Quantum mechanics and general relativity: you have probably already heard about this. It sounds complicated and dedicated to an elite of gifted scientists. Actually, far from all these considerations, these concepts are approachable and chiefly fascinating! So, let’s start our journey from the infinitely small to the infinitely big (and mutually!).

### Did you say quantum mechanics?

Here is a scaring word! But so, what is this famous theory? In fact, classical physics is able to describe our environment but doesn’t explain the microscopic behavior of atoms and particles. To explain in a not so boring way, in classical physics you have the particles (atoms for example) and the waves (light, electricity for example). In quantum mechanics we are able to have a superposition of these states: an atom can be at the same time a wave and a particle. We name this a “wave-particle”, and this superposition of states is named quantum state. For instance, a wave-particle can be at different places or at different speeds at the same time! Counter-intuitive, no?

### Schrödinger’s cat!

Conceived in 1935 by Erwin Schrödinger (1887-1961), this thought-experiment became really renowned because it allowed people to understand the paradoxical aspect of quantum mechanics. In this experience, a cat is located in a closed box, where there is a device which detects radioactive atoms and kills the cat if it detects that the atoms are disintegrated. In quantum mechanics, the atoms can be in different states at the same time: here, the radioactive atoms can be simultaneously intact and disintegrated. So we can say that the cat is at the same time dead and alive!

### General relativity: explain to me, Einstein!

Before Albert Einstein (1879-1955), the notions of gravitation came from Isaac Newton (1642-1727) who considered that the gravitation was coming from attractive and repulsive forces. By developing various reflections, Einstein leads to a new vision of gravity, which will replace the one from Newton: general relativity! According to him, Earth, for example, is not spinning around the Sun because this one is exerting a force on Earth, but because of a perturbation of spacetime. Let’s have an easy and famous popularization example. You start by stretching an elastic fabric. After, you put a stone somewhere on the fabric and logically the fabric will bend where there is the stone. In fact, the fabric is the representation of spacetime and the stone represents the sun for example. Now, try to roll a ball in a straight line close to the stone and you’ll see that the path will be deflected: not because of a force but because of the bending of the fabric (this is an easy way to understand the famous expression “curvature of spacetime”).

Also, for Einstein, the time doesn’t have an invariant value: space and time are not different entities anymore, but just one, “spacetime”, with 4 dimensions (3 for space and 1 for time): this is notion of relativity. Space, like time, can be deformed by many things (like our previous fabric).

### Theory of everything: crazy dream!

The theory of everything is a theory which has not been discovered yet. Ultimate goal of physics nowadays, this theory aims to explain and describe, from only one fundamental law, all the phenomena in the nature, from the smallest element to the whole Universe. Unbelievable, isn’t it? Yes, but not so easy… Indeed, it’s needed to unify both previous theories, but the question of gravity remains insolvable. To be quick, there is an opposition about this in the nature of quantum mechanics and general relativity.

More than the physical field, such a theory would have a strong influence on many philosophical questions. “Why the laws of nature are what they are?” or “why is there something rather than nothing?”.

So, there is many interesting questions and we can imagine that the answers will bring us again a whole world of new questions. Maybe it’s the definition of the human being: finding answers and having lots of questions?

*——————————————
Tanguy Payet*

Sources:

http://www.cea.fr/comprendre/Pages/matiere-et-univers.aspx

https://www.science-et-vie.com/archives/le-reve-fou-de-la-theorie-du-tout-31479

https://fr.wikipedia.org/wiki/Chat_de_Schr%C3%B6dinger

https://www.futura-sciences.com/sciences/definitions/physique-mecanique-quantique-844/

https://www.futura-sciences.com/sciences/definitions/physique-relativite-generale-3572/

A Brief History of Time – Stephen Hawking