Suppose you have two particles, particle A and particle B. Suppose these two particles can interact in some way such that if particle A does something, it will cause particle B to change state.
Perhaps if particle A emits a smaller particle which strikes B, particle B will spin in a different direction. We will call that change in direction “event C”.
Logic would dictate that if particle A and particle B are to interact to cause event C, then some kind of physical action must occur. Particle A must take some action which then causes event C. A will have to emit some particle or vibrate some physical connection between particle A and particle B or somehow affect some kind of physical interaction.
Otherwise, how else could particle A cause particle B to change the direction of its spin? By non-physical means? Using an abstraction? I think not…
Well, we might ask how quickly can particle A cause event C?
Does event C happen 1 second after particle A does something? 1 millionth of a nanosecond? A femtosecond? Some other very short period which is very difficult to measure or fathom?
We know that it is not instantaneous. How do we know that? Because a literally instantaneous event refers to something that happens without any motion or change. But, if there is no motion/change, then how can particle A cause event C?
There has to be some motion of A or some other kind of change which causes event C to happen (see our discussion of time to help see why this is the case).
What is more, we know that particles A and B have to interact. And interaction is a kind of motion/change. There cannot be an interaction that occurs without motion/change, that is a contradiction in terms.
So, we can conclude that nothing is literally instantaneous.
The principle of locality states that for one particle to cause an effect on another particle, there must be some kind of physical interaction within the immediate vicinity. Be it interaction through some physical field or the exchange of particles. Or whatever kind of physical interaction might be permitted.
Relativity puts a speed limit on that interaction. Whatever physical interaction is taking place, it cannot occur at greater than the speed of light. And certainly not instantly. However, we have already ruled out instantaneous action without having to resort to relativity.
So, according to the principle of locality, for any kind of interaction between particles or anything else, there must be a physical interaction at a range close enough so that the speed of light is not exceeded.
This forbids things like two particles interacting at faster than light across a galaxy. Or interacting with something causing an instantaneous effect on anything else. And no interaction may go faster than the speed of light.
Well, realism and locality might seem quite reasonable. What could the problem be? In the next part of this series, we will see some of these issues.
It should come as little surprise that quantum mechanics disagrees with both of these principles and finds a way to try rationalize how they must be false.
We shall see that according to quantum mechanics, particles can instantly affect one another across any distance you want. And at that, without any kind of physical interaction! And that the act of observing creates reality!
Tune into the next part of the series to read more about entanglement and some of its history.