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Score one for Albert Einstein. In May, NASA’s Fermi Gamma Ray Space Telescope and other satellites spotted a short gamma ray burst, an explosion that astronomers think happens when neutron stars collide. NASA scientists calculated the explosion took place in a galaxy 7.3 billion light-years away.
Of the many gamma ray photons Fermi’s LAT (large area telescope) 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, confirming Einstein’s theory is that all electromagnetic radiation-radio waves, infrared, visible light, X-rays and gamma rays-travels through a vacuum at the same speed.
“This measurement eliminates any approach to a new theory of gravity that predicts a strong energy dependent change in the speed of light,” Peter Michelson, principal investigator of Fermi’s LAT at Stanford University in Palo Alto, Calif., said in a statement. “To one part in 100 million billion, these two photons traveled at the same speed. Einstein still rules.”
Some of the new approaches to theories of gravity speculate space-time as having a sort of 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 energy.
“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.”
Scanning the entire sky every 3 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 project scientist Julie McEnery 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 systems where a neutron star rapidly orbits a hot, young star.
NASA’s Fermi Gamma Ray Space Telescope was launched a year ago and is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, along with contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.
“The Fermi team did a great job commissioning the spacecraft and starting its science observations,” Jon Morse, Astrophysics Division director at NASA headquarters in Washington, said. “And now Fermi is more than fulfilling its unique scientific promise for making novel, high-impact discoveries about the extreme universe and the fabric of space-time.”