Face Off: Building a Toy Universe to Pit Quantum Theory Against Gravity

You’d have to go to extremes to carry out an experiment on a black hole: the nearest is thousands of light years away, and it’s not clear if you could ever get much information out of one. That’s a shame for physicists who would love to get their hands on one because it’s an arena in which gravity and quantum physics face off. But Sorin Paraoanu, a quantum physicist at the Aalto University School of Science in Finland, has come up with a way to custom-build a toy environment in his lab to test what happens when these two fundamental descriptions of reality collide.

If Paraoanu succeeds, the result will be like "a lego game for theorists working on unifying gravity and quantum physics," he says.

Physicists would love to learn more about this clash in order to reconcile the two titanic theories into a single theory of quantum gravity. The classical, gravitational theory of general relativity is hugely successful at describing the behaviour of large-scale objects by thinking in terms of their warping of spacetime. And quantum theory is wonderful for describing behaviour in the world of atoms. But these two can come into conflict when trying to describe situations with high energy packed into a small space—like black holes.

Doing any experiments in this regime is very difficult, however. One solution is to create an analog for gravity that is more powerful, and bends space-time more, than gravity itself. This ensures that the gravity analog will come into conflict with quantum rules in situations far less extreme than a black hole—involving less energy and a larger space. Paraoanu plans to do this using a "metamaterial" made from a chain of superconducting quantum interference devices (SQUIDs), each of which can be fine-tuned to precisely control how the material’s refractive index changes along its length. This is mathematically equivalent to changing the speed of light through the material or, in the context of building a gravity analog, changing the curvature of spacetime that light moves through. "In these analog systems we can be more efficient than real gravity," says Paraoanu. "We can bend spacetime as if we were near a black hole."

Some popular models of quantum gravity propose that spacetime itself isn’t one classical continuum, but instead made up of quantized bits and pieces—and Paraoanu’s setup should be able to explore this notion. In the real world, one would have to examine impossibly small scales of space to see such a situation, zeroing down to 1.6×10⁻³⁵ m (for comparison today’s most powerful accelerator, the Large Hadron Collider, can only probe down to 10^-20 m). In Paraoanu’s toy world, he reckons such effects should be apparent on the perfectly-reasonable scale of about a micron (10^-6m).

Originally from Romania, Paraoanu did his PhD under physics Nobelist Anthony Leggett at the University of Illinois at Urbana-Champaign. Family life then took Paraoanu to Finland, where his post-doc helped to shift his career towards practical experiments that test the boundaries of quantum mechanics. That remains his focus today, in his position at Aalto.

Progress will take time. "We can’t say we have created this curvature in spacetime yet. We have ideas of how to do that. But for now we’re still on a flat spacetime," he says. For now, his group is using an FQXi grant of over $75,000 to help to better understand their metamaterial’s properties. In 2013, for example, Paraoanu and colleagues demonstrated that a tiny amount of radiation could be created by fluctuating the refraction index in an array of SQUIDs, as predicted by the dynamic Casimir effect (PNAS, 110, 4234-4238 (2013)). (Paraoanu spoke about this paper on the FQXi podcast.) The fluctuations created a difference in the number of virtual particles popping in and out of existence between two parallel plates and outside those plates, which in turn produced a detectable flash of energy in the form of microwaves.

The difficulty, Paraoanu adds, is in the technical details: his SQUIDs need to be made with greater precision than provided by commercial producers, and the signals they hope to detect are so tiny that they need to be very much amplified. "We have to fight with the background noise and be fast—measure before decoherence sets in," when quantum effects dissipate, he says. But, he adds, "It’s not so frustrating if you make progress one step at a time."

Other researchers are taking different approaches to creating toy environments where quantum theory and gravitational theory can collide, including looking at the propagation of sound in the weird, cold world of Bose Einstein condensates or at the propagation of tightly-focused lasers. FQXi member William Unruh of the University of British Columbia is attempting something similar with water waves.

Broadly, "artificial systems for quantum simulation are really useful," says Christopher Wilson at the University of Waterloo, who has also worked with SQUIDs to show a dynamic Casimir effect. The general principle of any such simulation is to mimic quantum effects in a way that can be more easily measured. Paraoanu’s focus specifically on quantum gravity is interesting, Wilson says. Other analog systems could be used to study chemistry, or any other quantum-scale phenomena.

Paraoanu hopes his work will give birth to a new field: "solid-state quantum gravitation." In the meantime, he says, getting funding for such fundamental, non-applied research is tough work. "The aim is to acquire deep knowledge of the fundamental laws of physics, which I believe is necessary if we want, in the long run, to provide solutions for the more practical problems of our civilization," he says. Paraoanu himself is driven by more philosophical questions about why the Universe is the way it is: besides physics, he also has a degree in philosophy. "It doesn’t quite help fix a vacuum pump in the lab," Paraoanu admits. "But it gives an interesting motivation."