Imagine a glass with a certain amount of water. Up to half his height, for example. There will be those who think that the glass is half empty and those who think that it is half full . Both are wrong, of course, because the glass is completely full . And if we were to drink the water it contains, it would still be completely full. Filled with nitrogen, oxygen, argon and other gases ; full of air Even if we managed to get all the air out of the glass, down to the last molecule and the last glass, it would still be full. This time full of photons , because being at a certain temperature (about 20 ºC if we have it inside the house) it will emit electromagnetic radiation. Light, for friends. In the same way that a star emits light or a very hot piece of metal has a reddish glow, this glass would emit light. Specifically infrared light, which our eyes are not capable of perceiving, but our night vision cameras are.
Well, the origin of this light is the temperature of the glass itself. The hotter it is, the more energetic the emitted light will be. Therefore we should be able to cool it until the emitted light has energy 0 . That is, until no light is emitted. We would achieve this by reaching the minimum possible temperature, known as “absolute 0”, which is equivalent to about -273 ºC, or 0 K (0 kelvin, without adding “degrees”). As far as we know, it is impossible to reach this temperature (at least in a finite number of steps, so to achieve it we would need infinite time). But hey, let’s suppose that after removing the water and the air from the glass we manage to skip the third law of Thermodynamics, the one that prevents us from reaching absolute 0 . In that case , how would our glass be, full or empty? Indeed, against all intuition, the glass will be full. I don’t know if “full” is the correct word to define the interior of the glass, but what is certain is that it will not be empty. It will contain something.
This is because, energetically speaking, it is profitable for the universe to fill the void of things rather than leaving it completely empty . An empty universe would have more energy than one filled with certain things. We call this state empty because at the time of naming it we thought it was really empty. Today we define the vacuum as the state with the lowest energy in a given region of space.
Before proceeding you need to understand two concepts . The first is that one of the many fields of study in physics is that of changes of state. A change of state is the transition between two different configurations that the same system can have and that transition usually takes place when there is a change in temperature. For example water when it freezes or evaporates, or a magnet when it exceeds its Curie temperature.
Pierre Curie , who won the Nobel Prize in Physics in 1903 together with Marie Curie and Henri Becquerel , discovered at the end of the 19th century that by heating a ferromagnetic body (a magnet, basically) enough , there comes a time when it loses its magnetism, it leaves from being a magnet , becoming so again when it cools down.
The second concept that I wanted to talk to you about is that the direction of a magnet, the direction of its magnetic field, is in principle random. That magnetic field is nothing more than the sum of the magnetic fields of each individual atom. When the magnetic field of all its atoms is aligned , that is, pointing in the same direction, the magnet will be in a stable configuration, in the lowest possible energy configuration . When this happens the different atomic magnetic fields will feed each other, giving the macroscopic magnetic field of the magnet. But any address is equally valid. After all, it doesn’t matter to the atoms of the magnet whether they point to the left or to the right.
Imagine that we had the magnet in a state in which the magnetic field of each atom pointed in a different direction . At the microscopic level , we would not detect a magnetic field , since the minifields would counteract each other. If we now “let go” of the magnet, so that it falls to a configuration of lower energy , these atomic fields will simply fall in a certain configuration and, in principle, randomly, being all aligned. This initial state, in which each atom points in a different direction, is achieved by taking the ferromagnetic material above its Curie temperature . By letting it cool, the magnet “falls” to the state where its atomic magnets line up, regaining its magnetism.
Similarly, if you heat up the universe enough, it will have enough energy (remember Einstein’s famous formula, the one that tells us that energy and mass are equivalent: E=mc2) enough to create different kinds of particles. You can create particles of different types and quantities depending on the energy you have. It will be able to create quarks, electrons, gluons, photons… you name it. And when you let it cool, the universe will have no choice but to drop to the lowest-energy configuration possible . But this setup won’t be one where all the particles annihilate each other and nothing is left, no.
Suppose for a moment that the up and down quarks (“u” and “d”), the constituents of protons and neutrons, were massless particles. So if they didn’t interact with each other, it would cost the universe exactly zero energy to fill space with them. But the thing is even better, because they interact by attracting each other, that is, their minimum energy state tends to bring them together , so it will take less energy for the universe to fill the space with uyd quarks than to leave it empty. It will not be able to fill it completely with these particles, because then principles of quantum physics would come into action that prevent it. There will be a particular density of quarks that will be optimal . And there will be countless possible mixes of these types of particles. The universe will not care which mix to choose , there will not be a preferred state because all of them will have less energy than leaving the universe empty. That is, as the universe cools down and falls to a state of minimum energy, it will have different states to choose from, all equivalent.
Sounds familiar to you, doesn’t it? That’s what happened with the magnet. Well, any of these states to which the universe falls spontaneously when it cools down, will be the energy of the vacuum. It will be those things that fill the void. Of course in our universe, the u and d quarks do not have mass 0, but they do have a very small mass, so the whole explanation given is complicated, but it is still valid.
REFERENCES:
S.E. Rugh & H. Zinkernagel , 2000, The Quantum Vacuum and the Cosmological Constant Problem, Studies in History and Philosophy of Modern Physics, vol. 33 (2002), 663-705, DOI:10.1016/S1355-2198(02)00033-3
F. Wilczek, 1994, 1012 degrees in the shade, The Sciences, Jan./Feb. pp. 22-30,
