Superstring theory attempts to unify
gravity with quantum mechanics by
describing particles and forces as tiny
vibrating strands and loops.
For string theory to say anything
about how the forces arise, physicists
have to figure out how all those extra
dimensions roll up, or “compactify,” into
the four familiar ones.
String theory also conjures up a
shadow population of partner particles
for all of the ones currently known to
exist — a notion called supersymmetry.
In fact, supersymmetry may be necessary to join the strong, weak and electromagnetic forces, so it is important even
if string theory isn’t correct.
For example, most versions of string
theory require that the universe have
10 or 11 dimensions — nine or 10 of space
and one of time, rather than the four
that people experience: up-down, front-back, left-right and past-future.
“The forces are unified in 11 dimensions, but they split apart when you go
to four dimensions,” says Gordon Kane,
a physicist at the University of Michigan
in Ann Arbor.
When forces collide
Many physicists have high hopes that the
LHC will find evidence for both supersymmetric particles and extra spatial
dimensions.
“Even if we don’t go out to the other
dimensions, in some sense the other
dimensions can come to us,” says Harvard
physicist Lisa Randall.
Working in the 1990s with colleague
Raman Sundrum, now at the University
of Maryland in College Park, Randall
showed that it might be possible to detect
the decay of a gravity-carrying particle
coming from an extra dimension. Finding such a particle at the LHC would both
verify the existence of extra dimensions
and suggest why gravity is much weaker
than the other three forces.
“I think it would be somewhat surpris-
Back to one
one of the enduring
puzzles in physics is
why gravity — which
guides matter on the
scale of planets and
galaxies — is so much
intrinsically weaker
than the other three
forces. in the moments
just after the Big Bang,
some researchers
think, the forces may
have been united as
one, separating into
forces with differing
strengths as temperatures decreased.
Relative strength of force
Gravity
Temp. (kelvins)
1015
Age of the universe (sec)
10–12
Recent era
Strength of the four forces back in time
Strong nuclear force
Electromagnetism
Weak nuclear force
Uni;ed
force
10–35
1029
10–43
1032
Distant past
ing,” Randall says. “But this is one of the
things we could find, and this is one of
the things they should be looking for.”
Most physicists think it’s more likely
that the LHC will find evidence for super-
symmetric partners of the particles in
the Standard Model. Which partners
appear, and their properties, would put
some helpful constraints on how the
universe compactifies the 11 dimensions
predicted by string theory.
For example, if the lightest super-
particle turned out to be the wino, the
superpartner of the weak force–carrying
W boson, that would be consistent with
a version of string theory known by the
pithy moniker “M-theory compactified
on 7-D manifold of G2 holonomy.”
Such supersymmetric particles may
already have been observed, in fact — not
on Earth, but in space. Some of the dark
matter that is thought to make up more
than 80 percent of the matter in the
universe could be composed of super-
symmetric particles left over from the
universe’s earliest moments (see Page
24). In the last fe w years two space-based
instruments, the Fermi Gamma-ray
Telescope and the Italian-led PAMELA
mission, have seen evidence of dark
matter in the Milky Way in the form of
gamma rays and antimatter that could
have been produced by supersymmetric
particles colliding.
Because the LHC and any future colliders can carry physicists only so far
back toward the moment just after the
Big Bang, science’s understanding of a
unified theory is ultimately going to have
to come from exploring the vastness of
the universe. Some physicists wonder
if such a strategy, which relies on finding and interpreting clues left behind by
nature, can produce results comparable
to the high-precision experimental data
that led to the Standard Model during
the 20th century.
But string theory is not 20th century
science — in fact, string theorist Edward
Witten has described it as “21st century
physics that fell accidentally into the
20th century.” Now that the 21st century
has arrived, it’s string theory’s time to be
put to the test. s
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april 23, 2011 | science news | 27
