The gamma-ray photons – which had been racing across the universe for the last 7.3 billion years -arrived at NASA’s orbiting Fermi Gamma-ray Space Telescope within nine-tenths of a second of each other.  The photons – originating from a short gamma-ray burst generated by the collision of two neutron stars - provided rare experimental evidence about the structure of space-time.

“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 photons.  “To one part in 100 million billion, these two photons travelled at the same speed,” he said, “Einstein still rules.”

In Einstein’s vision of a unified space-time, he suggested that in the vacuum of space, all forms of electromagnetic radiation – gamma rays, radio waves, infrared, visible light and x-rays – travelled at the same speed, no matter how energetic.  Newer theories of gravity suggest space-time is a shifting, frothy structure, which should slow down higher energy gamma-ray photons, relative to lower energy ones.

“Physicists would like to replace Einstein’s vision of gravity – as expressed in his relativity theories – with something that handles all fundamental forces,” Michelson said, “There are many ideas but few ways to test them.”

The Standard Model of particle physics is considered to have unified three of the four forces: electromagnetism, the “strong force” holding nuclei together inside an atom, and the “weak” force, responsible for radioactive decay, but gravity is the odd one out.  No one has been able to develop a model bringing all four fundamental forces together in a theory of how the universe works.