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7.3 Billion Years Later, Einstein’s Theory Prevails
By DENNIS OVERBYE
Astronomers said Wednesday that a race halfway across the universe had ended in a virtual tie. And so the champion is still Albert Einstein — for now.
The race was between gamma rays of differing energies and wavelengths spit in a burst from an exploding star when the universe was half its present age. After a journey of 7.3 billion light-years, they all arrived within nine-tenths of a second of one another in a detector on NASA’s Fermi Gamma-Ray Space Telescope, at 8:22 p.m., Eastern time, on May 9.
Astronomers said the gamma-ray race was one of the most stringent tests yet of a bedrock principle of modern physics: Einstein’s proclamation in his 1905 theory of relativity that the speed of light is constant and independent of its color, or energy; its direction; or how you yourself are moving.
“I take it as a confirmation that Einstein is still right,” Peter F. Michelson of Stanford, principal investigator for Fermi’s Large Area Telescope and one of 206 authors of a paper published online Wednesday in the journal Nature, said in an interview.
There is no evidence so far that the energy or wavelength of light affects its speed through space. That is important because of what it could say about the structure of space-time. Some theorists have suggested that space on very small scales has a granular structure that would speed some light waves faster than others — in short, that relativity could break down on the smallest scales.
Dr. Michelson and others emphasize that while the new Fermi results do not yet eliminate the prospect, further observations with more gamma-ray bursts could eventually verify or refute the hypothesis. That would have a major effect on physicists’ efforts to unify the Einsteinian gravity that governs outer space with the weird quantum laws that govern the inner space of the atom.
Mario Livio, an astronomer at the Space Telescope Science Institute in Baltimore, called the Fermi results an interesting effect but not revolutionary by any stretch. “The beauty of the experiment is not as much in what it achieves,” Dr. Livio said, “as in the fact that you can use astronomical observations to place some interesting limits on very fundamental physics.”
Quantum theory, as Einstein discovered to his chagrin, reduces life on subatomic scales to a game of chance in which elementary particles can be here or there but not in between. One consequence is that space-time itself should become discontinuous and chaotic when viewed at very close distances, the way an ocean that looks smooth from an airplane appears choppy and foamy up close.
This, the story goes, could have an effect on the propagation of light — or photons, as they are called in quantum-speak — slowing light with short wavelengths relative to light with longer wavelengths. The higher the energy of a photon, the shorter is its wavelength. One way to think about it is to envision the photons as boats on this choppy sea. The small ones, like tugboats, have to climb up and down the waves to get anywhere, while the bigger ones can slice through the waves and bumps like ocean liners, and thus go a little faster.
Until now such quantum gravity theories have been untestable. Ordinarily you would have to see details as small as 10-33 centimeters — the so-called Planck length, which is vastly smaller than an atom — to test these theories in order to discern the bumpiness of space. Getting that kind of information is far beyond the wildest imaginations of the builders of even the most modern particle accelerators, and that has left quantum gravity theorists with little empirical guidance.
“What’s really lacking,” Dr. Michelson explained, “is a laboratory experiment that tells us anything. So we have to use cosmology: we use the universe as the lab.”
The photons from GRB 090510, detected on May 9, ranged from 10,000 electron volts — the energy unit of choice in physics — to 31 billion electron volts, a factor of more than a million, in seven brief bursts over about two seconds.
The spread in travel time of 0.9 second between the highest- and lowest-energy gamma rays, if attributed to quantum effects rather than the dynamics of the explosion itself, suggested that any quantum effects in which the slowing of light is proportional to its energy do not show up until you get down to sizes about eight-tenths of the Planck length, according to the Nature paper, whose lead author was Sylvain Guiriec of the University of Alabama.
But Dr. Livio emphasized that this was only one of many classes of models. “It would be amazing that in effect we don’t need a quantum theory of gravity,” he said. “This only tells us where there are the dead ends.”
Indeed, other physicists said that even this model would not be ruled out until the size limit had been set much below the Planck size.
The good news, astronomers said, is that more data expected from Fermi could decide the question. As Lee Smolin, a quantum gravity theorist from the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, said, “So a genuine experimental test of a hypothesized quantum gravity effect is in progress.”
In the meantime, the last word belongs to Einstein, Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics wrote in an e-mail message paraphrasing a 1919 headline in The New York Times about observations that confirmed Einstein’s general relativity. “But the Nature story,” Dr. Kirshner wrote, “is ‘Einstein found right again. Heavens not askew! Savants not agog!’ ”
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