“EINSTEIN attacks quantum theory.” That was the headline in The New York Times on 4 May 1935. The world’s most famous scientist and two collaborators had discovered what they saw as a fatal flaw at the heart of our greatest theory of nature. They had found that particles separated by kilometres seemed to be able to interact instantaneously with each other. Albert Einstein called it “spooky action at a distance”.
Even though he had helped lay the foundations of quantum theory, Einstein felt it must be missing something. That spookiness just didn’t feel right – there must be something we weren’t seeing that could explain it. No idea this strange could be true, surely?
We now know that it is. That is the lesson from most of the past century of physics, as quantum theory, including spooky action at a distance, passed every experimental test thrown at it. At the tiniest scales, reality really is as strange as our best theory of the subatomic world suggests.
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What we haven’t figured out is why quantum theory is so strange. Physicists like me have long been examining its foundations in search of answers. Recently, these efforts have turned up a major surprise: a new hypothesis called “almost quantum theory” that is even more bizarre than the original. What really excites me is that we might be on the cusp of putting it to the test. If it passes, the newspapers will be reporting the scientific upset of the century.
Quantum theory deals with the subatomic world of particles, and it describes their behaviour with peerless accuracy. It is often spoken of as the most bulletproof scientific theory. But that doesn’t make its ideas any easier to digest. Among its strange facets is that subatomic particles can exist in a cloud of possible states called a superposition before they are measured – the counterintuitive nature of which is most famously captured by Schrödinger’s cat, the thought experiment about a feline that is simultaneously dead and alive. Then, there is the fact that light, say, can behave as both a particle and a wave.
But it is Einstein’s spooky action at a distance, more properly known as non-locality, that bamboozles us most. Take two particles, prepared using a special procedure known as quantum entanglement, and send them far apart. If you peek at one, you will immediately be able to discern some of the quantum properties of the other. It seems that they influence each other instantaneously over large distances, even though no influence takes place. “Spooky” really is the word.
The Bell test
To grasp non-locality more fully, it helps to consider an anecdote about odd socks first told by the Irish physicist John Bell, who greatly advanced our understanding of the quantum world. It was inspired by Reinhold Bertlmann, who worked with Bell in the late 1970s. Bell realised his colleague had a habit of wearing a different coloured sock on each foot. This meant that as soon as you saw that one of Bertlmann’s socks was pink, for instance, then you knew the other one wouldn’t be pink.
Bell thought that sounded suspiciously similar to entanglement. It made him wonder if entanglement was as odd as it seemed. The socks anecdote can be explained easily enough by Bertlmann’s choices as he dressed. Could the correspondence between entangled particles be similarly predetermined – thus explained by everyday, non-quantum physics?
Bell’s genius was to answer this question with what has come to be known as the Bell test. It involves entangling two particles and sending them far apart, to labs where they can be measured in two different ways. Each lab makes one measurement, not knowing which one the other lab has chosen, and uses that to predict things about the result of the other lab’s measurement. Think of it as the quantum version of looking at the pink sock and predicting that the other sock isn’t pink. They do this lots of times and count up the number of correct predictions. Bell showed that if entanglement can be explained by everyday, non-quantum physics, you would get the right answer in a Bell test no more than 75 per cent of the time. When the test is conducted on quantum entangled particles, however, the right answer emerges 85 per cent of the time.
Bell’s test, then, was a way to quantify how weird the correlations between quantum particles are – and it showed that they really do exceed anything we can explain using classical physics. This is what we really mean when we talk about “non-locality”.
Reading about this is what first got me interested in becoming a physicist. The fact you could ask such deep questions about reality and get a clear answer fascinated me. Now, Bell’s test is playing a key role in the development of a set of ideas even stranger than quantum theory.
These ideas had their genesis 30 years ago, when researchers wondered if there were a single principle at the heart of quantum theory. To see why that matters, compare quantum theory with Einstein’s theory of special relativity. This was built chiefly from the basic principle that nothing can travel faster than light. If quantum theory can be similarly derived from one principle, a kind of essence of quantum, it would not only be highly elegant, it might also show us where the weirdness ultimately springs from.
In 1994, Sandu Popescu at the University of Bristol, UK, and Daniel Rohrlich at Ben-Gurion University of the Negev in Israel were mulling this over. They came up with a potential theory of physics that mathematically formalised just two simple principles. First, no signals can go faster than the speed of light. Second, non-locality applies to reality. It all seemed routine. But they were in for a shock.
It turned out their idea, known as PR boxes, allowed for much stronger correlations than we observe. A Bell test would produce the right answer 100 per cent of the time. It seems obviously mistaken, but PR boxes started from reasonable assumptions, so why was it wrong? “It was a huge surprise,” says Mirjam Weilenmann at the Institute for Quantum Optics and Quantum Information in Austria.
This result went largely unnoticed for a time. “Their work appeared in a somewhat obscure journal,” says Matty Hoban at Quantinuum, a quantum computing company in Oxford, UK. But a little over a decade ago, some physicists began to investigate further.
One was Miguel Navascués, also at the Vienna institute. In 2009, he too decided to reformulate the rules of quantum theory, this time starting from the principle that nothing travels faster than light and a new principle called macroscopic locality. The latter says that, as we move from particle-sized objects to the larger, macroscopic world, the rules of classical physics emerge and non-locality vanishes. A Bell test under these assumptions showed the right answers for entangled particles must occur less than 100 per cent of the time. It suggested that PR boxes had gone off the rails because it left out the principle of macroscopic locality. There was now a feeling that this kind of research might inch us closer to finding the essence of quantum theory.
In the same year, a team led by Marcin Pawłowski at the University of Gdansk in Poland tried the reformulation trick again, this time starting from a single principle called information causality. This says that when two people exchange information, one can’t receive more than the other sent. This proved decisive. A Bell test performed under the resulting formulation would produce the right answers 85 per cent of the time, the maximum level of accuracy observed in real experiments.
Spooky action at a distance
This caused quite a stir. “Information causality was an enormous success, it was amazing,” says Navascués. Some thought that we might finally have hit on the essence of quantum theory. “People said maybe this principle encapsulates all of quantum mechanics,” says Navascués. But he wasn’t so sure. He didn’t think the authors had done enough to show that their framework could describe all the nuances of quantum physics, not least the other strange phenomena beyond non-locality.
So, Navascués, Hoban and their collaborators came up with yet another proposal in 2015. It misses out some of the information contained in quantum theory proper, which is why it has become known as almost quantum theory. But it seems to come with everything we know to be true about quantum theory baked in. What’s more, when you work through the result you would get from a Bell test under almost quantum theory, it again comes out as about 85 per cent. Navascués and his collaborators had achieved their aim of showing the flaws in information causality because that principle didn’t uniquely reproduce quantum theory.
It might seem a downer that information causality had been found wanting. But when you think it through, there is an exciting alternative: what if almost quantum theory is actually the true description of reality?
In almost all situations, it makes the same predictions as regular quantum theory. Yet there are some unusual instances where, in a surprising twist, it predicts that there would be correlations between particles that are stronger than plain vanilla quantum theory does. None of these situations has so far been experimentally investigated. So that leaves us in a historic position. We have a potentially viable theory of reality that we can’t rule out, and it suggests that, in some circumstances, quantum theory isn’t weird enough to do justice to reality.
As if that weren’t thrilling enough, there is another reason to get excited about almost quantum theory. One of the biggest missions of physics is to find a more unified description of reality. At the moment, our theories of gravity and the quantum world are separate beasts, and a promising way of uniting them would be to find a quantum version of general relativity. It turns out that almost quantum theory has a similar mathematical structure to one candidate for a theory of quantum gravity, known as the consistent histories formulation of quantum gravity. The building blocks of this hypothesis, proposed by Nobel prizewinner Murray Gell-Mann, correspond to sequences of particle interactions. The idea isn’t currently popular and this could all be a coincidence. Or it could be telling us something. “I thought it was a really cool connection,” says Hoban.
Quantum entanglement
It is vital that we find out if almost quantum theory stands up. But it won’t be easy. It predicts that, in certain situations, particles can have stronger correlations than we have ever observed. But, by definition, the systems of particles involved will be harder to control and work with. One way to put it to the test might be to conduct a version of the Bell test with three particles instead of two, says Ana Belén Sainz, also at the University of Gdansk. “I would love to see these experiments,” she says.
The only trouble is, we don’t yet know what kinds of particles would be best for such tests. Familiar ones like electrons or photons aren’t likely to be hiding much. But Navascués says there are systems of quantum particles that we have always struggled to control – particles like kaons, which are composed of quarks bundled together in an unusual way. He thinks these might be hiding post-quantum physics.
Another place to look for this is inside quantum computers, says Hoban. Within these machines, lots of particles interact in ways we can’t always understand. “I would love it if we start building these quantum computers and, suddenly, they’re not behaving as they should,” says Hoban. This could be a sign of almost quantum theory. Navascués agrees that looking at systems where large numbers of particles are interacting might be fertile ground. He is talking with a group of experimentalists in China to explore how they could design systems like this and test them.
If almost quantum theory turns out to be true, there will be major implications. The ability to entangle particles underpins quantum computing and quantum cryptography. Quantum computing promises a revolution by providing a totally new way to do calculations. Quantum cryptography offers a reliable way to secure communications and could form the basis of a quantum internet. If almost quantum theory is true and we can harness it, it could supercharge both technologies.
Even if all this turns out to be smoke and mirrors, the search for new principles of physics is valuable. The more we learn about quantum theory, the better the chance we might find a way to reconcile it with general relativity, Einstein’s theory of gravity. “Quantum theory is already super old compared to other theories, but there are so many new avenues people explore all the time,” says Weilenmann.
Speaking of Einstein, you have to spare a thought for him in all of this. He fervently hoped that spooky action at a distance was a flaw that would end up showing quantum mechanics was wrong. Little did he know that 90 years later we might be about to find an even spookier theory of physics.
Ciarán Gilligan-Lee is a physicist affiliated to University College London and Spotify, where he leads an AI research lab. Follow him on Twitter @quantumciaran
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