Nyah, nyah! Einstein's special relativity holds, so quantum gravity remains undetectable.

Einstein Trumps Quantum Gravity

For 5 years, physicists have hoped that a flaw in Einstein's special theory of relativity might reveal that space and time aren't smooth at the smallest scale, but fuzzy and foaming. Those hopes have been dashed by two independent measurements of cosmic gamma rays, which show that Einstein was right after all--and that current plans to detect the "quantum foam" of spacetime should fizzle.

The frothiness of space and time is predicted by many theories that attempt to meld Einstein's theory of gravity with quantum mechanics. Physicists hoped to detect it by finding a hole in Einstein's dictum that it is meaningless to say an object is moving or stationary relative to the universe. Known as Lorentz invariance, that principle implies that all particles of light, or photons, travel through empty space at the same speed regardless of how much energy they pack. In recent years, however, various quantum gravity theories have suggested that because of the underlying frothiness of spacetime, Lorentz invariance might not hold, in which case light of different wavelengths would travel at slightly different rates. Researchers might be able to measure the tiny speed differences by studying light from enormous extragalactic explosions known as gamma ray bursts--or so theorists predicted in 1998.

But the new studies put the kibosh on that tantalizing idea. Floyd Stecker, a theoretical astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and colleagues studied gamma rays from the hearts of the galaxies Markarian 421 and Markarian 501, some 450 million light-years from Earth. En route the rays pass through a haze of infrared photons that fill intergalactic space. If Lorentz invariance were violated, the gamma rays would zip right through the haze. According to special relativity, however, the highest energy gamma rays should collide with the infrared photons to make electron-antielectron pairs. This process should soak up gamma rays above a well-defined cutoff energy--just what the researchers observed, Stecker reports in a paper to be published in the journal Astroparticle Physics.

Gamma rays from the Crab Nebula also bear out Einstein's theory, gravitation theorist Ted Jacobson and colleagues at the University of Maryland, College Park, report in the 28 August issue of Nature. The rays come from extremely energetic electrons spiraling in the magnetic fields inside the gargantuan cloud of gas. If Lorentz invariance were violated, the electrons would slam up against a virtual speed limit slower than the speed of light. From the energy of the gamma rays, however, Jacobson and colleagues deduced that the electrons were traveling within a 10-billion-billionth of the speed of light--even stronger evidence that Einstein was right.

The results sink several quantum gravity theories, including some that predict the universe has extra, as yet undiscovered dimensions. Still, some theorists accentuate the positive in the negative results. "I think it's great," says Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada. "It means that physically plausible hypotheses are being confronted with experimental data."

Related sites
The preprint of Stecker's paper
The preprint of Jacobson's paper
A paper responding to the findings