Einstein 1 Quantum Gravity 0

This week's Nature is reporting

A strong astrophysical constraint on the violation of special relativity by quantum gravity
T. JACOBSON, S. LIBERATI & D. MATTINGLY
Department of Physics, University of Maryland, College Park, Maryland 20742-4111, USA

Correspondence and requests for materials should be addressed to T.J. (jacobson@physics.umd.edu).

Special relativity asserts that physical phenomena appear the same to all unaccelerated observers. This is called Lorentz symmetry and relates long wavelengths to short ones: if the symmetry is exact it implies that space-time must look the same at all length scales. Several approaches to quantum gravity, however, suggest that there may be a microscopic structure of space-time that leads to a violation of Lorentz symmetry. This might arise because of the discreteness or non-commutivity of space-time, or through the action of extra dimensions. Here we determine a very strong constraint on a type of Lorentz violation that produces a maximum electron speed less than the speed of light. We use the observation of 100-MeV synchrotron radiation from the Crab nebula to improve the previous limit by a factor of 40 million, ruling out this type of Lorentz violation, and thereby providing an important constraint on theories of quantum gravity.

What does this mean? In the review article Sean Carroll explains what is the issue here:

Jacobson et al. consider synchrotron radiation, emitted by electrons circling in a magnetic field, from the Crab nebula. To produce high-energy photons through synchrotron emission, the electrons must be moving close to the speed of light. If Lorentz invariance is violated, the maximum velocity for photons and for electrons can have a slightly different value, which imposes a cut-off on the frequency of synchrotron radiation that can be produced. Using observations of radiation at frequencies beyond this cut-off, Jacobson et al. are able to set the new stringent limit on Lorentz invariance. The crucial assumption made in their analysis is that the behaviour of photons and electrons can be described by an 'effective local field theory' at low energies. Such theories are well used in this area of physics, and this seems a reasonable assumption to make, but exceptions are known. So a window, albeit small, remains open for Planck-scale effects.

Einstein has been right a lot lately. First with his so-called blunder, concerning the cosmological constant (now called dark energy) and now special relativity has held up against new quantum gravity theories.