Extra dimensions make new room to tackle old mysteries
Since Fermi's theory of the weak interactions, a great mystery in
fundamental physics has been: why is even this most feeble subatomic
force so much stronger than gravity? In the context of modern Grand
Unified Theories, this disparity between forces is understood
in terms of a giant Desert in
energy scales stretching over seventeen orders of magnitude. The Desert
extends from the electroweak scale, currently being probed by the
highest energy accelerators, to the Planck scale, where gravity is
expected to become as strong as the other interactions. Planckian
energies probe miniscule distances of roughly 10^{33} cm, where quantum
gravitational effects are also supposed to come into play. Because
these phenomena take place at such high energies, there is no hope for
direct experimental tests of quantum gravity in the standard framework.
Leaving aside theoretical speculations about quantum gravity for a
moment, what do we know about gravitational interactions experimentally?
Due to its miniscule strength, we know surprisingly little: gravity has
only been directly measured down to distances of about a millimeter! All
of the above statements about the energy and
distance scales where gravity becomes strong are based on a theoretical
extrapolation of the inverse square law for gravity, over thirty orders
of magnitude, from a millimeter where it is actually measured, down to
the Planck length of 10^{33} cm.
Given the crucial way in which this
extrapolation shapes our thinking about the relation of gravity to the
other forces, it is important to scrutinize it.
In the last year, a new framework has been proposed for tackling the above
questions, challenging the old assumption of a large energy
Desert. Instead of altering the properties of particle physics at short
distances, the properties of gravity are altered. This idea,
proposed by Nima ArkaniHamed (now at U.C. Berkeley), Savas Dimopoulos
(Stanford) and Gia Dvali (NYU), postulates
that gravity becomes strong at the
electroweak scale, making quantum gravity accessible to the next
generation of particle accelerators.
The measured weakness of gravity
at distances longer than a millimeter is due to the presence of new
spatial dimensions in which gravitational force lines can spread out,
diluting its strength. The idea that there may be new spatial dimensions
in nature dates back to the 20's and is a central ingredient of modern string
theories. However, these dimensions are normally thought to be
rolled up into tiny circles about 10^{33} cm
big, making it impossible to detect them experimentally. The dimensions
in this new proposal are enormous by comparison, perhaps as
large as a millimeter. That we have not so far detected them is
because only gravity can propagate in these extra dimensions.
The particles and forces of which matter is composed are stuck to a
threedimensional ``wall" in the extra dimensions. Remarkably, this new
picture is not excluded by any known experimental observation,
surviving laboratory, astrophysical and cosmological constraints.
Many implications of this framework have been
explored intensively in the past months. Some of the most interesting
possibilities involve populating the extra dimensions with new particles
and parallel ``walls" where other universes live. Interactions between the
parallel universe and our own
have been used to provide explanations for many of the
outstanding mysteries in the Standard Model, providing possible answers to
questions such as: Why do the neutrinos have such tiny masses?
Why is the electron a million times lighter than the top quark?
Why is the proton so longlived? Why do the strong, electromagnetic and
weak forces seem to unify at ultrahigh energies? Given the radical revision
in the notion of what our spacetime looks like, the picture of very early
universe cosmology is also changed in interesting ways. Old ideas, such as
the inflationary universe, can be realized in terms of the dynamics of
the extra dimensions or the motion of parallel universes.
Very interesting variations on these ideas have also been proposed.
For instance, it may be that gravity itself can be trapped to a
threedimensional wall in four spatial dimensions. Among other things, this
proposal, put forward recently by Lisa Randall (MIT) and Raman Sundrum (now at
Stanford), allows for the possibility that the new dimension can be
infinitely large in extent.
One of the most interesting aspects of the above picture is that it predicts
remarkable new phenomena that will soon be tested experimentally. In this
framework, the next generation of particle accelerators, such as the Large
Hadron Collider (LHC) at CERN, should observe strong quantum
gravitational effects; for instance, the highenergy particle beam at
the LHC can cool by boiling off gravitons into the extra dimensions.
More exotic gravitational objects, such as small black holes, can also
be produced at LHC energies. If the true theory of quantum gravity at
short distances is string theory, new particles corresponding to
vibrations of the strings may be produced, as well as states where
strings wrap around new dimensions.
Another exciting aspect of this proposal is that, in some cases, it
predicts deviations from Newtonian gravity that may be observable in a
new generation of tabletop experiments measuring gravity at
submillimeter distances. The possible signals include observation of a
transition in gravitational force from the inverse square law to an
inverse fourthpower law, and new attractive or repulsive forces
anywhere between one and a million times stronger than gravity operative
at submillimeter scales. The first results from these important
experiments will become available in the next couple of years.
Nima ArkaniHamed
