Researchers look beyond space and time to cope with quantum theory

Physicists have proposed an experiment that could force us to make a choice between extremes to describe the behaviour of the Universe.

The proposal comes from an international team of researchers from Switzerland, Belgium, Spain and Singapore, and is published today in Nature Physics. It is based on what the researchers call a ‘hidden influence inequality’. This exposes how quantum predictions challenge our best understanding about the nature of space and time, Einstein’s theory of relativity.

“We are interested in whether we can explain the funky phenomena we observe without sacrificing our sense of things happening smoothly in space and time,” says Jean-Daniel Bancal, one of the researchers behind the new result, who carried out the research at the University of Geneva in Switzerland. He is now at the Centre for Quantum Technologies at the National University of Singapore.

Excitingly, there is a real prospect of performing this test.

The implications of quantum theory have been troubling physicists since the theory was invented in the early 20th Century. The problem is that quantum theory predicts bizarre behaviour for particles – such as two ‘entangled’ particles behaving as one even when far apart. This seems to violate our sense of cause and effect in space and time. Physicists call such behaviour ‘nonlocal’.

It was Einstein who first drew attention to the worrying implications of what he termed the “spooky action at a distance” predicted by quantum mechanics. Measure one in a pair of entangled atoms to have its magnetic ‘spin’ pointing up, for example, and quantum physics says the other can immediately be found pointing in the opposite direction, wherever it is and even when one could not predict beforehand which particle would do what. Common sense tells us that any such coordinated behaviour must result from one of two arrangements. First, it could be arranged in advance. The second option is that it could be synchronised by some signal sent between the particles.

In the 1960s, John Bell came up with the first test to see whether entangled particles followed common sense. Specifically, a test of a ‘Bell inequality’ checks whether two particles’ behaviour could have been based on prior arrangements. If measurements violate the inequality, pairs of particles are doing what quantum theory says: acting without any ‘local hidden variables’ directing their fate. Starting in the 1980s, experiments have found violations of Bell inequalities time and time again.

Quantum theory was the winner, it seemed. However, conventional tests of Bell inequalities can never completely kill hope of a common sense story involving signals that don’t flout the principles of relativity. That’s why the researchers set out to devise a new inequality that would probe the role of signals directly.

Experiments have already shown that if you want to invoke signals to explain things, the signals would have to be travelling faster than light – more than 10,000 times the speed of light, in fact. To those who know that Einstein’s relativity sets the speed of light as a universal speed limit, the idea of signals travelling 10,000 times as fast as light already sets alarm bells ringing. However, physicists have a getout: such signals might stay as ‘hidden influences’ – useable for nothing, and thus not violating relativity. Only if the signals can be harnessed for faster-than-light communication do they openly contradict relativity.

The new hidden influence inequality shows that the getout won’t work when it comes to quantum predictions. To derive their inequality, which sets up a measurement of entanglement between four particles, the researchers considered what behaviours are possible for four particles that are connected by influences that stay hidden and that travel at some arbitrary finite speed.

Mathematically (and mind-bogglingly), these constraints define an 80-dimensional object. The testable hidden influence inequality is the boundary of the shadow this 80-dimensional shape casts in 44 dimensions. The researchers showed that quantum predictions can lie outside this boundary, which means they are going against one of the assumptions. Outside the boundary, either the influences can’t stay hidden, or they must have infinite speed.

Experimental groups can already entangle four particles, so a test is feasible in the near future (though the precision of experiments will need to improve to make the difference measurable). Such a test will boil down to measuring a single number. In a Universe following the standard relativistic laws we are used to, 7 is the limit. If nature behaves as quantum physics predicts, the result can go up to 7.3.

So if the result is greater than 7 – in other words, if the quantum nature of the world is confirmed – what will it mean?

Here, there are two choices. On the one hand, there is the option to defy relativity and ‘unhide’ the influences, which means accepting faster-than-light communication. Relativity is a successful theory that researchers would not call into question lightly, so for many physicists this is seen as the most extreme possibility.

The remaining option is to accept that influences must be infinitely fast – or that there exists some process that has an equivalent effect when viewed in our spacetime. The current test couldn’t distinguish. Either way, it would mean that the Universe is fundamentally nonlocal, in the sense that every bit of the Universe can be connected to any other bit anywhere, instantly. That such connections are possible defies our everyday intuition and represents another extreme solution, but arguably preferable to faster-than-light communication.

“Our result gives weight to the idea that quantum correlations somehow arise from outside spacetime, in the sense that no story in space and time can describe them,” says Nicolas Gisin, Professor at the University of Geneva, Switzerland, and member of the team.

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4 thoughts on “Researchers look beyond space and time to cope with quantum theory”

  1. Scoff if you want, but… while working with mainframes in the late ’70s, I discovered that something about my presence made them fail. At first, simply walking into the computer room caused the mainframes to crash. Experimentation showed that I could repeatably cause the effect at a distance by imagining ‘static’ at them. A few years later, while working with a DEC VAX-11/780 in a Tempest shielded room, I conducted another experiment. With the door open, thinking static at it worked as expected, but with the door closed it did not. I concluded that the effect was therefore electromagnetic in nature, and that the EM was traveling from me to the computer. Deconstructing this model with lateral thinking, I realized there was an unstated assumption: that the EM traveled from my location to that of the mainframe through the intervening space. I then tried again, but this time imagining that the EM arose within the closed Tempest room, so that it did not have to pass through the walls. The effect worked, and the computer stalled. My conclusion was that our 3-space consensus reality was an illusion of some sort. (Later tech was not susceptible, unfortunately.)

  2. I would like to explore the possibility of an instantaneous effect. Suppose a bounded and continuous universe contained waves of infinite speed. These waves would be bounded in the frequency domain by the extent of the universe. Suppose these waves were reflected by the bounds of the universe as waves often are in our world. These instantaneous waves would then form standing wave patterns as they would be directed both to and from the bounds of the universe. Given some assumptions about the size and shape of this imaginary universe, what would these standing wave patterns look like?

  3. As in the experiments proving Bell’s Theorem, this new experiment will merely confirm what is well known – that some sort of correlation between the particles can exist. But relativity should not have to be discarded on this account – light and signals cannot travel faster than (c). What can and should be discarded is the notion that nature’s behavior is fundamentally probabilistic. Probability was invoked to explain the particle-wave duality – how a ‘photon particle’ for example can behave like a wave – flitting about seemingly at random. But what if Planck’s objection to Einstein’s idea that the photon is a particle (not merely a quantum of energy spreading like a wave) is correct? Eric Reiter ( see unquantum.net also fqxi.org/community/forum/topic/1344 ) has now proven experimentally that the point photon concept is simply wrong! Quantum probability is not ‘real’ – physicists should reconsider and develop alternative explanations and interpretations that have been readily available for decades. Reiter points out for example that Compton himself gave an alternative wave explanation to his effect, not just the one that is cited as ‘proving’ the photon is a particle.

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