Quantum Entanglement & Spooky Action at a Distance

Does quantum entanglement make faster-than-light communication possible?
First, I know this video is not easy to understand. Thank you for taking the time to attempt to understand it. I've been working on this for over six months over which time my understanding has improved. Quantum entanglement and spooky action at a distance are still debated by professors of quantum physics (I know because I discussed this topic with two of them).

Does hidden information (called hidden variables by physicists) exist? If it does, the experiment violating Bell inequalities indicates that hidden variables must update faster than light - they would be considered 'non-local'. On the other hand if you don't consider the spins before you make the measurement then you could simply say hidden variables don't exist and whenever you measure spins in the same direction you always get opposite results, which makes sense since angular momentum must be conserved in the universe.

Everyone agrees that quantum entanglement does not allow information to be transmitted faster that light. There is no action either detector operator could take to signal the other one - regardless of the choice of measurement direction, the measured spins are random with 50/50 probability of up/down.

Special thanks to:

Nigel, Helen, Luke, and Simon for comments on earlier drafts of this video.

[shantanuraikwar]

"To understand spin, imagine a ball which is spinning, except it's not spinning and it's not a ball."

[xXPvPSkillerXx]

i am in a superposition of understanding this video and not understanding it at the same time...

[bengriffiths]

I remember watching this video when it was released as a first year uni student. Now I work with quantum entanglement on a daily basis and this video was one of the things that piqued my interest in this field

[asp]

The particles may not be actually spinning but my head certainly is.

[JerseySlayer]

I've watched this explained 50 times, 50 different ways, and I still only understand 50% of it.
And for some reason, it's still interesting.

[tiqvahone]

He lost me when he said "They're not actually spinning of course, they just have angular momentum and direction".

[RahimRahmat]

I just sat down, listen intently to the contents of this video, and suddenly 9 minutes have passed without me knowing it and the video ends. What a brilliant presentation.

[matteloht]

Heisenberg and Schrödinger get pulled over for speeding.
The cop asks Heisenberg "Do you know how fast you were going?"
Heisenberg replies, "No, but we know exactly where we are!"
The officer looks at him confused and says "you were going 108 miles per hour!"
Heisenberg throws his arms up and cries, "Great! Now we're lost!"
The officer looks over the car and asks Schrödinger if the two men have anything in the trunk.
"A cat, " Schrödinger replies.
The cop opens the trunk and yells "Hey! This cat is dead."
Schrödinger angrily replies, "Well he is now."

[markmd]

In order to understand that we don't understand entanglement, we should first understand that we don't understand spin.

[JackMcClauren]

"In order to understand it we must first understand spin. All fundamental particles have a property called spin. No, they're not actually spinning." You lost me. (0:41)

[sinny_rl]

I remember watching this video when it came out back when I had just finished primary school, I barely understood anything. Now rewatching it, as I'm in the midst of my Quantum Mechanics course in Uni, it's both nostalgic and satisfying finally being able to make sense of these concepts. Amazing video, thank you!

[jjathan]

the spooky thing is, you're using yourself as a particle

[WelshGuitarDude]

I really wish there was a good video explanation of what a particle spin is....ive never understood it.

[veritasium]

Thank you to those trying to make sense of this! For clarification:

1. We know the entangled particles must have undefined spins before we measure them because if they didn't they would sometimes give the same spin when measured in a direction perpendicular to their well-defined spins (and they never do).

2. We know the entangled particles can't have hidden information all along about which spin they will give in different directions because if they did we would measure different results at the two detectors >5/9ths of the time and we don't - we only get different results 50% of the time.

3. We can't use this behaviour to communicate faster than light because we can only pick the direction to measure in, we can't force the spin to be up or down - and it will be random with 50/50 probability. When the two detectors pick the same direction to measure in the results at one detector will be random but the opposite random of those measured at the other detector, which is a bit spooky.

[Sander]

It's very complicated but I find this fascinating. It's one of those educational video's that you need to watch a couple of times before you start to understand.

[kavi]

This is the best explanation I've seen of Bell's theorem, excellent video

[pollytheparrot]

I don't understand any of this, but since it's technically teaching me something, I don't feel bad about not starting my paper on Shakespeare.

[markomus]

Particleman... Particleman... does whatever a particle can. Spins around...up or down. Gets entangled. Traverses town...

[johannaverplank]

I've always struggled to understand the experiments you mentioned regarding measuring entangled particles, and I found your visual representations to be very helpful.

[buddhamack]

Now I just need a video explaining this video