Quantum Entanglement & Spooky Action at a Distance

If quantum-entangled particles exist (they do) you can use them to coordinate in a way that's impossible within a "hidden variable" model of the universe. You can dig into this by searching for "bell's inequality" or "bell test experiments", there are a few different experiments that have been done many, many times. There's a pretty good Veritasium video that explains one such experiment in non-physicist terms: [VIDEO]

I can't recommend Veritasium and CGP Grey enough. They are amazing YouTube Channels! Check out this video where Veritasium explains Quantum Entanglement. [VIDEO] It amazes me how he explains something so difficult in less than 10 minutes!

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 [VIDEO]

Check out [VIDEO] It's my favorite video for explaining how we know that the state is not determined until it has been measured. It's not just a decision we thought up on paper. It is real and well proven.

[VIDEO] is a good overview that I really like for Bell's inequality. Worth a watch if you have the time.

I don't think that explanation is clear. It takes a bit more work to get something which proves the experimental difference. One of the better explanations I've seen is regarding measuring spin with three state detectors: [VIDEO] This would give specific probabilities to certain outcomes (not just 50/50) IF there were a local hidden variable. (Note: the word local is important there. It means information encoded and carried by or with the particles themselves.)

If you watch for more than 4 minutes, this 9 minute video explains how Bell proved that the particles can't have been given opposite "ball colors" at the time they became entangled. [VIDEO] It's quite clever how he proved it, and that it's actually possible to rule out "information that we're just not aware of".

> 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 [VIDEO] - less technical and more visual.

Your example is akin to the Hidden variable theory (in the example the hidden variable will be the fact the two letters are copies). Bell's inequality tests have ruled out the hidden variable theory. Veritasium has a good video explaining Bell's inequality - [VIDEO]

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: [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.