Sir

The unification of quantum mechanics with gravity is the most pressing question in theoretical physics today. However, experimental feedback to the theorists has been sorely lacking. Astrophysicists are now beginning to probe the behaviour of gravity at quantum (microscopic) scales.

For example, Igor G. Mitrofanov (Nature 426, 139; 200310.1038/426139a ) described a possible constraint on a leading theory, loop quantum gravity, based on the polarization of high-energy radiation from astrophysical sources. The high-energy photons have to travel cosmological distances to reach us, allowing small effects of quantum gravity to reveal themselves. This specific constraint depends on the reported detection of polarization from a γ-ray burst, which has yet to be confirmed, hence Mitrofanov cautioned readers to await confirmation of this measurement before concluding that loop quantum gravity is not viable.

There is no need to wait. The constraint on the polarization of γ-rays applies equally to the polarization of X-rays, for which there are 30-year-old measurements. The X-ray polarization of the Crab nebula, a thousand-year-old remnant of an exploded star, was first measured by Novick and collaborators1 in 1972 and confirmed by a different instrument four years later2. The observed X-ray polarization from the Crab nebula is in strong conflict (χ<10-4, see note below) with the predictions of loop quantum gravity, if the effects of quantum gravity depend linearly on photon energy.