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 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—ones 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 Scientific American‘s February 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 earlier this 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 than long-wavelength ones to the microscopic quantum structure of spacetime, 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.
Theoretical physicists have long debated whether quantum gravity would alter photon speed, and most were not surprised by the negative result. But what’s important is the change of mindset. Experimenters and observers care less about what we should see than what we can see. These are people who love to build stuff. If they can build some gizmo that might bring gravity and quantum mechanics into contact, they’ll do it, whatever the theorists might say. They take an “if you build it, something will come” attitude. Historically, physics has been well-served by going out to look at nature with a minimum of prejudice.
The latest brainstorm is to apply techniques from quantum optics and related disciplines, which manipulate photons of light and other particles in order to build encrypted communications links, develop the components of a quantum computer, and study matter at extremely low temperatures. The tool of this trade is an interferometer, an apparatus that probes the wave nature of particles. It consists of a particle source, a particle detector, and two paths to get from one to the other. Being quantum, a particle goes both ways. That is to say, the wave corresponding to the particle splits in two, travels the distance, and fuses back together again. The relative length of the paths (or anything else that differentiates them) determines whether the waves will mutually reinforce or cancel and therefore what the detector will detect.
At first glance, these setups are the last place you’d go to look for quantum gravity. They are decidedly low-energy experiments, usually conducted on lab benches the size of dining-room tables. There is nary a gamma ray or accelerated particle to be found. But Moyer’s cover story describes how an interferometer can serve as an extremely precise ranging instrument. Any change in the paths’ relative length, as you might expect if spacetime is roiled by quantum fluctuations, will register at the detector.
Last spring, a team of physicists in Vienna led by Çaslav Brukner explored another use of interferometers: to see whether quantum particles truly obey gravity as Einstein conceived it. This isn’t quantum gravity, per se—the particles are quantum, but gravity behaves in a strictly classical way. Nonetheless, it is a fascinating case of how the two theories interact. You might think that the gravity on a single particle is way too feeble to measure, but an interferometer can manage it. You set it up so that the two paths are at different heights and therefore experience a different gravitational potential, which registers at the detector.
The Vienna team proposed sending not just any particle through the interferometer, but one that acts like a miniature clock—marking time by rotating or decaying. General relativity predicts that clocks run slower the deeper they get into a gravitational field, which, in this experiment, would act to wash away the wave nature of the particle altogether. The fading-away of the wave properties would be the unmistakable fingerprint of general relativity and a stepping-stone to quantum gravity. Current interferometers lack the necessary precision to look for this effect, but it is just a matter of time. (Sorry, couldn’t resist.) For more, see the authors’ own blog post and their paper in Nature Communications last fall.
It it also possible that quantum gravity could modify Heisenberg’s famous uncertainty principle. As Sabine Hossenfelder at Backreaction described last Wednesday, gravitational effects may set a minimum length that anything in nature could ever have, which means that no matter how much momentum imprecision you’re willing to accept, a position measurement could never be more precise than the minimum length. Experiments like this one could use tiny mirrors and springboards to pick up that effect.
Still another approach suggested by the ever-inventive Viennese is to define quantum gravitational ideas in concrete rather than abstract terms. Theorists think that quantum fluctuations in spacetime might make cause-effect sequences ambiguous, with the practical consequence of changing the types of correlations physicists observe in the lab. But the Viennese suggest thinking about it the other way round: Physicists observe certain types of correlations in the lab and, from these, draw conclusions about spacetime.
Some such correlations—those that muddle cause and effect—would be be inexplicable in ordinary physics. When quantum effects enter into play, “spacetime” loses some of the most basic features we associate with it, such as the notion that objects reside in certain places at certain times. In the Viennese scenario, you lose the ability to tell a story: One thing happened, then another, then another. It becomes a Dadaist jumble.
This approach hasn’t lent itself to a specific experiment yet, but is generally inspired by the experimentalist mindset. In this, it follows a trail blazed by Einstein himself, who developed his theories of relativity by thinking of abstract ideas in a concrete way. Even when experimenters can’t build actual experiments, their feet-on-the-ground mentality provides a fresh look at some of the hardest problems in modern science.
Editor’s picks for further reading
FQXi: Journeying through the Quantum Froth
Are cosmic rays revealing the quantum nature of spacetime?
FQXi: Table-Top Tests for Quantum Gravity
In this podcast, discover how scientists are probing quantum gravity using quantum optics.
Perimeter Institute: Lectures on the experimental search for quantum gravity
Watch a series of scientific lectures on experimental probes of quantum gravity.