Quantum Gravity Tests Coming Soon
Quantum mechanics (QM) is arguably the most rigorously validated scientific theory we have. But it can’t be the ultimate theory of the physical world because it doesn’t dovetail with general relativity (GR), Einstein’s theory of gravity. Can the two theories be reconciled—or will one of them have to be replaced by something else?
The standoff between QM and GR has persisted for many decades. Some leading physicists would make GR supreme. Physics Nobel laureate Anthony Leggett from the University of Illinois has voiced the suspicion that if you push QM hard enough, it will break down, and something else will take over. Another Physics Nobel laureate, Roger Penrose of the University of Oxford in the UK, wrote in 1989 that “one must strongly consider the possibility that quantum mechanics is simply wrong when applied to macroscopic bodies” [1].
Most physicists think that Leggett’s “something else” will be a marriage of QM and GR: a quantum theory of gravity. No one knows what such a theory would look like, and current efforts to develop one—string theory is the best-known example—don’t yet make experimentally testable predictions.
All attempts to quantize GR have so far produced unsatisfactory answers, says Igor Pikovski of Stevens Institute of Technology in New Jersey. But he and others are asking a potentially simpler question: not what is the quantum theory of gravity but whether gravity is quantum mechanical at all.
It is not a given that gravitational fields must be quantized, even though the other three fundamental forces of nature (the electromagnetic force and the strong and weak nuclear forces) are described in quantum-mechanical terms. “The question is really open [as to] whether gravity is quantum or classical,” says Sougato Bose of University College London. This question, Pikovski says, “is the biggest challenge of modern physics.”
Bose says that, in contrast to searches for a quantum theory of gravity, simply asking if gravity is quantized might not only have an unambiguous answer, but it could be experimentally answerable in the not-too-distant future. And if gravity turns out not to be quantized, Pikovski says, then “something very fundamental must break when gravity interacts with quantum systems. So there is a big puzzle that, no matter how it is resolved, will revolutionize physics.”
Bose and Pikovski are among those now planning experiments that they hope will deliver an answer before too long. These experiments require stretching or pushing beyond current limits on measurement precision, but the researchers say that the techniques are well established; all that is needed is to refine existing methods. “It’s an exciting time,” Pikovski says.
Two Sides of the Equation
In general, GR is used to predict the behavior of very big and massive objects, like stars and galaxies, whereas QM describes very small ones, such as atoms and fundamental particles. But problems may then arise when the very massive meets the very small. For example, for black holes, GR predicts that matter can collapse to an infinitesimal point called a singularity, but QM sets a minimum size—the Planck length—beyond which classical ideas of spacetime break down.
But we don’t need to peer inside black holes to see the tension between the two theories. That tension is evident already in Einstein’s GR equation that relates the mass and energy of an object to the spacetime curvature that the object generates. Curvature appears in the equation as a continuous and thus classical variable, but we know from QM that energy and mass generally require a quantum description that leads to discrete values.
Theorists have dealt with this apparent inconsistency in various ways. Penrose thinks that if GR is classical, it could “break” QM. For example, imagine that a quantum object is in a superposition state that allows it to be observed in more than one location. Because of the object’s mass, this situation demands a superposition of multiple spacetimes, each curved in a different way. Penrose argues that for an object with enough mass, the energy needed to sustain such a situation would become too great for the superposition to remain stable. So the more massive the object, the more likely it is that gravity will “enforce” classicality by causing the superposition to “collapse” to a single observable state, thus losing its quantum nature.
Another possibility is that mass, though quantized, can behave as though it has an effective value that is a continuous, classical parameter, and this number controls curvature—which then also remains classical. Such “hybrid theories” have few advocates currently, but they are an option in principle (see Viewpoint: Might There Be No Quantum Gravity After All?) [2].
Most researchers believe that gravity must be quantized. If it were fundamentally a classical phenomenon, says cosmologist Claudia de Rham of Imperial College London, then any object with mass, such as an electron, interacting via gravity, should enable us to “precisely infer [its] position and momentum in a way forbidden by quantum physics.” She says that such gravity-based pinpointing would wreak havoc with quantum calculations, causing the estimated probabilities of observing some measurement outcomes to become negative or greater than 100%, or even complex numbers.
“From this perspective,” de Rham says, “the quantum nature of gravity is not merely a possibility but an absolute necessity, without which the laws of nature as we know them would crumble.” Kathryn Zurek of Caltech agrees: “We believe that gravity is quantized because we believe physics makes sense!” However, most researchers agree that the idea must be tested. After all, Zurek says, “physics is an empirical science.”
Gravitational Superpositions
In 2017 Bose and his colleagues—and another team working independently—proposed an experiment to test the quantum nature of gravity using quantum entanglement, a phenomenon in which the interaction of quantum objects makes them interdependent (see Synopsis: A Test of Gravity’s Quantum Side) [3, 4]. Bose and colleagues have argued that entanglement between two objects is only possible if they interact via quantum fields; classical fields cannot generate entanglement (although there may be limited exceptions [5]).
The researchers’ idea was to place each of two masses in a superposition of locations and to make sure that the masses interact only through gravity. To create such spatial superpositions, they suggest using a technique called Stern-Gerlach interferometry: The passage of a quantum particle through an interferometer causes its position to spread out into a wave that can then be split and directed along two different paths and recombined as a superposition [6]. The team showed theoretically that if entanglement is found between two such superposed masses, then gravity must be quantized.
One challenge for this experiment is to couple two quantum objects solely via gravity and to ensure that there is no electromagnetic interaction between them. Using uncharged objects can help, but they can still interact electromagnetically through the Casimir effect, which results from one object “feeling” fluctuations in the electron clouds of the other. That influence can extend up to a few micrometers or so.
And because gravity is very weak compared to the other forces, it will only couple two relatively well separated objects significantly if they are fairly massive. However, the more massive an object is, the harder it is to place it in a well-defined superposition.
Bose and his co-workers estimated that objects with masses of around 10–14 kg—say, diamond crystals a few tens of nanometers across—should be in the sweet spot: big enough to feel one another’s gravity yet small enough to be placed in well-defined quantum states. If held in laser-based optical tweezers about 100 µm apart, a pair of such nanocrystals should be large enough to experience a gravitational interaction about 10 times greater than that of any residual Casimir force, the team found.
The researchers imagined giving each of these nanocrystals a single quantum spin, produced by a single defect in each crystal lattice called a nitrogen-vacancy center. The crystals could be placed in spatial superpositions so that each can be thought of as having two simultaneous locations with different distances, and thus different gravitational attractions, to the other crystal’s locations. According to standard QM—and assuming that gravity is quantized—these conditions would automatically cause entanglement of the two spins. This entanglement would produce correlations that could be observed in spin measurements.
In 2024, Bose and colleagues came up with an alternative version of this experiment, where a measurement would be made on one of the nanocrystals, placed initially in a superposition, using the other as a detector [7]. The “quantumness” of the gravitational force would be revealed if the detection disturbed the superposition.
Both scenarios are very challenging, and Bose estimates that it may take more than ten years before one of them is realized experimentally. The biggest hurdle is putting such relatively large objects into quantum superpositions. The nanocrystals would need to be cooled to the point where they exclusively occupy the lowest quantum energy state of motion. Researchers accomplished such cooling in 2020 using 150-nm-wide silica crystals [8]. Coaxing such ground-state objects into a superposition remains hugely challenging, although researchers have recently reported doing just that for nanoparticles of sodium containing about 7000 atoms [9].
Seeking Gravitons
Pikovski and his co-workers have a different proposal for probing the putative quantum nature of gravity [10]. If gravity is a quantum field, it would have an associated particle called the graviton that mediates the force, just as the electromagnetic force is carried by the photon. “Recently, my group showed [theoretically] that one can detect single gravitons through quantum sensing,” he says.
The idea is that a mass might emit or absorb individual gravitons at its natural resonant frequency, stimulated by a pervasive background buzz of gravitational waves from astrophysical sources. “If the resonator has the same frequency as the passing wave, a graviton can be absorbed, and the graviton energy is transformed into vibrations,” Pikovski says. The team would see a discrete jump in vibrational energy. The concept is similar to that of the large metallic cylinders developed in the 1960s to detect gravitational waves (see Focus: A Fleeting Detection of Gravitational Waves) [11]—except that Pikovski’s experiment would be adapted to detect single gravitons.
A single graviton in the ambient gravitational field has almost no chance of being absorbed and detected, Pikovski says. “But passing gravitational waves have a huge number of gravitons, around 1036, so there is a chance that just one of them will be absorbed.” The researchers calculate that the ideal mass of the resonator is a few kilograms. Despite the immense challenge, Pikovski says that several research teams are interested in tackling the experiment. It took 50 years for the LIGO project, which detected gravitational waves in 2015, to go from a proposal to a detection, and “I am quite confident that [this experiment] can be done much faster, given today’s technology,” he says.
Zurek, meanwhile, has proposed yet another way to prove that gravity is quantized: by using the scattering of a light beam to look for quantum fluctuations in spacetime, in an experiment called GQuEST (see Focus: New Strategy in the Hunt for Quantum Gravity) [12]. She says that these quantum fluctuations are analogous to the fluctuations that continuously produce particle–antiparticle pairs in the vacuum. If gravity is quantized, then quantum-gravity fluctuations would generate tiny gravitational waves that would ruffle spacetime over a wide range of frequencies.
It was widely thought that such effects would be far too weak to see in the laboratory because the fluctuations (if they exist) would show up only on distance scales of around 10–35 m. So “when I first started working on this in 2017, people were upset,” Zurek says. “We were challenging the conventional wisdom about how gravity works.” The key to her detection concept was that some fluctuations would be the collective result of many gravitons—enough for potentially observable consequences.
The GQuEST experiment will look for the scattering of individual photons in a laser beam by these collective gravitational-wave modes. Zurek and colleagues hope to use an interferometer with arms about 7 m long to spot such scattered photons one by one. Of course, there are other sources of photon scattering. But quantum-gravity-induced scattering would have characteristic features, such as a particular dependence on the size of the apparatus and on the angle between the interferometer arms.
The GQuEST interferometer is now being built at Caltech, but getting to the required sensitivity is going to be difficult, so the experiment’s start date is uncertain. Zurek says that the theory is also still a work in progress: “We have not computed every single detail of the observable effects. I still don’t understand everything about this problem.”
Yet Pikovski is optimistic about the prospects for such studies. “I can imagine that experiments accessing quantum features of gravity will materialize within even a few years or a decade, especially with new ideas and modifications now being explored.”
Quantum Gravity in the Sky
Seeing a fingerprint of quantum gravity is not just about high-precision experiments in the lab. “One of the most promising ways to prove the quantum nature of gravity is to look for its signature in the sky,” de Rham says. She and others believe that if gravity is quantized, then quantum fluctuations should make spacetime rough at the quantum scale. This roughness should have left an imprint on the cosmos from the Universe’s earliest moments. Specifically, these quantum fluctuations should have generated a background of primordial gravitational waves that imposed a characteristic pattern of polarization in the cosmic microwave background (CMB) radiation [13].
Efforts to map this polarization are in progress [14]. But spotting (or ruling out) the pattern anticipated from primordial gravitational waves is challenging because the radiation is affected by interactions—especially with dust—during its trip across the Universe. In 2014 the collaboration using the BICEP2 telescope in Antarctica prematurely claimed to have seen primordial gravitational waves, but the observed polarization was subsequently revealed to have been produced by dust in our Galaxy (see Viewpoint: A Clearer View of a Dusty Sky) [15].
De Rham says that hopes of identifying the true primordial polarization of the CMB—and checking whether it fits with a quantum picture of gravity—now hinge on observations by instruments such as the balloon-borne SPIDER microwave detector [16] and the telescopes of the Simons Observatory in Chile [17]. If we’re lucky, she adds, such data might even offer hints about the properties of the graviton—for example, whether it has mass. The lack of information about such a basic question shows how little researchers currently understand about a quantum description of gravity. But with so many efforts now underway to tackle this issue at the heart of modern physics, it’s reasonable to hope that some answers may arrive soon.
–Philip Ball
Philip Ball is a freelance science writer in London. His next book, The Man Who Broke Reality, a biography of Niels Bohr, will be published in December.
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