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Testing Fundamental Physics
GLAST covers an energy range that will allow science team members to make sensitive tests of fundamental physics, perhaps finding violations to some of the field’s most cherished tenets. But as GLAST Deputy Project Scientist Julie McEnery of NASA’s Goddard cautions, "This is not guaranteed science; this is somewhat speculative."

For example, GLAST will be able to test whether light travels at the same speed in a vacuum regardless of wavelength. According to Albert Einstein’s special theory of relativity, all electromagnetic radiation should travel at the same speed, which has been measured to be 299,792,458 meters (186,282.4 miles) per second. In other words, high-energy gamma-ray photons should zip across space at exactly the same speed as low-energy radio photons.

But some models of quantum gravity, which attempt to merge Einstein's general theory of relativity with quantum mechanics, predict that extremely high-energy gamma rays could travel at a slightly different speed than other forms of light. According to quantum mechanics, space-time becomes turbulent at tiny scales, as quantum fluctuations cause virtual particle-antiparticle pairs to continually form and annihilate. If quantum fluctuations also produce tiny black holes, as suggested by some versions of quantum gravity, very-high-energy gamma rays have such short wavelengths that they might actually "feel" this quantum turbulence, which could slightly boost or retard their velocity.

"GLAST may be able to test this prediction by running a very long race of 10 billion light-years," says GLAST Project Scientist Steve Ritz of NASA Goddard. If very-high-energy gamma rays from GRBs preferentially arrive at Earth slightly ahead of or behind low-energy gamma rays, this could indicate a violation in the principle that all light travels at the same speed in a vacuum. Even if GRBs tend to release high-energy gamma rays slightly before or after low-energy gamma rays, GLAST could notice that the lag time gets larger as GRB distances increase. "If this happens, and if we can exclude more mundane astrophysical explanations, this would be a huge discovery," says Ritz. "GLAST would truly carry us beyond Einstein."

GLAST could conceivably see the evaporation of tiny black holes - weighing around 10^14 grams (100 million tons) — that formed moments after the Big Bang. At that time, the density variations in the Universe might have been high enough to allow small regions to collapse gravitationally into small black holes. Nobody knows whether such primordial black holes actually formed, but if they did, some might still be around in the Universe today. As first described by Stephen Hawking and others in 1974, black holes theoretically have finite lifetimes because they radiate away their mass in a quantum process. They literally evaporate into ordinary particles. The evaporation rate increases as the black-hole mass decreases, which explains why Hawking radiation is unobservable from stellar-mass black holes. At the end of their lives, tiny black holes undergo a runaway explosion into a shower of gamma rays and other particles. It’s possible that GLAST could detect gamma rays emitted by exploding black holes, and that would be a spectacular confirmation of the connection between quantum mechanics and general relativity.

The LAT might also pick up a strange phenomenon predicted by quantum theory, but which has only been observed in the laboratory: photon splitting. A very-high-energy gamma-ray photon could literally split into two lower-energy photons by tapping into a surrounding energy reserve, such as a neutron star's magnetosphere. Compton Gamma-ray Observatory observations of the pulsar B1509-58 provided tantalizing hints for this process. GLAST observations of this and other pulsars could provide strong evidence that photon splitting is actually occurring in nature.

by Robert Naeye