That is essentially what Fedrizzi’s team tested. The group measured polarization and other features in a beam of photons and found a level of overlap that could not be explained by the ignorance models. The results support the alternative view that, if objective reality exists, then the wavefunction is real. “It’s really impressive that the team was able to address a profound issue, with what’s actually a very simple experiment,” says Andrea Alberti, a physicist at the University of Bonn in Germany.

The conclusion is still not ironclad, however: because the detectors picked up only about one-fifth of the photons used in the test, the team had to assume that the lost photons were behaving in the same way7. That is a big assumption, and the group is currently working on closing the sampling gap to produce a definitive result. In the meantime, Maroney’s team at Oxford is collaborating with a group at the University of New South Wales in Australia, to perform similar tests with ions, which are easier to track than photons. “Within the next six months we could have a watertight version of this experiment,” says Maroney.

But even if their efforts succeed and the wavefunction-as-reality models are favoured, those models come in a variety of flavours — and experimenters will still have to pick them apart.

One of the earliest such interpretations was set out in the 1920s by French physicist Louis de Broglie8, and expanded in the 1950s by US physicist David Bohm9, 10. According to de Broglie–Bohm models, particles have definite locations and properties, but are guided by some kind of ‘pilot wave’ that is often identified with the wavefunction. This would explain the double-slit experiment because the pilot wave would be able to travel through both slits and produce an interference pattern on the far side, even though the electron it guided would have to pass through one slit or the other.

In 2005, de Broglie–Bohmian mechanics received an experimental boost from an unexpected source. Physicists Emmanuel Fort, now at the Langevin Institute in Paris, and Yves Couder at the University of Paris Diderot gave the students in an undergraduate laboratory class what they thought would be a fairly straightforward task: build an experiment to see how oil droplets falling into a tray filled with oil would coalesce as the tray was vibrated. Much to everyone’s surprise, ripples began to form around the droplets when the tray hit a certain vibration frequency. “The drops were self-propelled — surfing or walking on their own waves,” says Fort. “This was a dual object we were seeing — a particle driven by a wave.”

Since then, Fort and Couder have shown that such waves can guide these ‘walkers’ through the double-slit experiment as predicted by pilot-wave theory, and can mimic other quantum effects, too11. This does not prove that pilot waves exist in the quantum realm, cautions Fort. But it does show how an atomic-scale pilot wave might work. “We were told that such effects cannot happen classically,” he says, “and here we are, showing that they do.”

“We were told that such effects cannot happen classically, and here we are, showing that they do.”

Another set of reality-based models, devised in the 1980s, tries to explain the strikingly different properties of small and large objects. “Why electrons and atoms can be in two different places at the same time, but tables, chairs, people and cats can’t,” says Angelo Bassi, a physicist at the University of Trieste, Italy. Known as ‘collapse models’, these theories postulate that the wavefunctions of individual particles are real, but can spontaneously lose their quantum properties and snap the particle into, say, a single location. The models are set up so that the odds of this happening are infinitesimal for a single particle, so that quantum effects dominate at the atomic scale. But the probability of collapse grows astronomically as particles clump together, so that macroscopic objects lose their quantum features and behave classically.

One way to test this idea is to look for quantum behaviour in larger and larger objects. If standard quantum theory is correct, there is no limit. And physicists have already carried out double-slit interference experiments with large molecules12. But if collapse models are correct, then quantum effects will not be apparent above a certain mass. Various groups are planning to search for such a cut-off using cold atoms, molecules, metal clusters and nanoparticles. They hope to see results within a decade. “What’s great about all these kinds of experiments is that we’ll be subjecting quantum theory to high-precision tests, where it’s never been tested before,” says Maroney.

Parallel worlds

One wavefunction-as-reality model is already famous and beloved by science-fiction writers: the many-worlds interpretation developed in the 1950s by Hugh Everett, who was then a graduate student at Princeton University in New Jersey. In the many-worlds picture, the wavefunction governs the evolution of reality so profoundly that whenever a quantum measurement is made, the Universe splits into parallel copies. Open the cat’s box, in other words, and two parallel worlds will branch out — one with a living cat and another containing a corpse.

Distinguishing Everett’s many-worlds interpretation from standard quantum theory is tough because both make exactly the same predictions. But last year, Howard Wiseman at Griffith University in Brisbane and his colleagues proposed a testable multiverse model13. Their framework does not contain a wavefunction: particles obey classical rules such as Newton’s laws of motion. The weird effects seen in quantum experiments arise because there is a repulsive force between particles and their clones in parallel universes. “The repulsive force between them sets up ripples that propagate through all of these parallel worlds,” Wiseman says.

Using computer simulations with as many as 41 interacting worlds, they have shown that this model roughly reproduces a number of quantum effects, including the trajectories of particles in the double-slit experiment13. The interference pattern becomes closer to that predicted by standard quantum theory as the number of worlds increases. Because the theory predicts different results depending on the number of universes, says Wiseman, it should be possible to devise ways to check whether his multiverse model is right — meaning that there is no wavefunction, and reality is entirely classical.

Because Wiseman’s model does not need a wavefunction, it will remain viable even if future experiments rule out the ignorance models. Also surviving would be models, such as the Copenhagen interpretation, that maintain there is no objective reality — just measurements.

But then, says White, that is the ultimate challenge. Although no one knows how to do it yet, he says, “what would be really exciting is to devise a test for whether there is in fact any objective reality out there at all.”