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AT FIRST, it looks like an ordinary mirror. But it’s not. It is “half-silvered”. Half of the light that hits it is reflected. The other half passes straight through.

This is not in itself extraordinary. Any time you look out of a window and see the room you are sitting in partially reflected, you’re seeing a similar effect. Special beam-splitting mirrors are part of any teleprompter, and you can buy them online without breaking the bank if you really want to.

It’s what they do to the individual photons of light that’s the strange thing. Peer too closely, and these looking glasses might destroy your very perception of reality. They could leave you unsure where or even who you are, and make you question whether you exist at all. They might even so skew your notions of cause and effect that they leave you wondering whether you, rather than the mirror, are to blame for all this. The question is whether science gets any more profound than what happens at a half-silvered mirror. “I don’t think it does,” says , a physicist at Imperial College London.

The culprit, as usual when we find ourselves assailed by doubt and racked with existential fear, is quantum theory. Quantum theory is our best stab yet at delivering a picture of how material reality works at the smallest scales, and its predictions have been confirmed time and time again by experiment. It’s just that the reality it describes seems to bear little relation to… well, reality.

For a start, quantum reality is unpleasantly random. Take the atoms of something as apparently real as us. According to quantum theory, when in isolation they are never definitely in any one place. There is only ever a certain probability of finding an atom at point X, a different probability of finding it at Y, and yet another of finding it at Z. As long as you don’t ask where it is, an atom exists in a “superposition” of all the possible places it might be. Ask the question – make a measurement – and the atom will reveal itself to be somewhere, but you won’t necessarily be able to predict where.

That weirdness reaches its apogee at the half-silvered mirror. Let light hit the mirror in the right way, and it is not just the light beam that is split, but individual photons. They become in effect two photons. One of them passes through the mirror, and the other is reflected.

Each of these photons has particular properties, for example spin – a quantum-mechanical quantity that can be envisaged rather like a rotation in space. But something very odd happens when you decide to measure these spins one after the other. You can do this over and over again, each time measuring the two spins relative to different things – the lab floor, the direction of the prevailing wind outside, the direction in which a fly is walking across the ceiling above. After a while, a chill runs down your spine. A pattern emerges: each time, the outcome of the second measurement depends on how you chose to make the first measurement.

This is something we cannot explain with normal, classical conceptions of reality. It is entanglement: the ability of quantum objects that were once related to apparently influence each other’s properties when subsequently separated, even by a long way (see diagram). Spooky action at a distance, in Einstein’s word.

It is tempting to seek succour in some “normal” physical explanation for this, as Einstein did, and so maintain our standard perceptions of reality. There must be some undetected influence that flies between the two photons. Something physical must pass from one to inform the other of the information that has been extracted.

Whatever form that influence might take – a photon, some other exchanged particle or perhaps a type of wave? – a good guess is that it will not travel faster than the speed of light. Thanks to Einstein’s relativity, that is always seen as a kind of fundamental speed limit to any kind of usable information flying through the universe. Having that limit prevents all sorts of unpleasant consequences. “We would have weird situations, weird violations of causality, if there were superluminal signalling,” says Rudolph. Any faster-than-light channel might also be open to hijacking for nefarious purposes: you could use it to transmit information backwards in time. Allow violations of relativistic causality, and we could all be lottery millionaires.

Hidden physical influences of the less outlandish sort, which obey relativity, can be tested for relatively easily. First you separate two entangled photons by a huge distance. The second photon is sent away – to the (ISS), say – with instructions to carry out a measurement at a precise time. An instant before that measurement occurs, you measure the first photon. Time it right, and there is not enough time for any influence to travel between the two, even at the speed of light.

Shaken and stirred

Nobody has yet done the ISS test, but we have done similar things many times on Earth. Each time, when the report of the second measurement comes back, the weird influence has still been felt. The second photon responds to measurements as if it were aware of what happened to the first. Experiments performed by and his colleagues at the University of Geneva in Switzerland in 2008 showed that any spooky influences travelling 18 kilometres through a fibre-optic network must be travelling at a minimum of 10,000 times the speed of light (). The experiments have also been done over hundreds of kilometres in free air with similar results, and there are ambitious plans to repeat them in space.

So where does this leave us? Perhaps shaken by these strange tales of unexplained correlations, you might also be stirred to accept another explanation, however far-fetched it seems at first. Relativity only forbids an influence propagating above light speed when it carries information. So what if some weird phenomenon unknown to physicists could break relativity, connect two entangled particles, while being information-free?

We have even less idea what that sort of influence might look like. Chances are it doesn’t matter: since last year, this escape route back to normality has also been blocked off. Together with Gisin and others, at the University of Geneva worked through what would happen within a network of four senders and receivers that could synchronise their measurements of entangled photons. In this theoretical set-up, influences could travel through space-time at whatever speed they liked, just as long as they contained no information.

And it failed to reproduce reality. There was no way any physical mechanism of any stamp could produce the quantum correlations seen in experiments unless hidden influences within the network could also send information at above light speed (). If we trust in relativity, that leaves us with a problem. “It pushes the weirdness further than we thought,” says Bancal. “You go to try to find the causes of these correlations, but somehow they’re just not there.”

“You go to try to find the causes of these strange quantum correlations, but somehow they’re just not there”

Gisin is even more forthright in his conclusion. For him, it means that the dimensions of reality we move in cannot possibly contain the explanation for a more fundamental quantum reality. “There is no story in space and time that tells us how the correlations happen,” he says. “There must exist some reality outside of space-time.”

Unless there is something fundamental that we have wrong. Violations of relativity are frowned upon because they violate our ideas of causality. We humans are suckers for causal order, looking back in time to trace the cause of any event. Even more basically, we are determined determinists, blithely assuming that every event actually has a cause. That seems to work reasonably well in our large-scale everyday world, but when it comes to the nitty-gritty of the underlying quantum reality, can we be so sure?

Theorist and his colleagues at the University of Vienna in Austria recently set out to investigate whether quantum systems are subject, in theory, to the same causal laws as the rest of us. They started off from the classic situation in which two independent observers, Alice and Bob, make a measurement on a photon. The twist Brukner and his team added was quantum uncertainty, a principle that fundamentally constrains the amount of information you can extract from a quantum system – including information about time.

Brukner describes the scenario they uncovered as akin to having Alice walk into a room and find a message written by Bob. She erases it, and writes a reply – then Bob comes in to write the original message that Alice has just replied to. In effect, just as quantum particles can be in two or more places at once, so seemingly can this particle be in two or more moments at once. The system can be simultaneously in the states “Alice came into the room before Bob” and “Bob came into the room before Alice”. “We cannot say whether Alice’s measurement is ahead of Bob’s measurement, or the other way round,” says Brukner ().

“Just as quantum particles can be in two places at once, so seemingly can they be in two moments at once”

Brukner is already thinking about ways to test the results of these theoretical calculations in experiment, but it won’t be easy, he says. Given the delicate nature of quantum states, any attempt to measure a quantum-mechanical superposition of causal orders destroys that superposition, collapsing it into a definite causal order.

Even without experimental confirmation, though, he thinks the conclusion is clear. “Causal order is not a fundamental property of nature,” he says. Causality is only restored when the parameters of the experiment are tweaked to make the particle behave more like familiar, classical particles. That leads him in the same sort of direction as Gisin. We live in space-time, and experience causal order within it, yet causal order is not apparently fundamental to quantum theory. If we accept quantum theory as the most fundamental description of reality that we have, it means that space-time itself is not fundamental, but emerges from a deeper, currently inscrutable quantum reality.

Spontaneous universe

If we accept quantum theory, that is. All the havoc quantum theory wreaks with cherished notions of reality, relativity and causality raises a natural question: is quantum theory itself the problem? For all its successes, perhaps all that randomness, uncertainty and spooky influence is just because quantum mechanics is incomplete. As currently formulated, at least, it might simply not supply all the information we need to explain why things are as they are. An analogy might be made with the laws of thermodynamics. They provide a foolproof, high-level description of how things work – heat always passes from the hotter to the cooler – while saying nothing about the underlying dynamics of individual atoms that makes that happen.

To investigate this possibility, Roger Colbeck and of the Swiss Federal Institute of Technology (ETH) in Zurich have taken a look at what would happen in those classic Alice-and-Bob-type experiments if an underlying theory were to provide an additional, arbitrary amount of information about the correlations between two entangled particles. Do the outcomes of the measurements look any less random and unpredictable?

The short answer is no. In any situation where both Alice and Bob can independently choose the type of measurement they make on their particle, additional information doesn’t make their predictions of what will happen in experiments any more accurate than if they use quantum theory. The mysterious unpredictability of quantum mechanics has nothing to do with incomplete information, it seems.

“The randomness is intrinsic,” says Colbeck. Deep down, the universe is spontaneous. Fundamentally, there is no reason why a quantum particle has the properties it does: there is no hidden influence, no cast-iron cause and effect, no missing information. Things are as they are; there is no explanation.

“Some people find this very depressing,” Colbeck says. So depressing, in fact, that it leads them to question an even more fundamental assumption about reality and our relation to it. It lies in a little clause in the way most investigations of quantum reality and quantum measurements, including Colbeck and Renner’s, are set up. Let’s go back to the first experiment, the one with the photons at the half-silvered mirror. To measure the direction of the photons’ spins, you must first choose something to measure them relative to – the lab, the wind, the fly on the ceiling. Your choice influences the outcome of the measurement. But what if it is not actually your choice? What if something else were forcing your hand, making you perform the experiments such that the correlations always appear?

In thrall to ourselves

This takes us into the domain of human free will, a slippery territory where philosophers are usually more abundant than physicists. It sounds vaguely loopy, yet some serious physicists think that a lack of free will – that we are participants in something of a cosmic puppet show – might be the best way to save us from all the weirdness and loss of relativity and causality implied by quantum correlations.

“One solution might be that there is no free will – we are all participants in something of a cosmic puppet show”

Nobel laureate of the University of Utrecht in the Netherlands, for example, is one who finds the idea of quantum correlations that defy notions of space and time “difficult to buy”. He thinks the answer might instead lie in an extreme form of determinism in which human minds are set on a trajectory of choices, such as what to make a quantum measurement relative to, from which they are powerless to deviate.

Others are less impressed. “Invoking conspiratorial correlations among all the brains, measuring instruments, and subatomic particles in the universe to make it ‘look like’ quantum mechanics is true is vastly stranger than the thing it’s supposedly trying to explain,” says , a quantum physicist at the Massachusetts Institute of Technology. In essence, he says, there is little difference between invoking something like that and invoking a superhuman deity.

Rudolph doesn’t have an answer – no one does. But he reckons the problem is that we are still hopelessly anthropocentric. The growing disconnection between our experience of the world and the results of quantum experiments, he says, are simply a modern version of the ever-more complex epicycles that Ptolemy and those who followed him used to explain the motions of the heavenly bodies. The problem back then was that we could only see the planets as revolving around Earth; it took Copernicus to turn things around, and suddenly all was plain and simple.

Perhaps we have constructed theories such as relativity and quantum theory with a similarly limited view, in thrall this time to a sense of space and time that might not exist beyond ourselves. “We think time and position and so on are important variables for describing the world because we evolved to perceive them,” says Rudolph. “But whatever is going on down there doesn’t seem to worry about them at all.”

So there you have it. When the light shines on that the half-silvered mirror, what we see is hardly a reflection of the world as we would like to know it. Reality, relativity, causality, free will, space and time: they can’t all be right. But which ones are wrong?

This article appeared in print under the headline “Reality check”