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Well, I know that the title might sound provocative, but that's precisely its main aim.

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The subject of this talk is "vacuum", which I describe as enigmatic. But don't worry,

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because this blank slide does not mean "here, that's vacuum, end of the lecture",

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it is more complex than that. The essential point, as you can imagine, is

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to explain the concept of "vacuum", its the definition, from the point of view of a

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physicists and of cosmology, which differ from other senses the word

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may have, some of them colloquial: power vacuum, vacuum cleaner...

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So the first thing I am going to do is to look for a definition that

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doesn't correspond accurately to the one we consider, but it is the closest

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one from the point of view of Science and Engineering. So the definition is

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as follows: "Vacuum is what remains when

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a space is void of molecules and atoms". In this context,

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I mean, using this definition we can talk of high vacuum,

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vacuum system,  vacuum pump ...

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The first thing I am going to do is to show you how near we are of

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achieving this vacuum experimentally. And we are actually a bit distant

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Firstly, I will start by something that could be estimated by any student,

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any high school student, which is the number of molecules found in a cubic metre

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(air or fluid). It is an extraordinary huge figure, septillions [U.S scale] of molecules,

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so the starting point is quite far from a "molecule-free" vacuum.

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But obviously, the experimental development eventually achieves the creation of

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some devices that will allow us to reach high vacuum levels, erasing some Zeros

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from that previous figure; but there still remain quite a lot of them, that is

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trillions of molecules. We haven't been able to cross that point and

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experimentally get closer to that "molecule-free" limit.

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If we take a look at our surroundings, we can see that even at the galaxy level,

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if consider a point far from the stars but

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inside the galaxy, the number of molecules found in a cubic meter is still high,

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althought much smaller: 5 hundred thousand molecules

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in a cubic meter. But there is something that is quite near to that vacuum:

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the average considering the whole universe, which can be estimated

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by calculating how many molecules, in fact, how many

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protons and neutrons (each molecule is constituted by several protons and

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neutrons) there are. So the result obtained after estimating

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the average number of protons and neutrons,

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referred to as baryons when grouped, is less than 1, so we could

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say that we are somehow nearer to the limit of reaching vacuum.

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But we needed to change and to clarify the definition,

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when we found out that atoms, despite their name, are divisible.

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They are made of particles: protons, neutrons,electrons... this allows us to

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erase atoms, destroy them. If we heat them enough we can

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delete these atoms and divide them into protons and neutrons,

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or electrons, nucleus. That is what created the need to alter the definition

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for vacuum from the one that I said before to this one, in which words like

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"molecule" or "atoms" are replaced by "fundamental particles". So, considering this modified concept,

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we understand as "vacuum" the place free from fundamental particles.

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Excerpt from the conference held

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November the 5th, 2015, at the Residencia de Estudiantes,

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during the sessions "Los límites de la Física Fundamental"

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Full video available at: www.eddaddeplata.org