Does an exciting but controversial new model of quantum gravity reproduce
Einstein's theory of general relativity? Scientists at Texas A+M University
in the US explore this question in a paper appearing in Physical Review Letters
and highlighted with a Viewpoint in the August 24th issue of Physics.
"If it ain't broke, don't fix it," sums up fairly well how many scientists
have viewed Einstein's theory of general relativity. The theory, which Einstein
developed in the early 20th century, says that matter curves spacetime, and
it is this curvature which deflects massive bodies – an effect that we
interpret as the influence of gravity. The theory has been tested to extremely
high accuracy and without it, our satellite global positioning system would
be off by about 10 km per day.
Despite the success of general relativity, one of the most important problems
in modern physics is finding a theory of quantum gravity that reconciles the
continuous nature of gravitational fields with the inherent 'graininess' of
quantum mechanics. Recently, Petr Horava at Lawrence Berkeley Lab proposed such
a model for quantum gravity that has received widespread interest, in no small
part because it is one of the few models that could be experimentally tested.
In Horava's model, Lorentz symmetry, which says that physics is the same regardless
of the reference frame, is violated at small distance scales, but remerges over
longer distance scales
The team at Texas A&M, which includes Hong Lu, Jianwei Mei and Christopher
Pope, report their investigations into how the modifications proposed in Horava's
theory will broadly affect the solutions of general relativity. One aspect of
their study is that it leads to an important caveat, described by Horatiu Nastase
in a Viewpoint commentary in Physics (physics.aps.org). Lu et al.'s calculations,
explains Nastase, suggest that Horava's model only reproduces general relativity
on unobservable scales, "larger than the size of the Universe".
Lu et al.'s paper is an important contribution to testing the Horava model
and shows that a good deal of work remains to understand its full implications.
Posted August 24th, 2009
