On May 10, 2009, two photons reached the end of a 7.3-billion-year race at a detector on the Fermi Gamma Ray Space Telescope. First one photon blinked into the Large Area Telescope's detector ; then, 0.9 seconds later, the second photon crossed the finish line. The second photon had been beaten by a whisper; divide that truncated second by the 7.3 billion year journey, and you'll see that their traveling times differed by less than one part in one hundred million billion (that's 10-17 for fans of scientific notation).
Both photons were gamma rays, from the most energetic end of the electromagnetic spectrum, up to 300 billion times more energetic than visible light. They were from the same gamma-ray burst, a blast of radiation from the collapse of a massive star far in the past (and away from us in space.)
All this is to say, score another point for Einstein, the obscure patent clerk turned physics giant, and for the invariance of light speed.
In a vacuum, Einstein said, all electromagnetic radiation should travel at the same speed. Glass or water slows down light, but in a vacuum, even to an observer on a speeding rocket, it always travels at 299,792,458 meters per second.
Four years ago, things didn't look so good for Einstein. In 2005, the MAGIC telescope on the Canary Islands recorded the last-place finisher in another galactic race, 500 million years long, as lagging four minutes behind. This seemed to support some cosmologists who proposed that higher-energy photons might travel more slowly than lower-energy photons.
Einstein's general theory of relativity is immensely powerful, but it's still fundamentally classical. Einstein bristled at the very idea of quantum mechanics, which makes it meaningless to speak of a photon's properties before one observes that photon, collapsing the probabilistic wavefunction into a definite value (and rendering another property entirely unknowable, as in the case of position and momentum.) The onus on modern theoretical physics is to reconcile Einstein's picture of gravity with the other greatest theory of the twentieth century, quantum mechanics.
Quantum mechanics says that energy is quantized. Some proponents of loop quantum gravity think that space-time, too, comes in fundamental chunks. If you could shrink down so that 10-35 meters was a comfortable walking distance, they say, you wouldn't see that silky smooth "fabric" of space-time everyone talks about. Instead, you'd be tossed and battered by a swirling sea of Planck-scale foam.
Since higher-energy photons have shorter wavelengths, some cosmologists say, they might be more vulnerable to this fluctuating foam, while lower-energy photons' longer wavelengths would keep the sailing smooth. As Dennis Overbye at the New York Times puts it:
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.
On small scales these slight differences would be negligible. But after long distances, the tiny discrepancy in velocity would start to become visible. The lower-energy photons would begin to pull ahead, and the higher-energy photons would start to fall behind. Fermi, with its ability to make precise gamma-ray detections, is one of the few telescopes that can observe this kind of lag. The most recent results set a limit on how much a photon's energy could change its speed.
But how did MAGIC get such a different result? Rachel Courtland at New Scientist explains:
The MAGIC time delay may be down to an astrophysical process where particles are accelerated to enormous energies within the hearts of galaxies. Follow-up calculations after MAGIC's 2005 result showed that is possible to produce flares that release lower-energy radiation before higher-energy radiation, according to MAGIC collaborator Robert Wagner of the Max Planck Institute of Physics in Munich, Germany. "I think what we can say for the time being is quantum gravity effects cannot be the dominant effect," he says.
The man may not have believed in quantum mechanics, but for now, chalk up another point for old Einstein.