Question: how would you explain quantum physics in the most simplist way?

James M Monk answered on 22 Jun 2011:

Quantum mechanics is about probabilities. Instead of saying that a particle is definitely at a particular place, you have to say it has a certain probability to be in several places – the sum of all these probabilities is one (meaning the particle has to be somewhere). So at each point in space, we can assign a probability that the particle is there.

We can also ask what the particle’s momentum is, and it turns out that the same probability that describes its position in space also describes its momentum. The catch is that if the particle is very constrained in its position, perhaps because you just made a measurement of it, then the range of possible momenta is very large.

So the rules of QM tell you that if you observe a particle or system in a given position/momentum, what the probability is to find the particle in another position/momentum at a later time. It never really tells you exactly what is going to happen (as, say, Newton’s laws of motion do), but it does tell you the probability that something will happen.

It’s quite a hard thing to explain in only a few words. When I was a student I read a book called “Schrödinger’s Kittens” on the subject – it could be well worth reading something like that if you are interested.