IF YOU could empty the universe, what would be left over? The underlying structure of the cosmos is called space-time, and it is often likened to a fabric. But “space-time fabric is a science-fiction term”, says Jonathan Oppenheim, a physicist at University College London. There is no consensus about what it really means.
In classical physics, namely in Albert Einstein’s general theory of relativity, the fabric of space-time doesn’t exist on its own. Instead, space-time is intertwined with – and shaped by – mass and energy, giving rise to gravity. Most importantly, Einstein’s equations are continuous, so, in the classical view, the fabric must be smooth.
Advertisement
But today, most physicists think that space-time must abide by the rules of quantum mechanics, which govern the behaviour of subatomic particles and fields. In which case, it can be broken down into discrete chunks, or quantised. This would mean that, although space-time appears as a smooth background against which everything in the universe plays out, if you could zoom in sufficiently closely, you would see that it is actually made of something, just like everything else.
The problem is, we still have no evidence that space-time is quantised. It is difficult to prove it one way or another, because what you might imagine as the “pixels” of space-time – its most fundamental constituents – would be so vanishingly small that directly observing them would be impossible.
That leaves us with indirect observations. The good news is that physicists have devised a range of ingenious experiments that could finally settle the question of what space-time is made of, if anything, once and for all.
Slow neutrinos
When and his collaborators published preliminary results in June, they didn’t expect an uproar. But their claims shocked the physics community. “Either they didn’t understand or didn’t read the paper,” says Amelino-Camelia, a physicist at the University of Naples Federico II in Italy.
What they presented were details of their measurements of fundamental particles called neutrinos, which have mass but barely interact with other matter, often originating in distant galaxies. In a classical, non-quantised space-time, neutrinos should travel close to the speed of light. But some quantum theories of space-time predict that it imparts a minuscule amount of drag that would slow down neutrinos at rates depending on their energy.
Amelino-Camelia likens the effect to a glass prism slowing down different frequencies of light at different rates to separate them into a rainbow. Except space-time’s drag is far more subtle, so the neutrinos must travel across vast distances if we are to see the effect. “Thankfully, the universe is big enough,” he says.
Looking at neutrinos detected by the IceCube Neutrino Observatory, located in Antarctica, Amelino-Camelia and his colleagues analysed the direction of to uncover a handful that appeared to trace back to a common origin. The idea was that if these neutrinos arrived at different times, they must have been slowed down by varying amounts. They had done exactly that – so, by implication, we have seen evidence of quantised space-time.
At least that is the claim. Critics complained about how few data points were used to pinpoint this common origin and that the potential for errors in the measurements wasn’t shown. But Amelino-Camelia argued that the time difference between each neutrino detection and its corresponding explosion was so well known that the error margins were too tiny to display. Still, he says, more data is needed. “If our conclusions don’t fade away with more data, we will be very lucky,” he says. “And we will have lots to talk about.”
Quantum foam
“If I were to zoom in, space and time would be a jumble of fluctuations,” says , a physicist at the California Institute of Technology in Pasadena. “When you look over much bigger distances, those fluctuations average out, so what we see is smooth.”
Zurek’s work focuses on these tiny, hypothetical fluctuations in space-time, which would be the result of putative particles of gravity known as gravitons popping into and out of existence, as many known particles do. She calls it quantum foam, and she is interested in whether, in certain scenarios, it might be possible to see signs of its existence.
She thinks we could, as long as we live in a holographic universe. Roughly speaking, the holographic principle says that although our perception may be 3D, everything within the bulk of the cosmos can be said to emerge from a two-dimensional surface. In May 2022, Zurek showed that, if that is true of our universe, to the point where they become measurable.
On that basis, she has proposed an experiment. It starts with an interferometer, a device using laser light split between two paths that ultimately cross again, revealing interference patterns. In Zurek’s proposed set-up, the light would be able to nudge gravitons, which can also be thought of as space-time pixels, into moving together, as if they were one cohesive, fluctuating cloud. She dubs the giant cloud a “pixellon” and says it would be able to alter the trajectories of light around it, creating a signature in the interferometer.
Zurek cautions that such an experiment is probably a long way off. For one, she is still making sure her predictions don’t conflict with known physics. “The theory of gravity is an extremely delicate thing,” she says. And secondly, the most sensitive interferometer currently at our disposal, the Laser Interferometer Gravitational-Wave Observatory (LIGO) – the first to detect ripples in space-time called gravitational waves – may not be sensitive enough to detect the effects of quantum foam. “It wasn’t designed to do this kind of measurement,” says Zurek.
But even if her experiment is run at LIGO and nothing turns up, she has already started working with experimentalists to design a more sensitive interferometer that could.
Weighing protons
Particles of light, or photons, are massless, so we don’t normally think that gravity affects them. We should think again, says at the Australian National University in Canberra. Einstein showed energy and mass to be equivalent, meaning energetic photons also emit a weak gravitational field, like a mass would. For a sufficiently high photon energy, this warps space-time – an effect that could alter a photon’s path in a measurable way.
In June, Mehdi and his collaborators predicted that by driving photons to an extremely high energy, they would interact differently with quantised versus classical gravity. “The [quantised] space-time medium can generate weird effects,” says Mehdi.
In one proposed experiment to tap into this difference, a light beam would be split into two halves that are allowed to self-interact. When the two split beams are combined again, their interference patterns would show signatures unique to either quantum or classical gravity. Another set-up would look for a more subtle statistical effect. Due to interactions unique to quantised gravity, a rare event could occur: three photons could annihilate to create a single photon, with a frequency three times that of the originals.
Both experiments would require powerful lasers and special mirrors. “When I tell experimentalists about the [possible] results, I see their eyes popping in a good way,” says Mehdi. “But then I tell them they need to improve the laser power and detection efficiency, and that’s when their eyes pop in a bad way.” The truth is that Mehdi’s experiments are beyond our capabilities for the moment. However, advances in similar experiments, like gravitational wave detectors, mean the technology isn’t far off.
Entangled masses
If gravity is a quantum force, like the three other fundamental forces of nature, it should behave in a quantum way. One way to test that is to see if objects with mass are subject to entanglement, a quantum phenomenon in which the properties of particles are correlated such that measuring those of one particle influences the other, even when they are separated by vast distances.
For a long time, such a test seemed beyond us, largely because any measurements you make would collapse the quantum state of your masses, making observations impossible. But in 2017, Sougato Bose at University College London and his colleagues proposed a tabletop experiment that could do the trick.
The idea is to first put a relatively large mass, around a thousandth of a millimetre in diameter, into a quantum superposition. This is where it exists in multiple states at once until it is measured or observed, at which point it is said to “collapse” into a definite state. Then you bring in a second mass, also in a superposition, and let the two masses fall.
If gravity is quantum, you would expect to see gravitons temporarily materialise to entangle the two masses. “If the pairs could get entangled, then gravity must have been quantum in nature,” says Bose. “This is how we can verify the quantum nature of gravity.” If it is classical, on the other hand, the masses wouldn’t be gravitationally superposed and no gravitons would emerge to entangle them.
It is an exceedingly difficult experiment to pull off, not least because quantum states like superposition and entanglement are so delicate. “Even a single atom from the vacuum can collapse the quantum process,” says Bose. But we are getting better at avoiding this sort of collapse all the time, as we find new ways to carefully manipulate nanoscopic masses, so Bose is optimistic that his experiment will be possible within in the next five to 10 years.
Post-quantum gravity
If they had to bet on it, most physicists would wager against the true nature of space-time being classical. The other three fundamental forces are quantum, so why not gravity too? “But if you think about it, that doesn’t make a lot of sense,” says Oppenheim. “We have been unable to quantise gravity, despite more than a century of effort.”
Instead, Oppenheim is exploring the possibility that gravity isn’t quantum, at least not entirely. His idea is that it is some sort of hybrid, which he calls “post-quantum classical gravity”. This means that space-time and gravity can be classical while being consistent with everything else being quantised.
Many people think that you can’t combine a quantum and a classical system. But Oppenheim says you can, with a few tweaks to quantum theory and general relativity. For that to work, the interaction between quantum particles and the classical force of gravity must be unpredictable.
So, although it might appear that gravity is quantum in some cases, it may be that any apparent quantum behaviours we see in a gravitational field – a superposition, say – are actually just the result of uncertainty in the position of quantum particles within the field. In which case, measurements of a gravitational field won’t give away a particle’s position.
Oppenheim has proposed two ways to test this inspired by Henry Cavendish’s famous 1797 experiment, which measured the tiny gravitational force between two spheres. Oppenheim’s update requires a far greater level of precision, of course. And both of his proposals have the potential to throw up hints that he is onto something with post-quantum classical gravity, or else rule it out to suggest space-time is quantum after all.
The first experiment tests a model by Lajos DiÓsi and Roger Penrose about the role classical gravity plays on a mass in a quantum superposition. They theorised that for small masses, tiny quantum fluctuations in gravity would be detectable above the gravitational field. But for a big mass, the larger gravitational field would induce the mass’s quantum superposition to collapse. On the other hand, quantum gravity predicts that the mass’s superposition should persist, not collapse.
The second test is similar, but instead looks for the transition point of a mass switching from quantum to classical behaviour. For example, we know that photons can behave as a particle or a wave, but necklace beads, say, will show only classical behaviour. In other words, they only behave as particles. The question is, at what mass does this transition occur?
Oppenheim has calculated the transition points for a post-quantum classical universe, and is currently in talks with experimentalists. But it isn’t easy. “Because gravity is so weak, our measurements of it are very inaccurate,” says Oppenheim. “It could be fluctuating wildly and we wouldn’t know.”
Non-local effects
In 1959, physicists Yakir Aharonov and David Bohm proposed that classical electromagnetic equations don’t give the full picture: extra quantum effects must be included. A couple of years later, an experiment verified their prediction by finding that electrically charged particles suffer a “jolt”, even in the absence of an electromagnetic field. The most accepted explanation is something known as a non-local quantum effect.
But could this Aharonov-Bohm effect also work for particles in gravitational fields? If gravity is quantum, it should – and it is possible to test that. In January 2022, , now at Johns Hopkins University in Maryland, and his collaborators published results, based on this idea, that are perhaps the closest thing we have to proper evidence for quantum gravity.
Their atomic interferometry experiment began with a fountain of ultra-cold atoms that they split along two paths. One path passed by a large mass positioned in such a way that it didn’t exert a net gravitational force on the atoms, because its gravity cancelled itself out, leaving quantum gravity as the only possible source of interaction. The atoms on the other path travelled without any external influences.
“We wanted to know if the atoms can still tell whether or not the source mass is present,” says Overstreet. They could. And when the two paths were recombined, interference patterns showed that the first path of atoms had shifted compared with the second path. Overstreet says that even after accounting for all possible sources of error, the effect was still significant.
While they aren’t yet able to distinguish between post-quantum and quantum gravity, the researchers argue that the quantised version provides a more complete explanation of their demonstration – which paves the way for future experiments.
Overstreet now hopes to further at Fermilab, near Chicago, where a . This will act as a gigantic quantum sensor, with enough sensitivity to look for gravitational quantisation in – you guessed it – roughly five to 10 years.
żěè¶ĚĘÓƵ audio
You can now listen to many articles – look for the headphones icon in our app
Lyndie Chiou is a freelance writer based in California
