What Are Grand Unified Theories

“Symmetry, once broken, still leaves a trace. That trace is what we see in the laws of nature.”

– Salam on the role of symmetry breaking in unified theories.

What are Grand Unified Theories?

Grand Unified Theories (or GUT for short) are theories which try to merge the three of the four fundamental forces of nature – the strong nuclear force, the weak nuclear force and electromagnetism into one unified theory.

These GUTs are theories take us furthest back in time. Why? Because each one is an attempt to show that the three basic forces of nature – the strong, weak, and electromagnetic interactions – which govern the behaviour of all matter, were once unified into a single force (a so called “Superforce“) in the earliest moments after the Big Bang.

Who Proposed the GUT?

GUTs were not invented by one single person. These theories are a result of contributions from many physicists. But the idea of a unified theory was first proposed in 1974 by Howard Georgi and Sheldon Glashow, two American theoretical physicists at Harvard University. They proposed the first true GUT based on the symmetry group SU(5). This is widely considered the first modern GUT.

Why Unite all Forces into one?

The idea of uniting all 3 of the 4 fundamental forces into one did not come out of nowhere. Experiments have confirmed that at higher energies, the electromagnetic interaction and weak interaction merge into a single combined electroweak interaction.

Let’s recap on what the four fundamental forces are:

• Strong Nuclear Force – a fundamental force that binds protons and neutrons together inside the atomic nuclei.
• Weak Nuclear Force – responsible for the radioactive decay.
• Electromagnetic Force – The force between charged particles (positive and negative). It includes electric and magnetic effects. Responsible for electricity, magnets, and electronics and much more.
• Gravity– is the force of attraction between anything with mass. It holds planets in orbits, shapes galaxies.

Many scientists have conducted research and it is theorized that the super force existed only during the earliest moments after the Big Bang, specifically within $10^{-43} seconds, known as the Planck time.

Which Forces were Successfully Unified?

As of today, the following forces have been successfully unified in experiments:

• James Clerk Maxwell showed that electricity and magnetism were two aspects of one electromagnetic field.
• Sheldon Glashow, Abdus Salam, and Steven Weinberg have unified electromagnetism and the weak nuclear force. These three brilliant physicists built a mathematical model where both forces come from one unified symmetry. Electromagnetism + weak force = the electroweak force. This unification has been experimentally confirmed.

So by then, two of the four forces of nature were already unified — leaving the strong nuclear force (described by quantum chromodynamics, or QCD) as the next puzzle piece.

Why is Gravity not included?

As you may have noticed, there are 4 fundamental forces of nature, but GUTs focus only on the three, excluding gravity. Why?

Because of all the four forces, three of them can be described as quantum fields, whereas Gravity is described as a curvature of space-time, and not made of “quanta”, (it probably does have quanta, but we do not yet have a working quantum theory of gravity) like the other forces are structured. The three forces rely on quantum particle interactions, and gravity is not made of any particles. That is why we need to use Einstein’s General Relativity for gravity and we can use Quantum Mechanics to study the world of particles, and therefore, the remaining three fundamental forces.

There is a hypothesis that a quantum particle called the graviton exists, but it has never been observed. If it did exist, perhaps we will then have a shot at combining all four forces together.

As of Today

As of today we have not found a unified theory. There is no single, universally accepted equation for a Grand Unified Theory. Yet.

• Quantum Mechanics for the other three forces
• General Relativity for gravity

There is no successful theory to combine these two frameworks at the moment and it the physicist’s greatest problem. However, the goal of a unified field theory has led to significant progress in theoretical physics.

If a successful Grand Unified Theory will be found, this theory would need to include all particles that we know of in the Universe – such as leptons, quarks, photons, gluons, and the carriers of the weak interaction.

Resources

The following books helped with my research for this article:

• Nothing – New Scientist, Edited by Jeremy Web
• The Quantum Labyrinth – Paul Halpern
• Brief Answers to Big Questions – Stephen Hawking

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Thank you, I hope you enjoyed the article. Please note, I carefully research the topic before publishing, however, some facts could have changed since publishing this article, so I appreciate your understanding. All articles published are thoroughly researched and inspired by published books. The list of resources are published in every article. Please be kind, and have a nice day.