Extra dimensions make new room to tackle old mysteries

Since Fermi's theory of the weak interactions, a great mystery infundamental physics has been: why is even this most feeble subatomicforce so much stronger than gravity? In the context of modern GrandUnified Theories, this disparity between forces is understoodin terms of a giant Desert inenergy scales stretching over seventeen orders of magnitude. The Desertextends from the electroweak scale, currently being probed by thehighest energy accelerators, to the Planck scale, where gravity isexpected to become as strong as the other interactions. Planckianenergies probe miniscule distances of roughly 10^{-33} cm, where quantumgravitational effects are also supposed to come into play. Becausethese phenomena take place at such high energies, there is no hope fordirect experimental tests of quantum gravity in the standard framework.

Leaving aside theoretical speculations about quantum gravity for amoment, what do we know about gravitational interactions experimentally?Due to its miniscule strength, we know surprisingly little: gravity hasonly been directly measured down to distances of about a millimeter! Allof the above statements about the energy anddistance scales where gravity becomes strong are based on a theoreticalextrapolation of the inverse square law for gravity, over thirty ordersof magnitude, from a millimeter where it is actually measured, down tothe Planck length of 10^{-33} cm.Given the crucial way in which thisextrapolation shapes our thinking about the relation of gravity to theother forces, it is important to scrutinize it.

In the last year, a new framework has been proposed for tackling the abovequestions, challenging the old assumption of a large energyDesert. Instead of altering the properties of particle physics at shortdistances, the properties of gravity are altered. This idea,proposed by Nima Arkani-Hamed (now at U.C. Berkeley), Savas Dimopoulos(Stanford) and Gia Dvali (NYU), postulatesthat gravity becomes strong at theelectroweak scale, making quantum gravity accessible to the nextgeneration of particle accelerators.The measured weakness of gravityat distances longer than a millimeter is due to the presence of newspatial dimensions in which gravitational force lines can spread out,diluting its strength. The idea that there may be new spatial dimensionsin nature dates back to the 20's and is a central ingredient of modern stringtheories. However, these dimensions are normally thought to berolled up into tiny circles about 10^{-33} cmbig, making it impossible to detect them experimentally. The dimensionsin this new proposal are enormous by comparison, perhaps aslarge as a millimeter. That we have not so far detected them isbecause only gravity can propagate in these extra dimensions.The particles and forces of which matter is composed are stuck to athree-dimensional ``wall" in the extra dimensions. Remarkably, this newpicture is not excluded by any known experimental observation,surviving laboratory, astrophysical and cosmological constraints.

Many implications of this framework have beenexplored intensively in the past months. Some of the most interestingpossibilities involve populating the extra dimensions with new particlesand parallel ``walls" where other universes live. Interactions between theparallel universe and our ownhave been used to provide explanations for many of theoutstanding mysteries in the Standard Model, providing possible answers toquestions 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 long-lived? Why do the strong, electromagnetic andweak forces seem to unify at ultrahigh energies? Given the radical revisionin the notion of what our space-time looks like, the picture of very earlyuniverse cosmology is also changed in interesting ways. Old ideas, such asthe inflationary universe, can be realized in terms of the dynamics ofthe 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 athree-dimensional wall in four spatial dimensions. Among other things, thisproposal, put forward recently by Lisa Randall (MIT) and Raman Sundrum (now atStanford), allows for the possibility that the new dimension can beinfinitely large in extent.

One of the most interesting aspects of the above picture is that it predictsremarkable new phenomena that will soon be tested experimentally. In thisframework, the next generation of particle accelerators, such as the LargeHadron Collider (LHC) at CERN, should observe strong quantumgravitational effects; for instance, the high-energy particle beam atthe LHC can cool by boiling off gravitons into the extra dimensions.More exotic gravitational objects, such as small black holes, can alsobe produced at LHC energies. If the true theory of quantum gravity atshort distances is string theory, new particles corresponding tovibrations of the strings may be produced, as well as states wherestrings wrap around new dimensions.

Another exciting aspect of this proposal is that, in some cases, itpredicts deviations from Newtonian gravity that may be observable in anew generation of table-top experiments measuring gravity atsub-millimeter distances. The possible signals include observation of atransition in gravitational force from the inverse square law to aninverse fourth-power law, and new attractive or repulsive forcesanywhere between one and a million times stronger than gravity operativeat sub-millimeter scales. The first results from these importantexperiments will become available in the next couple of years.

-Nima Arkani-Hamed