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Seemed more than just lacking intuitive explanations. In entanglement for example, the answer is because it can't transfer information & you learn that just fin

by Herring 18y ago

Seemed more than just lacking intuitive explanations. In entanglement for example, the answer is because it can't transfer information & you learn that just fine in any intro to quantum class.

timcederman 18y ago
Thank you. This is also the reason why faster than light 'travel' can occur, if it doesn't transmit information. Entanglement is slightly mysterious though as what is the physical mechanism behind it?
- yummyfajitas 18y ago
  The physical mechanism is that quantum mechanics governs a probability distribution (the "wave" in "wave-particle duality") on configuration space, not "physical" space. Imagine you have two particles in 1 space dimension. The first particle has a position x (1 real number), the second particle has a position y (1 real number). "Physical space" would be the set of real numbers (R^1), while configuration space is the set of pairs of real numbers (x,y) (this set is R^2). The first number in the pair represents the position of particle 1, the second the position of particle 2. An entangled state is simply a probability distribution on R^2 which is not a product state (i.e. P(x,y) != P_1(x) P_2(y)). This means that knowledge of the position of particle 1 gives you (possibly incomplete) information about the position of particle 2. One example of this is "probability 1/2 that x=0, y=0, 1/2 that x=1, y=1". The minute you know x, you know y. That's what entanglement is. What makes it weird is the idea that "knowing x" (measuring it) changes things. An explanation of this is trickier. Still understandable in principle (not just 'follow the math'), but too long for this post.
  - ericb 18y ago
    >> too long for this post. How about another post, then? :-)
- deleted 18y ago
  [deleted]
DaniFong 18y ago
It's not exactly covered convincingly though. The reason it supposedly can't transfer information is because you'd have to measure it, and we assume that measurement strictly follows Born's rules, which is a whole other can of worms. Sometimes people talk about density matrices instead, but we only use those since they are consistent with Born's rules. Additionally, many physicists point out that what you seem to be doing when you perform a measurement is entangling the system with the many bazillion degrees of freedom in the measurement apparatus and the world at large. We don't know, exactly, where the separation between a 'measurement-like' physical process occurs, or even if there really is one. There isn't even a standard measure of entanglement for three or more particles. It's a pretty big mystery, still.
- Herring 18y ago
  er, ok then i'll rephrase - "we're mostly sure it can't transfer.." or "we've never been able to get it to" I doubt the writer meant it the same way you do. He was talking about the known things that hes not wrapped his mind around. In this case, explaining why the theories don't necessarily conflict should do.

by Herring 18y ago

Seemed more than just lacking intuitive explanations. In entanglement for example, the answer is because it can't transfer information & you learn that just fine in any intro to quantum class.

timcederman 18y ago

Thank you. This is also the reason why faster than light 'travel' can occur, if it doesn't transmit information. Entanglement is slightly mysterious though as what is the physical mechanism behind it?

yummyfajitas 18y ago
The physical mechanism is that quantum mechanics governs a probability distribution (the "wave" in "wave-particle duality") on configuration space, not "physical" space. Imagine you have two particles in 1 space dimension. The first particle has a position x (1 real number), the second particle has a position y (1 real number). "Physical space" would be the set of real numbers (R^1), while configuration space is the set of pairs of real numbers (x,y) (this set is R^2). The first number in the pair represents the position of particle 1, the second the position of particle 2. An entangled state is simply a probability distribution on R^2 which is not a product state (i.e. P(x,y) != P_1(x) P_2(y)). This means that knowledge of the position of particle 1 gives you (possibly incomplete) information about the position of particle 2. One example of this is "probability 1/2 that x=0, y=0, 1/2 that x=1, y=1". The minute you know x, you know y. That's what entanglement is. What makes it weird is the idea that "knowing x" (measuring it) changes things. An explanation of this is trickier. Still understandable in principle (not just 'follow the math'), but too long for this post.
- ericb 18y ago
  >> too long for this post. How about another post, then? :-)
deleted 18y ago
[deleted]

DaniFong 18y ago

It's not exactly covered convincingly though. The reason it supposedly can't transfer information is because you'd have to measure it, and we assume that measurement strictly follows Born's rules, which is a whole other can of worms. Sometimes people talk about density matrices instead, but we only use those since they are consistent with Born's rules. Additionally, many physicists point out that what you seem to be doing when you perform a measurement is entangling the system with the many bazillion degrees of freedom in the measurement apparatus and the world at large. We don't know, exactly, where the separation between a 'measurement-like' physical process occurs, or even if there really is one. There isn't even a standard measure of entanglement for three or more particles. It's a pretty big mystery, still.

Herring 18y ago
er, ok then i'll rephrase - "we're mostly sure it can't transfer.." or "we've never been able to get it to" I doubt the writer meant it the same way you do. He was talking about the known things that hes not wrapped his mind around. In this case, explaining why the theories don't necessarily conflict should do.