system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In quantum mechanics, what do they mean when they say a particle takes "every possible path"?

I'm referring to Feynman's sum of all paths, which is often mentioned in layman physics books but rarely fully explained. What counts as a "possible path"? Often it is described as a particle travelling from point A to B could include a path in which it heads off to Alpha Centauri and back again. I know quantum physics is very counter intuitive, but in what way are we supposed to picture the particle heading off to Alpha Centauri? What sent it in that direction? What would have caused it to return? And are we assuming it travels below light speed (in which case that path would take years) or does this it not matter if it moves faster than light since this path is not the actual one measured, but only incorporated mathematically (whatever that means)? In that case can we take it to the extreme and say the particle also took a path to the other side of the observable universe and back again?
gpt
A basic difference between classical physics and quantum theory is the fact that within the quantum world, certain predictions can only be made in terms of probabilities.

As an example,  take the question whether or not a particle that starts at the time A at the location A, will reach location B at the later time B.

In classical physics, an answer can be given depending on the particles velocity and the forces acting upon it, giving a simple yes or no answer. BUT,  In quantum theory, it is only possible to give the probability that the particle in question can be detected at location B at time B. Quantum mechanics  therefore considers all possibilities for the particle travelling from A to B. Not only the boring straight-line approach, but also the possibility of the particle turning loopings and making detours etc.