Jan 10 2022

## A Quantum Gravity Experiment

Perhaps the greatest challenge of current theoretical physics is to come up with a testable theory that unites the principles of general relativity with quantum mechanics. This has proven to be a very challenging problem, one that may take generations of physicists to crack. Right now physicists are mostly stuck in the theoretical realm, trying to come up with theories (like string theory) that may be internally mathematically consistent, but are challenging to falsify experimentally. However, Rana Adhikari, professor of physics at Caltech, and her colleagues are trying to come up with a way to do just that. Their approach derives from another weird concept within theoretical physics – that the universe may be pixelated, and may even be a hologram (three dimensions projected from a two dimensional surface).

For background, prior to the 19th century we comfortably lived in what we now call a classical universe. Our models of how the universe works were based upon our observations and experiments within the frame of macroscopic creatures living on the surface of a planet. Galileo and Newton developed, for example, laws of motion that defined how objects move and behave, including Newton’s theory of gravity. However, classical physics started to break down in the 19th century. For example, astronomers making more and more precise measurements of the orbits of the planets were finding that the orbit of Mercury was different than what our classical equations predicted. Those equations work extremely well, but there was something off about Mercury. Attempts at finding an explanation, such as a hidden planet on the other side of the sun, failed. Eventually we had to conclude that our classical equations were not quite right, or at least could not account for the special case of Mercury.

This is where Einstein comes in. First he proposed in theory of special relativity, which fixed some vexing problems in physics by proposing that the speed of light is an absolute constant regardless of frame of reference, and that it was space and time that are variables which can change based upon frame, specifically with respect to relative velocity. This was considered “special” relativity because it only referred to the speed of light. Einstein would have to work for years more before he was able to account for gravity in a theory of general relativity. His new equations not only solved the problem of Mercury’s orbit (it is close enough to the sun that relativistic effects from the sun’s gravity are measurable) but also made a large number of predictions. Over the last century Einstein’s theories have been confirmed to an extremely high degree.

At the same time physicists were working at the other end of the spectrum – at the scale of the very small, atomic and subatomic particles. While they were working out some of the basics, experiments starting producing contradictory results. The very short version is that at very tiny scales particles sometimes act as waves. These particles/waves also seem to have properties which are determined randomly, and further they can be entangled with each other even over great distances. This all seems like “quantum weirdness” but so far these interpretations have proven very robust experimentally. At their most fundamental level, matter and energy seem to be formed of entangled probability waves. That is – except for gravity.

Physicists are having a hard time coming up with a unifying theory that accounts for both quantum effects and general relativity effects at the same time. This means, in extreme situations like near black holes, we cannot predict how the universe behaves because we cannot account for how general relativity and quantum mechanics behave together, in situations where both are relevant simultaneously. We need a new theory of quantum gravity. This would not make general relativity or quantum mechanics wrong, just as general relativity did not make Newtonian mechanics wrong, but rather create a deeper theory that contains the older theories. In order to do this something new needs to be added to the equation. We need to figure out something about how the universe works at its most fundamental level.

This is where theories like string theory come into play. The idea of string theory is that, at an extreme subatomic scale (the smallest scale imaginable) perhaps the universe is made from multidimensional strings vibrating at different frequencies. Different frequencies equate to different fundamental particles. String theory also posits (in some of its forms) that the universe contains 10 dimensions, but six of those dimensions are super tiny, and only four are macroscopic (three spacial and one temporal dimension). This is basically a mathematical contrivance to make the equations work. But as we saw with Einstein’s theories, the mathematical cheats of of Lorentz turned out to be how the universe actually works. The universe is ultimately mathematical, so making the math work is a critical first step.

This finally gets back to Adhikari and colleagues. Their idea is this – if some version of string theory is correct, then that could mean that at its mostly fundamental (tinniest) level the universe is granular or pixelated. Spacetime is not infinitely smooth, but at high resolution (their example is a resolution in which a single atom is as big as a galaxy) we would seen the graniness of spacetime. If we could craft experiments that could see the universe at this scale we might be able to test some aspects of string theory. Their premise is that theoretical physics can only go so far, and eventually they need some experimental physics to connect their theories with the real world. When you want to see effects that are super tiny you turn to interferometry (such as LIGO which is setup to detect gravitational waves which are at the scale of the width of a proton).

Their experiment is further premised on the notion that gravitons are real. A graviton is a theoretical force carrying particle for gravity. We don’t know if gravitons exist, but if there is a theory of quantum gravity then it helps to treat gravity like the other forces, which all have force-carrying particles (like photons for electromagnetic waves). I don’t understand the math involved, but the broad brushstroke description is that they are proposing a design for an interferometer (laser beams crossing at a 90 degree angle) that could respond to fluctuations in gravitons, which would occasionally kick out a single photon from the interferometer and then be detected by a single-photon detector. The pattern of these fluctuations could then be a way to test aspects of string theory, and specifically to detect the pixelation of spacetime at the smallest of all possible scales.

Hopefully this will all work out as a way to add some experimental findings to string theory, or competing theories of quantum gravity. We need something to move beyond the standard model of particle physics and our current well-established theories. Physicists keep looking for ways to break these theories, but they are proving frustratingly robust (if limited). Perhaps this is another way to grind forward and start figuring out perhaps the most challenging scientific problem of our age.