The quantum experiment that could prove reality doesn’t exist

THERE is an old philosophy question about a tree in a forest. If it falls with nobody there to hear it, does it make a sound? Ask a quantum physicist and they might say the sound was there – but you couldn’t be sure the tree was.

Quantum mechanics has long pushed the boundaries of our understanding of reality at its tiniest. Countless experiments have shown that particles spread out like waves, for instance, or seem be in more than one place at once. In the quantum world, we can only know the likelihood that something will appear in one place or another – until we look, at which point it assumes a definite position. This troubled Albert Einstein. “I like to think that the moon is there even if I am not looking at it,” he said.

Now, a new class of experiments is putting Einstein’s conviction to the test, seeing if quantum weirdness stretches beyond the tiny world of quarks, atoms and qubits into the everyday world of tables, chairs and, well, moons. “If you can go from one atom to two atoms to three to four to five to a thousand, is there any reason why it stops?” says Jonathan Halliwell at Imperial College London.

These experiments are not just investigating whether there is a hard boundary between the quantum and classical worlds, but also probing the true nature of reality. If the work goes as some theorists expect, it might just kick the legs out from under one of our most firmly held beliefs: that things exist regardless of whether we are looking at them.

In 1935, Einstein came up with a thought experiment designed to reveal that quantum mechanics was an incomplete theory of reality that must, sooner or later, be replaced. Together with his colleagues Boris Podolsky and Nathan Rosen, he imagined a pair of particles entangled with each other so that whatever you do to one instantly affects the other. Measure, say, the position or velocity of one particle and it will reveal the position or velocity of the other without having to measure it. Now imagine placing these particles at opposite ends of the universe and performing the same measurement. At first glance, it seems that information is being transmitted between the particles faster than the speed of light.

Einstein argued that this “spooky action at a distance” was so absurd that the outcome of any entanglement experiments must be predetermined. Physicist John Bell was also uncomfortable with the ill-defined nature of quantum mechanics. In 1964, he devised a mathematical way to put this paradox to the test, called Bell’s inequality. If Einstein and his colleagues were right, then Bell’s inequality would be fulfilled.

Again and again, experiments have found that Bell’s inequality is violated. If you insist that reality behaves classically, as opposed to in a quantum way, then to account for entanglement and the violation of Bell’s inequality, “you have to assume that something happens faster than the speed of light”, says Vlatko Vedral at the University of Oxford. Take your pick, Einstein: quantum weirdness is a reality or information breaks the universal cosmic speed limit.

But that is only part of the story. Bell’s inequality addresses locality, the idea that the space between objects matters. It doesn’t answer the question of whether the moon is there when you aren’t looking. Realism says the position, speed, energy and other properties of particles can be reasonably well defined and performing a measurement on an object shouldn’t affect what the object does in the future. Instead, quantum mechanics adds uncertainty and superpositions, mixtures of many possible identities that collapse into one value when a measurement is taken.

Realism in macroscopic objects is called, you guessed it, macrorealism. When you look at the moon or measure how far away it is with a laser, you don’t change it – at least, not according to our common-sense view of the world. “Macrorealism is the fullest expression of classical reality,” says Halliwell. And, like Bell’s inequality, there is a test for it.

Testing reality

The Leggett-Garg inequality, devised in 1985 by Anthony Leggett and Anupam Garg, also looks for correlations between measurements to see whether quantum or classical rules are being followed. But instead of two particles separated in space, like Bell’s inequality, this deals with a single object over time. Because of this, Leggett and Garg realised they could, in theory, test the quantumness of very big objects. In other words, their inequality could tell us whether realism holds true in the everyday world.

In recent years, the first Leggett-Garg experiments have been carried out on simple quantum systems from superconducting fluids and photons to atomic nuclei and tiny crystals. These have demonstrated once again that the microscopic world is non-real. The trick with Leggett-Garg experiments is to make sure they are non-invasive, which means there needs to be a way of measuring a particle without disturbing it. That isn’t easy, but it can be done. And in each case, the researchers found that for every non-invasive measurement they could make, the system was in a superposition of states.

Now, it is time to test something bigger. “It all boils down to seeing how far we can push this,” says Urbasi Sinha at the Raman Research Institute in India. “We don’t really know.”

The largest things currently known to behave in a quantum way were observed by Markus Arndt and his colleagues at the University of Vienna, Austria, who were performing a different kind of experiment. In 2020, they used a double-slit set-up, passing objects through a slit one at a time to see if they behave like waves by forming interference patterns, to show that proteins obey quantum rules.

This approach has its problems. When you are working with big, complex objects, their quantumness quickly disappears as a result of interaction with the surrounding environment – a phenomenon called decoherence. Quantum states are fragile. They easily break under bombardment from gas molecules, stray photons of light and even delicate electric and magnetic fields. “Any quantum object can behave classically if you’re not treating it properly,” says Chiara Marletto at the University of Oxford. This is especially troublesome for double-slit experiments because it takes a long time to build up the double-slit interference pattern – time in which decoherence can run riot.

Leggett-Garg experiments are just as tricky. They have their own sources of decoherence, but researchers must also find ways to measure a system without disturbing it. Only by doing this can you say for sure whether the object is in a quantum superposition or not. “You have to do the measurement in a clever way,” says Sinha. “You’re trying to measure something, but on the other hand, you want to ensure that the act of measurement doesn’t leave any invasive mark.”

It is impossible to drag most quantum systems – which move in discrete steps – into the classical world, where movement is continuous. This makes it hard to examine quantum objects and the stuff we would usually think of as classical in the same experiment. But Sougato Bose, a theorist at University College London, has a plan. He proposes using an experimental set-up that can transcend the classical and quantum worlds.

The set-up he has in mind, a simple harmonic oscillator, comprises an object trapped inside a well, moving back and forth like a swinging pendulum. Precisely how it oscillates depends on whether it obeys quantum or classical rules. And as there is theoretically no limit to how big a simple harmonic oscillator can be, Bose and his collaborators hope to use it to take a leap into the macroscopic world – using a nanocrystal 100,000 times more massive than objects tested by Arndt’s team.

To do this, the researchers’ idea is to look for the swinging nanocrystal when they expect it to be exactly in the middle of the oscillator, on the border between left and right (see “Reality in the balance,”). “We don’t observe, and then we suddenly take a snapshot observation,” says Bose. But crucially, the detector will only be able to see one half of the oscillator. If it sees the nanocrystal, the researchers know it is in that side. If the detector doesn’t, they know it is in the other.

If the crystal is behaving in a classical way, it should be there half of the time on this first measurement. Then, after waiting the time it takes to complete half a swing and so return to the centre of the system, the researchers would measure again and would expect to see it half the time. But if the particle is quantum, the act of not seeing it in one half of the oscillator would collapse its so-called wave function – the mathematical description of the quantum state. Even though we don’t see the nanocrystal, we now know its position, and because of quantum uncertainty, this injects the particle with momentum and changes the way it is oscillating. By repeating measurements at set intervals, the researchers hope to be able to build up correlations that tell them whether the nanocrystal is behaving in a quantum way or classically. The trick in all of this is to throw out the measurements in which the nanocrystal is seen and only keep the ones in which it isn’t, so that the measurements are non-invasive.

Since Bose and his collaborators proposed the experiment in 2018, advances in the trapping and cooling of nanocrystals to avoid decoherence, alongside new precision lasers, mean the idea can now be realised. Teaming up with Hendrik Ulbricht, an experimentalist at the University of Southampton, UK, Bose plans to carry out the test on a nanocrystal made of about a billion atoms. “It’s a big jump,” says Ulbricht.

Only recently have lasers become sharp enough to determine which side of the trap the nanocrystal is oscillating in. Bigger particles are described by smaller waves and so for these nanocrystals, the laser must be able to resolve widths about the size of a water molecule. Ulbricht and Bose expect to have results within the next six months. If the experiment violates Leggett-Garg inequalities, it will break the concept of realism in macroscopic objects.

Still, even if this is the end result, it will be hard to convince everyone that the quantum world extends this far. For one, Leggett-Garg experiments actually test whether a system behaves classically; if it doesn’t, it is assumed to be behaving quantum-mechanically, but that might not truly be the case. Another stumbling block is the array of loopholes that might lead to Leggett-Garg inequalities being violated even though the system is behaving classically. Although measurements should be non-invasive, the practicalities open up so-called clumsiness loopholes. “The stubborn macrorealist cynic could always say: ‘Ah, the measurement itself disturbed the system’,” says Halliwell, who is dreaming up new methods of measurement to avoid such problems.

Brick walls

Then there is the collusion loophole, in which particles outside your experiment make it look like macrorealism is violated when it isn’t. And let’s not forget the detection loophole, where a detector’s inability to register every particle that comes its way can distort the result.

Researchers like Sinha are busy trying to close all the possible loopholes in Leggett-Garg experiments. Earlier this year, her lab carried out the most watertight test of macrorealism yet, according to Halliwell, in a system made of photons. “We have closed the remaining loopholes for now, but you can never claim that it’s completely loophole-free,” says Sinha.

Loophole-free tests of Bell’s inequality were only published in 2015, half a century after Bell’s original idea. Even now, eagle-eyed physicists keep pointing out possible new cracks in the design of these experiments. Ulbricht acknowledges that their experiment is likely to contain loopholes too. “There will be a very healthy and long debate, I’m sure,” he says.

No experiment has ever contradicted quantum mechanics. And there is no reason to think that quantum weirdness doesn’t apply to objects as big as the moon and beyond, so long as you isolate your system from the environment’s decoherence. “From a theoretical point of view, quantum theory doesn’t put any limitation on how large an object you can put in a quantum superposition,” says Marletto.

But Ulbricht hopes that these experiments might reveal a brick wall that no quantum system can go beyond. Such a wall between the quantum and classical worlds would save reality as we know it, offering a way for our common-sense world to appear out of quantum weirdness. “There could be a universal mechanism, which is turning all quantum systems into classical ones as soon as they hit the brick wall,” he says. One idea, suggested in 1987 by Lajos Diósi at the Wigner Research Centre for Physics in Hungary and Roger Penrose at the University of Oxford, is that our classical reality emerges through instabilities in the structure of space-time.

By testing whether quantum mechanics applies to bigger and bigger objects, Ulbricht says we can rule out some models of objective collapse theory, an extension of quantum mechanics that makes predictions about where the brick wall should be. Yet it will be impossible to be certain this wall exists, as the collapse might be caused by run-of-the-mill decoherence. “Sometimes, the environment may be very conspiratorial,” says Bose. Whether a brick wall or something else entirely, “finding a deviation from the predictions of quantum theory – whether or not you like quantum theory – is great, because we would be able to then try to find a new theory”, says Marletto. “People are frustrated that quantum theory is really good at being confirmed experimentally.”

So is the moon there when you don’t look, or is that tree in the forest even there to fall in the first place? As tests of Leggett-Garg inequalities creep into the truly macroscopic world, the answer, increasingly, is no. “If macroscopic realism is violated, then you can’t assume the moon is there,” says Halliwell. Reality as we think of it might not be real after all.

It is even possible that future Leggett-Garg inequalities would not only disagree with the rules of the classical world, but also break the so-far unbreakable quantum ones. “This would give you a glimpse into some kind of post-quantum world,” says Vedral. “It’s hard to imagine what this could be, but I think we’re going to find something even weirder.”

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