Einstein Ahead in a Tie

The two great achievements of twentieth-century physics are Einstein’s theory of General Relativity, which describes the space-time fabric of te universe; and the theory of quantum mechanics, which describes the decidedly strange world of sub-atomic particles.  (The Nobel Prize winner Richard Feynman once wrote, in his book The Character of Physical Law, “I think I safely say that nobody understands quantum mechanics.”, referring to its completely unintuitive nature.)

One of the basic principles of relativity is that the speed of light is a constant: it is the same for all wavelengths of light, and for all observers.  But some theories of quantum gravity have posited that space-time is “grainy” at very small length scales (on the order of the Planck length, 1.6×10-35 meters).   This in turn would imply that light of shorter wavelengths (equivalent to higher energy) would be slowed down compared to radiation of longer wavelengths.  (This is somewhat analogous to the observation that larger diameter wheels travel more easily over rough surfaces.)  The effect, however, would be very small, and therefore difficult to observe.

Now the New York Times is reporting that scientists working with the Fermi Gamma Ray Telescope have been able to track a burst of gamma rays, of differing energies and wavelengths, given off by an exploding star 7.3 billion light-years away, about half the width of the known universe..  The photons in the bursts had energy levels ranging from 10,000 electron volts (eV) to 32 giga-eV, roughly a million to one ratio.  They all arrived within a 0.9 second interval; that implies that, if the quantum graininess exists, it must not manifest itself at any distance larger than the Planck length.  (There are also articles on this in the New Scientist and Ars Technica.)

This is not a conclusive result, one way or the other.  But it is somewhat exciting, because it provides some of the first true experimental evidence that bears on the theoretical question, at the threshold of the scale at which the quantum effect, if it exists, should become detectable.  And better evidence may be forthcoming: the same issue of Nature that contains the letter reporting the Fermi result also reports the discovery of a new, even older gamma-ray source, from a time when the universe was only about 630 million years old.

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