The reason why you can not go through walls, according to science

The mutant superheroine of the Marvel universe, Kitty (ShadowCat) has the ability of "phase" that allows it to become intangible (and, therefore, to be able to cross walls). But why can not we - the common mortals - do the same?

Our body, like all the ordinary matter of the universe in its majority, is made up of atoms of different quantity and nature. Atoms are, in essence, a large empty space, since 99.9% of the mass of the atom resides inside the nucleus. So, taking into account that electrons only contribute 0.1%, it would seem plausible that being essentially an empty space, we could penetrate the walls. Do not?

For the fact that the nucleus of the atom is so heavy is the key: We are strong enough that the elements in our atoms can not pass through the empty spaces of other atoms, and vice versa. So the intangibility of Shadowcat is going to have to remain part of the fiction.

Everything has to do with the electrons that orbit the nucleus of an atom. The pictures of the atoms show electrons that 'buzz' around a nucleus in a nice ordered pattern, but that is not the case at all: they somehow swarm around it in something that looks more like a cluster of clouds.

To pass through another atom - and to get through objects - the electrons of the first atom would have to exist, albeit briefly, in the same atomic space as the electrons of the second atom. And, in a few words, this is impossible.

As formulated for the first time by the Austrian physicist Wolfgang Pauli in 1925, there can not be two fermions in the same quantum state within the same quantum system. That is, there can not be two electrons occupying the same space doing the same job. It happens like the legendary movie "The Immortals", there can only be one. It is what we know as Pauli's Exclusion Principle and it applies to all fermions. It also means that atoms are quite effective in preventing other atoms from entering their space.

This is what makes solid objects solid and prevents them from passing to each other.

Does that mean you can never touch? Let's say it's complicated.

In quantum mechanics this is sometimes explained as a repulsive force between the two fermions, and the popular interpretation of science is that this prevents atoms from touching other atoms. But the way in which the word "force" is used to describe these interactions does not translate into the way the word is used in the world at large.

According to research published in 2003 in the American Journal of Physics (published in full in arXiv), the word "force" is a poor analogy that has the potential to be misinterpreted by all of us but, for the time being, it does not we have a better one

Van der Waals forces

According to Philip Moriarty, professor of physics at the University of Nottingham (England) "contact" exists at the atomic level: it is the point at which the attractive force of Van der Waals (a type of intermolecular force that is created between molecules that makes that can generate attraction or repulsion and that allow substances to have different physical properties such as boiling point or density), balances Pauli's repulsion factor. But that is not necessarily the same as "touching".