Rare space experiment gives clues about the structure of the universe

Credit: NASA/Sanoma State University/Aurore Simonnet In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect.

University Park, Pa. — A physics experiment using a super-fast explosion in a galaxy 7.3 billion light-years away has given scientists rare experimental evidence about the fundamental structure of space and time.

The experiment was performed by a team that includes astrophysicists at Penn State, who used NASA's Fermi Gamma Ray Space Telescope to study particles from the explosion moving at nearly the speed of light. The experiment confirmed aspects of Einstein's theories of gravity, which unite space and time in the concept of space-time. The team's research is published in the current online edition of the journal Nature and will be published at a later date in the print edition. An image and more details are on the Web at: http://www.science.psu.edu/alert/Meszaros10-2009.htm

"The next major goal is to fuse quantum mechanics with gravity into a single quantum gravity theory," said Peter Meszaros, the holder of the Eberly family chair in astronomy and astrophysics at Penn State, a professor of physics there, and a member of the team that did the physics experiment with the Fermi telescope.  "Physicists would like to replace Einstein's vision of gravity -- as expressed in his relativity theories -- with something that handles all fundamental forces," said Peter Michelson, principal investigator of Fermi's Large Area Telescope, at Stanford University.

Scientists have constructed many models to fit their ideas for the new theories, but they have few ways to test these models with physical experiments.

The opportunity to test these models occurred on May 9 at 8:23 p.m. U.S. Eastern time, when Fermi and other satellites detected the "short" gamma ray burst, designated GRB 090510, in the act of ejecting particles at 99.99995 percent of the speed of light. Astronomers say this type of explosion likely occurred in the distant galaxy during an annihilating collision between neutron stars.

Many approaches to new theories of quantum gravity picture space-time as having a shifting, frothy structure at physical scales trillions of times smaller than an electron. Some models predict that such a foamy structure would cause higher-energy gamma rays to move slightly more slowly than photons at lower energy.

"Such models would violate Einstein's postulate that all electromagnetic radiation -- radio waves, infrared rays, visible light, X-rays, and gamma rays -- travel through a vacuum at the same speed," said Meszaros. "But different versions of quantum gravity predict different degrees of violation of this postulate, and we need to separate the wheat from the chaff."

Of the many gamma-ray photons detected by Fermi from the 2.1-second burst, two had energies differing by a million times. Yet after traveling some seven billion years, the pair of photons arrived just nine-tenths of a second apart.

"This measurement eliminates any approach to a new theory of gravity that predicts a strong energy-dependent change in the speed of light," Michelson said. The long-distance experiment showed that "To one part in 100 million billion, these two photons traveled at the same speed. "Einstein still rules," Michelson said.

As a result of the new space experiment, Meszaros further explained, "Any viable theory of quantum gravity must be one that predicts either a weaker violation of the speed-of-light constancy than that which we measured, or none at all."

In addition to Mészáros, other Penn State scientists on the research team include Xuefeng Wu, a research associate, and Kenji Toma, a postdoctoral scholar.