Today, two great theories, each extremely complete, allow to explain ALL possible physical phenomena. Problem: they are incompatible. For more than a century, scientists have been trying to unify them into a single elegant theory that would be able to describe our entire universe. This is the Theory of Everything.
Why does the Moon revolve around the Earth and the Earth around the Sun? What is the nature of light? What is all the matter around us made of? To answer these fundamental questions and a thousand others, in short to describe the functioning of our world, physicists have not ceased to develop over the centuries ever more precise theories, as the people of the “Lord of the Rings” have forged the different rings of power.
What are the two irreconcilable theories?
The first is quantum physics, which we have already talked about here (put the link to the Pop Science video). This theory, also called the standard model, explains all the phenomena that occur at the microscopic scale, that is to say the functioning of three of the four fundamental forces of nature:
The electromagnetic force, which roughly explains the behavior of light
The strong nuclear force, which binds the elementary particles within atoms and therefore allows all matter to exist.
The weak nuclear force, that is radioactivity.
Note in passing that it is to quantum physics and to all its brilliant craftsmen that we owe among other things: electricity, the laser, computers, nuclear power plants…
Next to quantum physics, the other major theory is general relativity.
Developed by Albert Einstein in the early 20th century, it describes the world on a large scale.
With this theory, Einstein explains the nature of gravity, the fourth fundamental force of nature. He revealed that it is the consequence of the curvature of space-time. It is to this theory that we owe… the GPS!
Quantum physics as well as general relativity have been validated many times by experiment with success and with extreme precision.
But these two magnificent theories do not work together.
When physicists try to add gravity to the equations of quantum physics, the mathematics no longer makes sense. They say that the equations collapse.
You will say to me: SO WHAT? We have a good theory for the infinitely small, a good theory for the infinitely large, isn’t that enough? Well no, it is not enough!
Because we absolutely need to marry the two to describe phenomena where the infinitely large and the infinitely small meet.
For example the Big Bang, when the whole universe was contained in an infinitesimal point, and the black holes, these tiny objects in size and yet massive as hundreds, thousands, millions of suns.
If physicists could unify quantum physics and general relativity into a theory of everything, they might be able to answer some very existential questions: what was there before the Big Bang? Are black holes gateways to other regions of space, or to other universes?
The theory of everything in physics is the Grail, the ultimate quest, the answer to the secrets of the universe, the good that triumphs against the evil!
So where are we in this quest that has been going on for over a century? When will this unified theory of the universe be available? Not for the moment. It is because unifying these two theories is diabolically complex.
In the 80’s, we thought we could unify the 4 forces with the string theory.
This theory states that each elementary particle is the result of a specific mode of vibration of a kind of 1-dimensional object in the shape of a string. A bit like a guitar string whose mode of vibration determines the note.
In this model, four different modes of vibration generate the 4 fundamental forces. And that’s it.
The only problem is that string theory implies a bizarre 11-dimensional universe, but above all, it has a multitude of variants on which physicists do not agree. But an elegant theory of everything should put everyone in agreement.
But the quest continues. Today, it mobilizes hundreds of researchers all over the world, more motivated than ever to get their hands on the precious Grail of physics and to decipher the secrets of the universe.