Quantum Entanglement & Spooky Action at a Distance

> it's both simply because it hasn't been measured ... Meaning it's either one or the other A common misconception about quantum superposition is that the particle "has" some state, and we only use superposition to model that we "dont know". This is called the hidden-variable theory ( [link] ). It's been proven that this hidden variable theory can't be true because of [link] . This is completely non-intuitive. See this video for a laymen's explanation: [link] (you may also need to watch the prev. video about the EPR paradox [link] ). Edit: actually, this is a much better video [this video] - less technical and more visual.

You understood correctly that quantum mechanics cannot be based on some hidden variable we haven't or cannot find. (Edit: oops, I re-read your comment and it seems you say the exact opposite thing? The point is that Bell's theorem rules hidden variables OUT. Here's an easily digestible video about this: [this video] ) About probability: no one knows. It might be that there is a "wave function collapse" that has a probabilistic outcome. But it might be that there is no such a thing. Quantum mechanics that remain in the "quantum" realm are not probabilistic, it's only when you cross over to the classical world. So it might be equally well, that the Everett interpretation (so-called "many worlds interpretation") is true, and the probability is something physicist Sean Carroll calls "self-locating uncertainty": [link] That is, there are multiple "yous" that experience different outcomes, and you don't know which branch of the wavefunction you find "yourself" in, which "you" you are.

So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? This outside observer would simply observe that the particle has spin "up" or "down" but would never know that it was entangled earlier by someone else. This observer would never know it was in a superposition unless they had knowledge that someone had previously done this. Only the person who entangled the particle can clue the observer in on the fact that they were entangled. I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. You only know about its current state. There is nothing in your observation that shows "hey the particle just changed from superposition to spin up" Thanks for the replies! I watch "word science festival" and similar videos and I am fascinated by the quantum world but I have wondered these questions for a while and didn't know anyone who could answer them. I saw this great Veritasium video about measuring quantum particles but it still left me with the questions above [this video]