Q&A with David Rideout: Testing Reality in Space

In 2016, China plans to launch a satellite dedicated to quantum experiments. Along with other planned international space experiments, this will open up a new arena for probing how the rules of quantum mechanics—which govern the behavior of particles on the smallest scales—combine with the laws of general relativity, which controls how spacetime bends across cosmic distances. David Rideout, a physicist at the University of California, San Diego, recently brought together leading theoretical physicists at the Perimeter Institute in Waterloo, Canada to design a series of experiments that could be carried out in space to test the nature of reality—as he explains to Colin Stuart.

Why have you been interested in looking at the relationship between quantum physics and general relativity?

My initial interest was in understanding gravity and the central open question in gravitational physics is how it might be reconciled with quantum theory. However, this task of reconciliation is made extremely difficult by the lack of relevant experimental results. There are plenty of experiments which test gravitational physics and quantum theory individually, however virtually none which simultaneously probe both regimes.

Currently there is great interest in building a global satellite network for quantum communication, and this provides an opportunity for the first time to test quantum phenomena at scales at which gravity becomes relevant. So the interest in technologies such as quantum key distribution is providing opportunities to advance our understanding of fundamental physics.

Where is that funding coming from?

The Institute for Quantum Computing (IQC) received money from the Canadian Space Agency to develop the technology to establish a quantum communication channel between the Earth’s surface and satellites in low Earth orbit or beyond. Motivated by that project, we set up a series of meetings at the Perimeter Institute to discuss what sort of tests of fundamental physics we could conduct using this technology. Funding from FQXi then made it possible for us to write up the discussions in the form of a paper.

How useful was it to get a lot of different minds to collaborate and attack those fundamental physics questions from different angles?

The meetings brought together experimental groups from the IQC, engineers from COM DEV, a satellite company in Cambridge, Ontario, and theorists from the Perimeter Institute, to talk about ideas for experiments that can realistically be performed within a time frame of several years or more. It was exciting to get so many physicists and engineers sitting together talking about practical ideas regarding how we might test numerous theoretical ideas by experiment. Such a dialogue is crucial for both theorists and experimentalists, to encourage them to seriously consider how to connect current theory with practical experiments. Any time that multiple groups can dialogue, it creates powerful cross-pollination which pushes the frontiers of science. It was a privilege to coordinate this effort.

A lot of the tests involve ideas about quantum entanglement. What is quantum entanglement?

Consider a love triangle in which Christine likes either Alex or Bob, and sends each a letter of acceptance or rejection. When one opens his letter he will immediately know the contents of the other letter as well. Now imagine the letters are quantum, and that the outcome depends on the manner in which the envelope is opened. An envelope can be cut across the long end or the short end. If the state of the letters sent by Christine exhibit quantum entanglement, then the outcome of Bob’s letter can depend upon the manner in which Alex opens his envelope. This is the "spooky action-at-a-distance" which led Einstein to doubt that quantum mechanics was a complete theory.

An experiment setting a distance record for quantum entanglement was carried out in 2012 in Tenerife by Anton Zeilinger and his team. Why would it be useful to do this experiment again in space?

Those experiments operated over a distance of 143 kilometres, but if people want to exchange quantum information between cities widely separated on the Earth’s surface, which are often thousands of kilometers apart, then atmospheric losses will prevent the effective direct transmission of single photons over such a long distance. However, since the density of the atmosphere diminishes the closer you get to space, satellite transmission appears to be a practical way forward. There are also a number of alternative quantum theories which predict a deviation in entanglement over greater distances, so experiments at larger distances can test these theories.

In the paper you mention perhaps one day conducting these entanglement tests between two fast moving satellites. What might that unveil about the nature of time?

Quantum mechanics and relativity are based on two different conceptions of time. In quantum mechanics, a particle is mathematically described by its wavefunction. Before measurement, this wavefunction encompasses a myriad of different possible configurations—multiple possible locations for the particle to be found, for instance. Upon measurement, this wavefunction is said to "collapse" into one set configuration from those possibilities—and this collapse occurs at one precise time. Relativity, by contrast, tells us that different observers can disagree on when in time an event occurred.

So the motivation of the fast moving observers experiment is that each observer would have a different notion of what that moment in time is, according to special relativity. If the two satellites that are making the measurements are approaching each other at relativistic speeds, then an observer on each satellite would have the opinion that their measurement took place before the measurement of the other observer. If we wanted to take quantum mechanics literally then there is an open question—a paradox of sorts—as to what would happen in this situation. Future experiments could test this paradox and see how nature behaves in such a scenario.

That involves time dilation in special relativity, created by the relative motion of two observers. General relativity also predicts that clocks in different gravitational fields can run at different rates. What about testing the gravitational time dilation predicted by general relativity?

That’s actually one of the simpler tests to carry out. It is called a COW experiment (Colella–Overhauser–Werner experiment). You could test general relativity by using a interferometer between the Earth’s surface and a satellite in low Earth orbit. An interferometer combines light waves traveling along two different paths such that they interfere destructively with each other. Then if one path changes length compared to the other by a fraction of the wavelength, the difference will be detected as a change in intensity of the combined waves. Due to the satellite being at such a high altitude, the rate at which time passes on the satellite will be different from that on the Earth and that will affect the phase lag in the interferometer, which could be measured. This effect has been detected with GPS satellites, however it would provide an independent test of general relativity. If the interferometer is sufficiently sensitive it may be able to detect higher order effects which arise because the Earth rotates and has topographic features such as mountains.

General relativity also introduced the world to the notion of spacetime, a smooth fabric that pervades the universe and gives rise to gravity as it warps and bends around heavy objects. But some candidate models of quantum gravity, which attempt to unite general relativity and quantum mechanics, suggest that if you zoom in, you will find that spacetime is granular, or "quantized." How could quantum satellites be used to probe the nature of this fabric?

There might be hope of finding a signature of the quantum nature of space and time. There is a great deal of expectation that our theory of gravity will break down when we reach very small scales (around the Planck scale, 10^-35 meters). There is a significant amount of evidence to suggest we should expect a discrete nature of spacetime at those scales. That discreteness should have some effect on the propagation of polarised photons through spacetime.

Is that something we are likely to see with satellites in low Earth orbit?

Well, it turns out that the effect is extremely small and is even difficult to see on cosmological scales, so it seems extremely unlikely that we’ll see it at scales of Earth orbit. However, it is worth keeping these tests in mind because they push theorists to think carefully about how some of these theories might be tested in the future and how some of these emerging technologies—such as quantum satellite communication—could conceivably lead to important developments in fundamental physics. Really this is the first time that quantum experiments at the scale of Earth orbit and beyond have been considered.