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FQXI ARTICLE

August 2, 2015

Q&A with David Rideout: Testing Reality in Space

Satellite experiments could investigate the boundaries of quantum physics and relativity.

September 19, 2013

David Rideout

University of California, San Diego

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.

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.

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.

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.

Reality Lab

This artist’s conception shows NASA’s Tracking and Data Relay Satellite-K

communication satellite, launched earlier this year. Future satellites could allow

the exchange of quantum information across the globe.

Credit: NASA/Goddard Space Flight Center

Quantum mechanics and relativity are based on two different conceptions of time. In quantum mechanics, a particle is mathematically described by its

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’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.

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

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.

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STEVE AGNEW wrote on June 5, 2015

That space and motion are emergent versus matter and time as emergent is indeed a chicken and egg proposition. However, normally matter and time are not conjugate variables and so space and momentum take precedence in both intuition and science. A two dimensional time is what allows time to then have expectation values of time delay and decay and decay time is therefore one of those time dimensions from which space emerges.

As regards to permeability and permitivity, the magnetic and...

That space and motion are emergent versus matter and time as emergent is indeed a chicken and egg proposition. However, normally matter and time are not conjugate variables and so space and momentum take precedence in both intuition and science. A two dimensional time is what allows time to then have expectation values of time delay and decay and decay time is therefore one of those time dimensions from which space emerges.

As regards to permeability and permitivity, the magnetic and...

JOHN R. COX wrote on May 31, 2015

Steve,

Be careful, too, in assuming space is nonexistent. There is no experimental means to solve the emergent question, 'what came first - the chicken or the egg?' The common misconception of what spacetime means rests on the taking of time and space separately, as macroscopically apparent, post a break in symmetry. While it is quite true that Minkowski did not identify at what scale that break occurs, which leads to singularity without an empirical limit applied, that is no different...

Steve,

Be careful, too, in assuming space is nonexistent. There is no experimental means to solve the emergent question, 'what came first - the chicken or the egg?' The common misconception of what spacetime means rests on the taking of time and space separately, as macroscopically apparent, post a break in symmetry. While it is quite true that Minkowski did not identify at what scale that break occurs, which leads to singularity without an empirical limit applied, that is no different...

STEVE AGNEW wrote on May 31, 2015

Careful...you have implicitly assumed that space first exists for mass to occupy. First, define space, then define matter. There is no reality for an empty universe.

**Akinbo Ojo replied on May. 28, 2015 @ 09:58 GMT: ***"But this we can tell. ALL without exception that has mass occupies some region of space. Thus 'extension' is fundamental to having the property which we call 'mass'."*

What if it was matter that existed before space? Space would then simply emerge as a...

Careful...you have implicitly assumed that space first exists for mass to occupy. First, define space, then define matter. There is no reality for an empty universe.

What if it was matter that existed before space? Space would then simply emerge as a...

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