Observations from NASA’s Fermi Gamma-ray Space Telescope are hinting that the highest-energy gamma rays travel through empty space at a little less than the speed of light — unlike any other form of electromagnetic radiation. If future observations bear this out, it will rock the foundations of modern physics and perhaps point the way to a "theory of everything" that would help unify the twin pillars of 20th-century physics: Einstein’s general theory of relativity and quantum mechanics.
The Fermi satellite, formerly known as the Gamma-ray Large Area Space Telescope (GLAST), launched in June 2008. Its purpose is to the study the extreme universe — exploding stars, cosmic jets, annihilating particles, and other stuff that we wouldn't want happening near Earth. Soon after launch, Fermi started picking up distant gamma-ray bursts (GRBs), powerful explosions usually triggered by dying stars.
On September 16, 2008, Fermi recorded the most intrinsically powerful GRB observed to date. The burst (known as GRB 080916C) took place 12.2 billion years ago, as told by the redshift of its spectral lines. Intriguingly, the highest-energy gamma rays from this GRB arrived a little later than the lower-energy gamma rays. The higher a gamma ray photon’s energy, the shorter its wavelength. These high-energy photons, detected by Fermi’s Large Area Telescope (LAT), have wavelengths one-thousandth the size of an atomic nucleus.
As predicted by quantum mechanics, and as verified by countless laboratory experiments, space-time becomes turbulent at very tiny scales, as "virtual particles" — electrons and positrons, protons and anti-protons, for instance — pop in and out of existence for fleeting moments: faster than the Heisenberg Uncertainty Principle would allow them to be individually seen. This process should extend all the way down to tiny black holes 10–33 centimeter wide popping in and out of existence in just 10–44 second. In other words, at such a fine scale, space-time itself should become foamy and indeterminate.
But does this happen, really?
According to some theories that attempt to unify quantum mechanics with general relativity, very-short-wavelength gamma rays will begin to "feel" the fine-scale space-time turbulence, which would act to retard their velocity. In other words, if these theories accurately describe nature, high-energy gamma rays would travel slightly slower than the speed of light. You can think of the space-time foam, with all its tiny "hills" and "valleys," as presenting a longer pathway to a small particle than to a larger one that "sees" space as being smooth and flat.
The effect of the space-time foam on something as large (by comparison) as a high-energy gamma photon would be extremely minuscule. It would be nearly impossible to measure in a laboratory experiment. But as Fermi project scientist Steve Ritz (NASA/Goddard Space Flight Center) notes, GRBs give us a chance to conduct the experiment in a laboratory billions of light-years wide. A delay of a few seconds in a 12-billion year journey is a pretty minuscule effect, showing up only in the 16th or 17th decimal place, but it should be easily seen in the arrival times of a fast-acting gamma-ray burst.
The September 16th burst is the most inherently powerful one observed to date, and it was easily seen by Fermi at many wavelengths. With its ability to detect very-high-energy gamma rays and pin down their sky coordinates, Fermi is uniquely suited to carry out this experiment.
The 16.5-second delay for the highest-energy photon observed in this burst is consistent with some of the theories of quantum gravity, which is an exciting development. But before Fermi’s scientists uncork their champagne bottles, they must rule out alternative explanations.
For instance, maybe bursters really do emit their highest-energy photons a few seconds after the rest. Ruling this out will require observing many more GRBs — in particular, to see whether the amount of the delay scales with the distance of the burst, and not with any intrinsic characteristic of the event itself. "Burst emissions at these energies are still poorly understood, and Fermi is giving us the tools to figure them out," says LAT lead scientist Peter Michelson of Stanford University, whose team reports its results in the February 19th Science Express.
Over the next few years, Fermi will detect more and more strong bursts, including at the very shortest wavelengths. If Fermi sees a time lag for high-energy gamma rays that becomes larger with increasing distance in a direct, linear fashion, it would be compelling evidence that these theories of quantum gravity are indeed telling us something profound about nature at its most fundamental scale. If that happens, Fermi scientists may do more than just uncork the champagne; they can start reserving themselves a round-trip ticket to Stockholm.
"This one burst raises all sorts of questions," says Michelson. "In a few years, we'll have a fairly good sample of bursts, and we may have some answers."
On a very different front, scientists have proposed that a strange, irreducible form of noise seen in the GEO600 gravitational-wave detector in Germany is an indirect effect of the same space-time foam.