Zombie universe

Did the quantum world freeze into weirdness at the big bang, asks Jon Cartwright
AS ENDINGS go, it is a bit of an anticlimax. As the universe enters old age, its stars burn out. Slowly, the temperature across the cosmos reaches equilibrium. With no heat flowing, thermodynamic laws make it impossible to transfer energy in a useful way. Nothing interesting or productive happens any more. Everything creaks to a standstill. This “heat death” of the universe was a favoured topic of the gloomier sort of 19thcentury physicist. These days, we console ourselves that, if it is to happen, it will not be for many, many multiples of the current age of the universe. Antony Valentini, a theoretical physicist at Clemson University in South Carolina, is less sanguine. For the past two decades, he has championed the idea that something like heat death has already happened – not in our layer of reality, admittedly, but on an underlying level that we are hard-pressed to see. Fundamental physics is not short of eccentric and unworkable proposals, and it is easy to dismiss such a bold suggestion. But there are aspects of Valentini’s idea that make some of his peers believe he might just be on to something. Just as a thermodynamic heat death would prevent us from doing anything useful with energy in the distant future,  if Valentini’s “quantum death” has happened, it could explain our puzzling inability to fully get to grips some of aspects of nature – those to do with quantum behaviour. “He’s well respected and taken seriously,” says Carlo Rovelli of Aix-Marseille University in France. Now Valentini thinks he may have seen the first evidence for this theory, etched in the afterglow of the big bang. Strange as it might seem, quantum death might breathe new life into our understanding of reality. It is almost 90 years since luminaries of theoretical physics, among them Albert EDWARD KINSELLA Einstein, Niels Bohr, Werner Heisenberg and Erwin Schrödinger, gathered in Brussels to try and make sense of the bizarre results then emerging from atomic physics. At the 1927 Solvay conference, it was becoming clear that subatomic entities such as electrons could appear both as localised particles or as fuzzy, spread-out waves. Which one you saw depended on how you measured them. Quite generally, quantum objects seemed to exist in a haze of indecision before anyone observed them. The newly apparent quantum property of spin, for instance, could take one of two values, conventionally termed “up” and “down”. Until you tried to pin down the spin of something like an electron, it seemed to have both values at once, randomly choosing which guise to reveal only at the final moment. This fuzziness, it later became clear, even extends from one quantum entity to another. If two electrons are born together, measuring one appears to instantaneously alter the state of the other, regardless of whether they are separated by metres, kilometres or even light years. Einstein in particular was not a fan of this “entanglement”, damning it with the phrase “spooky action at a distance”. Shut up and calculate Mathematically, though, all of this was no problem. As the pioneers of quantum physics showed, a quantum system, be it a single electron or an entangled pair, could be described by a “wave function” containing information on all the system’s possible properties such as spin. Just like tossing a die, you couldn’t be certain which side of the wave function would show itself when you made a measurement. But a deft mathematical trick – simply squaring the wave function – made it possible to calculate the probability “ Quantum death could explain our puzzling inability to get to grips with quantum reality” that different sides would show up. Such sleights of hand have allowed us to build solid technologies on fuzzy quantum foundations, from lasers to computers, solar cells and nuclear reactors. But we are left wondering what it all means. Before measurement, is an electron really a smearedout cloud of probability, everything and nothing at once, as the wave function suggests? And how can it know what its partner is doing on the other side of the room – or galaxy? After the 1927 meeting, most physicists settled on a common answer. Known fondly as the “shut up and calculate” school of thought, it is more formally the Copenhagen interpretation, after the institution at which Bohr, its prime mover, worked. It says that quantum mechanics is just a tool to help us predict the goings-on of the world, rather  than necessarily being a description of reality itself. It works – just don’t ask why. Valentini is not the first to think this is a cop-out. The 1927 meeting gave air to several rival interpretations – a fact often glossed over, he says, because historical accounts were written mainly by advocates of the victorious Copenhagen interpretation, such as Heisenberg and Bohr. “The standard history you read in the textbooks is very skewed,” he says. Valentini’s visions of quantum death began with one of those early rival theories. It was the brainchild of the French physicist Louis de Broglie, and unlike the Copenhagen interpretation makes a clear statement about what is real in the quantum world. Every particle exists in a definite location and with definite properties at all times, and is guided by an equally real “pilot wave”. Entangled electrons are linked by a pilot wave so that a wiggle at one end – the manipulation of one of the electrons during a spin measurement, for instance – causes an instantaneous wiggle at the other, changing the other electron’s properties, too. What we see as spooky action at a distance is the result of a complex, tangled web of pilot waves linking things on a level hidden from view. The attraction of pilot-wave theory à la de Broglie is that it makes exactly the same predictions as standard quantum mechanics, and so like that theory agrees with all experimental results to date. But this is a double-edged sword: it means there is also no way to test whether it is a better description of reality than quantum mechanics. Given that the theory suggests mysterious, inscrutable layers of reality, most physicists have preferred to stay tight-lipped in Copenhagen. But something left Valentini unsatisfied. Einstein’s reservations notwithstanding, there is now no serious doubt spooky action exists: we exploit connections between entangled particles to create virtually uncrackable techniques for transferring information securely. The strange thing is that, although we know any “communication” between the particles must occur at many thousands of times the speed of light, we can’t exploit the connection to actually send messages that fast. This central feature of quantum theory is, for us, strangely redundant. Einstein’s castiron rule that nothing physical moves faster than light speed remains. The reason, Valentini first realised in the early 1990s, lies in the probabilities that come from squaring the wave function. Do this calculation for many pairs of entangled electrons, and each electron in the pairs will turn out to be spin-up in exactly half of the instances, and spin-down in the other half. This equal split is crucial. Were it anything else – 100:0, 80:20, or even 51:49 – tampering with the electrons on one side could induce a noticeable change in those on the other that would count as an instant transfer of information (see diagram, right).  Such a 50:50 split seems an extraordinarily finely poised state for the universe to assume. In Valentini’s eyes, it cried out for a physical mechanism to make it just so. Orthodox quantum mechanics says the universe runs purely on probability: it simply is the way it is with no reason to believe it hasn’t always been like that. Pilot waves would make things a little different. This is where thermodynamics comes in. Look at individual molecules in a canister of gas, and they are likely all to be at the same temperature and spread out over the available volume, in the same sort of deathly equilibrium the whole universe will some day seek out. Equally, experience tells us it is unlikely the molecules started out like this. When first injected into the canister, they would have been in some non-equilibrium

Out of the valley of death

Allow information to travel faster than light speed, and almost anything becomes possible. You can communicate instantaneously across galaxies, tap even the most secure quantum network and perhaps even build number-crunchers so powerful they would surpass even our wildest dreams of a super-powerful quantum computer. In Antony Valentini’s model of quantum death (see main story), such mayhem would have been over within the first instant of the universe’s birth. But perhaps, as Valentini proposed in 2001, some particles managed to pull away from the mob in the aftermath of the big bang, and so avoid sudden quantum death. To do this, such relic particles would have had to be incredibly elusive – every sort of particle that we have detected so far, even the slippery neutrino, interacts with other matter too strongly. One possibility is the graviton, the hypothetical, massless particle responsible for the force of gravity. The chances of detecting one of these in the foreseeable future is next to zero, given the vast detectors we must already build to bag a handful of neutrinos. A more likely possibility is a partner of the graviton, the gravitino. While these are also thought to be near impossible to detect directly, they could decay into other particles, such as photons, that satellite observatories might spot. These decay products ought to inherit the quirks of the universe before quantum death. Catch a bunch, entangle them and share them between two distant parties, and you would have a recipe for faster-than-light communication.

state, perhaps concentrated near the inlet. In the same way, Valentini argues,  the big bang produced a universe in a nonequilibrium quantum state. In those first moments, particle properties could have  been highly ordered, with all the up spins  in one place, and all the down spins in  another. But the intense heat and violence made for an extraordinarily tangled web of pilot waves that would naturally want to  “ The tangled early universe would naturally have wanted to relax” relax into a simpler state. Within a tiny fraction of a second, the pilot waves piloted the universe there. Unlike the long throes of thermodynamic death, quantum death came in an instant. Valentini’s extension of de Broglie’s pilotwave theory shows that those suspicious equal-probability splits, and our related inability to make use of spooky action at a distance, are not conspiracy or fine-tuning. They are the natural end for a universe starting out in any other quantum configuration. Most physicists have no truck with any situation in which information can be transferred faster than light speed, which would have been the case in Valentini’s early universe. It would overturn cherished concepts that have proved their worth over the past century, such as the idea that time is relative, with no one clock beating out the pulse of the universe. “To me it’s very worrisome,” says theorist Daniel Sudarsky of the National Autonomous University of Mexico. “It would change our whole conception of the nature of space-time.” Relics from the time before quantum death might live on in our current universe, potentially still causing mayhem with our notions of what physics does and doesn’t allow (see “Out of the valley of death”, left). Whether that is the case or not, Valentini thinks he has seen clues that reinvigorate the whole idea of quantum death. Soon after the big bang, while in the throes of quantum death, the universe is thought to have undergone a short-lived burst of breakneck expansion known as inflation, which amplified tiny differences in density  to give the seeds of the stars and galaxies we see today. Earlier this month, researchers at the BICEP2 experiment at the South Pole reported that they had found evidence of inflation imprinted in patterns of polarisation in the cosmic microwave background left over from the big bang. In 2007, Valentini predicted that inflation would also have magnified any density fluctuations that hadn’t yet reached quantum equilibrium. Their odd distributions should be imprinted in the microwave background as a slight loss of power at longer wavelengths. Last year, data from the European Space Agency’s Planck satellite gave the first conclusive evidence that such a power deficit exists (arxiv.org/abs/1303.5075). “It’s good news,” says Valentini. “Qualitatively, it fits.” Not everyone agrees. Glenn Starkman, of Case Western Reserve University in Cleveland, Ohio, says there were already hints of a power deficit in data from Planck’s predecessor, NASA’s Wilkinson Microwave Anisotropy Probe. Valentini would have known about this, he says, which makes his prediction retrospective. “He needs to come up with a distinguishing prediction for something we haven’t already measured,” he says. Valentini takes on board such scepticism. Together with his Clemson colleague Samuel Colin, he is working on a more detailed prediction about how features in the cosmic microwave background should vary with its wavelength. This would concern not only power, but also anisotropy – a lopsidedness in the distribution of fluctuations across the sky. “It’d be like killing two birds with one stone,” he says. “With two pieces of evidence, there will be a considerably stronger case.” Meanwhile, Valentini appeals to common sense. After all, thermodynamics allows us to perform useful work – it is only after the prophesied heat death that nature will get lazy. Does it not make sense that our inability to exploit what quantum theory offers is the result of an analogous quantum death? “What has always puzzled me is that there’s this kind of conspiracy,” says Valentini. “On one hand, quantum theory seems to be fundamentally non-local – something is going on faster than light. On the other, you can’t actually use it to send a signal. And just intuitively, it seems to me that there is something going on behind the scenes.”  ■