It’s a question that I’m often asked by old family friends and people who I haven’t seen for a while: “what is it you do nowadays?”. Usually the people who ask me this do not have a strong background in theoretical physics, so to avoid beginning a mini-lecture about what exactly it is I do, I usually just reply: “I study theoretical physics”.

Some will stop questioning me there, give me a respectful nod, and then proceed to move the topic of conversation to something else.

Others however are more inquisitive, they’ll press me further to describe exactly what I mean when I say that I do “theoretical physics”. To this I’ll respond that I wish to go into string theory research. At this point there are two ways the conversation can go. Either the person I’m talking to will have read some popular science and will vaguely know the meaning of what I’ve just told them, or they’ll have watched some Big Bang Theory and compare me to Sheldon Cooper.

Most times the conversation will then disperse away from my area of research, but there are some people who will want me to explain in more detail what string theory actually is and what impact it has on the world.

To this question there is no quick answer, so at the risk of boring my audience I launch into a soliloquy that is as concise as I can muster.

To anyone who is confused about what it is exactly that theoretical physicists do, here is a brief yet broad summary of what we’re trying to achieve.

## Quantum Theory and General Relativity

We can think of modern theoretical physics as being split into two different ways of thinking. There’s the quantum mechanical way of thinking about the world, and the relativistic way of thinking about the world.

We can also characterise the world in terms of four fundamental forces:

- Gravity – governs how massive bodies attract one another
- Electromagnetism – governs everything to do with electricity, magnetism, light and the forces that hold molecules and atoms together
- Strong Nuclear Force – governs the forces holding nuclei together
- Weak Nuclear Force – governs the forces behind the breaking up of particles in radioactive decay

The quantum mechanical way of thinking deals with phenomena of the world on the smallest of scales. When we think quantum mechanically, we think of each of the four forces above as operating by the exchanging of particles called “bosons” or “force carriers”.

For example in electromagnetism we have the photon; the photon is a particle of light, but it also serves as the particle that transmits the force of repulsion that two particles of the same charge would feel. The photon is kind of like a messaging system between two particles. The particles exchange photons between them and tell each other how to act.

On the other hand, the relativistic way of thinking is very different. When we think relativistically we think on large, not small scales. Instead of thinking of forces as being transmitted by bosons, we explain them in terms of the shape of space. When we think about the force of gravity that acts between the Earth and the Sun, we explain it by appealing to the fact that the mass of the Sun causes the space around it to change shape, like placing a bowling ball on a sheet. The Earth moves on an elliptical orbit because it follows straight lines on this curved space.

## The Holy Grail of Theoretical Physics

The aim of physics can be broadly summarised as “the unification of the forces”. What we want to achieve is the development of a framework that encompasses all of the four forces under one mathematical structure. Maxwell’s equations of electromagnetism provide one of the first historical examples of this unification that physicist’s strive for.

Before Maxwell, electricity and magnetic phenomena were plentiful and varied. One could tell that these phenomena were related, but in mathematical terms they were disparate, just a random collection of laws and relationships.

Maxwell’s place in scientific history was thus assured when he discovered the mathematical framework which unified all these different phenomena and explained how they arose from a set of just four mathematical equations:

The aim of modern physics then is to achieve a similar thing to what Maxwell did, but on a grander scale. The aim is to discover some single mathematical structure with which we can explain the workings of all four of the fundamental forces simultaneously.

The trouble is in reconciling the two very different ways of thinking that I described earlier.

To date, the quantum way of thinking has successfully unified three of the four fundamental forces under one mathematical structure. We can understand electromagnetism, the weak force, and the strong force in terms of quantum field theory. Electromagnetism is mediated by photons, the weak force by the W and Z bosons, and the Strong force by particles called gluons.

Gravity however has resisted explanation in terms of the quantum way of thinking. Particles such as the graviton have been hypothesised, but no evidence for their existence has been found.

## The Approaches

There are two approaches to take from here. One approach is to say that we should think relativistically about the world and try and explain the forces in terms of geometry. The other way thinks we should develop a theory of gravity from a quantum perspective.

Quantum theories of gravity try to make gravity submit to the quantum way of thinking. As we have bosons for the other fundamental forces, it is thought that gravity may have a boson called the graviton which transmits the gravitational force. The problem with this however is that the addition of gravitons into quantum field theory leave the theory “non-renormalisable” – which basically means that some of the predictions of the theory lead to infinite, physically meaningless answers.

On the other hand, String Theory attempts to explain the four forces in the same way that gravity is explained – in terms of the shape and geometry of space. Where gravity is explained as being the shape of space in the four dimensions of space and time, a force like electromagnetism would be explained in terms of vibrations in an extra fifth dimension that we cannot see.

String theory then suggests the existence of other dimensions which we have never detected before because they’re wrapped up into a tiny size.

There are other approaches still, Supersymmetric theories try to exploit the symmetries in the Standard Model of Physics and develop them mathematically.

*The bottom line is that we want to try and unify the four fundamental forces of nature, but the jury is out on which approach to doing this will end up successful.*

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