It is remarkable that some of the hardest problems in physics and cosmology can be expressed in very simple terms. For example, current theories do not tell us why the feeble subatomic force responsible for weak interactions is 17 orders of magnitude stronger than gravity. This is a huge disparity. Why is the world like this? It means that quantum gravity is not accessible above length scales of 10^-33 cm. Such a miniscule distance cannot be probed or the consequences of quantum gravity experimentally tested. Only in the very early universe are energies on the Planck scale encountered, which is why particle physicists and cosmologists have placed such emphasis on understanding conditions immediately after the Big Bang. A further consequence of gravity's weakness is that the inverse square law of attraction has only been verified at distances greater than 1 mm. In the Standard Model of particle physics it is assumed we can descend 30 orders of magnitude down the ladder and still be in a 1/r² gravitational field.

   For Science Watch, Savas Dimopoulos (Stanford University) explains the motivation for the research. "Given the crucial way in which this extrapolation shapes our thinking about the relation of gravity to the other forces, it is important to question it, which is precisely what is done in the new framework."

   The new thinking proposes that gravity becomes strong at the electroweak level. Using a simple idea going back to the 1920s, Dimopoulos, Nima Arkani-Hamed (Stanford Univ.) and Gia Dvali (ICTP, Trieste, Italy) propose that there are additional spatial dimensions on the scale from ~ 1 fermi up to ~ 1 mm. This is a huge departure from string theory, where the extra dimensions are rolled into tiny circles 10^-33 cm across. With two additional dimensions gravity follows a 1/r⁴ law at scales below ~ 1 mm, and 1/r² at macroscopic distances. The transition occurs because above ~ 1 mm the gravitational flux lines penetrate the extra dimensions diluting their strength.

   Dimopoulos adds, "We are proposing enormous new dimensions, perhaps as large as a millimeter, which have gone unnoticed because only gravity can propagate in them. The particles and forces of which matter is composed are stuck to a 3-D wall in the extra dimensions." In this picture the Standard Model is embedded on a 4-D manifold inside six dimensions, and the graviton propagates freely in all dimensions. This can lead to highly intriguing physics as particles are emitted into the extra dimensions, thus leaking away energy.

   The two Hot Papers are boldly impressive in discussing the confrontation of the theory with physical data: there are no known violations of laboratory, astrophysical, or cosmological data. But new tests of the theory are not far away because its quantum gravitational effects kick in on a scale 5 to 15 orders of magnitude larger than in string theory. The Large Hadron Collider (LHC) at CERN should observe strong quantum gravitational effects; for example, the high-energy 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 the TeV energies available with LHC. In fact the LHC now becomes a quantum-gravity machine, which can look into these extra dimensions of space through apparent violations of energy conservation, as well as the appearance and disappearance of particles from extra dimensions.

   The deviations from Newtonian gravity predicted in this theory are accessible to the "table-top" experimenters who are measuring gravity at sub-mm distances. The giveaway would be observing the transition from inverse-square to inverse fourth-power behavior, or seeing new attractive or repulsive forces much stronger than Newtonian gravity coming into play. 

Dr. Simon Mitton is the Senior Fellow of
St Edmund’s College, University of Cambridge, UK