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.
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 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 lengths, 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 Časlav 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.
This type of experiment, first done in 1975 using neutrons, confirms that Newton’s law of gravitation applies equally to planets and particles. Later experiments, notably by Steven Chu, Nobel laureate and U.S. Secretary of Energy, aimed to go a step farther and hunt for distinctive features of general relativity, beyond those of Newton’s theory. They claimed to find them, but others were wary. Ironically, Chu’s leading skeptic was none other than a fellow recipient of the 1997 Nobel, Claude Cohen-Tannoudji.
The Vienna team bypassed the controversy by proposing a modified experiment. It would send 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 do more than differentiate two paths of unequal height; it would 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.
Yet another approach builds on efforts to see distinctive quantum effects in systems of ever increasing size. The Viennese physicists, working with a colleague in London, reasoned that there are two ways to achieve high energy and therefore probe quantum gravity. You can either pack a lot of energy into a single particle or you can assemble a huge number of low-energy particles and coax them into behaving collectively like one big particle.
The proposed experiment involves a tiny mirror on a tiny spring. By shining light on the mirror, you damp it down until the contraption reaches its minimum possible energy, at which point it acts like a single quantum. With a mass of 20 micrograms, it would have as much total energy (via E=mc2) as the most powerful lone particle imaginable. By continuing to shine light on the mirror, you have complete control over its position and momentum. The team suggests running the device through a cycle: reposition it slightly, then give it a velocity, then return it to its original position, then bring it to a stop. Even though the mirror is back where it started, it is not exactly the same as it was before—the quantum wave corresponding to the mirror has shifted slightly. By analogy, when a car engine goes through a cycle, it returns to its same internal state, but leaves you farther down the road.
Technically, the residual shift is a consequence of quantum noncommutativity—the fact that the order of operations makes a difference to a quantum system. Repositioning, then changing velocity, is not the same as changing velocity, then repositioning. Noncommutativity underpins the famous Heisenberg uncertainty principle, whereby you can’t measure both the position and momentum of something with perfect precision; you need to make a tradeoff.
What makes this interesting is that quantum gravity could modify the 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. The mini-mirror experiment would pick that up.
Still another approach suggested by the ever-inventive Viennese, which hasn’t lent itself to a specific experiment yet, but is generally inspired by the experimentalist mindset, 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.
The nice thing about this inversion is that you can imagine observing correlations that aren’t explicable in spatiotemporal terms—for instance, correlations that can’t be placed in a causal sequence, not even in principle. Per the usual style of quantum information theorists, the team expresses its idea in the form of a game. Suppose two players, Alice and Bob, are in two booths, each equipped with a red and a green button and a red and a green light; when Alice presses a button, the corresponding light comes on in Bob’s booth, and vice versa.
Each player flips a coin. The object of the game: to guess the outcome of the other person’s coin toss. They have to make their guesses before they flip their coins. In normal spacetime, the game unfolds in a causal sequence. One of the players has to go first—say, Alice. Her red and green lights are dark, since Bob hasn’t had a chance to press any button yet, so the best she can do is guess his outcome. She sends her own outcome to Bob, so that at least he always gets the right answer. Overall, they get both outcomes correct 75% of the time.
But imagine that the button and light are correlated independently of who goes first. Then, Alice’s light does go on and she can make an educated guess about Bob’s outcome. If you extend quantum mechanics to cover this situation, you can calculate the odds of winning: about 85%, better than they could achieve when everything is neatly ordered.
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. That is such a bizarre and abstract concept, even for theoretical physicists, that any way to visualize it counts as progress. So 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.
This is the unabridged version of a post on NOVA's blog, The Nature of Reality.
Image credit: Fermilab