Conventional wisdom has it that putting the words “quantum gravity” and “experiment” in the same sentence is like bringing matter into contact with antimatter. All you get is a big explosion; the two just don’t go together. The distinctively quantum features of gravity only show up in extreme settings such as the belly of a black hole or the nascent universe, over distances too small and energies too large to reproduce in any laboratory. Even alien civilizations that command the energy resources of a whole galaxy probably couldn’t do it.
Physicists have never been much for conventional wisdom, though, and the dream of studying quantum gravity is too enthralling to give up. Right now, physicists don’t really know how gravity works—they have quantum theories for every force of nature except this one. And as Einstein showed, gravity is special: it is not just any old force, but a reflection of the structure of spacetime, on which all else depends. In a quantum theory of gravity, all the principles that govern nature will come together. If physicists can observe some distinctively quantum feature of gravity, they will have glimpsed the underlying unity of the natural world.
Even if they can’t crank up their particle accelerators to the requisite energies, that hasn’t stopped them from devising indirect experiments—experiments that don’t try to swallow the whole problem in one gulp, but nibble at it. My award-winning colleague Michael Moyer describes one in February’s cover story, and lots of others are burbling, too. Rather than matter and antimatter, “quantum gravity” and “experiment” are more like peanut butter and chocolate. They actually go together quite tastily.
An example came out at the American Astronomical Society meeting in Austin last month. Robert Nemiroff of Michigan Technological University presented his team’s study of extremely high-energy, short-wavelength cosmic gamma rays. The idea, which goes back to the late 1990s, is that short-wavelength photons may be more sensitive to the microscopic quantum structure of spacetime than long-wavelength ones, just as a car with small tires rattles with road bumps that a monster truck doesn’t even feel. The effect might be slight, but if the photons travel for billions of years, even the minutest slowdown or speed-up can appreciably change their time of arrival. Nemiroff’s team focused on gamma-ray burst GRB 090510A, observed by the Fermi space telescope. It went off about 7 billion years ago, and photons of short and long wavelength arrived at almost the same time—no more than about 1 millisecond apart. Any speed difference was at most one part in 1020, implying that quantum gravity hardly waylaid these photons at all.
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