- #1
kochanskij
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Superluminal signals using quantum entanglement
Please consider this thought experiment. Would this provide a means for communicating faster than light or is there an error somewhere in it?
SUMMARY: Person B can instantly know whether or not Person A did measurements on his particles by looking for an interference pattern with his entangled partner particles.
DETAILS:
A source sends entangled particle pairs in opposite directions. Each particle and its partner are in a superpostion of spin-up and spin-down states. As a particle reach Person A, he can choose to do a spin measurement or let it go by unmeasured. If he let's it go by, then its entangled partner will remain in a superposition. If he does a measurement, then its partner will instantly collapse to a definite spin state. Person B wants to know if his particle is in a superposition or a definite spin state. (He doesn't care if it is spin-up or spin-down.) So he puts a device in its path that uses a magnetic field to deflect it upward if its spin is up, downward if its spin is down, and both ways if it is in a superposition. The device brings the two paths out through slits that are very close together. Just as in the standard double slit experiment, if the particle followed both paths, it will interfere with itself and hit a screen at a spot allowed by the interference pattern mathematics.
Person B can not see an interference pattern with only one particle, so let's use batches of 1000 particles each. Person A receives particles one at a time every millisecond. He let's 1000 pass by to transmit a "1". He measures 1000 in a row to transmit a "0". Person B let's 1000 entangled partner particles pass through his double slit device, one at a time, and hit a screen. If he sees interference fringes, he writes down a "1". If he sees two spots behind the slits but no fringes, he writes down a "0". He then clears the screen and does the same thing with the next batch of 1000 particles.
The two scientists could be any distance apart, even many light-years. The collapse of the B particle's wavefunctions will occur instantly when (and if) person A does a measurement. So a binary message could be sent faster than light. This experiment could actually be done in a modern physics lab using high speed electronic devices.
Where is the error in this plan? Or could information really be transmitted faster than light, in violation of special relativity?
Please consider this thought experiment. Would this provide a means for communicating faster than light or is there an error somewhere in it?
SUMMARY: Person B can instantly know whether or not Person A did measurements on his particles by looking for an interference pattern with his entangled partner particles.
DETAILS:
A source sends entangled particle pairs in opposite directions. Each particle and its partner are in a superpostion of spin-up and spin-down states. As a particle reach Person A, he can choose to do a spin measurement or let it go by unmeasured. If he let's it go by, then its entangled partner will remain in a superposition. If he does a measurement, then its partner will instantly collapse to a definite spin state. Person B wants to know if his particle is in a superposition or a definite spin state. (He doesn't care if it is spin-up or spin-down.) So he puts a device in its path that uses a magnetic field to deflect it upward if its spin is up, downward if its spin is down, and both ways if it is in a superposition. The device brings the two paths out through slits that are very close together. Just as in the standard double slit experiment, if the particle followed both paths, it will interfere with itself and hit a screen at a spot allowed by the interference pattern mathematics.
Person B can not see an interference pattern with only one particle, so let's use batches of 1000 particles each. Person A receives particles one at a time every millisecond. He let's 1000 pass by to transmit a "1". He measures 1000 in a row to transmit a "0". Person B let's 1000 entangled partner particles pass through his double slit device, one at a time, and hit a screen. If he sees interference fringes, he writes down a "1". If he sees two spots behind the slits but no fringes, he writes down a "0". He then clears the screen and does the same thing with the next batch of 1000 particles.
The two scientists could be any distance apart, even many light-years. The collapse of the B particle's wavefunctions will occur instantly when (and if) person A does a measurement. So a binary message could be sent faster than light. This experiment could actually be done in a modern physics lab using high speed electronic devices.
Where is the error in this plan? Or could information really be transmitted faster than light, in violation of special relativity?