The Graviton – The Final Fundemental Force Carrier

In Physics, there are considered to be four fundamental forces of nature. Four forces, which define every interaction we have with the world around us. Every time you go to pick something up, or tap your phone screen, or even simply stand up, you utilise these four forces in comibination. It might seem alien, but it is true, every macro scale force is a sum of innumerable individual fundamental force contributions. However, these forces aren’t all made equal…

The four forces are, Electromagnetism, Gravity, and the Strong and Weak interactions. These forces cannot be reduced down further, just like the atom was considered once to be the smallest building block of all matter (which ended up not being true, but that is beside the point), these four are the smallest building blocks of all forces.

What makes scientists so sure that these forces are fundamental though? what if, just like with the atom, we discover in the future that these too are comibinations of smaller forces. Well, unlike macro forces like friction, the fundamental forces are characterised by their carrier particles. Every time one of these forces occur, it is represented by an exchange of quantum particles. For the electromagnetic force, the carrier is photons, for the weak force, W and Z Bosons, and for the strong force, gluons. Gravity however is an outlier.

The Force carrier of Gravity, is the graviton, but unlike the other three forces, gravity’s particle has yet to found. It is currently purely hypothetical, with no experimental evidence for it. It is hypothesised to be a massless particle, similar to a photon, but due to gravity being a much weaker interaction compared to electromagnetism, the actual detection of the graviton, if it exists, would be very difficult indeed. The issue of detection practicality is not a new one for gravity research. Many might remember the landmark detection of gravitational waves in 2016 by LIGO, a detection which came nearly a full 100 years after Einstein predicted them. The difficulty of measuring gravitational phenomena is due to how weak the force is. Outside of incredibly large masses, gravity has a relatively small effect, and it scales with one over the seperation of the effected masses squared, which means it falls off very quickly.

So what do we do? Does the graviton not exist? Or will we never know? Well we aren’t completely in the dark, The detection of gravitational waves was indeed a landmark one, and we can learn alot about a would be graviton from them. For the graviton to be massless as expected, gravitational waves should propagate at the speed of light in a vacuum, current evidence suggests that this may not be the case, but this is far from a closed book.

So what does this mean, scientists have yet to be able to find a force carriering particle for gravity, and theres little evidence backing up current theories. But, does it matter? Well gravity not having a particle suggests that it isnt a quantised force like the other three, and indeed, it’s quantum mechanics where the issues with our understanding of gravity truly rear their head. attempts to combine general relativity and quantum mechanics to study the effects of gravity on the quantum scale simply break down. It’s the hope of some scientists that the answer to this incompatibility is the graviton, and that its discovery, and subsequent plugging of the gap in knowledge we have on quantum gravitation, would allow us to unify general relativity and quantum mechanics.

And so, the search goes on, infact as recent as last month there has been some new evidence for graviton like particles in the form of chirol graviton modes in semiconductors. Will we one day find our final fundamental force carrier? We shall have to wait and see.

References:

First experimental evidence for graviton-like particles – https://cosmosmagazine.com/science/physics/graviton-particle-experimental-evidence/

Image – https://www.ligo.caltech.edu/page/what-are-gw