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Let's imagine the following thought experiment,

Two particles, A and B are entangled on the Earth. As decided, Alice (a friend of Bob) stays on the Earth with the A and Bob travels with the B to a light years distant planet named X. There was a contract between Alice and Bob that, at some point of time in future if Bob finds the B's spin in UP direction, he will destroy the planet X otherwise he won't. Now, at the point of time in future, when Alice measures the A's spin and finds it DOWN that instantaneously means Bob is destroying the planet X (because, Bob finds B's spin UP).

Now, for particularly this thought experiment, it appears to me that, Alex is receiving the information that "Bob is destroying the planet X" faster than light speed. What am I missing?

Note: as a newbie to the field of quantum physics, I might be missing very basic stuff. Nevertheless, I am requesting to audience to suggest me to improve my query if it has any area of improvement before downvoting/closing the post because, this questing the really genuinely sticking my head.

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    $\begingroup$ She does not get the information, she just supposes bob did as he sad some time before, but he may be dead, or ill or lost the explosive, so she does not really know. Same with anything someone tells you he will do it at some specific time in the future at same distance. you think to know ist at the time he promised to do it, so information in a distance without any time. but it is no information. $\endgroup$
    – trula
    Commented Oct 9, 2022 at 15:53
  • $\begingroup$ @trula you’re bringing new constraints. With the same logic none of the “information” is supposed to be called an information as there would be still uncertainty. For example, a scientific instrument that is transferring information may be malfunctioned even though it was proven millions of time prior to the experiment and peer reviewed. At what certainty we can call something an information or not? $\endgroup$
    – Sazzad Hissain Khan
    Commented Oct 9, 2022 at 16:01
  • $\begingroup$ No malfunctioning is not against having an information, but not be able to see or otherwise really know something has happened makes it just a supposition, where your instrument may give you a false information which is different. $\endgroup$
    – trula
    Commented Oct 9, 2022 at 16:23
  • $\begingroup$ @trula I think you didn’t get my last point right. Any new constraint you bring can be countered by adding new properties to the thought experiment system. OK, let me rephrase, if in this thought experiment, Bob is immoral and absolutely honest and powerful. Now, would that be considered as an information? $\endgroup$
    – Sazzad Hissain Khan
    Commented Oct 9, 2022 at 16:54
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    $\begingroup$ The entanglement seems completely irrelevant here. If Bob tells Alice he will destroy planet X as soon as he gets there, you have exactly the same issue. Or he leaves behind an envelope stating his intention, which Alice opens at the time she expects him to arrive. $\endgroup$
    – WillO
    Commented Oct 9, 2022 at 20:54

1 Answer 1

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The answer lies in the definition of information. If Alice measures its particle in the "up"state, she knows that Bob's particle is in the "down" state. However, this is not considered to be information. The idea of saying that information can not travel faster than light contains the concept that only one person knows something, and that this information is communicated to a second person. Therefore, even if Alice and Bob agree that Bob performs a definite action depending on the outcome of his experiment, e.g.

  • if Bob obtains "up" in his measurement, he will destroying the earth, and
  • if Bob measures "down", he will destroying the sun,

Alice does not gain any new information by taking the measurement. The communication of the information, which experimental result has which consequence, took place before the measurement was performed. Alice and Bob simply correlated the faith of an object to a random event. The fact that the result of the measurement is random incorporates the "informationless property". If Alice and Bob measure a sequence of entangle particle pairs, each observer obtains a random sequence. The fact that the two random sequences are correlated does not change the fact that each sequence is random.

In order to communicate information between Alice and Bob, one of them has to imprint his/her information onto the particle. However, if Alice manipulates her particle to be definitely in the "up"-state, and then performs her measurement, the entanglement between the two distant particles is lost. While Alice measures her particle in the "up" state, Bob measures his particle in an arbitrary state, and not necessarily in the "down" state. Therefore, by loosing the entanglement the correlation of the two measurements was lost. Again, no information is communicated.

This considerations also apply to the so called weak measurements. Unfortunately, Physics does not provide "free lunches".

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  • $\begingroup$ “The fact that the two random sequences are correlated does not change the fact that each sequence is random” - but at the same time its also true that randomness of an event that we know a correlation with other decreases over an event of which we have no idea. Isn’t it? $\endgroup$
    – Sazzad Hissain Khan
    Commented Oct 9, 2022 at 17:35
  • $\begingroup$ @SazzadHissainKhan: I edited the text to put more emphasise onto the communication aspect. Your argue is similar to "if I know the state of two particles, I know more than if I know the state of a single particle". Therefore, you are focusing your attention onto the "amount of the information". I believe this is misleading. $\endgroup$
    – NotMe
    Commented Oct 9, 2022 at 21:29
  • $\begingroup$ Thanks for your time. TBH, I’m focusing that part because particularly that parts bothering in my head. If amount of information is more than surely there has been information gain. Maybe, my cognition couldn’t get the definition of information right yet. $\endgroup$
    – Sazzad Hissain Khan
    Commented Oct 10, 2022 at 2:07

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