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Physicists Find a Way to See the ‘Grin’ ofQuantum GravityA recently proposed experiment would confirm that gravity is a quantum force.

By Natalie Wolchover

In 1935, when both quantum mechanics and Albert Einstein’s general theory of relativity wereyoung, a little-known Soviet physicist named Matvei Bronstein, just 28 himself, made the firstdetailed study of the problem of reconciling the two in a quantum theory of gravity. This “possibletheory of the world as a whole,” as Bronstein called it, would supplant Einstein’s classicaldescription of gravity, which casts it as curves in the space-time continuum, and rewrite it in thesame quantum language as the rest of physics.

Bronstein figured out how to describe gravity in terms of quantized particles, now called gravitons,but only when the force of gravity is weak — that is (in general relativity), when the space-timefabric is so weakly curved that it can be approximated as flat. When gravity is strong, “the situationis quite different,” he wrote. “Without a deep revision of classical notions, it seems hardly possible toextend the quantum theory of gravity also to this domain.”

His words were prophetic. Eighty-three years later, physicists are still trying to understand howspace-time curvature emerges on macroscopic scales from a more fundamental, presumablyquantum picture of gravity; it’s arguably the deepest question in physics. Perhaps, given the chance,the whip-smart Bronstein might have helped to speed things along. Aside from quantum gravity, hecontributed to astrophysics and cosmology, semiconductor theory, and quantum electrodynamics,and he also wrote several science books for children, before being caught up in Stalin’s Great Purgeand executed in 1938, at the age of 31.

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Public domain

The Soviet theoretical physicist Matvei Petrovich Bronstein (1906-1938), a pioneer of quantum gravity researchwhose work remains largely unknown in the west.

The search for the full theory of quantum gravity has been stymied by the fact that gravity’squantum properties never seem to manifest in actual experience. Physicists never get to see howEinstein’s description of the smooth space-time continuum, or Bronstein’s quantum approximation ofit when it’s weakly curved, goes wrong.

The problem is gravity’s extreme weakness. Whereas the quantized particles that convey the strong,weak and electromagnetic forces are so powerful that they tightly bind matter into atoms, and canbe studied in tabletop experiments, gravitons are individually so weak that laboratories have nohope of detecting them. To detect a graviton with high probability, a particle detector would have tobe so huge and massive that it would collapse into a black hole. This weakness is why it takes anastronomical accumulation of mass to gravitationally influence other massive bodies, and why weonly see gravity writ large.

Not only that, but the universe appears to be governed by a kind of cosmic censorship: Regions ofextreme gravity — where space-time curves so sharply that Einstein’s equations malfunction and thetrue, quantum nature of gravity and space-time must be revealed — always hide behind the horizonsof black holes.

“Even a few years ago it was a generic consensus that, most likely, it’s not even conceivably possibleto measure quantization of the gravitational field in any way,” said Igor Pikovski, a theoreticalphysicist at Harvard University.

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Now, a pair of papers recently published in Physical Review Letters has changed the calculus. Thepapers contend that it’s possible to access quantum gravity after all — while learning nothing aboutit. The papers, written by Sougato Bose at University College London and nine collaborators and byChiara Marletto and Vlatko Vedral at the University of Oxford, propose a technically challenging, butfeasible, tabletop experiment that could confirm that gravity is a quantum force like all the rest,without ever detecting a graviton. Miles Blencowe, a quantum physicist at Dartmouth College whowas not involved in the work, said the experiment would detect a sure sign of otherwise invisiblequantum gravity — the “grin of the Cheshire cat.”

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Gavin W Morley

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A levitating microdiamond (green dot) in Gavin Morley’s lab at the University of Warwick, in front of the lens usedto trap the diamond with light.

The proposed experiment will determine whether two objects — Bose’s group plans to use a pair ofmicrodiamonds — can become quantum-mechanically entangled with each other through theirmutual gravitational attraction. Entanglement is a quantum phenomenon in which particles becomeinseparably entwined, sharing a single physical description that specifies their possible combinedstates. (The coexistence of different possible states, called a “superposition,” is the hallmark ofquantum systems.) For example, an entangled pair of particles might exist in a superposition inwhich there’s a 50 percent chance that the “spin” of particle A points upward and B’s pointsdownward, and a 50 percent chance of the reverse. There’s no telling in advance which outcomeyou’ll get when you measure the particles’ spin directions, but you can be sure they’ll point oppositeways.

The authors argue that the two objects in their proposed experiment can become entangled witheach other in this way only if the force that acts between them — in this case, gravity — is aquantum interaction, mediated by gravitons that can maintain quantum superpositions. “If you cando the experiment and you get entanglement, then according to those papers, you have to concludethat gravity is quantized,” Blencowe explained.

To Entangle a DiamondQuantum gravity is so imperceptible that some researchers have questioned whether it even exists.The venerable mathematical physicist Freeman Dyson, 94, has argued since 2001 that the universemight sustain a kind of “dualistic” description, where “the gravitational field described by Einstein’stheory of general relativity is a purely classical field without any quantum behavior,” as he wrotethat year in The New York Review of Books, even though all the matter within this smooth space-time continuum is quantized into particles that obey probabilistic rules.

Dyson, who helped develop quantum electrodynamics (the theory of interactions between matter andlight) and is professor emeritus at the Institute for Advanced Study in Princeton, New Jersey, wherehe overlapped with Einstein, disagrees with the argument that quantum gravity is needed todescribe the unreachable interiors of black holes. And he wonders whether detecting thehypothetical graviton might be impossible, even in principle. In that case, he argues, quantumgravity is metaphysical, rather than physics.

He is not the only skeptic. The renowned British physicist Sir Roger Penrose and, independently, theHungarian researcher Lajos Diósi have hypothesized that space-time cannot maintainsuperpositions. They argue that its smooth, solid, fundamentally classical nature prevents it fromcurving in two different possible ways at once — and that its rigidity is exactly what causessuperpositions of quantum systems like electrons and photons to collapse. This “gravitationaldecoherence,” in their view, gives rise to the single, rock-solid, classical reality experienced atmacroscopic scales.

The ability to detect the “grin” of quantum gravity would seem to refute Dyson’s argument. It wouldalso kill the gravitational decoherence theory, by showing that gravity and space-time do maintainquantum superpositions.

Bose’s and Marletto’s proposals appeared simultaneously mostly by chance, though experts said

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they reflect the zeitgeist. Experimental quantum physics labs around the world are putting ever-larger microscopic objects into quantum superpositions and streamlining protocols for testingwhether two quantum systems are entangled. The proposed experiment will have to combine theseprocedures while requiring further improvements in scale and sensitivity; it could take a decade ormore to pull it off. “But there are no physical roadblocks,” said Pikovski, who also studies howlaboratory experiments might probe gravitational phenomena. “I think it’s challenging, but I don’tthink it’s impossible.”

Courtesy of Sougato Bose

Sougato Bose, a physicist at University College London, leads a team of researchers who plan to experimentallyaccess quantum gravity.

The plan is laid out in greater detail in the paper by Bose and co-authors — an Ocean’s Eleven castof experts for different steps of the proposal. In his lab at the University of Warwick, for instance, co-author Gavin Morley is working on step one, attempting to put a microdiamond in a quantumsuperposition of two locations. To do this, he’ll embed a nitrogen atom in the microdiamond, next toa vacancy in the diamond’s structure, and zap it with a microwave pulse. An electron orbiting thenitrogen-vacancy system both absorbs the light and doesn’t, and the system enters a quantumsuperposition of two spin directions — up and down — like a spinning top that has some probabilityof spinning clockwise and some chance of spinning counterclockwise. The microdiamond, laden withthis superposed spin, is subjected to a magnetic field, which makes up-spins move left while down-spins go right. The diamond itself therefore splits into a superposition of two trajectories.

In the full experiment, the researchers must do all this to two diamonds — a blue one and a red one,

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say — suspended next to each other inside an ultracold vacuum. When the trap holding them isswitched off, the two microdiamonds, each in a superposition of two locations, fall vertically throughthe vacuum. As they fall, the diamonds feel each other’s gravity. But how strong is their gravitationalattraction?

If gravity is a quantum interaction, then the answer is: It depends. Each component of the bluediamond’s superposition will experience a stronger or weaker gravitational attraction to the reddiamond, depending on whether the latter is in the branch of its superposition that’s closer orfarther away. And the gravity felt by each component of the red diamond’s superposition similarlydepends on where the blue diamond is.

In each case, the different degrees of gravitational attraction affect the evolving components of thediamonds’ superpositions. The two diamonds become interdependent, meaning that their states canonly be specified in combination — if this, then that — so that, in the end, the spin directions of theirtwo nitrogen-vacancy systems will be correlated.

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After the microdiamonds have fallen side by side for about three seconds — enough time to becomeentangled by each other’s gravity — they then pass through another magnetic field that brings thebranches of each superposition back together. The last step of the experiment is an “entanglementwitness” protocol developed by the Dutch physicist Barbara Terhal and others: The blue and reddiamonds enter separate devices that measure the spin directions of their nitrogen-vacancy systems.(Measurement causes superpositions to collapse into definite states.) The two outcomes are thencompared. By running the whole experiment over and over and comparing many pairs of spinmeasurements, the researchers can determine whether the spins of the two quantum systems arecorrelated with each other more often than a known upper bound for objects that aren’t quantum-mechanically entangled. In that case, it would follow that gravity does entangle the diamonds andcan sustain superpositions.

“What’s beautiful about the arguments is that you don’t really need to know what the quantumtheory is, specifically,” Blencowe said. “All you have to say is there has to be some quantum aspectto this field that mediates the force between the two particles.”

Technical challenges abound. The largest object that’s been put in a superposition of two locationsbefore is an 800-atom molecule. Each microdiamond contains more than 100 billion carbon atoms —enough to muster a sufficient gravitational force. Unearthing its quantum-mechanical character willrequire colder temperatures, a higher vacuum and finer control. “So much of the work is getting thisinitial superposition up and running,” said Peter Barker, a member of the experimental team basedat UCL who is improving methods for laser-cooling and trapping the microdiamonds. If it can bedone with one diamond, Bose added, “then two doesn’t make much of a difference.”

Why Gravity Is UniqueQuantum gravity researchers do not doubt that gravity is a quantum interaction, capable of inducingentanglement. Certainly, gravity is special in some ways, and there’s much to figure out about theorigin of space and time, but quantum mechanics must be involved, they say. “It doesn’t really makemuch sense to try to have a theory in which the rest of physics is quantum and gravity is classical,”said Daniel Harlow, a quantum gravity researcher at the Massachusetts Institute of Technology. Thetheoretical arguments against mixed quantum-classical models are strong (though not conclusive).

On the other hand, theorists have been wrong before, Harlow noted: “So if you can check, why not?If that will shut up these people” — meaning people who question gravity’s quantumness — “that’sgreat.”

Dyson wrote in an email, after reading the PRL papers, “The proposed experiment is certainly ofgreat interest and worth performing with real quantum systems.” However, he said the authors’ wayof thinking about quantum fields differs from his. “It is not clear to me whether [the experiment]would settle the question whether quantum gravity exists,” he wrote. “The question that I have beenasking, whether a single graviton is observable, is a different question and may turn out to have adifferent answer.”

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Courtesy of Chiara Marletto

Chiara Marletto, a quantum physicist at the University of Oxford, proposed an indirect way to confirm that gravity isa quantum force.

In fact, the way Bose, Marletto and their co-authors think about quantized gravity derives from howBronstein first conceived of it in 1935. (Dyson called Bronstein’s paper “a beautiful piece of work”that he had not seen before.) In particular, Bronstein showed that the weak gravity produced by asmall mass can be approximated by Newton’s law of gravity. (This is the force that acts between themicrodiamond superpositions.) According to Blencowe, weak quantized-gravity calculations haven’tbeen developed much, despite being arguably more physically relevant than the physics of blackholes or the Big Bang. He hopes the new experimental proposal will spur theorists to find outwhether there are any subtle corrections to the Newtonian approximation that future tabletopexperiments might be able to probe.

Leonard Susskind, a prominent quantum gravity and string theorist at Stanford University, sawvalue in carrying out the proposed experiment because “it provides an observation of gravity in anew range of masses and distances.” But he and other researchers emphasized that microdiamonds

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cannot reveal anything about the full theory of quantum gravity or space-time. He and his colleagueswant to understand what happens at the center of a black hole, and at the moment of the Big Bang.

Perhaps one clue as to why it is so much harder to quantize gravity than everything else is that otherforce fields in nature exhibit a feature called “locality”: The quantum particles in one region of thefield (photons in the electromagnetic field, for instance) are “independent of the physical entities insome other region of space,” said Mark Van Raamsdonk, a quantum gravity theorist at theUniversity of British Columbia. But “there’s at least a bunch of theoretical evidence that that’s nothow gravity works.”

In the best toy models of quantum gravity (which have space-time geometries that are simpler thanthose of the real universe), it isn’t possible to assume that the bendy space-time fabric subdividesinto independent 3-D pieces, Van Raamsdonk said. Instead, modern theory suggests that theunderlying, fundamental constituents of space “are organized more in a 2-D way.” The space-timefabric might be like a hologram, or a video game: “Even though the picture is three-dimensional, theinformation is stored in some two-dimensional computer chip,” he said. In that case, the 3-D world isillusory in the sense that different parts of it aren’t all that independent. In the video-game analogy,a handful of bits stored in the 2-D chip might encode global features of the game’s universe.

The distinction matters when you try to construct a quantum theory of gravity. The usual approachto quantizing something is to identify its independent parts — particles, say — and then applyquantum mechanics to them. But if you don’t identify the correct constituents, you get the wrongequations. Directly quantizing 3-D space, as Bronstein did, works to some extent for weak gravity,but the method fails when space-time is highly curved.

Witnessing the “grin” of quantum gravity would help motivate these abstract lines of reasoning,some experts said. After all, even the most sensible theoretical arguments for the existence ofquantum gravity lack the gravitas of experimental facts. When Van Raamsdonk explains his researchin a colloquium or conversation, he said, he usually has to start by saying that gravity needs to bereconciled with quantum mechanics because the classical space-time description fails for black holesand the Big Bang, and in thought experiments about particles colliding at unreachably highenergies. “But if you could just do this simple experiment and get the result that shows you that thegravitational field was actually in a superposition,” he said, then the reason the classical descriptionfalls short would be self-evident: “Because there’s this experiment that suggests gravity isquantum.”

Correction March 6, 2018: An earlier version of this article referred to Dartmouth University.Despite the fact that Dartmouth has multiple individual schools, including an undergraduate collegeas well as academic and professional graduate schools, the institution refers to itself as DartmouthCollege for historical reasons.

This article was reprinted on TheAtlantic.com.