One of the most surprising properties of elementary particles is that they can be in several places at the same time, at least until they are observed. This happens because particles behave also as waves and, just like those, they can spread, diffract and interfere. A typical case of this is an electron going through a double slit, like in the picture below.
The typical set-up for the double slit experiment involves an electron gun, two slits and some screen where the electrons can be detected. You can shoot the electrons one by one and then look at the pattern that their impacts show on the screen.
At first, you don’t see much:
But later, a pattern starts to emerge.
This pattern will look familiar to readers who know about wave interference. It is something that typically happens when we send a wave, like a water wave, through two slits and see where the crests and the troughs form. Here you can see a video of the interference pattern of two waves:
However, the electrons are clearly particles: each electron crashes at a particular location, not everywhere on the screen. Even so, the pattern that they create as a whole is the same as that of two waves interfering. It is almost as if what behaved as a wave was the probability of finding the electron there.
This is, in fact, what happens. Before being observed, electrons behave as something called a “probability wave”: it is like a regular wave, but cannot be observed. When you observe the electron, it stops behaving like a wave and starts behaving like a particle, taking up a definite position just like a regular particle. However, until you observe it, the electron or, at least, its associated probability wave, can be in several places at once.
It is possible to take this experiment even further. For example, we could place an electron detector at the slits to find out through which one it really went. This should clear out the confusion and get rid of all that nonsense about the electron going through both slits. If you do this, you will find that the electron does choose one slit instead of going through both: however, the interference pattern will also disappear! Since you observed the electron, it stopped behaving as a wave and started doing what you would expect a bullet to do.
So are electrons particles? Are they waves? I find the question doesn’t really make sense. We humans are used to seeing rocks and water waves, but that’s not what the universe is made of. This is not the “real” stuff. The real stuff is electrons, which are both waves and particles. They are wavicles. It may seem bizarre to use because we have nothing familiar to compare it with. But nature couldn’t care less about what we find familiar. Nature does whatever it wants and it is up to us to get rid of our prejudices about reality and listen to what she has to say.