Hunting for Higher Dimensions
Experimenters scurry to test new theories suggesting that extra dimensions are detectable
By P. Weiss
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Energy
spike from a gluon stands alone because a graviton has fled into extra dimensions,
taking energy with it. This simulation models an experiment planned for the
Tevatron accelerator, slated to start up again in 2001. (Maria Spiropulu/Harvard U.) |
Only 2 years ago, the idea of extra dimensions inhabited a nebulous region somewhere between physics and science fiction.
Many
physicists had already begun to see the up-and-coming string theory as the
next major step for theoretical physics. In that theory, everything in the
universe is composed of tiny loops, or strings, of energy vibrating in a
space-time that has six or seven extra dimensions beyond the seemingly endless
three standard dimensions of space and one of time. Conveniently, however,
those extra dimensions are compactified, as physicists say, crumpled up in
a space so small as to be unobservable.
The idea that extra dimensions
might be larger—perhaps detectable—was something that scientists mostly talked
about "late at night, after a lot of wine," says Gordon L. Kane, a theorist
from the University of Michigan in Ann Arbor. Kane therefore felt he was
walking on the wild side when he penned a fictional news story about experimenters
discovering extra dimensions.
Kane's story, which appeared in the May 1998 Physics Today,
was one of three winners of an essay contest sponsored by that magazine.
Basing his tale on some innovative theorizing published in 1990 by Ignatius
Antoniadis of the École Polytechnique in Palaiseau, France, Kane wrote of
peculiar sprays of particles yielding "startling data." He set his experiments
in 2011 at a European accelerator, known as the Large Hadron Collider (LHC),
which is currently under construction.
The results could imply the existence of one or two extra spatial dimensions, the story stated, "a surprise to everyone."
Even
by the time his article came out, however, the possibility no longer seemed
quite as surprising as it had when he wrote it a few months earlier. Between
the submission of Kane's story and its publication, two theoretical studies
had come out that suddenly pushed the idea of relatively large extra dimensions
into the spotlight.
One study came from a team at CERN, the European
Laboratory for Particle Physics in Geneva where LHC is being built. It examined
the consequences of extra dimensions being 10,000 trillion times larger than
the extra dimensions of string theory are typically imagined to be. At the
larger size, still only about one-ten-thousandth the size of a proton, the
extra dimensions might produce effects detectable by the current generation
of particle accelerators or their immediate successors, such as LHC, the
researchers found.
The other study argued that certain types of
extra dimensions could be even larger, as grand as a millimeter. They might
then be accessible not only in colliders but in small-scale, table-top experiments
as well, say researchers at Stanford University and the International Center
for Theoretical Physics (ICTP) in Trieste, Italy.
Today, teams
of experimentalists in both the United States and Europe are searching for
the signatures of extra dimensions. The hunt for such indicators "is certainly
one of the best chances of making a very spectacular discovery in the next
couple of years," says Joseph Lykken of the Fermi National Accelerator Laboratory
in Batavia, Ill.
Meanwhile, the wave of novel, extradimension
theory continues to roll on. In the latest splash, researchers have proposed
extra dimensions of infinite size.
Imagining any of these extra
dimensions isn't easy. Depending on how many extra dimensions there are,
physicists say, they might curl into a simple loop or sphere or bend into
a tortuous 6-dimensional pretzel popular in string theory. Every point in
the traditional, apparently 4-dimensional universe is then a tiny, multidimensional
volume. Theorists suggest that an extra dimension might be on the order of
10-35 meter.
Physicists also measure
the extra dimensions in terms of the energy needed to probe them. A particle
accelerated to 1 trillion electron volts (TeV) has, according to standard
arguments from quantum mechanics, a wave aspect with a wavelength of about
2 x 10-19 m. It can therefore explore facets of
the subatomic world on that scale. Doubling the energy means seeing features
half that size, and so on. So far, the smallest length scale observable with
accelerators is a little greater than 10-19 m.
The
idea of extra dimensions dates back to at least the 1920s. At that time,
physicist Oskar Klein, building upon work by mathematician Theodor Kaluza,
added a curled-up fifth dimension to the familiar universe in an ingenious
but unsuccessful attempt to unite the forces of electromagnetism and gravity.
Physicists
believe that the four forces—electromagnetic, weak, strong, and gravitational—were
joined as a single superforce at the time of the Big Bang. In theory, they
could merge only if the forces were about the same strength under conditions
of high energy. However, gravity is much weaker than the others.
As
some researchers today explore extra dimensions, they are on the lookout
for implications regarding unification of the four forces. Other scientists
striving for models that unify the forces have found extra dimensions a useful
tool.
Testing unification theories directly appears to be impossible,
however, since the phenomenon would only occur at energies in the range of
1013 to 1016 TeV. The highest-energy collisions achieved in accelerators today approach only 1 TeV.
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Oskar
Klein (left) proposed in the 1920s that hidden spatial dimensions might influence
observed physics. He poses with physicists George Uhlenbeck (middle) and
Samuel Goudsmit in 1926 at the University of Leiden, the Netherlands. (AIP Emilio Segrč Visual Archives) |
CERN
theorists Keith R. Dienes, Emilian Dudas, and Tony Gherghetta wondered what
would happen if they uncurled one or more of the extra dimensions in string
theory to 10-19 m, the largest size that would
not already have been detected. To their surprise, they discovered that the
three nongravitational forces could unify in the energy range of 1 TeV. This
unification could then be observed directly in LHC and indirectly in less-powerful
colliders.
They posted their study on the physics archive (http://xxx.lanl.gov/abs/hep-ph9803466) maintained by Los Alamos (N.M.) National Laboratory in March 1998.
For
physicists, an energy of 1 TeV was already a landmark. Both theory and experiment
had established that a mixing of the electromagnetic and weak forces begins
to take place a little below that energy level. Physicists have been troubled
because unification of even three forces requires much higher energies. They
refer to this puzzle as the hierarchy problem.
Scientists at Stanford
University and ICTP used extra dimensions in their attempt to solve the hierarchy
problem. They focused first on gravity and looked for a way to make it comparable
in strength to the other forces at an energy of about 1 TeV.
They
accomplished that feat by hypothesizing extra dimensions that affect only
gravity and are as large as 1 mm. Only a yawning gap in the scientific record
makes such extra dimensions feasible. While physicists have probed the other
forces of nature down to nearly 10-19 m, they've made extensive measurements of gravity only down to about 1 centimeter.
To
describe extra dimensions that would affect gravity alone, the Stanford-Trieste
researchers made use of entities known as branes. Those complex, membranous
objects, which can have many spatial dimensions themselves, have become a
central part of string theory. In some versions of the theory, the universe
itself is a brane with three spatial dimensions—a 3-brane—moving through
a higher-dimensional space-time.
String theory dictates that any
extra dimensions outside a brane affect only gravity. In other words, just
the force-carrying particles of gravity, called gravitons, could travel in
the space-time beyond the brane, leaving the other forces confined to the
brane. By contrast, extra dimensions associated with the brane influence
all the forces.
Therefore, even if gravity boasts an intrinsic
strength similar to that of the other three forces, because it diffuses throughout
the external space-time, also called the bulk, its apparent strength in the
3-brane universe is much reduced.
Any extra dimensions affecting
gravity would alter Isaac Newton's inverse-square law, which holds that objects
attract each other with a force inversely proportional to the square of the
distance between them. The theorists calculated that one extra dimension
in the bulk would have a scale of 100 million kilometers—about the distance
from Earth to the sun. That option isn't feasible because Earth's orbit obeys
the inverse-square law.
If there were two extra dimensions, however,
each would have a scale of 0.1 to 1.0 mm—large enough to be detectable but
small enough not to be ruled out by tests of the inverse-square law to date.
With more extra dimensions, the length scale shrinks far below the millimeter
range.
Combining both approaches, "you wind up with a very compelling
picture," says Dienes, a CERN team member, now at the University of Arizona
in Tucson. "These two scenarios together lower all the fundamental high-energy
scales of physics."
Inspired by these proposals, experimenters
are looking for signs of extra dimensions both at accelerators and in gravitational
laboratories.
Most of the accelerator searches have begun in the
past year, says Kingman Cheung of the University of California, Davis. Before
that, researchers had been translating the theorists' proposals into concrete
predictions. Cheung presented a summary of ongoing and proposed searches
last December at the Seventh International Symposium on Particles, Strings,
and Cosmology '99 (PASCOS '99) conference at Tahoe City, Calif.
To
find extra dimensions of the type studied by the CERN group, experimenters
are on the alert for what they call Kaluza-Klein towers, which are associated
with carriers of the nongravitational forces, such as the photon of electromagnetism
and the Z boson of the weak force. Excitations of energy within the extra
dimensions would turn each of these carriers into a family of increasingly
massive clones of the original particle—analogous to the harmonics of a musical
note.
"I like to think of these Kaluza-Klein states as echoes off the fifth dimension," Dienes says.
Because
these towers tend to magnify the strengths of the forces, their influence
might even be detected at energies below those at which the towers themselves
become apparent, researchers say.
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Some
theorists envision the universe as multidimensional space-time embedding
a membranous entity, called a brane, also of multiple dimensions. Diagram
depicts familiar 3-dimensional space (time not shown) as a vertical line.
At every point along line, one extra dimension curls around with a radius
(r) of no more that about 10-19 meter. Perpendicular to every point of the brane extends the bulk, another extra dimension. (Adapted from Dienes et al., Nuclear Physics B) |
Going
back through the data from an earlier run of CERN's Large Electron-Positron
Collider (LEP), researchers have found no evidence of such extradimensional
influences at up to an energy of 4 TeV, Cheung told Science News. The CERN team's extra dimensions must therefore be smaller than 0.5 x 10-19
m. The towers might become detectable in 6 or 7 years, when the completed
LHC will be able to probe energies of up to 14 TeV, he says.
Gravity
doesn't lend itself to measurement in accelerators because the other forces
overwhelm its tiny influence on particle interactions. "The graviton is so
weakly interacting, it doesn't enter the picture," Cheung says.
Instead,
physicists typically make precision measurements of gravity by using extremely
delicate experiments, named after the 18th-century scientist Henry Cavendish,
that determine the force between two suspended masses. At very small separations,
however, electrostatic influences and molecular interactions known as van
der Waals forces again swamp the gravitational effects.
By conducting
Cavendish experiments with extremely sensitive equipment, at least two teams
of scientists are testing for millimeter-scale extra dimensions. If those
dimensions exist, gravity in the submillimeter range would increase not according
to Newton's inverse-square law but in inverse proportion to the fourth power
of the separation.
Researchers at Stanford University led by Aharon
Kapitulnik have developed a micromachined cantilever that reacts to the gravitational
tug of an arm swinging back and forth 80 micrometers beneath it. A laser
detects motion in the cantilever, which is chilled to 4 kelvins to reduce
random thermal motion.
The experimenters intend to measure not
only gravity but also van der Waals and other short-distance forces. However,
because of the hubbub over extra dimensions right now, "we are neglecting
all other experiments," Kapitulnik says.
Similarly in Boulder,
Colo., a tungsten strip resembling a diving board weighing a few grams sits
in a vacuum over another strip of tungsten. As a motor rapidly wiggles the
diving board up and down, scientists look for motion in the strip below.
A next-generation instrument operating at 4 K will eventually replace the
current room-temperature version, says John C. Price of the University of
Colorado, who leads the effort.
Given the dearth of knowledge
about gravity in the subcentimeter range, the group is looking for any kind
of deviation from expectations, not just extradimensional effects, he says.
Nonetheless, the excitement about extra dimensions helps spur the group on,
Price says.
If the strength of gravity takes a sharp turn upward
at around 1 TeV, as the Stanford-Trieste scenario implies, an opportunity
opens for testing this theory also in accelerators. Collisions at such energies
could produce gravitons in large numbers, and some of these particles would
immediately vanish into the extra dimensions, carrying energy with them.
Experimenters would look for an unusual pattern of so-called missing energy
events.
This and more subtle effects of extra dimensions could
show up at existing accelerators, such as LEP and the Tevatron at Fermilab,
only if the dimensions have scales nearly as big as a millimeter. The powerful
LHC will greatly improve the chances for detecting missing energy events
and other prominent extradimension effects.
Despite his award-winning
literary fling 2 years ago, Kane has soured on large extra dimensions. He
remains a firm believer in six or seven extra dimensions, he says, but only
at about 10-35 m. The theory is cleaner that way,
he argues, with just the three familiar, very large spatial dimensions, and
the rest reduced to the scale of strings themselves.
"If I was trying to win a contest today, I'd write on something else," he says.
By
contrast to Kane's insistence on small extra dimensions, one pair of researchers
recently came up with an argument for extra dimensions of unlimited extent,
similar in size to the familiar dimensions. These scientists noted that the
3-brane, like any other object with energy or mass, would warp space-time
and thereby confine gravitons to a region just slightly larger than the brane.
The
warping would also localize extra dimensions' effects on Newton's inverse-square
law of gravity to subcentimeter distances not yet explored. Such localization
allows the dimensions themselves to stretch indefinitely, argue Lisa Randall
of the Massachusetts Institute of Technology and Princeton University and
Raman Sundrum of Boston University. This novel idea, described in the Dec.
6, 1999 Physical Review Letters, has many implications and may suggest
new indicators of extra dimensions. The work has already sparked dozens of
journal and online articles.
Whether or not large extra dimensions
actually show up in the laboratory, researchers are sparing no effort to
push the limits of one hidden dimension on which everyone agrees: imagination.
From Science News,
Vol. 157, No. 8, February 19, 2000, p. 122.
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