#1442 Why can’t I walk through a wall?

Why can’t I walk through a wall? We can’t pass through a wall because of the Pauli Exclusion Principle and because of electromagnetic repulsion.

If you think about it on the microscopic level, there is a huge amount of space between the atoms in anything. They are like planets in the solar system. So, if there is that much space, you would think we would be able to slide our atoms between the atoms in the wall and come out on the other side. But it doesn’t quite work like that. If you tried to run through a wall, you would end up with a broken nose and probably a concussion. You would still be on this side of the wall. A single particle, on the other hand, could pass through the wall because of something called quantum tunneling. Quantum tunneling is when a particle passes through a barrier as a wave rather than as a particle. We can’t do this, and if we could, we probably couldn’t put ourselves together again on the other side of the wall.

The atoms in my fingers and the atoms in the keys of the keyboard I am typing on are different and are separate. There is no way that they can combine. There might be space between the atoms, but all atoms have electrons around them, and those take up space. They don’t orbit nicely, like in a textbook diagram, but swarm around the nucleus at different levels. This mist of electrons is called a probability cloud, and it is formed by where the electrons might be, not by where they actually are.

Electrons, along with neutrinos and other subatomic particles, are fermions. And no two fermions can exist in the same place at the same time. This is called the Pauli Exclusion Principle, named after an Austrian physicist, Wolfgang Pauli. Fermions (electrons) have a set amount of energy, and they exist in a specific energy state. If you started to move through the wall, your atoms with their probability cloud of electrons with energy would have to pass through the probability clouds of the atoms in the wall with their energy, and that cannot happen. The electrons in each atom would be forced into a higher energy state by the energy of the other atom, and that can’t happen because those bands are already filled with electrons. There is a steep rise in energy as you bring the two atoms together, and the only way you can overcome this is to add an enormous amount of energy, such as in nuclear fusion.

The second thing that stops you from walking through the wall is electromagnetic repulsion. Electrons are always negatively charged. The electron cloud around the nucleus of the atoms in your face and the electron cloud around the nucleus of the atoms in your wall are both negatively charged. We know that like charges repel each other, and the electrons in the wall repel the electrons in your face. You can bring them together, but you have to have an inordinate amount of energy, and even then, you wouldn’t go through the wall so much as become part of the wall.

That is not to say that particles cannot pass through a wall. They can, but only single particles, and only on the quantum scale, and only some of the time. This is called quantum tunneling, and it is where particles can pass through a barrier rather than going over it. It happens because on the quantum scale, particles are both waves and particles. They are moving waves until you observe them, and then they become stationary particles. This is Heisenberg’s uncertainty principle, where you can know the velocity of something or its position, but not both at the same time. Imagine it like a ball rolling on the floor. You can measure how fast the ball is rolling, but if you try to pin the location of the ball down to one point, you can no longer tell how fast it is moving. You can only ever know the velocity or the location. Quantum particles are waves, which means they can pass through things like a wall with no trouble. If you have a quantum wave and observe it, the particle will freeze. Sometimes the particle will be on this side of the wall, and sometimes the particle will be on the other side of the wall. There is an equal probability that it will be on either side. This is called quantum tunneling.

Interestingly, the particle travels to the other side of the wall almost as quickly, no matter how thick the wall is. If you have a wall that is 10 cm thick, a wall that is 10 km thick, and a wall that is 10,000 km thick, the particle will get to the other side of the walls in about the same time. That makes it seem that quantum tunneling through a barrier can allow a particle to travel faster than light, which we know is impossible. No scientist yet knows how they can do this, but they are trying to find out. And this is what I learned today.

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Sources

https://en.wikipedia.org/wiki/Quantum_tunnelling