Conjuring a Neutron Star from a Nanowire

Some hundred years ago, famous escape artist Harry Houdini walked through a brick wall in a New York theatre. The wall stood on a carpet to ensure there were no trap doors. Houdini took his place on one side of the wall, a screen was placed over him and a second screen was put on the other side of the wall. Houdini waved his hands above the screen and said, "Here I am." Then he vanished and almost instantly reappeared on the other side. The audience was too stunned to applaud.

This was a clever trick; Houdini appeared to do something that is clearly impossible. Or was it? Quantum mechanics, the set of rules that governs the subatomic world, theoretically allows objects to pass through barriers that they should not be able to traverse. This quantum tunneling effect has been demonstrated for small particles. Physicists have also confirmed that subatomic particles can have a wave-like nature and can exist in superposition states of being in two places at once. But despite ample proof for the theory, Houdini-sized objects stubbornly refuse to follow suit.

Now, Keith Schwab, a physicist at the California Institute of Technology in Pasadena, wants to identify why the large-scale classical world is governed by different rules to the quantum realm. The key, Schwab thinks, may be that gravity nudges particles out of their fragile quantum states, forcing them to adhere to more familiar classical rules. If that’s correct, then to fully understand the natural world, physicists will need to come up with a theory of quantum gravity that reconciles quantum mechanics with Einstein’s theory of gravity, general relativity. In the meantime, Schwab and his colleagues have devised a set of tests that bring quantum mechanics and gravity together in the lab to help them.

Theoretical physicists have been trying for decades to solve the mismatch between quantum mechanics and gravity. It’s been a particularly hard struggle because there are no direct tests of quantum gravity to guide them. That’s because quantum gravity is usually thought to only apply in the most extreme conditions, where huge amounts of gravity are confined into small spaces—such as, in the heart of a black hole. Even the most powerful particle accelerators cannot probe the quantum gravity regime. "What’s desperately needed is any insight from experiments," says Schwab. He and his colleagues aim to rectify that with the aid of an FQXi grant of just over $44,000.

Ripples of Inspiration

Schwab’s interest in the interplay between quantum mechanics and gravity was sparked in the late eighties when he overheard researchers at Caltech who were talking about the LIGO experiment, which is designed to detect gravitational waves—ripples in the fabric of spacetime itself that are predicted by Einstein’s theory to be generated by cataclysmic cosmic events, such as collisions between black holes. What fired Schwab’s imagination was that in order to pick up signs of events on the cosmic scale, the researchers needed to make very precise measurements in the lab, which requires looking for such small-scale changes in their measuring apparatus that quantum effects needed to be considered. It made Schwab think about ways to see physics effects beyond the scales in which they are usually thought to hold sway.

That, in turn, inspired Schwab to see if it was possible to spot quantum behaviour in mechanical structures that are large enough to see under a microscope. "The thing that we were interested in 15 years ago was whether it is possible that collections of small particles—like a billion atoms linked together to form a little piece of solid—could have quantum properties, meaning: could you see quantum physics at large length scales," says Schwab. (See "Quantum Upsizing.")

Schwab’s team has since confirmed that you can: "The punch line of that line of thought is that, yes, you do need quantum mechanics to describe the motion of these mechanical structures properly," he says. At quantum scales, objects never completely come to rest, but always vibrate slightly. "You can manipulate their quantum fluctuations," Schwab explains. "You can pretty much do everything you would expect to do quantum mechanically with a relatively large object."

Such tests do not probe quantum gravity, however. To be able to marry quantum mechanics with gravity, we need more than just the ability to see quantum phenomena at larger length-scales. We also need to include a vast gravitational field—one that is stronger than we are used to on Earth.

Together with theoretical physicist Miles Blencowe, at Dartmouth College in New Haven, Schwab uses the power of the principle of equivalence to create circumstances which physicists can use to investigate the quantum mechanical behaviour of small mechanical structures in much higher gravitational fields. The principle, which lies at the heart of Einstein’s theory, dictates that gravity and acceleration are equivalent. That means that by shaking his mechanical devices (in this case a nanowire) back and forth at very high frequencies—about a billion times a second—Schwab can achieve accelerations of about 100 million times the acceleration of the Earth’s gravitational field, mimicking the arenas in which quantum gravity comes into play.

Quantum Acceleration

"If we have devices that are in quantum states and we have this enormous acceleration, then we can start to have laboratory experiments where we can study the connection between gravitation and quantum mechanics," says Schwab. "It’s like setting up a quantum physics experiment on the surface of a neutron star."

Schwab and Blencowe have just submitted an article to a peer-reviewed journal in which they explore some of the consequences for the quantum wave nature of atoms under these extreme g-forces (Katz et al, arXiv:1409.2137). "The physics of what happens to the atoms gets really interesting when the vibrating wire is itself in a quantum state (that is, is in a superposition of different amplitudes) inducing a sort of quantum acceleration for the atom," says Blencowe.

One consequence could be that the quantum wave nature of the atoms is destroyed by the high g-forces, collapsing their quantum properties and making them behave classically. This could also enable the experiments to teach us something about quantum gravity. "The vibrating wire in a quantum superposition of different amplitudes may be somewhat analogous to quantum fluctuating space-time and therefore be a possible analogue for certain quantum gravity effects, giving us some insights into the latter," says Blencowe.

Schwab and Blencowe are not the only physicists looking into table-top tests that combine quantum effects with gravity. Igor Pikovski, of Harvard University, and FQXi member Caslav Brukner and their colleagues recently published a paper in Nature Physics looking at how general-relativistic effects could affect quantum systems (Pikovski et al, Nature Physics (2015)). In particular, they have been investigating time dilation—the slowing of clocks near heavy objects. The team has calculated that even the weak time dilation effect on our planet due to Earth’s gravity could be large enough to disrupt the quantum properties for molecules and larger objects.

Pikovski is excited about Schwab’s work. "I believe Schwab’s experiments are of great importance as they can help us to better understand the interplay between quantum theory and gravity," says Pikovski. "This is a very exciting time to conduct research in this field as significant experimental progress is just around the corner."

Schwab hopes that others will also be inspired to come up with new experimental questions that take advantage of the extraordinary conditions that can easily be created in the laboratory. "The basic message we want to put out into the community is that we can do this and we want smarter people to think about it and design ideas," he says. "They will come up with the killer ideas!"