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through both slits and interfering with itself. Try to detect the photons’ path through one or the other slit, however, and the interference pattern disappears. Not only is light’s 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’s not just light. Experiments soon confirmed all the quantum particles that make up material Speaking reality have this dual nature, too. Finding that reality’s true character is slippery is still a big step away from saying it doesn’t exist when we aren’t looking. Yet this is exactly what orthodox quantum in ripples mechanics says. In this picture, often called the Copenhagen interpretation after the Unseen influences may explain Danish city where it took shape, a quantum object is represented by a mathematical wave the mysteries of quantum reality, function that allows us to make probabilistic predictions of what we will find when we says Anil Ananthaswamy measure things. Only on measurement does this wave function “collapse” to reveal “Reality’s nature is slippery, but that’s a big step from saying it doesn’t exist” N OCTOBER 1951, physicist David Bohm regardless of observers. Newton’s laws left the US for Brazil. Branded a communist of motion, for example, say we live in a something localised in space and time. In the Isympathiser, he had been arrested for clockwork, deterministic world that words of Werner Heisenberg, a pioneer of the refusing to testify to the US Congress. behaves in well-defined, predictable ways Copenhagen interpretation, “the idea of an Acquitted, he was still stripped of his Princeton independently from what we are doing. objective real world whose smallest parts exist professorship. His departure began an exile The thin end of the wedge came in objectively in the same sense as stones or trees that would last until his death, as a naturalised 1905, when Albert Einstein said that the exist, independently of whether or not we British citizen, four decades later. photoelectric effect, in which certain metals observe them… is impossible”. That remains The theory Bohm was nurturing as he left give out electrons when illuminated, can only the dominant view to this day. his native shores has spent even longer in the be explained if light is made up of quantum Yet it’s not the only possible interpretation cold. In part, that’s down to politics. But his particles – photons, as they came to be called. (see “The many guises of quantum theory”, ideas also seemed scientifically beyond the The thing was, light was known to be a page 31). De Broglie suggested another: that pale. Bohm proposed there was a hidden wave. In the early 1800s, Thomas Young had particles are real and have equally real waves reality to quantum theory, meaning its crazy done a version of the now classic double-slit associated with them. In this picture, when a predictions of a world that doesn’t exist until experiment, in which light is shone at two particle goes through one of the double slits, you choose to look at it are just that: crazy. parallel slits. The interference pattern formed its “pilot” wave goes through both, interferes That went against the established grain, and on a screen beyond is what we would expect with itself, and then guides the particle to a still does. But more than six decades on, Bohm if waves of light were spreading outwards location on the screen. is getting a fresh hearing, as new experiments from both slits – behaviour that seems De Broglie presented his ideas at the 1927 are hinting that he might have been on to impossible if it is made of single particles. Solvay Conference in Brussels, a legendary something. If so, some aspects of reality So which is it then – particle or wave? gathering of the early quantum greats. But he would become easier to fathom, while others Both, as versions of Young’s experiment had not developed the theory mathematically, would be harder to stomach. Forget standard have since confirmed. These involve light and it received a lukewarm reception. He quantum weirdness – the world Bohm revealed so dim that only one photon at a time passes quietly dropped the idea, becoming an is a more profoundly and mysteriously through the double slit. Each photon lands adherent of the Copenhagen interpretation. interconnected place than we ever imagined. on the screen at some seemingly random David Bohm was unaware of de Broglie’s It wasn’t always distasteful to suggest spot. Over time, however, these positions work when, in the early 1950s, he developed a that reality is, well, real. Before quantum turn out not to be random; rather, the mathematically solid theory in which a wave physics, our understanding was governed accumulated spots form an interference with properties identical to that of the wave by classical theories in which reality exists pattern, as if each photon were going sam chivers function guides particles around. “This >

28 | NewScientist | 8 April 2017 8 April 2017 | NewScientist | 29 wave is a pilot wave,” says physicist Sheldon more rigorous version of the experiment, Goldstein of Rutgers University in New Jersey, The many guises of with proper shielding from air currents. They “It choreographs the motion of the particles.” quantum theory found, once again, that the bouncing droplet Bohm’s theory made exactly the same creates a pilot wave that guides it on – and predictions as standard quantum theory. But Why does reality only seem to coalesce into a definite they discovered a second wave pattern. the fact that you could only predict outcomes Created by the interaction of the droplet with of experiments probabilistically was because state when you make a measurement? The answer the edge of the circular bath, this pattern in you lacked knowledge about the particles’ depends on your preferred view of the quantum world the droplet’s position emerges over time and initial state, not because nature doesn’t exist has properties that mimic the wave function. when you’re not looking. Bohm’s ideas made Copenhagen interpretation Quantum Bayesianism This is just as in de Broglie’s more complex de Broglie revisit and revise his own pilot- The “shut up and calculate” view: Quantum uncertainty is not intrinsic version of pilot-wave theory (Journal of Fluid wave theory. He developed a two-wave theory the quantum world does not exist to reality – it has to do with your own Mechanics, DOI: 10.1017/jfm.2016.537). in which every particle rides a pilot wave, in any meaningful sense without lack of knowledge about whatever you The bouncing droplet experiments have which in turn interacts with another wave measurements. are attempting to measure. allowed Couder’s and Bush’s teams to observe

that behaves like a wave function. T behaviour usually seen only in quantum Many worlds interpretation Information systems. For example, the statistics of the sh, MI u Make a measurement and the When you measure something, droplet’s seemingly chaotic movements hn B

Spooky influences o

d J universe splits, taking you into the you extract some physical form of bear an uncanny resemblance to those of Both of these pictures also explained another parallel world where you got the information from it, forcing it into a an electron moving inside a corral of atoms. central feature of the quantum world – the arris an result you did. high-definition state. “Now we have a macroscopic realisation of way “entangled” quantum objects seem to the physical picture suggested by de Broglie,

influence each other’s states instantaneously f Daniel H Objective collapse Bohmian mechanics and it exhibits many of the allegedly y o

at a distance. Standard Copenhagen quantum es Spread-out quantum states are Reality is guided by pilot waves; inscrutable features of quantum mechanics,” t mechanics provides no explanation for this r collapsing into definite states all the measurement just discovers what says Bush. “That’s a hell of a coincidence.” ou non-locality, or “spooky action”, as Einstein time. Your clodhopping measurement reality is up to, in the same way as Maybe – but there was still the problem of mages c dismissively referred to it. In the alternative I just helps things on their way. classical physics (see main story). those contradictory, surrealist particle paths picture, though, if particles are entangled, the alternative theory seems to allow. Last a common pilot wave guides them, and any year, a refined version of the double-slit change in the position or momentum of one a philosopher of physics at Columbia And there it has largely stayed, bar the odd It was a series of unlikely experiments which gave the droplet a horizontal as well as experiment conducted by Aephraim Steinberg particle instantly changes the pilot wave, thus University in New York City. It didn’t help that finding that, if anything, hindered its revival. involving oil droplets that started to change a vertical kick. The bouncing droplet started of the University of Toronto, Canada, and his influencing all the other particles. “The fact Einstein, then in his twilight years and a vocal In 1992, for example, a thought experiment some minds. In 2005, Yves Couder and wandering across the oil bath, guided by the colleagues suggested that might not be quite that Bohmian mechanics is non-local is not critic of quantum theory, dismissed it too. In a by physicist Marlan Scully of Texas A&M Emmanuel Fort at Denis Diderot University very wave that it had created and helped such a problem after all. Brace yourself, a defect of the theory,” says Roderich Tumulka, letter to Max Born, another quantum pioneer, University and his colleagues showed that in Paris stumbled upon a physical analogue sustain with each bounce. because this is where things get really weird. Goldstein’s colleague at Rutgers. “It is a feature he wrote: “Bohm believes (as de Broglie did, the theory made it possible for a particle to of pilot waves. They discovered that if they The interesting thing was what happened First, the researchers created pairs of that a true theory has to have.” by the way, 25 years ago) that he is able to be measured passing through one slit in a let a millimetre-sized droplet of silicone oil fall when this wave-particle system encountered photons with entangled polarisations. One In another world, Bohm’s work might have interpret the quantum theory in deterministic double-slit experiment, but then land on on to a bath of the same oil that was vibrating a barrier, a fraction of a millimetre below the photon of each pair was sent through the been seen as a breakthrough. But by the time terms… That seems too cheap to me.” the screen at a position that implied it had up and down, the droplet would bounce surface, with two gaps in it: a double slit. The double slit, which was designed so that if the the idea was published in 1952, he was already Bohmian mechanics entered the twilight passed through the other. “Tersely: Bohm indefinitely on the surface. And not just that: walking droplet went over one or the other slit, photon was vertically polarised it would go in exile. “A lot of the reception of Bohm’s zone of scientific theories – not quite dead, trajectories are not realistic, they are when it bounced the first time, it created a while its pilot wave went over both, and the through slit A, and if horizontally polarised theory is tied up with that,” says David Albert, but not really a live concern either. surrealistic,” they wrote. wave that it encountered on the next bounce, wave pattern that emerged on the other side through slit B. The second photon served guided the droplet on. The researchers as a probe: thanks to the entanglement, collected 75 such trajectories, and their measuring its polarisation was akin to Really surreal analysis suggested the formation of an knowing the polarisation of the photon Quantum double-slit experiments tell us that nothing is quite as it seems interference pattern on the far side of the passing through the slits, and thus which slits. Despite there only ever being one slit it must have gone through (see “Really Measurements at the slits detect single photons Allow the photons to reach the screen and an Now introduce an entangled "probe" photon that Take measurements at the screen, however, and particle-like droplet in the apparatus at surreal”, below left). passing through one slit or the other: light is made interference pattern develops over time: light tells us which slit its partner photon passed through. half the time they disagree: the state of the probe any time, its pilot wave was causing it to This set-up gave the team two bites at the of particles is a wave, and it passes through both slits Measure at the slits again, and the states of the two suggests the travelling photon went through one photons must agree slit, but its position on the screen implies it passed acquire seemingly wave-like behaviour. same cherry: they were able to determine the through the other – a seemingly surreal trajectory If you couldn’t see the wave, the pattern travelling photon’s position as it went through built up over time would make you think the apparatus, and could also measure the the droplets had gone through both slits polarisation state of the associated probe (Physical Review Letters, vol 97, p 154101). photon. They did this with tens of thousands It was clearly only an analogy, and attempts of photon pairs, and found that, on average, by other teams to repeat the work suggest at the moment a photon passed through that the supposed interference pattern might slit A, the probe photon would be vertically have been the product of air currents, as well polarised, as expected. But at the screen, LASER PROBE PHOTON as inadequate statistics. More recently, John things were a lot more ambiguous. When PROBE

INTERFERENCE PATTERN Bush and his colleagues at the Massachusetts a travelling photon was measured at a SCREEN PHOTON Institute of Technology have performed a position on the screen corresponding to >

30 | NewScientist | 8 April 2017 8 April 2017 | NewScientist | 31

COPY SUB PAGE SUB G_Quantum_comeback OK for press having passed through slit A, half the time the polarisation of the probe photon was horizontal – suggesting that the travelling photon had passed through slit B. These were seemingly surreal trajectories, unmasked (Science Advances, vol 2, p e1501466).

Reality regained What’s 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 T slit, and there is no contradiction. But look at sh, MI it the moment the moving photon hits the u hn B o

screen and, half of the time, its polarisation d J state has changed. This sort of non-locality is

admissible in standard quantum theory, but it arris an is baked into Bohm’s version. The experiment

is by no means a proof of Bohm’s theory, but it f Daniel H y o

shows that its prediction of surreal trajectories es t cannot be used to debunk it. r ou So Bohmian mechanics can and should mages c remain a contender, says Albert. “Any realist I picture is preferable to any anti-realist picture,” he says. But winning hearts and minds will Brain death showing that a Bohmian wave function can still be a struggle. For a start, Bohmian create structures or “foliations” in space-time, mechanics is formulated to replicate the Quantum mechanics gives a bizarre and that events on any one foliation are predictions of standard quantum mechanics: twist to that old trope about a monkey simultaneous, leading to non-locality. It’s experimentally, it’s almost impossible to tell at a typewriter with infinite time. the most sophisticated approach yet – but them apart. Also, the theory is mathematically Ditch the monkey, and consider also very much still a work in progress. fleshed out only for particles travelling far quantum fluctuations in an everlasting When Goldstein started learning standard slower than the speed of light. Quantum universe. They could at some point quantum mechanics in the 1960s, he was mechanics, in contrast, has been extended to spontaneously form anything, even a seduced by its mystery and spookiness, he embrace relativistic particles travelling close brain. If one such “Boltzmann brain” says – only to realise gradually that Bohm’s to the speed of light, and so forms the basis exists, it’s likely that many others do. ideas made more sense. Besides, he says, of quantum field theory and the standard In fact, if we live in such a universe, Bohm’s ideas have their own sense of mystery: model of particle physics. “Clean, worked-out it’s likely that our brains are this kind. the way in which every entangled particle Bohmian versions of those things do not That sounds nonsensical. “If a influences every other particle in the universe, exist,” says Goldstein. theory predicts that the majority of and the fact that the wave function is a new For David Kaiser, a physicist and observers are Boltzmann brains, that’s kind of entity. “You have still got romance,” historian of science at MIT, that may be bad for the theory,” says Roderich he says. “It’s in the right place now. the theory’s Achilles heel. “My aesthetic Tumulka at Rutgers University. Not misplaced.” concern is that it feels, in the original Standard quantum mechanics says In the end, though, it’s not about winning description at least, horribly non-relativistic, that an infinitely enduring universe over minds, but being open to the Bohmian anti-relativistic,” he says. exists in a “superposition” of all picture, says Steinberg. “The best thing Goldstein and his colleagues have been possible states, including those with experiments like ours can do is to remind trying since the mid-1990s to marry Bohm’s Boltzmann brains. But in Bohmian people that the interpretation exists,” he ideas with Einstein’s special relativity. mechanics (see main story) such a says. “People aren’t aware of it, and we The hardest part is to accommodate the universe evolves towards a static want to bring more attention to it.” Bush instantaneous interactions of Bohmian state. The probability of that state feels similarly about his walking-droplet mechanics. That’s at odds with relativity’s being one with Boltzmann brains is experiments. “That’s why I’m a believer in limit on how fast influences can spread – minuscule, and even if it is, nothing this venture, even if its sole result is to get namely, the speed of light. What’s more, is changing so the brain can’t be young people to question their views on relativity does not distinguish points in space functional. “It’s much more likely that quantum mechanics,” he says. n as being in any one present. Goldstein and there are no Boltzmann brains, and his colleagues have tried to get around this, then it stays that way,” says Tumulka. Anil Ananthaswamy is a consultant for New Scientist

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