What’s with this Wave Particle Duality?

I thought that I’ll make a small post about what “things being a particle an a wave at the same time” actually means so that the two people who actually do read my blog don’t get confused in the future.

When someone is trying to convey how quirky quantum mechanics is, they tend to bring up the dual nature of matter and radiation, and while it’s cool to think of things as both particles and waves, it kind of paints a false picture. You see, things are not both waves and particles, it would be more appropriate to think of them as neither waves, nor particles. But let’s not get ahead of ourselves, what even are particles and waves?

Without dirtying the water with too many details, we can call any localised object with a definite mass and position in space (more or less, please don’t go quantum on me), a particle. A wave on the other hand, is something which is not localised, that is cannot exist in a specific point in space and shows interference. What is interference you ask? It’s basically two or more waves coming together to make higher peaks and deeper valleys or totally cancel each other out, or something in between. It turns out that interference is a defining characteristic of waves. If something interferes, you can be pretty sure it’s a wave.

In 1801, Thomas Young’s double split experiment showed that light showed this phenomena of interference, putting an end (or so people then thought) to the century long debate about whether light is a wave or a particle. About a hundred years later, the photoelectric effect would be discovered, an effect which could not be explained if we thought of light as waves. So what was going on here?! Is light a wave or a particle? Sometime later the double slit experiment was done with electrons and surprise, surprise, they too showed interference! With the world they knew and loved collapsing around them, physicists of the twentieth century were lost, that is until a few amazing individuals came to the stage and tried to clean up the mess. They were mostly successful, but still left many questions for us to answer.

The only reason why people say things act like both waves and particles is because in certain experiments one thing might behave like a particle whereas in another experiment the same thing might behave like a wave. That’s why I don’t like people saying matter acts like both particles and waves. Matter has the properties of a particle in one experiment and that of waves in another. To label matter as “both wave and particle” would be claiming that we know something about what is happening and why is it happening even when we do not. It’s like seeing a bright light in the sky at night, and saying “I have absolutely no idea what that source of light is, hence it must be aliens visiting from outer space.”

NO! You can’t leap from one to the other like that. The only thing we can conclude is that matter is neither a wave nor a particle, at best it’s something in between.

As to why something like this even happens, no one really knows. Some say it’s a meaningless question, and ultimately we should nit question the things we cannot observe. Others say that what we conventionally think of as a particle, does not follow a single path in space when travelling, but all possible paths at the same time (“Why?” you ask, because quantum mechanics), and these particles which travel different paths (in parallel realities) interact with each other, causing interference. As crazy as that last sentence sounds, it has mathematics basking it up, you can read more about it here:

https://en.wikipedia.org/wiki/Many-worlds_interpretation

https://plato.stanford.edu/entries/qm-manyworlds/