Physics is tough. Want to spot a ripple in space-time? You just need a detector capable of seeing a length change less than one-millionth the size of an atom. Want to find a Higgs boson? No problem – so long as you have $7 billion, 14 years and 6000 scientists to hand. Still, one experiment is so hard as to make even the cheeriest physicist gulp: testing the idea that gravity is quantum.
A theory of quantum gravity is the outstanding goal of modern physics. It would reconcile two currently incompatible pillars of our description of the universe: general relativity, our large-scale theory of gravity; and quantum mechanics, our microscopic account of nature’s other fundamental forces. Individually, these have been thoroughly tested, always passing with flying colours. Yet try to combine them, and things fall apart. If we could show that gravity is quantum in nature, perhaps by finding a quantum particle of it, the problem would be all but solved. However, even our most powerful detectors don’t come close to the extraordinarily high energies thought to be needed to find these so-called gravitons.
Advertisement
Not long ago, the late theorist Freeman Dyson echoed the mood among many physicists when he argued that quantum gravity might simply be untestable. But recently, some have begun to claim that it may not be so. If true, we could soon see the first hints of how the two most fundamental theories of nature relate to each other. “It seems to me that, technologically speaking, the time is opportune,” says , a theorist at the University of Oxford.
Uniting physics
The two pillars of modern physics have never been likely bedfellows. On one side, general relativity says that the universe is made of a continuous and predictable space-time, which results in a particular force – gravity – when it flexes around massive objects. On the other side, quantum mechanics says that all of the universe’s matter and forces should be made of indivisible particles with an odd property – they can’t decide exactly where they are.
For decades, physicists have muddled along with both theories. Broadly speaking, general relativity excels for everything that is very big, when gravity dominates, while quantum mechanics rules over everything very small, when nature’s other forces take over. But, ultimately, both can’t be true: nature cannot be simultaneously continuous and made of indivisible chunks; it cannot be both predictable and random. Nowhere is this more evident than at the big bang, when everything in the universe was compacted into an infinitesimally small point with infinite gravity. Anyone hoping to understand that extreme event has no choice but to attempt a reconciliation.
Bringing gravity under the framework of quantum mechanics – “quantising” gravity – has been the biggest project in fundamental physics for more than half a century. There are several ideas that claim the capability to resolve it, the most promising being string theory, which replaces fundamental particles with vibrating strings. Because strings are naturally spread out over multiple higher dimensions, the big bang would no longer be a pure singularity, so the tension that arises between general relativity and quantum mechanics at infinitesimal scales is diluted. An alternative means of reconciliation is loop quantum gravity, which attempts to build space-time itself out of indivisible quantum units. Both amount to a quantum version of gravity – and both are immensely hard projects mathematically – but what makes them harder still is the lack of any way to check them empirically. As far back as 1957, the theorist Richard Feynman said that the “one serious difficulty” is the lack of experiments testing for signatures of quantum gravity. “Furthermore,” he stated, “we are not going to get any experiments.”
Behind his assertion was some fairly basic logic. We know that all the other forces governed by quantum mechanics are transmitted by indivisible particles: photons for the electromagnetic force, which governs light and the basic chemistry of matter; gluons for the strong force, which sticks together protons and neutrons inside atoms; and W and Z bosons for the weak force, which enables certain particles to radioactively decay. If gravity has the same underlying theory as these forces, it should also be carried by its own particle: a graviton. True, the graviton might be a front for something more fundamental – the hum of a string in string theory, for example, or the excitation of space-time in loop quantum gravity. But whatever the theory, something like a graviton should almost certainly appear.
The trouble is, the weaker the force, the more rarely its particle interacts with other particles. And gravity is a very weak force – 10 trillion trillion times weaker than the force actually known as the weak force. We might think gravity is strong, seeing it pull an apple from its branch to Earth’s surface, but the gravitational attraction between a pair of apples is scarcely perceptible. That is why the prospect of spotting a graviton in an experiment has always been unimaginably slim. According to conventional theory, the effects of quantum gravity can be felt only at unthinkably high energies. Even the Large Hadron Collider – the hugely powerful particle accelerator that detected the Higgs boson – is one thousand million million times too weak to see quantum gravity in action. In the early 2000s, Dyson said you might need detectors so big that they would collapse in on themselves, forming a black hole.
The ping of a graviton
No wonder it came as a shock last year, when a group led by at Stockholm University in Sweden suggested that it could be possible to . The experimental blueprint involved a microscopic metal bar, like a tiny tuning fork. First the bar is cooled close to absolute zero, so that its atoms all adopt the same behavioural state. Next it is illuminated by a laser, ever so slightly nudging this collective state into one that is – in hazy quantum fashion – both resonating and not resonating. Set up like this, the researchers claimed, the bar will fully resonate upon even the tiniest of disturbances – even the ping of a single graviton.
It sounds too good to be true – and for some other physicists, it is. Several months before the Pikovski group published its paper, theorist at the Lawrence Berkeley National Laboratory in California and his colleagues revisited the question of whether gravitons are detectable using the technology designed to capture gravitational waves.
These waves are ripples in space-time that are occasionally produced in distant cataclysmic events, such as the merging of black holes, but, like gravitons, were considered undetectable for most of the past century. It was only in 2015 that one was spotted for the first time, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US measured a minuscule disturbance in space-time equal to less than one millionth of an atom’s width.
Even the Large Hadron Collider is far too weak to see quantum gravity in action
If gravitational waves can be detected, asked Carney and colleagues, why not gravitons? In principle a gravitational wave is just a bunch of gravitons, and the researchers calculated that technology developed in the past decade has already become sensitive to waves small enough to constitute . But here is the catch: any signal would be identical to that from a classical, non-quantum gravitational wave that just happens to be very small. Carney argues that the same ambiguity would bedevil any click recorded in a bar-resonator experiment, as proposed by Pikovski’s group. “You hit it with some energy, and it starts moving,” he says. “That doesn’t tell you if the gravitational field is quantised.”
There is disagreement as to what a bar-resonator experiment could actually show. Some, including Carney, believe it cannot say anything useful about the quantum nature of gravity. Others, such as Pikovski and Vedral, argue it could strongly suggest that gravity is quantised, just not in a foolproof manner. Either way, most agree that a clear-cut signal of a graviton is, alas, an impossible dream. “For an unambiguous proof of quantisation we’ll probably have to wait another 100 years or more,” says Pikovski.
So were the cynics right in the end? Not quite. After all, there are other hallmarks of quantum mechanics besides quantisation itself. One is entanglement: whenever two quantum objects interact, some of their properties should become instantaneously correlated. In 2017, Vedral and his collaborator at Oxford, and, independently, a team led by at University College London, outlined a way to exploit this phenomenon. Their proposal is to prepare two masses in a quantum state such that their positions are uncertain, then isolate them from all other forces – and wait. If, after a time, the positions of the masses begin to correlate with one another, they must have become entangled by gravity. Ergo, gravity must be quantum.
It isn’t quite as simple as it sounds. In fact, Feynman once considered a more primitive version of this experiment, only to dismiss it as “phenomenally difficult”. Bigger masses are harder to put in quantum states, but you can’t go too small, or else the gravity between the objects becomes too weak to measure. Worse, our everyday world is saturated with photons in the form of light, heat and radio waves, which all risk drowning out any subtle correlations. Yet technology has come a long way since Feynman’s day, when quantum experiments consisted of a few atoms at best. In 2019, a team led by at the University of Vienna in Austria put some 2000 atoms at once in a quantum state – a milestone on the way to gravitational entanglement. “There are three or four experimental groups who are racing to do it,” says Vedral.
We are a way off, though – these masses are at least a million times too small for gravity to have a measurable effect. , who leads a different group at the University of Vienna, hopes to perform a viable experiment in 15 years, but he admits that this is a self-imposed deadline. (“I’m forced to retire in 20,” he says.) He also points out that, strictly speaking, a foolproof setup would demand the masses be prepared far apart from one another in an effort to prevent meddling by any external effects, such as light passing between the two masses. Unless they are far enough apart, we can’t say that only a quantum theory of gravity could explain their entanglement without violating relativity. But gravity is so weak as to be practically zero between masses that are well-separated. “That sort of experiment is impossible in any realistic time frame,” says Aspelmeyer. “Let’s say before the next catastrophic meteor hits the Earth.”
So gravitons can’t be directly detected, and any sign of gravitational entanglement will take 15 years or more – and might still then be uncertain. It certainly seems anticlimactic. But all is not lost. For there is a third way – one that turns the question around. What if gravity isn’t quantum?
There are good reasons to think gravity is categorically different from the other forces. For starters, it is the only force that affects everything. In general relativity, this is because gravity is equivalent to the very curvature of space-time, not attracting objects to one another but rather creating a slope in the fabric of space-time that they simply slide down. In this sense, gravity is not really a force like the others – it is more like a geometrical mirage.
Testing quantum gravity
Not wanting to mess with this neat description, some theorists have tried to find ways to preserve classical space-time, setting gravity apart as the only fundamental force that isn’t quantum. In some hypotheses, it would even break down the quantum nature of objects above a certain mass, explaining why our everyday world appears classical. The ideas within this so-called “semi-classical” approach are various, but as a group led by at University College London , they must all have one thing in common: some kind of gravitational randomness, or noise.
To understand why, imagine a mass that, according to standard quantum mechanics, is in two positions at the same time. Does the mass’s gravity originate from one position, or the other? If gravity isn’t a quantum force, but an innate feature of space-time, it cannot originate from both – there can be only one space-time, after all. If the mass’s position is indefinite, but space-time is definite, the best space-time can ever do is guess where to bend.
The point isn’t how gravity does this, Oppenheim says, only that if it is a fundamental feature of space-time, it has to do it somehow. And that means that if gravity itself isn’t quantised, any precise experiment involving mass ought to be fundamentally limited in its possible accuracy – some noise will always be present. “It holds for any theory in which space-time is fundamentally classical, so it’s very broad,” says Oppenheim.
Crucially, unlike the graviton and the entanglement experiments, tests of this idea can begin right now. In fact, they already have. In 2021, Aspelmeyer’s group made a few gold beads, each just a couple of millimetres across. They attached a bead to each end of a toothpick-sized rod, so as to look like a miniature dumbbell, and suspended the rod horizontally on a spring. Next, they oscillated a third bead close by, to see if its gravity would make the others shift. They detected an acceleration equal to about one hundred-billionth of that due to Earth’s gravity – the smallest source of gravity yet recorded.
This measurement is not precise enough yet – Oppenheim reckons it needs to be about a thousand times better to tell us anything useful about the nature of gravity. But we are closing in fast. Aspelmeyer’s group is already finalising an upgraded experiment based on masses 10,000 times smaller, using computer-chip manufacturing techniques. Results from this type of test can be combined with data from the ongoing experiments on gravitational entanglement, to “squeeze” the noise parameter and see if gravity really does butt up against a classical limit.
How long will this take? No one knows for sure. But what was once thought impossible – actually testing whether gravity is quantum, rather than just pondering it – looks to be finally within sight. “This is now a question for experimenters,” says Oppenheim. “Nature doesn’t care what theorists think.”
