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I have seen a bunch of duplicates of this question and I’m sorry if this is a true duplicate, but all the other duplicates have super long and complicated answers that I don’t understand.

I just want to know if you change the state of one entangled particle if it would change the state of the other.

Like for example if there was a pair of entangled particles and the first one’s state was 1 and the other’s was 2. If something changed the first one’s state to 2, would it change the second one’s to 1 no matter the distance instantaneously. Now by the way, I haven’t measured anything in this example. Something which is not me changes the state of the particle and I haven’t measured anything.

The reason I say I have not measured anything is because I heard somewhere that if you measure the entangled particle, you end the entanglement. I don’t know if this is true, so I said it just in case.

Again I would really like a super simple answer.

Qmechanic
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Anish Kommireddy
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3 Answers3

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It depends on the interpretation you choose to believe. In no-collapse interpretations like the Everett (Many Worlds) interpretation, the answer is 'no'. Nothing changes about the entangled partner. Nothing travels faster than light. But the Everett interpretation has the weird (to many) consequence of billions of unobservable alternate realities.

In interpretations with wavefunction collapse, like the Copenhagen interpretation, the answer is 'yes', but the change is experimentally unobservable. So you can't use it for faster-than-light communication. The switch from entangled to collapsed is information (the quantum states are quite different), which definitely has to be transferred. But you can only detect the difference through the correlations between the entangled systems, which have to be passed around at lightspeed or less.

If something changed the first one’s state to 2, would it change the second one’s to 1 no matter the distance instantaneously.

This is the big problem - there is no such thing as 'instantaneously' in special relativity. What is 'instantaneous' for one observer may be either forwards or backwards in time for another observer. To someone watching your experiment from a moving vehicle, the result appears to arrive at its destination before it sets off.

This does not discourage believers in collapse interpretations! Because the consequences of the collapse are experimentally unobservable, it doesn't matter that it involves backwards-in-time effects. No bad consequences will result. It's just weird.

Simple version:

If you believe wavefunctions collapse, the answer is 'yes'.

If you believe wavefunctions don't collapse, the answer is 'no'.

Scientifically, you are allowed to believe either - it's your choice.

Nullius in Verba
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  • Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. – Community Jan 14 '23 at 09:02
  • Bohm-Brogle interpretation is also no-collapse, but it is non-local, so there also it changes instantly like in Copenhagen. – Anixx Jan 14 '23 at 14:13
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This depends on the interpretation of quantum mechanics, whether it is local or non-local. The other answer says it depends on whether the interpretation has collapse, but it is not true.

  • In Copenhagen interpretation (collapse, non-local, indeterministic), yes. The state changes instantly via any distance.

  • In MWI interpretation (no collapse, local, deterministic), no.

  • In Bohm interpretation (no collapse, non-local, deterministic), yes.

Et cetra.

Anixx
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In simple terms, entanglement exists in the every day world too.

As an example, if you know that brothers, John and Mike, in your home town, and hear that one of them has gone to New York , if you meet Mike, you know that John has gone to New York. Their relationship as brothers entangles them, and information gathered from one of the brothers applies to the other by logic.For twins, if you ask one the age, you immediately know the age of the other.etc.

In a mathematical model for particles , the various values of spin and other quantum numbers of particles are correlated through the mathematical relations and by conservation laws (similar to that John and Mike are always brothers). The particles are "entangled" with the quantum mechanical wave-function describing the pair of particles in your question.

So, if you measure (or magically know) the spin of one particle, from conservation laws you know the spin of the other particle without measuring it.

anna v
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    This is not a good example. Classical correlations are not entanglement. – Tobias Fünke Jan 14 '23 at 09:26
  • @TobiasFünke Our language originated in classical dimensions, and then used by analogy in quantum mechanical relations. The OP asked for simplicity. – anna v Jan 14 '23 at 13:56
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    I understand the intention; however, at least IMHO, it is not a good example because it does not capture the non-classical nature of entanglement. I know the OP wanted an easy explanation, but "as simple as possible but not simpler" applies. – Tobias Fünke Jan 14 '23 at 16:37
  • I am not deleting this because it expresses my view of how the everyday word "entanglement" has come to be used in physics. I accept that a large number of readers do not get it, but that is fine. – anna v Jan 22 '23 at 05:02