Einstein's theory prevails, says key experiment

Racing across the universe for the last 7.3-billion years, two gamma-ray photons arrived at NASA's orbiting Fermi Gamma-ray Space Telescope within nine-tenths of a second of one another. The dead-heat finish might stoke a debate among physicists over Einstein's theory of relativity because one of the photons possessed a million times more energy than the other.

In Einstein's vision of the structure of space and time, unified as space-time, all forms of electromagnetic radiation – gamma rays, radio waves, infrared, visible light and X-rays – are reckoned to travel through the vacuum of space at the same speed, no matter how energetic. But in some of the new theories of gravity, space-time is considered to have a "shifting, frothy structure" when viewed at a scale trillions of times smaller than an electron. Some of those models predict that such a foamy texture ought to slow down the higher-energy gamma-ray photon relative to the lower energy one.

Even in the world of high-energy particle physics, where a minute deviation can sometimes make a massive difference, nine-tenths of a second spread over more than 7 billion years is so small that the difference is likely due to the detailed processes of the gamma-ray burst rather than confirming any modification of Einstein's ideas.

“This measurement eliminates any approach to a new theory of gravity that predicts a strong energy-dependent change in the speed of light,” said Peter Michelson, professor of physics at Stanford University and principal investigator for Fermi's Large Area Telescope (LAT), which detected the gamma-ray photons on May 10. “To one part in 100 million billion, these two photons traveled at the same speed. Einstein still rules,” he told Science Daily.

The two photons provided rare experimental evidence about the structure of space-time. Whether the evidence will prove sufficient to settle any debates remains to be seen. The photons were launched on their pan-galactic marathon during a short gamma-ray burst, an outpouring of radiation likely generated by the collision of two neutron stars, the densest known objects in the universe.

A neutron star is created when a massive star collapses in on itself in an explosion called a supernova. The neutron star forms in the core as matter is compressed to the point where it is typically about 10 miles in diameter, yet contains more mass than our sun. When two such dense objects collide, the energy released in a gamma-ray burst can be millions of times brighter than the entire Milky Way, albeit only briefly. The burst that sent the two photons on their way lasted 2.1 seconds.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.