During its first year of operation, NASA's Fermi Gamma Ray
Space Telescope found new, unexpected sources of the highest energy of light
and showed that Einstein was right about the speed of light.
Fermi is a new observatory launched last year that looks for
gamma rays, created by some of the most violent explosions in the universe.
Some of these observations have given scientists more
insight into Einstein's theories of relativity by showing effects of
high-energy light that can't be replicated in a lab.
"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, or LAT, at Stanford University in Palo Alto, Calif. "There are many ideas, but few ways to test them."
Einstein rules
Many approaches to new theories of gravity picture
space-time as having a shifting, frothy structure at physical scales trillions
of times smaller than an electron. Some models predict that the foamy aspect of
space-time will cause higher-energy gamma rays to move slightly more slowly
than photons at lower energies.
Such a model would violate Einstein's edict that all
electromagnetic radiation radio waves, infrared, visible light, X-rays and
gamma rays travels through a vacuum at the same speed the speed of light.
On May 10, 2009, Fermi and other satellites detected a
so-called short gamma ray burst, designated GRB 090510. (Astronomers think this
type of explosion happens when neutron stars collide.) Ground-based studies
show the event took place in a galaxy 7.3 billion light-years away.
Of the many gamma ray photons Fermi's LAT detected from the
2.1-second burst, two possessed energies differing by a million times. Yet
after traveling some seven billion years, the pair 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. "To one part in 100 million billion, these
two photons traveled at the same speed. Einstein still rules."
Setting records
Fermi's secondary instrument, the Gamma ray Burst Monitor,
has observed low-energy gamma rays from more than 250 bursts. The LAT observed
12 of these bursts at higher energy, revealing three record setting blasts.
GRB 090510 displayed the fastest observed motions, with
ejected matter moving at 99.99995 percent of light speed. The highest energy
gamma ray yet seen from a burst 33.4 billion electron volts or about 13
billion times the energy of visible light came from September's GRB 090902B.
Last year's GRB 080916C produced the greatest total energy, equivalent to 9,000
typical supernovas.
Scanning the entire sky every three hours, the LAT is giving
Fermi scientists an increasingly detailed look at the extreme universe.
"We've discovered more than a thousand persistent gamma
ray sources five times the number previously known," , said Julie McEnery,
Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
"And we've associated nearly half of them with objects known at other
wavelengths."
Blazars distant galaxies whose massive black holes emit
fast-moving jets of matter toward us are by far the most prevalent source,
now numbering more than 500. In our own galaxy, gamma ray sources include 46
pulsars and two binary star systems where a neutron star rapidly orbits a hot,
young star.
The discoveries of Fermi's first year of observations are
detailed in the Oct. 29 issue of the journal Nature.
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