Why can't we walk through walls if atoms are mostly empty space?

In movies, people phase through walls like ghosts — think Vision from "Avengers" or Harry Potter going through Platform 9¾. It looks effortless. But in the real world, trying that trick would just leave you with a bruised nose and a lot of questions.

One question, for instance, might be why can't we walk through walls? Atoms, which are the building blocks of matter, are mostly empty space. The tiny nucleus — which is about 100,000 times smaller than the whole atom — sits at the center, while the electrons orbit far away. So why do solid objects feel so ... solid?

There are two physics concepts that make walking through solid materials impossible: electrostatic repulsion and the Pauli exclusion principle, experts told Live Science.

Classically, an atom has a nucleus, which is made of protons and neutrons, and electrons that move around it. The positive charge of protons and the negative charge of electrons pull toward each other, holding the atom together.

But in quantum mechanics, the electron doesn't move in a neat circle. Instead, it forms a kind of cloud — a fuzzy area where it might be. This is called "a probability cloud," Raheem Hashmani, a doctoral student in physics at the University of Wisconsin-Madison, told Live Science. This cloud doesn't move. It just sits there, showing the places where the electron is most likely to be found.

The cloud makes the outskirts of the atom negatively charged. "If I try to walk through a wall, the atoms in my body are going to see the [ones] in the wall, and they are going to repel each other," Steven Rolston, a physicist at the University of Maryland, told Live Science.

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That is called electromagnetic repulsion — like when you try to push the same poles of two magnets together. When walking through a wall, the electrons are interacting through electromagnetic waves. These waves are part of the forces that prevent atoms from overlapping and why solid matter stays and feels solid.

But what if atoms were pushed even closer together?

That's where the Pauli exclusion principle comes in. It states that certain particles, called fermions, can't share the same energy state or be in the same place at the same time. Electrons are fermions, so in this case, the terms are interchangeable.

"When those clouds of electrons start to come near each other, they overlap, which means that two electrons might be sharing the same physical space," Hashmani explained. "Under Pauli's exclusion principle, this is not allowed."

Both concepts — the Pauli exclusion principle and electromagnetic repulsion — prevent atoms from occupying the same space. Without them, solid matter as we know it wouldn't hold its shape. In liquids and gases, atoms have more freedom to move, but the same rules still apply. They just keep atoms from overlapping, not from moving around.

However, even if it is nearly impossible for objects to pass through one another, quantum mechanics always offers an interesting answer: Technically, there's a tiny chance it could happen.

Particles like electrons don't behave like tiny solid balls. Instead, they also act like waves, and those waves can sometimes stretch past physical barriers.

Let's say a wave representing a particle hits a wall — a barrier it doesn't have enough energy to cross. In classical mechanics, it would just bounce off. But in quantum mechanics, the wave doesn't stop suddenly, Hashmani said. Instead, it starts to exponentially decay as it enters the barrier. If the wall is thin enough, that wave might still have a small presence on the other side. And because the wave represents the probability of where the particle might be, there's a tiny chance the particle will show up on the other side. This is called quantum tunneling.

Still, the probability of a whole person passing through a wall "would be something like 1 in 10 to the power of 10 to the power of 30," Hashmani said. "If you put that in a calculator, it'll effectively give you zero. No calculator on the planet is going to give you something that's not zero. That's how infinitesimally small the probability is."

Rolston agreed. "It's about as close to zero as you can get, but it is not zero," he said. "It's so infinitesimally tiny that I'm sure it wouldn't happen in the age of the universe."

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