Physics Asked on April 12, 2021
I know there are a lot of similar question but I don’t believe this to be a copy. I understand that if two people lived far away they could not transfer information through quantum entangled particles because forcing a particle into a particular spin breaks the entanglement and simply observing the particle to collapse the other part of the pair will give a perfectly random result. But what about using entanglement to sort of indirectly coordinate plans from far away:
I know this is wrong somehow and uses a childish interpretation of the idea of communicating information but this is just to make this as clear as possible: lets say the year is 3050, there are 2 leaders of an allied war who want to attack a planet, they are currently on opposite sides of the planet and have 2 plans they can decide upon, 1) both attack from the east and west at once or north and south at the same time. Using an atomic clock the leaders coordinate to check the state of quantum entangled particles (or a qubit, doesn’t matter) at 12pm. If the qubit collapses as a (1/0) they go with plan A while (0/1) means plan B.
I believe that this does not constitute faster than light communication because both plans were conceived ahead of time and the particle or quantum state was just used as a random number generator, but it still seems as though the plan of attack was being transferred.
My questions are:
Going off of WillO's answer, while this scheme would work it would be no more effective than using a printer and two pieces of paper. Yes, your scheme is different in that it involves quantum nonlocality, but nevertheless it does not constitute faster-than-light communication because no information is being transferred between the two leaders. Their respective observations are correlated, but are nevertheless random. Hence, there's no problem. Is it weird? Yes. Is it a threat to causality? No. :)
Correct answer by Mark Morales II on April 12, 2021
Alternatively, they could have a computer print out two copies of the same random number, stuff the copies in their pockets, consult them when it's time to attack, and attack from one direction or another depending on whether the number is odd or even.
Why does this not constitute FTL communication? If the answer to that is clear to you, then you've answered your own question.
Answered by WillO on April 12, 2021
I think the most simple answer is that the leaders told each other that they would measure the particles at 12 pm. Hence, information was transmitted ahead and there is no communication (information exchange) at the time of the measurement.
Answered by Martin Vesely on April 12, 2021
The information is inherently contained in the particle pair themselves. Therefore, information only moves as quickly as the particles themselves do. What is not included in the description of your scenario is how the entangled particles originate. The particles cannot become entangled unless they are in very close proximity to begin with. Thereafter, they must move at slower-than-light speeds to their respective destinations. Even though the final state of the observable is not known until the observation occurs, I think it is fair to say that the information about the final state is implicitly encoded in the entanglement, rather than the observation.
In this sense, I think the paper printing scenario really is apt. Here, we would say that the same printer must print both pieces of paper, and the paper has the special property that once you look at it, the information is lost. This is exactly equivalent to saying that the information is not there until you look at it, from the perspective of an observer. That is, an observer cannot tell the difference between the printer deciding the outcome at the time of printing vs. the papers magically manifesting the same value at the time of observation. Even so, the printer analogy makes it clear that the information is created at "entanglement time" (i.e., print time), and thus, the information speed is simply the speed at which the papers move about, not the speed at which they are observed.
Answered by Lawnmower Man on April 12, 2021
You can replace the atomic clock with two boxes of lunch packed in a dark room.
The modified story:
The leaders mum cooked lunch in the previous day, chicken and salad. However, when packing, the lights went off, so she doesn't know which pot is which. She makes sure she gives her children lunch from the same pot. Using an the lunch boxes the leaders coordinate to check the state of their lunch (or the lunchbox, doesn't matter) at 12pm. If the lunch is chicken (1/0) they go with plan A while salad (0/1) means plan B.
Does that sounds like a plan? It does. You can even implement the above version of it now, hopefully not in a planet-attacking way, without quantum equipment.
Answered by zabop on April 12, 2021
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