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IN OCTOBER 1951, physicist David Bohm left the US for Brazil. Branded a communist sympathiser, he had been arrested for refusing to testify to the US Congress. Acquitted, he was still stripped of his Princeton professorship. His departure began an exile that would last until his death, as a naturalised British citizen, four decades later.

The theory Bohm was nurturing as he left his native shores has spent even longer in the cold. In part, that鈥檚 down to politics. But his ideas also seemed scientifically beyond the pale. Bohm proposed there was a hidden reality to quantum theory, meaning its crazy predictions of a world that doesn鈥檛 exist until you choose to look at it are just that: crazy.

That went against the established grain, and still does. But more than six decades on, Bohm is getting a fresh hearing, as new experiments are hinting that he might have been on to something. If so, some aspects of reality would become easier to fathom, while others would be harder to stomach. Forget standard quantum weirdness 鈥� the world Bohm revealed is a more profoundly and mysteriously interconnected place than we ever imagined.

It wasn鈥檛 always distasteful to suggest that reality is, well, real. Before quantum physics, our understanding was governed by classical theories in which reality exists regardless of observers. Newton鈥檚 laws of motion, for example, say we live in a clockwork, deterministic world that behaves in well-defined, predictable ways independently from what we are doing.

The thin end of the wedge came in 1905, when Albert Einstein said that the photoelectric effect, in which certain metals give out electrons when illuminated, can only be explained if light is made up of quantum particles 鈥� photons, as they came to be called.

The thing was, light was known to be a wave. In the early 1800s, Thomas Young had done a version of the now classic double-slit experiment, in which light is shone at two parallel slits. The interference pattern formed on a screen beyond is what we would expect if waves of light were spreading outwards from both slits 鈥� behaviour that seems impossible if it is made of single particles.

So which is it then 鈥� particle or wave? Both, as versions of Young鈥檚 experiment have since confirmed. These involve light so dim that only one photon at a time passes through the double slit. Each photon lands on the screen at some seemingly random spot. Over time, however, these positions turn out not to be random; rather, the accumulated spots form an interference pattern, as if each photon were going through both slits and interfering with itself.

鈥淩eality鈥檚 nature is slippery, but that鈥檚 a big step from saying it doesn鈥檛 exist鈥�

Try to detect the photons鈥� path through one or the other slit, however, and the interference pattern disappears. Not only is light鈥檚 nature fundamentally ambiguous, but its guise seems determined by what we choose to measure. And as a young French physicist named Louis de Broglie proposed in 1924, it鈥檚 not just light. Experiments soon confirmed all the quantum particles that make up material reality have this dual nature, too.

Finding that reality鈥檚 true character is slippery is still a big step away from saying it doesn鈥檛 exist when we aren鈥檛 looking. Yet this is exactly what orthodox quantum mechanics says. In this picture, often called the Copenhagen interpretation after the Danish city where it took shape, a quantum object is represented by a mathematical wave function that allows us to make probabilistic predictions of what we will find when we measure things. Only on measurement does this wave function 鈥渃ollapse鈥� to reveal something localised in space and time. In the words of Werner Heisenberg, a pioneer of the Copenhagen interpretation, 鈥渢he idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them鈥� is impossible鈥�. That remains the dominant view to this day.

Yet it鈥檚 not the only possible interpretation (see 鈥�The many guises of quantum theory鈥�). De Broglie suggested another: that particles are real and have equally real waves associated with them. In this picture, when a particle goes through one of the double slits, its 鈥減ilot鈥� wave goes through both, interferes with itself, and then guides the particle to a location on the screen.

De Broglie presented his ideas at the 1927 Solvay Conference in Brussels, a legendary gathering of the early quantum greats. But he had not developed the theory mathematically, and it received a lukewarm reception. He quietly dropped the idea, becoming an adherent of the Copenhagen interpretation.

David Bohm was unaware of de Broglie鈥檚 work when, in the early 1950s, he developed a mathematically solid theory in which a wave with properties identical to that of the wave function guides particles around. 鈥淭his wave is a pilot wave,鈥� says physicist of Rutgers University in New Jersey, 鈥淚t choreographs the motion of the particles.鈥�

Bohm鈥檚 theory made exactly the same predictions as standard quantum theory. But the fact that you could only predict outcomes of experiments probabilistically was because you lacked knowledge about the particles鈥� initial state, not because nature doesn鈥檛 exist when you鈥檙e not looking. Bohm鈥檚 ideas made de Broglie revisit and revise his own pilot-wave theory. He developed a two-wave theory in which every particle rides a pilot wave, which in turn interacts with another wave that behaves like a wave function.

Spooky influences

Both of these pictures also explained another central feature of the quantum world 鈥� the way 鈥渆ntangled鈥� quantum objects seem to influence each other鈥檚 states instantaneously at a distance. Standard Copenhagen quantum mechanics provides no explanation for this non-locality, or 鈥渟pooky action鈥�, as Einstein dismissively referred to it. In the alternative picture, though, if particles are entangled, a common pilot wave guides them, and any change in the position or momentum of one particle instantly changes the pilot wave, thus influencing all the other particles. 鈥淭he fact that Bohmian mechanics is non-local is not a defect of the theory,鈥� says , Goldstein鈥檚 colleague at Rutgers. 鈥淚t is a feature that a true theory has to have.鈥�

In another world, Bohm鈥檚 work might have been seen as a breakthrough. But by the time the idea was published in 1952, he was already in exile. 鈥淎 lot of the reception of Bohm鈥檚 theory is tied up with that,鈥� says , a philosopher of physics at Columbia University in New York City. It didn鈥檛 help that Einstein, then in his twilight years and a vocal critic of quantum theory, dismissed it too. In a letter to Max Born, another quantum pioneer, he wrote: 鈥淏ohm believes (as de Broglie did, by the way, 25 years ago) that he is able to interpret the quantum theory in deterministic terms鈥� That seems too cheap to me.鈥� Bohmian mechanics entered the twilight zone of scientific theories 鈥� not quite dead, but not really a live concern either.

And there it has largely stayed, bar the odd finding that, if anything, hindered its revival. In 1992, for example, a thought experiment by physicist Marlan Scully of Texas A&M University and his colleagues showed that the theory made it possible for a particle to be measured passing through one slit in a double-slit experiment, but then land on the screen at a position that implied it had passed through the other. 鈥�,鈥� they wrote.

It was a series of unlikely experiments involving oil droplets that started to change some minds. In 2005, Yves Couder and Emmanuel Fort at Denis Diderot University in Paris stumbled upon a physical analogue of pilot waves. They discovered that if they let a millimetre-sized droplet of silicone oil fall on to a bath of the same oil that was vibrating up and down, the droplet would bounce indefinitely on the surface. And not just that: when it bounced the first time, it created a wave that it encountered on the next bounce, which gave the droplet a horizontal as well as a vertical kick. The bouncing droplet started wandering across the oil bath, guided by the very wave that it had created and helped sustain with each bounce.

The interesting thing was what happened when this wave-particle system encountered a barrier, a fraction of a millimetre below the surface, with two gaps in it: a double slit. The walking droplet went over one or the other slit, while its pilot wave went over both, and the wave pattern that emerged on the other side guided the droplet on. The researchers collected 75 such trajectories, and their analysis suggested the formation of an interference pattern on the far side of the slits. Despite there only ever being one particle-like droplet in the apparatus at any time, its pilot wave was causing it to acquire seemingly wave-like behaviour. If you couldn鈥檛 see the wave, the pattern built up over time would make you think the droplets had gone through both slits ().

It was clearly only an analogy, and attempts by other teams to repeat the work suggest that the supposed interference pattern might have been the product of air currents, as well as inadequate statistics. More recently, John Bush and his colleagues at the Massachusetts Institute of Technology have performed a more rigorous version of the experiment, with proper shielding from air currents. They found, once again, that the bouncing droplet creates a pilot wave that guides it on 鈥� and they discovered a second wave pattern. Created by the interaction of the droplet with the edge of the circular bath, this pattern in the droplet鈥檚 position emerges over time and has properties that mimic the wave function. This is just as in de Broglie鈥檚 more complex version of pilot-wave theory ().

The bouncing droplet experiments have allowed Couder鈥檚 and Bush鈥檚 teams to . For example, the statistics of the droplet鈥檚 seemingly chaotic movements bear an uncanny resemblance to those of an electron moving inside a corral of atoms. 鈥淣ow we have a macroscopic realisation of the physical picture suggested by de Broglie, and it exhibits many of the allegedly inscrutable features of quantum mechanics,鈥� says Bush. 鈥淭hat鈥檚 a hell of a coincidence.鈥�

Maybe 鈥� but there was still the problem of those contradictory, surrealist particle paths the alternative theory seems to allow. Last year, a refined version of the double-slit experiment conducted by Aephraim Steinberg of the University of Toronto, Canada, and his colleagues suggested that might not be quite such a problem after all. Brace yourself, because this is where things get really weird.

First, the researchers created pairs of photons with entangled polarisations. One photon of each pair was sent through the double slit, which was designed so that if the photon was vertically polarised it would go through slit A, and if horizontally polarised through slit B. The second photon served as a probe: thanks to the entanglement, measuring its polarisation was akin to knowing the polarisation of the photon passing through the slits, and thus which slit it must have gone through (see 鈥淩eally surreal鈥�).

This set-up gave the team two bites at the same cherry: they were able to determine the travelling photon鈥檚 position as it went through the apparatus, and could also measure the polarisation state of the associated probe photon. They did this with tens of thousands of photon pairs, and found that, on average, at the moment a photon passed through slit A, the probe photon would be vertically polarised, as expected. But at the screen, things were a lot more ambiguous. When a travelling photon was measured at a position on the screen corresponding to having passed through slit A, half the time the polarisation of the probe photon was horizontal 鈥� suggesting that the travelling photon had passed through slit B. These were seemingly surreal trajectories, unmasked ().

Reality regained

What鈥檚 happening? In a word, non-locality. The experiment shows that the moving photon is constantly changing the polarisation of the probe photon. Look at the probe photon at the moment the moving photon goes through a slit, and there is no contradiction. But look at it the moment the moving photon hits the screen and, half of the time, its polarisation state has changed. This sort of non-locality is admissible in standard quantum theory, but it is baked into Bohm鈥檚 version. The experiment is by no means a proof of Bohm鈥檚 theory, but it shows that its prediction of surreal trajectories cannot be used to debunk it.

So Bohmian mechanics can and should remain a contender, says Albert. 鈥淎ny realist picture is preferable to any anti-realist picture,鈥� he says.

But winning hearts and minds will still be a struggle. For a start, Bohmian mechanics is formulated to replicate the predictions of standard quantum mechanics: experimentally, it鈥檚 almost impossible to tell them apart. Also, the theory is mathematically fleshed out only for particles travelling far slower than the speed of light. Quantum mechanics, in contrast, has been extended to embrace relativistic particles travelling close to the speed of light, and so forms the basis of quantum field theory and the standard model of particle physics. 鈥淐lean, worked-out Bohmian versions of those things do not exist,鈥� says Goldstein.

For , a physicist and historian of science at MIT, that may be the theory鈥檚 Achilles heel. 鈥淢y aesthetic concern is that it feels, in the original description at least, horribly non-relativistic, anti-relativistic,鈥� he says.

Goldstein and his colleagues have been trying since the mid-1990s to marry Bohm鈥檚 ideas with Einstein鈥檚 special relativity. The hardest part is to accommodate the instantaneous interactions of Bohmian mechanics. That鈥檚 at odds with relativity鈥檚 limit on how fast influences can spread 鈥� namely, the speed of light. What鈥檚 more, relativity does not distinguish points in space as being in any one present. Goldstein and his colleagues have tried to get around this, showing that a Bohmian wave function can , and that events on any one foliation are simultaneous, leading to non-locality. It鈥檚 the most sophisticated approach yet 鈥� but also very much still a work in progress.

When Goldstein started learning standard quantum mechanics in the 1960s, he was seduced by its mystery and spookiness, he says 鈥� only to realise gradually that Bohm鈥檚 ideas made more sense. Besides, he says, Bohm鈥檚 ideas have their own sense of mystery: the way in which every entangled particle influences every other particle in the universe, and the fact that the wave function is a new kind of entity. 鈥淵ou have still got romance,鈥� he says. 鈥淚t鈥檚 in the right place now. Not misplaced.鈥�

In the end, though, it鈥檚 not about winning over minds, but being open to the Bohmian picture, says Steinberg. 鈥淭he best thing experiments like ours can do is to remind people that the interpretation exists,鈥� he says. 鈥淧eople aren鈥檛 aware of it, and we want to bring more attention to it.鈥� Bush feels similarly about his walking-droplet experiments. 鈥淭hat鈥檚 why I鈥檓 a believer in this venture, even if its sole result is to get young people to question their views on quantum mechanics,鈥� he says.

This article appeared in print under the headline 鈥淪peaking in ripples鈥�