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NATURE gives rise to weird and wonderful things: dancing plants, sailing stones, pink flamingos. But no one, except perhaps on a hallucinogenic trip, has seen a flamingo melt into a wave or split itself into multiple copies. And that may be the weirdest thing of all, since our best theory of nature seems to suggest those things could happen.

That theory is quantum mechanics. Despite its spectacular success accounting for the bizarre behaviour of subatomic particles, it鈥檚 not clear how, or even if, it can explain why much larger bodies don鈥檛 behave in a similarly strange way. This is one reason why Einstein, among others, never accepted quantum theory as the ultimate description of nature.

Now a new experiment has seen a hint that these quantum critics may be right. The result must still be corroborated by many other tests, some now getting under way, but there鈥檚 no overstating the significance if it is shown to be correct. 鈥淚t would be revolutionary,鈥� says physicist and Nobel laureate at the University of Illinois at Urbana-Champaign. 鈥淚t would shatter the notion that quantum mechanics is the whole story about the physical world.鈥�

The real problem with quantum mechanics is simply stated. 鈥淲hat the hell is it about?鈥� says physicist of Rutgers University in New Jersey. Quantum mechanics describes subatomic particles using undulating mathematical objects called wave functions, which evolve smoothly over time. A particle described by a wave function is more potentiality than point. It exists in superposition, meaning, roughly, that it is smeared out in space or is in many places at once. How can such particles be the building blocks of solid stuff like flamingos?

The traditional answer comes from the Copenhagen interpretation of quantum mechanics. It claims that observations, or measurements, are the key. They cause wave functions to collapse and macroscopic properties such as position and velocity to crystallise from the microscopic murk. Schr枚dinger鈥檚 cat exemplifies the process: the Copenhagen interpretation suggests it exists in purgatory, in a half-alive, half-dead state until measurement determines its fate.

But there鈥檚 a catch. 鈥淣o one has ever been able to define a measurement,鈥� says physicist Philip Pearle of Hamilton College in Clinton, New York. And as physicist John Stewart Bell once wrote, 鈥淲hat exactly qualifies some physical systems to play the role of 鈥榤easurer鈥�? Was the wave function of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for some better qualified system鈥� with a PhD?鈥�

Quantum theory co-founder Niels Bohr, an arch-advocate of the Copenhagen interpretation, answered this 鈥渕easurement paradox鈥� by getting philosophical. 鈥淚t is wrong to think that the task of physics is to find out how nature is,鈥� he said. 鈥淧hysics concerns what we can say about nature.鈥�

Bohr believed quantum mechanics means that physics can鈥檛 answer questions about things that can鈥檛 be seen 鈥� a stance called anti-realism, or 鈥渟hut up and calculate鈥�. In this view, quantum paradoxes stem from a futile attempt to picture reality. You can imagine, but never see, Schr枚dinger鈥檚 cat in its dead-and-alive state, so it is pointless to speculate about the reality of the situation. It鈥檚 enough to be satisfied by the success of quantum mechanics in describing the world and enabling new technologies, from lasers to computers.

Philosopher of physics at the University of Oxford doesn鈥檛 buy that. 鈥淚f quantum theory doesn鈥檛 tell us what goes on inside molecules and atoms,鈥� he says, 鈥渢hen we better find another theory that does.鈥�

Alternative interpretations of quantum mechanics that preserve reality do exist. Saunders subscribes to the 鈥渕any worlds鈥� interpretation, which eliminates the measurement paradox by trading Copenhagen鈥檚 collapse of the wave function for a process in which the universe sprouts ever-multiplying branches to accommodate all the possibilities the wave function embodies. In this picture, Schr枚dinger鈥檚 cat lives in some worlds and dies in others. Goldstein, meanwhile, is an advocate of Bohmian mechanics. This adds in some mathematical flourishes to argue that superpositions are only apparent, meaning that particles always have well-defined positions and velocities, and can never spawn half-alive, half-dead animals of any kind.

Both these interpretations offer a view of the reality underneath quantum mechanics, but the pictures they paint are as different as a flamingo and a dancing plant. What鈥檚 worse is that both they and Copenhagen make identical predictions. So no experiment can choose between the three 鈥� another reason why physicists are inclined to slough off the whole business of explaining quantum reality to philosophers.

A small minority is rebelling. 鈥淯sually when there are paradoxes it鈥檚 not ingenious interpretations that solve them,鈥� says of the Institute for Advanced Study in Princeton, New Jersey. 鈥淚t鈥檚 just a fact that the laws of physics have a limited domain, which we don鈥檛 know the boundaries of. When you go beyond the boundaries, you find new physics.鈥�

Since the motivation for such new physics is the dissimilarity between the quirky microscopic world and the mundane macroscopic one, such a missing link, if any exists, must be hiding somewhere in the middle: between the scale of a molecule and the macro world.

According to Adler, Pearle and other rebels, such a link might be provided by a physical mechanism that can collapse wave functions without measurement. Models based on this 鈥渙bjective collapse鈥� were first developed in the 1970s and 1980s. In the most popular version, the missing link is a kind of noise that ripples continuously throughout space, much like an electromagnetic field. It 鈥減ings鈥� particles, prompting their wave functions to collapse rather as if there were a matrix of tiny detectors densely dotting the cosmos.

That might explain a lot. Subatomic particles such as protons would be small enough to slide through the matrix for many years without getting pinged. But macroscopic things would bump so frequently against the matrix that they could survive in superposition for no more than a sliver of a second before being pinged to their rightful place in macro reality.

Such a pervasive yet unobserved physical entity sounds far-fetched, but physics often makes progress by postulating things that sound preposterous yet turn out to be real. Antimatter, for example, popped up in equations before it was found in cosmic rays. The proposal of the Higgs boson as a missing link in existing theories of particle physics prompted a 50-year hunt for it, which was ultimately successful.

And unlike other solutions to the measurement problem, this alternative cause of collapse can be put to the test. One way is to take particles in superposition and pass them through a pair of parallel slits. This classic quantum experiment shows the smeared-out nature of single particles: they diffract at both slits and interfere as if they were a wave, producing an alternating pattern of light and dark on a screen placed behind.

Quantum theory says this can be done with arbitrarily large particles in flight for arbitrarily long times. Collapse theory says that big enough particles over long enough times will be pinged out of wavy behaviour and back into mundane dust, flattening the peaks and valleys of the interference pattern.

Quantum la-la land

Molecules as massive as 10,000 protons have already been made to interfere with no sign of objective collapse. Extrapolating from that tells us that any such effect must be weak enough to allow a dust speck, about a million billion times more massive than a proton, to stay in superposition for at least 10^-18 seconds. To solve the measurement problem and allow the macro world to emerge quickly enough from the micro one that the transition is inconspicuous, the collapse noise must limit dust鈥檚 time in quantum la-la land to at most 10^-7 seconds.

That leaves a vast range of 11 orders of magnitude for the quantum lifetime of dust. Angelo Bassi at the University of Trieste in Italy calls it the 鈥済rand desert鈥� and is one of a handful of physicists exploring it (see聽below). With a new kind of test, they are making surprisingly quick work of shrinking its boundaries.

The test is based on the way objective collapse shakes up particles. 鈥淭hey should move more than what you expect from standard quantum mechanics,鈥� says Bassi. This effect was unwittingly put to the test in 2016 by an experiment to check equipment for sensing gravitational waves (see 鈥�Einstein鈥檚 golden test鈥�). It returned a negative result, and reduced the range of times within which objective collapse can happen to just nine orders of magnitude.

Soon after, though, a humble tabletop test went one order of magnitude further into the desert 鈥� and may have struck oil. Questioning quantum mechanics is a rebellious act, so Andrea Vinante at the University of Southampton, UK, did his experiment without funding, borrowing a high-tech refrigerator and drafting friends to make a metal bead the size of a blood cell. He fixed the bead to a 0.5-millimetre-long silicon cantilever and cooled it to a fraction of a degree above absolute zero while monitoring its motion.

Temperature and vibration are one and the same, so Vinante and his collaborators, including Bassi, should have been able to quieten the bead鈥檚 quivering as much as they liked by lowering its temperature. Instead, they found a limit beyond which the trembling didn鈥檛 lessen any more. They published their results in Physical Review Letters late last year, describing 鈥溾��.

Could it have been collapse noise? Adler is cautiously optimistic: more than a decade ago, he suggested that collapse noise would have a similar magnitude to the observed effect. 鈥淚f I were to make a bet, my bet is that a noise will be found at the enhanced value that I proposed,鈥� he says. 鈥淏ut I could be wrong. Nature decides.鈥�

Vinante himself is just cautious. 鈥淭here are other possible subtle explanations which are very difficult to test,鈥� he says. For example, that part of the apparatus may not have been cooled to the measured temperature and so could have produced errant vibrations.

We could soon know if that is the case. The European Commission has recently awarded a 鈧�4.4 million grant to the first major experiment dedicated to objective collapse. Known as TEQ, for , the collaboration includes Bassi and Vinante among its members. The experiment, based in Southampton, will take three years to set up. It will cool and levitate virus-sized glass spheres in a magnetic field, monitoring their motion with lasers and improving the sensitivity of Vinante鈥檚 cantilever by two to three orders of magnitude. Since collapse noise is proportional to mass, the experiment will use spheres of different masses to help distinguish it from less exciting kinds.

A confirmed collapse signal would wipe untold blackboards clean at a stroke. String theory and other attempts at a 鈥渢heory of everything鈥� that would unify Einstein鈥檚 general relativity with quantum theory rely on our current conception of quantum mechanics being right. Realism would be saved, but the nature of reality would depend on the source of the collapse noise, for which an epic new search would begin. A whole new connection to general relativity could emerge: one suggestion from Roger Penrose at the University of Oxford and others is that gravity could be the entity that triggers wave function collapse.

If TEQ and other tests rule out objective collapse, new physics could still solve the measurement problem. The solution, says Leggett, just might require 鈥渃oncepts which right now we simply don鈥檛 anticipate鈥�.

In that case, interference experiments could be back in the picture 鈥� but this time in space. These experiments directly test superposition and should therefore see any effects capable of solving the measurement paradox. The advantage of doing them in space is that large masses take longer to interfere than small ones. On Earth, large masses fall and hit the ground before they can, but in space they can be left to fall pretty much indefinitely. The MAQRO () experiment was proposed for this purpose by Rainer Kaltenbaek at the Austrian Academy of Sciences in 2010. After years of seeming uninterested in the idea, the European Space Agency is now evaluating it.

And what if none of these experiments reveals anything new? Then the proponents of various existing interpretations will continue to press their case. Goldstein, for example, is hopeful that Bohmian mechanics will lead to some theoretical breakthrough, while Saunders believes that putting macroscopic objects in superpositions in the future will win sceptics over to his view of branching universes. 鈥淚t鈥檚 not easy to believe in many worlds,鈥� he says. 鈥淔amiliarity helps.鈥�

Leggett isn鈥檛 having it. If no new physics is found, he says, the only consistent approach is to give up on realism altogether and retreat to something like Bohr鈥檚 view. 鈥淓nd of story,鈥� he says. Perhaps 鈥� or the debate will rage on, fuelled by our desire to understand reality and our different opinions about what version of it is most believable.

This article appeared in print under the headline 鈥淩eality鈥檚 whispers鈥�