Arising from: Jacobson, T., Liberati, S. & Mattingly, D. Nature 424, 1019–1021 (2003)

Quantum gravity may cause the vacuum to act as a non-trivial medium (space–time foam), which alters the standard Lorentz relation between the energy and momentum of matter particles, thereby modifying their dispersion relations. Jacobson, Liberati and Mattingly1 argue that synchrotron radiation from the Crab nebula imposes a stringent constraint on any modification of the dispersion relation of the electron that might be induced by quantum gravity, but their analysis does not constrain any modification of the dispersion relation of the photon2,3. Such quantum-gravity effects need not obey the equivalence principle4 in the sense of being universal for all matter particles, as exemplified by quantum-gravity models in which photons are the only standard-model particles able to ‘see’ special quantum-gravity configurations that modify their dispersion relations. This implies that photons may be the only sensitive probe of quantum-gravity effects on particle dispersion relations, and the results of Jacobson et al. do not exclude all possible modifications of dispersion relations, even if they are suppressed by only a single power of the Planck mass (the characteristic quantum-gravity scale) — contrary to some subsequent interpretations of their results.

Enjoying our latest content?
Log in or create an account to continue

  • Access the most recent journalism from Nature's award-winning team
  • Explore the latest features & opinion covering groundbreaking research
Access through your institution

or

Sign in or create an account
Continue with Google
Continue with ORCiD