Is Light a Particle or a Wave?

There is a video on YouTube which has Deutsch explain the famous quantum double slit experiment, which is about particle-wave duality. Is light a particle, or a wave? You pass it through a slit depending on whether there's an observer and interference or not. It ends up in a wave pattern, ends up as individual photons, and this is the famous experiment which has baffled people for a long time and caused them to revise their world view.

The one that led Einstein to say, "God does not play dice with the universe." Correct, Einstein was a realist at the time when the founders of quantum theory were trying to develop a good explanation of what precisely was going on with these experiments in quantum theory. Einstein rejected all of them on the basis that they weren't realistic, and he was right to do so because none of them made any sense. To this day, none of the other alternatives make any sense.

Now, Einstein didn't know about the multiverse. We had to wait until Hugh Everett in the 1950s was able to devise a simple, realistic way of understanding quantum theory. But if I go back to this idea of the double slit experiment, it is often claimed that particles have a duality to them. Sometimes they're particles, and sometimes they're waves. The electron, for example, given certain experiments, will behave like a particle, and in other experiments, it behaves like a wave.

People who hear this think, "Well, okay, that kind of explains what's going on." For example, in the photoelectric effect, you shine a light at electrons, which literally means you're firing a photon, a particle of light, at an electron, and you can knock the electron out of the atom. This is supposed to be proof positive that light in the form of photons and electricity in the form of electrons are both particles because they're bouncing off one another, and this is what particles do.

Waves don't do that. You watch water waves at the beach; you'll see they pass through each other; they don't bounce off one another. Waves will bounce off particles, but they won't bounce off each other. Prior to Young's twin slit experiment, we actually relied upon Newton's ideas of light, and Newton's idea was that light was corpuscular, as he said, which means made of particles.

Then Young came along, and he shone a light through two slits cut into a piece of paper. What you find when you project that light onto another sheet of paper is not just two beams of light; you find what's called an interference pattern where the light has interfered with itself, in the same way that when waves pass through small apertures or natural geological gaps, they will interfere with one another. They produce crests in some places and troughs in others; they can cancel each other out. This was supposed to be proof to some of the early physicists that light, in fact, was a wave.

Now we get to quantum theory, and we find that things we thought were certainly particles, like electrons, when we do the same experiment with them, they interfere with one another. So it appears as though we've got particles acting like waves and waves acting like particles. The resolution to this is not to admit nonsense. This is what is often explained in quantum theory lectures at the undergraduate level: that you have to accept that something like a photon is born as a particle, it lives as a wave, and then it dies again as a particle, which is nonsense.

The reason it's nonsense is because the photon doesn't know that it's alive and it's dead. It doesn't know what experiment it's participating in.