THE quantum realm has always seemed worlds apart from general relativity, Einstein’s theory of gravity. One rules at the atomic scale and smaller; the other reigns supreme across the cosmos. This is one reason why physicists are wrestling in their efforts to meld quantum theory and relativity into a theory of everything that shows how the universe works at a fundamental level.
So far, all the attention has focused on schemes that come into play under the high-energy conditions that existed just after the big bang. The trouble is, experimenting with such theories is incredibly difficult. “The tests for it are way off,” says Roger Penrose at the University of Oxford. “You have to build an accelerator the size of the solar system – that’s not on the cards at all.”
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Perhaps, though, the quantum world has more in common with relativity than we think. According to Penrose, we’ve actually been doing experiments for decades that combine quantum theory and gravity. With a few tweaks, they might offer a different way to the revelations we seek. “It looks a much more promising route to the truth about how the universe actually works.”
How can this be? Well, until now the interplay between some of the best known oddities of physics has largely been ignored. Take the fact that atoms and small molecules can exist in two places at once, a phenomenon known as superposition. This famous ability is a key feature of quantum reality and has been demonstrated in countless experiments. Yet it turns out that some fundamental questions about the role of gravity in superposition have never been asked, let alone answered.
Asking those questions, and finding out the answers, would enable us to open the door on an understanding of the universe in its entirety. They might also shed light on one of the biggest mysteries of science: what causes the transition between the weird quantum world and the everyday “classical” reality we experience. What were once impossible-looking puzzles might not be too far from a solution. “We’re way closer than we’ve ever been,” says Cisco Gooding of the University of British Columbia (UBC) in Vancouver.
The enigma is, in many ways, quite simple to lay out. Here’s a starter: general relativity says that mass distorts space and time in a way that causes things around it to feel the attractive force we know as gravity (see diagram below). So, in superposition, is an atom’s mass creating two distinct distortions in space-time – and thus exerting a gravitational pull on itself?
Here’s another: special relativity says that an atom moving through space will have a unique experience of the flow of time. This phenomenon is known as time dilation. But if a moving atom is in superposition, the time dilation must occur along two different paths at once, and will be different on each path. So when the superposition ends and the two become one again, have they aged differently?
More fundamentally, it is questionable whether general relativity even allows superposition. “There’s a conflict here,” says Penrose. “You can’t have a superposition of two gravitational fields: that’s illegal.”
It is worth pointing out that we have seen nothing wrong with general and special relativity so far: experimental tests show they are correct.
Quantum collapse
Quantum theory works similarly well, even though its ideas are odder. Superposition, for example, works because of a phenomenon called quantum coherence. This is what allows quantum objects to split their existence, characteristics and properties between spatial locations, different kinds of movement or even between different particles entirely.
The real problem is that these two theories only work separately. Quantum theory has nothing to say about the properties of the space and time its particles pass through. And relativity says nothing about the properties of those particles. When you try to combine them, they simply butt up against one another. “How do you go from one to the other?” says of Dartmouth College in Hanover, New Hampshire. “We don’t have a quantum theory of gravity.”
And that is why the new generation of experiments in “gravitationally induced decoherence” can’t come soon enough for Penrose and others.
“ Fundamental questions about gravity’s role have never even been asked”
Decoherence is the name quantum researchers give to the falling apart of quantum coherence. You can put an atom – even a lot of atoms – in a spatial superposition, for instance, but it doesn’t last. Eventually, the superposition “collapses”, and the atom is suddenly in only one place.
The classic way to investigate quantum superposition is to fire an atom at a screen with two slits, also known as an interferometer. The atom can go through either, but experiments show that if no one is measuring which slit it passes through, it will actually go through both. The result is an “interference pattern” that forms in a detector placed behind the slits. This reveals a series of well-defined patches where the atoms appear to hit the detector, alternated with blank spaces where no atom seems to land. The only explanation for such a pattern is that each atom splits in two, with one part going through each slit, then interfering before it reaches the detector.
Even weirder things happen if you equip this interferometer with another detector sited so that it can tell which slit the atom went through. The mere presence of this detector causes decoherence and destroys the interference pattern. It seems that the atom only behaves oddly when no one – or nothing – is looking.
Ticking atoms
There are many ideas for why such a thing might happen. Most are to do with information loss: reading the atom’s path forces the atom to choose one path or the other and prevents it taking both. Experiments have shown that there doesn’t even have to be a detector: heating the atom up, so that it emits thermal photons that could be used to infer its position, seems to be enough to weaken the interference pattern.
No one really knows what to make of this. It is made even worse by the discovery that large collections of atoms seem to be unable to exist in superposition. We have made interference patterns with molecules composed of 800 atoms, but the more massive they get, the shorter-lived the superposition. This has led some to suspect that gravity might be the real reason why massive collections of atoms – including us – are not quantum.
Testing this idea is far from easy because superpositions of atoms are such delicate things. But our ability to protect them from heat, vibrations and other disturbances has come on leaps and bounds, meaning we can start to get to get to grips with gravity’s role.
For instance, Gooding and Bill Unruh, also at UBC, are planning to look at how an atom in a superposition experiences time as it flies through different paths in an interferometer and then recombines to produce an interference pattern. An atom can be thought of as a tiny oscillator, a bit like a clock’s pendulum. Send an atom into an interferometer and “it’s a little clock that is ticking differently, and when it comes back together, those two clocks don’t necessarily agree with each other,” says Gooding. “We should see some sort of clash between each of their individual notions of time evolution.” That should be enough to degrade the interference pattern in predictable and detectable ways.
Igor Pikovski at Harvard University has another plan based on time anomalies. Working with at the University of Vienna in Austria, they think we could put a clock in a superposition of two different heights from the ground. That would mean the two parts of the superposition exist in different parts of the Earth’s gravitational field.
According to general relativity, clocks run faster in a weaker gravitational field. That’s why, over your lifetime, your head ages 300 nanoseconds seconds more than your feet. For a one-atom clock in a superposition, this creates a problem – as the two times diverge, the atom will be forced back to being at one height or the other. “The fact that the atom records different time in different places gives away information on the atom’s position,” Pikovski says. “This destroys the coherence.” In other words, time dilation due to gravity can explain why we do not see quantum superpositions in our everyday world.
This can be tested using “atomic fountain” techniques that push atoms upwards through microwave fields to create ultra-accurate interferometers. It will involve some tweaks to existing experiments, but not too many. “It’s not something that can be done now, but it’s possible soon,” Pikovski says.
Other experiments under development involve a different kind of superposition. Dirk Bouwmeester at Leiden University in the Netherlands, and Markus Aspelmeyer at the University of Vienna are independently making mirrored cantilevers. These structures look rather like springboards that exist in two configurations at once. When a photon in superposition hits the mirror, it can put the cantilever into a superposition of being both vibrating (as if a diver had just left the springboard) and undisturbed.
This was first achieved a few years ago. Now Penrose believes that each part of the superposed springboard should create so much gravity for the other that they collapse back into one. “It’s hand-waving to a degree,” he says, “but something certainly goes wrong.”
The challenge now for Bouwmeester and Aspelmeyer’s teams is to make the superpositions last long enough to investigate the decohering effects of gravity. One of the problems with the diving boards is that it is hard to disconnect them from their environment. This results in superpositions collapsing because of vibrations transmitted through the apparatus, rather than gravity.
Making and studying superpositions of large objects – large in quantum terms anyway – is new territory for researchers. And not surprisingly, there are other ideas for why reality ceases to be quantum at larger scales. One suggestion is that we need to revise quantum theory itself. A souped-up version of it says that superpositions are impossible for objects composed of more than a certain number of particles because of a phenomenon called spontaneous localisation, which suggests that the distribution of mass – its density – is what matters.
We may find out that particular answer fairly soon. Markus Arndt’s group at the University of Vienna has been repeating the double-slit interferometer experiment with ever larger objects. Arndt believes that spontaneous localisation would kick in with particles of a mass of somewhere between 100,000 amend 100 million atomic mass units in his experiment (one amu is equivalent to 1/12th the mass of a carbon-12 atom).
Watch the individual molecules appear in the quantum movie below:
Video: Quantum movie reveals single molecules
We need to rule out spontaneous localisation before pointing the finger at gravity. “It’s difficult to promise when this will be, but hopefully before the end of the year,” he says. They are currently superposing objects of 10,000 amu, and working towards 100,000 – the lower limit of where spontaneous localisation is thought to happen. We will then have a nail-biting passage through to 100 million amu, when we would be able to rule it out.
Theory of everything
Of course, experiments will be the final arbiter of this. And in the end, all these techniques have their own challenges. “Nobody on the planet can actually do any of this yet,” Arndt says. It’s still a good time for theorists in this field, he quips. “We can’t constrain them.”
So, really, all the pressure is on the experimenters. Not that they are guaranteeing any quick solutions. Aspelmeyer reckons there is a long road ahead – and that’s just the bit he can see. “It could be long, very long or impossible,” he says.
And people disagree about how much such work will illuminate the search for a theory of everything. Many believe that such a theory is as distant a prospect as ever. But Gooding is an optimist. One of the good things about these experiments, he says, is that they don’t involve making anything up. We’ve tested general relativity and quantum mechanics under these conditions, and they work. “If we can demonstrate something from just those theories then we have a good reason to believe that effect is real,” he says. “If we demonstrated it from string theory there would always be a nagging thought in your mind that maybe it’s something to do with the assumptions of string theory.”
He thinks that we could have answers within 10 years. And that is very good progress. Until fairly recently, it didn’t look as if we’d ever be able to test gravity’s quantum interactions. “It’s now looking like something that’s actually doable, not something that has absolutely no chance of ever being observed,” Gooding says.
Leader: “After a century of relativity, a new view of gravity”
This article appeared in print under the headline “The secret life of reality”
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