## WHAT DOES GRAVITY LOOK LIKE?

Einstein’s theory of relativity described gravity as the distortion of space and time—which bend and stretch based on the masses of objects within them as well as the energy released from the phenomena. A few years later however, we gained awareness of the confusing world of quantum physics as physicists discovered the existence of very small particles—which were later found to affect even the biggest, most powerful phenomena in the universe.

This led to the discovery of force-carrier particles, or bosons, behind three of the fundamental forces governing the universe: the electromagnetic field has photons, the strong nuclear force has gluons, and the weak force is carried by W and Z bosons. This leaves gravity out. Physicists hypothesize that, if the other three fundamental forces have a corresponding quantum theory, there must be a particle behind gravity too.

In an attempt to marry gravity with quantum theory, physicists came up with a hypothetical particle—the graviton. The graviton is said to be a massless, stable, spin-2 particle that travels at the speed of light.

We know that general relativity and quantum theories cannot both be right, because they are mathematically incompatible. Physicists today believe that quantum mechanics is more fundamental and that gravity does need to be quantized, even if we don’t exactly know how – string theory is one possible solution. If so, then yes, gravitons exist in the same sense that photons do. So most physicists strongly believe they do.

The funny point – somewhat a philosophical question – comes when you ask the question “can I detect a single graviton”. The answer to that seems to be ‘no’ for various complicated reasons. So if you cannot detect it, is that a reason to say ‘it does not exist’?

Einstein’s theory of general relativity (GR) implies the existence of gravitons, because GR predicts the existence of classical gravitational waves. This existence of gravitons is implied even if the underlying quantum theory of spacetime is radically different from GR—for example, string theory or loop quantum gravity, both of which are quite different from GR. The reason is that any underlying theory has to reproduce GR in an appropriate limit. So a “graviton” might not be a fundamental particle (just like the pion is composite), but the graviton has to emerge at the same time as GR emerges from the underlying theory. In fact, any reasonable modification to GR which is consistent with present observational data also implies the existence of gravitons.

Gravitons were not observed and are unlikely to be observed anytime soon (or not so soon) because the gravitational interaction is so weak.

If gravitons do not exist, that means that gravity is not a quantum field theory. This is difficult (though not necessarily impossible) to reconcile with the fact that all other forms of matter are described by a quantum field theory (the Standard Model of particle physics.)

One possible resolution may be that (as we indeed suspect anyway) QFT is just an “effective” theory, and a deeper underlying theory allows these quantum fields and gravity to coexist.

Another possibility is that gravity is not a fundamental field but one that emerges from the statistical properties of matter (Entropic gravity) and thus does not need to be quantized.

The graviton remains hypothetical, however, because at the moment, it’s impossible to detect. Although gravity on a planetary scale is strong, on a small scales it can be very feeble. So much so that when a magnet attracts a paperclip, it pulls against the gravitational force of the entire planet, and still wins. This means that a single graviton—if it exists—is very, very weak. One study even argues that it’s impossible to detect a single graviton unless we measure them in planet-sized magnitudes, using a universe-sized detector.

## STRING THEORY AND THE HUNT FOR GRAVITONS

The Kaluza-Klein theory hypothesized that gravity may actually only seem weak from where we exist because it actually has the ability to go through more than three dimensions at once, and therefore spreads itself out thinly. This idea, after decades of being widely ignored due to mathematical inconsistencies, when revisited and refined eventually led to what is now known as string theory—currently the most promising contender in establishing a quantum theory of gravity. It is also our best hope in unifying quantum theory and general relativity, the biggest problem in physics today.

In string theory, mathematical calculations point to a universe with ten dimensions of space and one dimension of time, but the other spatial dimensions are far too small for us to see, even with microscopes. Several teams of physicists today are on the hunt for the graviton but, so far, all hopes of ensnaring the hypothetical particle have left us empty-handed.

There are many things about quantum physics that we don’t understand and understanding particles and the laws that govern them can help us wield the powers that quantum phenomena hypothetically possess. Proving the existence of a particle that would help make sense of it all is a dream, and remains that as of now.

As it stands, we are far from definitively proving it exists. As Fermilab senior physicist Don Lincoln wrote in a post: “Gravitons are a theoretically reputable idea, but are not proven. So if you hear someone say that ‘gravitons are particles that generate the gravitational force,’ keep in mind that this is a reasonable statement, but by no means is it universally accepted. It will be a long time before gravitons are considered part of the established subatomic pantheon.”