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King E

Can quantum particles communicate in the past?

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The quantum entanglement and delayed choice quantum eraser variation to the double slit experiment has given very strange results. So if a particle is detected then the pattern is changed. Does the detected particle communicate with its entangled pair in the past? or there is another explanation to it?

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Posted (edited)

This can be considered using the CPT-symmetry of the quantum field theory.

Edited by Kuyukov Vitaly

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Neither one of the entangled particles has a defined state, before the measurement is made, so why would any communication have to propagate backwards in time ?

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2 hours ago, King E said:

The quantum entanglement and delayed choice quantum eraser variation to the double slit experiment has given very strange results. So if a particle is detected then the pattern is changed. Does the detected particle communicate with its entangled pair in the past? or there is another explanation to it?

Retrocausality is a common interpretation of this. In the same way that faster-than-light communication is an interpretation of entanglement.

But those are just (human-level) descriptions. Interpretations can give us some insights into what is going on. And different interpretations can be be better in some contexts than others. But they don't tell us what is "really" happening. (Nothing does, of course.)

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Posted (edited)
On 5/30/2020 at 12:26 PM, King E said:

The quantum entanglement and delayed choice quantum eraser variation to the double slit experiment has given very strange results. So if a particle is detected then the pattern is changed. Does the detected particle communicate with its entangled pair in the past? or there is another explanation to it?

Communication with the past in your causal light cone is no problem, nor is it a paradox, even for classical mechanics, if I'm not mistaken. Notice: All systems are affected by their past. And because physics microscopically is reversible to a great extent (CP violation excluded,) I surmise you could argue that something that you see as the present affecting the past, what really is is you learning about aspects of the state that weren't obvious to you before (see below in relation to @MigL's argument.)

The same question turned upside down is: Does the future affect the present? That would sound more paradoxical, but it's not. If you are within the future causal cone, there is no reason why you couldn't say in some sense that the future "affects" the past. It would be a contorted way of saying it, but it would be OK, I think, e.g.: What the Earth is today "determines" what the Earth was more than 4 billion years ago when it collided with planet Theia to form the Earth-Moon system.

The real problem would be communication with your "unreachable present." That is, outside your causal light cone. I don't think that's possible and I don't think that's what QM is saying. But if you feel confused, you're not alone. Some 5 years ago I saw a paper, accepted for publication on PRL, and it was about the possibility of sending signals based on the observable to be measured.

All the problems people are having for decades (and still are) are to do with (1st): QM not allowing you to think of certain properties A and B at the same time (incompatibility,) even if you devise extremely clever ways to measure A in subsystem (1) and property B in subsystem (2) and then use conservation laws to infer the incompatible property in the other subsystem to circumvent Heisenberg's uncertainty principle. And (2nd): States being in general non-separable. But I wouldn't want to go off topic.

And quantum erasing really has to do with removing the effects of a measurement AFAIK, not with affecting the past. I would have to review down to the basics to say more.

I agree with @Strange that retrocausality is a human-level perception of the results of the experiment. Something similar happens with so-called "non-locality."

It goes more in the direction of not knowing the state precisely, as @MigL suggests, if I understand him correctly.

And I don't understand @Kuyukov Vitaly's point. But he may well have a valid one. I do intuit he's thinking really farther afield.

Edited by joigus
mistyped

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36 minutes ago, joigus said:

Communication with the past in your causal light cone is no problem, nor is it a paradox, even for classical mechanics, if I'm not mistaken. Notice: All systems are affected by their past. And because physics microscopically is reversible to a great extent

Does that mean fluid dynamics is applicable in the quantum realm?

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Posted (edited)
7 minutes ago, dimreepr said:

Does that mean fluid dynamics is applicable in the quantum realm?

You're a very bad person!!! Fluid dynamics is an all-scale-coupling spherical harmonics mixing mess of a system of equations. Don't bring trouble here, you dark spirit!!!

I was talking micro-causality and micro-retrocausality.

Although now that I think of it, my Earth example wasn't very micro. Ooops.

Edited by joigus
addition

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