. "But I could also have a cold brane," he said.
"That would be like a cold, empty universe. The brane still has some energy
density, but there's no particles living there."
And while the term brane derives from membrane - a two-dimensional surface -
branes could also exist in every possible dimension. A string is a
"1-brane," for one-dimensional object. Brane worlds (like the one we might
live in) must by necessity be "3 plus 1" branes - three dimensions of space
plus one of time. But you can just as easily have a pair of 10-dimensional
branes bounding an 11-dimensional universe.
For now, no one knows whether the building blocks of the ultimate theory
will be strings or branes. "You can't really say," Polchinski said. "It's
kind of Zen-like, but in a very precise way."
Ultimately, brane worlds will stand or fall, like all science, on the twin
tests of consistency and experiment. Whatever bizarre brane worlds may exist
in some larger dimensional landscape, they can't change what we perceive.
The stars can't slip off into hyperspace. The cat can't be disturbed from
the couch. Physics has to answer to nature as we know it.
Experimental evidence could come in the next decade from two very different
realms. A new particle collider under construction in Europe could reach
high-enough energies to produce, say, a five-dimensional "particle" of
gravity - a telltale sign of brane worlds beyond. This particle might be
detected as energy missing from a collision because it "leaks" into an extra
At the same time, cosmologists are figuring out ways to read the signature
of extra dimensions in the microwaves that pervade space as the afterglow of
the Big Bang; the effects would be subtle but detectable, with a new
generation of satellites.
"We just have to keep hoping that nature will be kind," Cline said.
In the end, there's always the chance that all these ideas will turn out to
be too, well, off-the-wall. "Who knows?" said University of Chicago
physicist Sean Carroll. But even if brane worlds aren't real, Carroll said,
"they will have taught us a useful lesson that we should have known all
along, which is that we don't have a clue to what's going on."
Polchinski, for one, believes that branes are probably real, even though he
isn't sure where the idea will lead. "It's possible that nature doesn't work
that way," he said. "But it's so rich with possibilities, if it's not good
for this, it's probably good for something else."