Washington, October 29 Two gamma-ray photons that raced across the Universe for the last 7.3 billion years to arrive at NASA's orbiting Fermi Gamma-ray Space Telescope within nine-tenths of a second of one another have given scientists proof of Albert Einstein's special theory of relativity.

The dead-heat finish may stoke the fires of debate among physicists over Einstein's special theory of relativity because one of the photons possessed a million times more energy than the other.

But for Einstein's theory, that's no problem.

In his 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 added. (ANI)