Imagine a glass with a certain amount of water. There will be those who think the glass is half empty and those who think 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. This little exercise in imagination is the explanation of the vacuum of space.
The vacuum of space explained in a way that everyone can understand
Filled with nitrogen, oxygen, argon and other gases, the glass is full of air. Even if we managed to get all the air out of the glass, down to the last molecule, it would still be full. This time full of photons, because being at a certain temperature it will emit electromagnetic radiation. 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 able to perceive, 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 light emitted will be. Therefore, we should be able to cool it until the emitted light has energy 0. That is, until no light is emitted.
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We would achieve this by reaching the lowest possible temperature, known as “absolute 0”, which is equivalent to approximately -273 ºC, or 0 K. As far as we know, it is impossible to reach this temperature. Suppose that after removing the water and air from the glass we manage to bypass the third law of Thermodynamics, the one that prevents us from reaching absolute 0. In this case, how is our glass, full or empty? Indeed, against all intuition, the glass will be full. I don’t know if “full” is the right word to describe the inside of the glass, but it sure won’t 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 leave 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 the naming, we thought it was indeed empty. Today we define vacuum as the lowest energy state in a given region of space.
The two concepts that explain how the Universe fills the void
The two concepts that will help you understand more easily
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 occurs when a change in temperature occurs. For example, water when it freezes or evaporates, or a magnet when it exceeds its Curie temperature.
Pierre Curie, who won the Nobel Prize for Physics in 1903 along with Marie Curie and Henri Becquerel, discovered at the end of the 19th century that when a ferromagnetic body, basically a magnet, is sufficiently heated, there comes a time when its it loses its magnetism, becoming a magnet after it cools.
The second concept is that the direction of a magnet, the direction of its magnetic field, is basically 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 various atomic magnetic fields will feed into each other, giving the macroscopic magnetic field of the magnet. Imagine that we had the magnet in a state where the magnetic field of each atom was pointing in a different direction.
At the microscopic level, we would not detect a magnetic field , because the minifields would counteract each other. If we now “let go” the magnet so that it falls into a lower energy configuration, these atomic fields will simply fall into a certain configuration and basically randomly, all aligned. This initial state, where each atom goes in a different direction, losing its magnetism because it would pass the Curie temperature. Allowing it to cool, the magnet “falls” into the state where its atomic magnets align, regaining its magnetism.
Similarly, if you heat the universe enough, it will have enough energy to create different types of particles. You can create particles of different types and amounts depending on the energy you have. It will be able to create quarks, electrons, gluons, photons… and when you let it cool, the universe will have no choice but to descend to the lowest possible energy configuration. But this configuration will not be one where all the particles annihilate each other and nothing is left, no.
Suppose for a moment that quarks, 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, meaning their minimum energy state tends to bring them together, so it will take less energy for the universe to fill space with quarks than to- leave it empty.
It will not be able to fill it completely with these particles, because then the principles of quantum physics would come into play that prevent it. There will be a certain density of quarks that will be optimal, and there will be countless possible mixtures of these types of particles.
The universe will not care which mixture it chooses, there will be no preferred state because they will all have less energy than leaving the universe empty. That is, as the universe cools and falls into a minimum energy state, it will have different states to choose from, all equivalent.