3 ms·

You have two particles, A and B, which can be in state 0 or 1. When you measure particle A, you get random value 0. B is instantaneously set to 1. Or the other

You have two particles, A and B, which can be in state 0 or 1. When you measure particle A, you get random value 0. B is instantaneously set to 1. Or the other way around. But you can't communicate using this, because you can't control the random value you get, thus there is no way to "modulate" the other end.

umvi 7y ago
Why can't we control the state of a particle (i.e. force toggle a particle from state 0 to 1 or vice versa)? Is there some fundamental law of physics preventing this or is it just because we don't have a method of observing the state of a particle without changing it?
- boublepop 7y ago
  Observation is a tricky word in quantum mechanics. You’re thinking about observing without changing like you would perhaps observe your neighbors through their windows without causing any change to their current state. However the way you make a measurement in qm is by causing the small subsystem to become correlated with a huge external system, so that for instance the spin of your one small particle is now 100% correlated to a thermal reservoir of maybe 10^23 other particles, such that a led lights green if the particle was spin-up and red if it was spin down. That’s like observing your neighbor by blasting down their home with a cannon firing a continues stream of other people at them, tearing the brick walls to shreds and observing the splatter patterns to tell after the fact if your neighbors where home or not. Now the tricky thing here is that by definition, your “observer” has to 100% correlated with the state of the system, otherwise the measuring device isn’t accurate. But the device is constraining the state of your neighbors to either “home” or “not-home” in order to be certain in the outcome. So there no room left for details like “home and playing chess” or “home cooking dinner”, and obviously those details also fairly quickly get lost once they see their walls get torn to shreds by your observer person cannon. Now this all might make it sound like we just need more delicate measurement devices, but that’s not the case. Any device that can 100% correlate to the state of the subsystem has to force the subsystem into a definite state. So the question isn’t “can you observe a particle without changing it” it’s “can you create a device that tells you either ‘yes the particle is spin up’ or ‘yes the particle is spin down’ with 100% certainty and no other outcomes, which however does not force the system to actually be either up or down?”. This obviously is not possible.
- 19870213 7y ago
  We can change it (light polarizing crystals for example), but that breaks the entanglement of the individual particles.
- khkjrhwer 7y ago
  You can force a particle state to 0 or 1, but this will break the entanglement with the other particle, and the other one will no longer be correlated with yours. It has been proven mathematically that FTL communication of information using entanglement is impossible (under our current understanding of physics, of course). It's called a "no-go theorem": https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
- fsh 7y ago
  It can be proven from the theory of quantum mechanics that communication via entanglement is impossible. No currently accepted physical theory allows communication faster than the speed of light.

umvi 7y ago

Why can't we control the state of a particle (i.e. force toggle a particle from state 0 to 1 or vice versa)? Is there some fundamental law of physics preventing this or is it just because we don't have a method of observing the state of a particle without changing it?

boublepop 7y ago
Observation is a tricky word in quantum mechanics. You’re thinking about observing without changing like you would perhaps observe your neighbors through their windows without causing any change to their current state. However the way you make a measurement in qm is by causing the small subsystem to become correlated with a huge external system, so that for instance the spin of your one small particle is now 100% correlated to a thermal reservoir of maybe 10^23 other particles, such that a led lights green if the particle was spin-up and red if it was spin down. That’s like observing your neighbor by blasting down their home with a cannon firing a continues stream of other people at them, tearing the brick walls to shreds and observing the splatter patterns to tell after the fact if your neighbors where home or not. Now the tricky thing here is that by definition, your “observer” has to 100% correlated with the state of the system, otherwise the measuring device isn’t accurate. But the device is constraining the state of your neighbors to either “home” or “not-home” in order to be certain in the outcome. So there no room left for details like “home and playing chess” or “home cooking dinner”, and obviously those details also fairly quickly get lost once they see their walls get torn to shreds by your observer person cannon. Now this all might make it sound like we just need more delicate measurement devices, but that’s not the case. Any device that can 100% correlate to the state of the subsystem has to force the subsystem into a definite state. So the question isn’t “can you observe a particle without changing it” it’s “can you create a device that tells you either ‘yes the particle is spin up’ or ‘yes the particle is spin down’ with 100% certainty and no other outcomes, which however does not force the system to actually be either up or down?”. This obviously is not possible.
19870213 7y ago
We can change it (light polarizing crystals for example), but that breaks the entanglement of the individual particles.
khkjrhwer 7y ago
You can force a particle state to 0 or 1, but this will break the entanglement with the other particle, and the other one will no longer be correlated with yours. It has been proven mathematically that FTL communication of information using entanglement is impossible (under our current understanding of physics, of course). It's called a "no-go theorem": https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
fsh 7y ago
It can be proven from the theory of quantum mechanics that communication via entanglement is impossible. No currently accepted physical theory allows communication faster than the speed of light.