6 ms·

> Our current understanding of physics says that nothing can travel faster than the speed of light, and yet, with quantum teleportation, information appears to

> Our current understanding of physics says that nothing can travel faster than the speed of light, and yet, with quantum teleportation, information appears to break that speed limit. The word “information” could be interpreted to mean communication faster than the speed of light, which is important to note is not made possible by entanglement. Entanglement can’t be used for faster-than-light communication because although observing the spin of particle A affects the spin observed for particle B, the spin you observe is effectively random, so there’s no way to know that the results match up unless you compare them with another means of communication, one that is at most the speed of light. This article explains it way better: https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-reasons-quantum-entanglement-doesnt-allow-faster-than-light-communication/ https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-r...

umvi 7y ago
However, the article claims: > Changing the properties of one particle will cause the other to instantly change too Is this true? Doesn't that mean if we can control the spin of one particle on command, the other will instantly change according to our control? So basically, FTL communication is possible iff controlling the spin of a particle is possible.
- boublepop 7y ago
  Think of it this way. We have a pair of gloves. By the nature of pairs of gloves one is right and one is left. We mix them randomly and put them both in closed boxes. Then we classically ship one box at below speed of light to Mars. Now someone on mars if they open their box will “instantaneously” know at above speed of light correlation which glove was in our box back on earth. But no information was transmitted through the revealing itself, and in fact you would need a classical below speed of light bit stream to accompany the quantum glove boxes if you want to use it for communication. In reality there’s some extra concepts to the quantum states because they act as though the selection of which glove is in which box doesn’t happen until we force the gloves into a definite state through opening the box. But that doesn’t change the nature of there being no information transfer. You can do the same sort of above speed of light signaling fully classically. If you for instance consider a lighthouse with a single beam switching between two far away points. One location will know instantly the signal the other receives, but since you aren’t controlling the lighthouse moving back and forth you can’t transfer information through the signal.
- khkjrhwer 7y ago
  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.
- kradroy 7y ago
  FTL communication has not yet been observed/discovered. The parent's last sentence explains the restrictions in this case. >the spin you observe is effectively random, so there’s no way to know that the results match up unless you compare them with another means of communication, one that is at most the speed of light. The term for this is called "quantum teleportation" and for each qubit that is "teleported" there are two "classically communicated" bits that are required for information to be transmitted.
- russdill 7y ago
  When you measure the spin of a particle, you can only get 2 possible answers, up or down. The up or down is with respect to the angle you measure the spin. You could measure it with respect to the z direction, the x direction, half way in between, etc. The important part about entangled particles is if you choose to measure two entangled particles in the same direction you will always get the opposite answer. Note that once you do such a measurement, the particles are no longer entangled and you're gonna need another timmy. Teleporting the quantum state involves transferring the entanglement of a particle here to a particle over there. This is done by using another pair of entangled particles. It's not possible to just transmit quantum state by purely classical means. I'll also note that the "transmitted instantly" part depends on your interpretation of quantum physics.
  - umvi 7y ago
    > I'll also note that the "transmitted instantly" part depends on your interpretation of quantum physics. Ok, the way I interpret it is that entangled particles are like 2 hard boiled eggs that have collided. It is well known that if you roll 2 hardboiled eggs into each other, only one will crack. So basically, if I roll two hardboiled eggs into each other and then hide them in paper bags, the eggs are now "entangled" and if I separate the bags by 1 million miles, I can observe the state of my egg and know instantly what the state of the other egg is. For example, if I peer into my bag and see my egg is not cracked, I know that the other egg is indeed cracked! However, if I change the state of my egg (crack it) then obviously nothing happens to the other egg because the eggs are no longer "entangled". There is no "instant transmission" happening here. All the properties of the eggs were set in stone the moment the eggs collided and thus it doesn't matter how far away you move them because the "entanglement" (collision) event already happened. This is not a very useful property of hardboiled eggs and so I fail to see how this could be useful by scaling down the eggs to photons.
    - syrrim 7y ago
      That's the 'local hidden variable' model that was disproven by Bell in the sixties.
- Gravityloss 7y ago
  Veritasium explains it quite well. https://youtu.be/ZuvK-od647c https://youtu.be/ZuvK-od647c

umvi 7y ago

However, the article claims: > Changing the properties of one particle will cause the other to instantly change too Is this true? Doesn't that mean if we can control the spin of one particle on command, the other will instantly change according to our control? So basically, FTL communication is possible iff controlling the spin of a particle is possible.

boublepop 7y ago
Think of it this way. We have a pair of gloves. By the nature of pairs of gloves one is right and one is left. We mix them randomly and put them both in closed boxes. Then we classically ship one box at below speed of light to Mars. Now someone on mars if they open their box will “instantaneously” know at above speed of light correlation which glove was in our box back on earth. But no information was transmitted through the revealing itself, and in fact you would need a classical below speed of light bit stream to accompany the quantum glove boxes if you want to use it for communication. In reality there’s some extra concepts to the quantum states because they act as though the selection of which glove is in which box doesn’t happen until we force the gloves into a definite state through opening the box. But that doesn’t change the nature of there being no information transfer. You can do the same sort of above speed of light signaling fully classically. If you for instance consider a lighthouse with a single beam switching between two far away points. One location will know instantly the signal the other receives, but since you aren’t controlling the lighthouse moving back and forth you can’t transfer information through the signal.
khkjrhwer 7y ago
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.
kradroy 7y ago
FTL communication has not yet been observed/discovered. The parent's last sentence explains the restrictions in this case. >the spin you observe is effectively random, so there’s no way to know that the results match up unless you compare them with another means of communication, one that is at most the speed of light. The term for this is called "quantum teleportation" and for each qubit that is "teleported" there are two "classically communicated" bits that are required for information to be transmitted.
russdill 7y ago
When you measure the spin of a particle, you can only get 2 possible answers, up or down. The up or down is with respect to the angle you measure the spin. You could measure it with respect to the z direction, the x direction, half way in between, etc. The important part about entangled particles is if you choose to measure two entangled particles in the same direction you will always get the opposite answer. Note that once you do such a measurement, the particles are no longer entangled and you're gonna need another timmy. Teleporting the quantum state involves transferring the entanglement of a particle here to a particle over there. This is done by using another pair of entangled particles. It's not possible to just transmit quantum state by purely classical means. I'll also note that the "transmitted instantly" part depends on your interpretation of quantum physics.
- umvi 7y ago
  > I'll also note that the "transmitted instantly" part depends on your interpretation of quantum physics. Ok, the way I interpret it is that entangled particles are like 2 hard boiled eggs that have collided. It is well known that if you roll 2 hardboiled eggs into each other, only one will crack. So basically, if I roll two hardboiled eggs into each other and then hide them in paper bags, the eggs are now "entangled" and if I separate the bags by 1 million miles, I can observe the state of my egg and know instantly what the state of the other egg is. For example, if I peer into my bag and see my egg is not cracked, I know that the other egg is indeed cracked! However, if I change the state of my egg (crack it) then obviously nothing happens to the other egg because the eggs are no longer "entangled". There is no "instant transmission" happening here. All the properties of the eggs were set in stone the moment the eggs collided and thus it doesn't matter how far away you move them because the "entanglement" (collision) event already happened. This is not a very useful property of hardboiled eggs and so I fail to see how this could be useful by scaling down the eggs to photons.
  - syrrim 7y ago
    That's the 'local hidden variable' model that was disproven by Bell in the sixties.
Gravityloss 7y ago
Veritasium explains it quite well. https://youtu.be/ZuvK-od647c https://youtu.be/ZuvK-od647c