Being in two places at the same time isn鈥檛 easy for mere humans (Image: Chen Liu/EyeEm/Getty)
It鈥檚 been holding us back as well as holding us down. A previously overlooked effect of gravity on quantum systems could be messing up quantum experiments. If confirmed, it suggests that some quantum studies may be impossible to perform on Earth.
No matter how hard you try, you can鈥檛 be in two places at once. But if you鈥檙e an electron, popping up in multiple places is a way of life. The laws of quantum mechanics tell us that subatomic particles exist in this superposition of states until they are measured and found to be in just one 鈥� when their wave function collapses.
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Chasing cats
So why can鈥檛 we do the same party trick as an electron? It seems that once something gets large enough, it loses its quantum properties, a process known as decoherence. That鈥檚 mainly because larger objects interact with their environment, which forces them into one position or another. Erwin Schr枚dinger famously pointed out the absurdity of large-scale superposition with the example of a cat that is both dead and alive.
But that hasn鈥檛 stopped physicists from trying quantum experiments by isolating objects from external influences. In 2010, a team at the University of California, Santa Barbara, placed a strip of metal 60 micrometres long into a superposition for a few nanoseconds, cooling it to just above absolute zero to shield it from temperature fluctuations.
The hope is that more precise experiments could place larger objects, such as a virus, into a superposition, getting us closer to Schr枚dinger鈥檚 mythical cat. But now it looks like there is a more fundamental obstacle: gravity.
In the gravitational field
General relativity, Albert Einstein鈥檚 sweeping reassessment of gravity that celebrates its centenary this year, tends to be ignored by quantum physicists. 鈥淯sually people don鈥檛 look much at it because gravity acts on very large scales,鈥� says of Harvard University. 鈥淭hey think there are probably not many effects that are relevant.鈥�
Now Pikovski and his colleagues have calculated what happens when you do quantum experiments in Earth鈥檚 gravitational field. They say a quirk of relativity called time dilation could be making large systems lose their quantum nature.
One of Einstein鈥檚 predictions is that gravity slows down time. For massive objects, the effect can be extreme, as shown in the film Interstellar, where an hour on a planet orbiting a black hole is equivalent to seven years on Earth.
But it also affects you. Lab experiments with atomic clocks have revealed that your head ages slightly faster than your feet, because of the tiny differences in gravitational field strength.
Pikovski鈥檚 calculations show that molecules placed in a superposition should also experience this time difference, and it can disrupt their quantum state. This happens because the bonds between atoms in a molecule act like springs and constantly vibrate. If a molecule is in a superposition of two states that are at different heights from the ground, each state will vibrate at a different rate, destroying the superposition.
Height matters
Decoherence happens faster as more particles are added to the system. For example, take an experiment attempting to place 1 gram of carbon 鈥� about 1023 atoms 鈥� in a superposition of two states. If they could be separated vertically by 1聽micrometre, Pikovski says Earth鈥檚 gravitational field will cause the experiment to decohere in a millisecond, even if nothing else interferes. 鈥淓ven completely isolated systems are somehow affected by their own composition,鈥� he says.
This doesn鈥檛 happen to smaller particles like electrons, because they don鈥檛 have moving parts, so are unaffected by time dilation, which explains why they happily remain in superposition until we measure them.
For the moment, Pikovski鈥檚 idea remains theoretical, but he says it could be tested with atomic clocks, which use the regular frequencies of energy emitted by atoms to keep time. If you could place an atomic clock in a vertical superposition, it would tick at two rates simultaneously, forcing it to decohere.
鈥淭his effect is relatively small, so the equipment needs to be a little better than currently,鈥� says Pikovski, but he thinks it could be done within a few years.
鈥淚 think it鈥檚 brilliant, I love it,鈥� says of the University of Oxford. He is working on tests of large-scale superposition involving vibrating nanotubes, and thinks he might be able to adapt his experiment to test Pikovski鈥檚 theory. 鈥淚f it is right, it might be a serious contribution to the mismatch between our everyday experience and the exquisitely tested theory of quantum mechanics.鈥�
Leaving Earth behind
If the effect is confirmed, it suggests that physicists may have to leave the clutches of Earth鈥檚 gravity to perform extreme quantum experiments.
鈥淚t would probably be easier to do a quantum experiment of large systems on the moon or in space than it is on Earth,鈥� says Pikovski, because lower gravity would slow decoherence. 鈥淚f people really want to do macroscopic superposition, eventually you will have to go to space.鈥�
鈥淲ouldn鈥檛 that be nice?鈥� says Briggs. 鈥淲e might get some of my bench-top experiments out into orbit.鈥� One location could be the International Space Station, where astronauts already run low-gravity experiments.
But others aren鈥檛 convinced. 鈥淚t is a tiny effect and I don鈥檛 think it will force us into space any time soon,鈥� says of the Nordic Institute for Theoretical Physics in Stockholm, Sweden. The effect is orders of magnitude below what could affect current experiments, she says.
Observers everywhere
Doing experiments in space has its own issues, says of the University of Zurich, Switzerland. Working in low gravity is difficult, expensive and equipment must be hardened against cosmic radiation, but it鈥檚 not totally unreasonable, she says. 鈥淢aybe in the future, the dominant effect will be quantum decoherence due to gravity.鈥�
Time dilation could also solve a thorny philosophical issue. Some physicists worry that quantum mechanics requires a conscious observer to collapse a wave function. Would a quantum cat in a box stay both dead and alive forever if no one checked on it? Could the cat itself collapse the wave function?
Gravitational decoherence solves the issue, says Pikovski. Even if you perfectly isolated a system from the effects that cause decoherence normally, gravity鈥檚 effect on the cat would collapse the system. 鈥淭ime dilation induces this kind of observer,鈥� he says.
Preserving the superposition
But this doesn鈥檛 entirely remove the problem, says of the University of Queensland in Australia. There should be no time dilation if both states are at the same height. 鈥淚n principle, it is still possible to preserve a superposition if sufficient control over the quantum systems can be achieved,鈥� he says.
Thinking more about how gravity and quantum effects interact, as Pikovski鈥檚 team has done, could lead to the ultimate prize in modern physics. The mathematics behind general relativity and quantum mechanics produce nonsense solutions in situations where both are important, like the singularity at the heart of a black hole.
Those attempting to develop a unified theory of quantum gravity study how the maths breaks and try to fix it, but struggle to do experiments to confirm their ideas 鈥� black holes are hard to come by in the lab.
鈥淭his approach can contribute to a better understanding of the interplay between quantum mechanics and gravity,鈥� says Pikovski. 鈥淚t allows you to build up understanding of phenomena that take place where both theories really matter.鈥�
The work may be about how quantum physics and gravity work together, but that doesn鈥檛 mean it鈥檚 a route to a theory of everything, says of Aix-Marseille University, France. 鈥淨uantum gravity refers to the quantum properties of space-time, not the quantum behaviour of matter in space-time, which is something for which we have perfectly credible theories,鈥� he says.
But Ralph thinks it is a good start. 鈥淭he predicted effect does combine quantum mechanics and general relativity in a non-trivial way,鈥� he says. 鈥淭here are virtually no experiments so far that test whether this way of doing calculations is correct, thus experiments testing these sorts of predictions are very important 鈥� even if they just confirm that we are on the right track.鈥�
Journal reference:
Clarification, 17 June 2015: Since this article was first published, one of Igor Pikovski鈥檚 quotes has been changed at his request to make it clear that the effect described in his paper is related to quantum mechanics and gravity, not to a unified theory of quantum gravity.
