I read this stuff in my spare time (because I'm fascinated by it!) and have no actual physics background, so please bear with me as I try to ask this question that has been bothering me: Right now I'm reading a book by Brian Greene called 'The Fabric of the Cosmos'. In his chapter on 'Entangling Space' he describes the probability wave - which I've heard the term before - but am only now seeing what it is. This quote (paraphrased) from the book is really confusing me: That last line really throws me. We know that a hydrogen atom has 1 electron, and we can't KNOW it's position before we measure it. So if that's the case how do we KNOW that it is in an orbit around the proton unless we actually look? It looks like understanding probability waves goes a long way to understanding QM, so I'd like to try and understand this as best as possible and I'd very much appreciate any assistance.