Physics of the Future: How Science Will Shape Human Destiny and Our Daily Lives by the Year 2100

roughly the size of a grain of sand.
    (We sometimes amaze our students with a simple demonstration. We take a Geiger counter, place it in front of a student, and put a harmless radioactive pellet in back. The student is startled that some particles pass right through his body and trigger the Geiger counter, as if he is largely empty, which he is.)
    But if we are largely empty, then why can’t we walk through walls? In the movie
Ghost,
Patrick Swayze’s character is killed by a rival and turns into a ghost. He is frustrated every time he tries to touch his former fiancée, played by Demi Moore. His hands pass through ordinary matter; he finds that he has no material substance and simply floats through solid objects. In one scene, he sticks his head into a moving subway car. The train races by with his head sticking inside, yet he doesn’t feel a thing. (The movie does not explain why gravity does not pull him through the floor so he falls to the center of the earth. Ghosts, apparently, can pass through anything except floors.)
    So why can’t we pass through solid objects like ghosts? The answer resides in a curious quantum phenomenon. The Pauli exclusion principle states that no two electrons can exist in the same quantum state. Hence when two nearly identical electrons get too close, they repel each other. This is the reason objects appear to be solid, which is an illusion. The reality is that matter is basically empty.
    When we sit in a chair, we think we are touching it. Actually, we are hovering above the chair, floating less than a nanometer above it, repelled by the chair’s electrical and quantum forces. This means that whenever we “touch” something, we are not making direct contact at all but are separated by these tiny atomic forces. (This also means that if we could somehow neutralize the exclusion principle, then we might be able to pass through walls. However, no one knows how to do this.)
    Not only does the quantum theory keep atoms from crashing through one another, it also binds them together into molecules. Imagine for the moment that an atom is like a tiny solar system, with planets revolving around a sun. Now, if two such solar systems collided, then the planets would either crash into one another or fly out in all directions, causing the solar system to collapse. Solar systems are never stable when they collide with another solar system, so by rights, atoms should collapse when they bump into one another.
    In reality, when two atoms get very close, they either bounce off eachother or they combine to form a stable molecule. The reason atoms can form stable molecules is because electrons can be shared between two atoms. Normally, the idea of an electron being shared between two atoms is preposterous. It is impossible if the electron obeyed the commonsense laws of Newton. But because of the Heisenberg uncertainty principle, you don’t know precisely where the electron is. Instead, it’s smeared out between two atoms, which holds them together.
    In other words, if you turn off the quantum theory, then your molecules fall apart when they bump into one another and you would dissolve into a gas of particles. So the quantum theory explains why atoms can bind to form solid matter, rather than disintegrate.
    (This is also the reason you cannot have worlds within worlds. Some people imagine that our solar system or galaxy might be an atom in someone else’s gigantic universe. This was, in fact, the final scene in the movie
Men in Black,
where the entire known universe was in fact just an atom in some alien’s ball game. But according to physics, this is impossible, since the laws of physics change as we go from scale to scale. The rules governing atoms are quite different from the rules governing galaxies.)
    Some of the mind-bending principles of the quantum theory are:
     
    • you cannot know the exact velocity and location of any particle—there is always uncertainty
    • particles can in some sense be in two places at the same time
    • all particles exist as mixtures of different states simultaneously; for example, spinning particles can be mixtures of particles whose axes spin both up and down simultaneously
    • you can disappear and reappear somewhere else
    All these statements sound ridiculous. In fact, Einstein once said, “the more successful the quantum theory is, the sillier it looks.” No one knows where these bizarre laws come from. They are simply postulates, with no