3 ms·

Isn't this distinction exactly what the article is about? By saying ahead of time, "one ball is red, the other is blue", you're describing a hidden-variables t

Isn't this distinction exactly what the article is about? By saying ahead of time, "one ball is red, the other is blue", you're describing a hidden-variables theory of entanglement. It may be unknowable (before measurement) which color the ball in your left hand is, but it has a color. But Bell's theorem provides a very measureable counterexample to this type of explanation of entanglement. Sure, in the article they talk about electron spins instead of ball colors, but the analogy is that there isn't a well defined "color of the ball" before it's measured. Of course, the analogy breaks down a bit: electron spin can be measured in multiple axes with somewhat complicated interactions.

tylerhou 5y ago
> By saying ahead of time, "one ball is red, the other is blue", you're describing a hidden-variables theory of entanglement. No, consider the case of neutral pion decay, which emits one spin up electron and one spin down electron. We can clearly say ahead of time one electron will be spin up, and the other will be spin down. But there is no hidden variable that determines which. If there were a hidden variable, then knowledge of that hidden variable would let you predict which electron is spin up (which ball was red). In the macroscopic world, the hidden variable might be the state of my brain when it chose which hand to grab which ball. But if you replaced me with a robot, and that robot used the measurement of a quantum event (such as an electron's spin) to determine which ball to choose, then there is no hidden variable.
- ithkuil 5y ago
  This a good source for learning this stuff for real instead of pop-sci approximations: https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2013/ https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-s...
- kgwgk 5y ago
  > No, consider the case of neutral pion decay, which emits one spin up electron and one spin down electron. No, it does emit two electrons with total spin zero which is not the same thing. > We can clearly say ahead of time one electron will be spin up, and the other will be spin down. Let’s imagine that one was really up and the other was down. But you decide to measure instead the spins along a perpendicular axis. You would expect to find no correlation between them. However, what you actually see is that if you measure both spins along any (common) axis they will point in opposite directions. It doesn’t make any sense to say that before any measurement one was up and the other down. The red and blue balls analogy is very misleading and has nothing to do with entanglement.
  - chii 5y ago
    > The red and blue balls analogy is very misleading and has nothing to do with entanglement. this is exactly why classical analogies should not be used to describe quantum entanglement - it gives the layperson a wrong impression. Those analogies makes it easy for the layperson to imagine the hidden variable hypothesis, which is proven to be wrong.

tylerhou 5y ago

> By saying ahead of time, "one ball is red, the other is blue", you're describing a hidden-variables theory of entanglement. No, consider the case of neutral pion decay, which emits one spin up electron and one spin down electron. We can clearly say ahead of time one electron will be spin up, and the other will be spin down. But there is no hidden variable that determines which. If there were a hidden variable, then knowledge of that hidden variable would let you predict which electron is spin up (which ball was red). In the macroscopic world, the hidden variable might be the state of my brain when it chose which hand to grab which ball. But if you replaced me with a robot, and that robot used the measurement of a quantum event (such as an electron's spin) to determine which ball to choose, then there is no hidden variable.

ithkuil 5y ago
This a good source for learning this stuff for real instead of pop-sci approximations: https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2013/ https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-s...
kgwgk 5y ago
> No, consider the case of neutral pion decay, which emits one spin up electron and one spin down electron. No, it does emit two electrons with total spin zero which is not the same thing. > We can clearly say ahead of time one electron will be spin up, and the other will be spin down. Let’s imagine that one was really up and the other was down. But you decide to measure instead the spins along a perpendicular axis. You would expect to find no correlation between them. However, what you actually see is that if you measure both spins along any (common) axis they will point in opposite directions. It doesn’t make any sense to say that before any measurement one was up and the other down. The red and blue balls analogy is very misleading and has nothing to do with entanglement.
- chii 5y ago
  > The red and blue balls analogy is very misleading and has nothing to do with entanglement. this is exactly why classical analogies should not be used to describe quantum entanglement - it gives the layperson a wrong impression. Those analogies makes it easy for the layperson to imagine the hidden variable hypothesis, which is proven to be wrong.