Question about photon entanglement and causality

[cgsanderson]

TL;DR

Entanglement can make effect precede its cause, seemingly. What happens if the effect is rigged to prevent the cause?

I have read that if two photons are entangled, and one photon is sent through a 2-slotted paper and is unobserved, then the second is sent through and is observed, the earlier photon is found to have behaved as though it were observed right along with the second one, seemingly defying conventional wisdom that cause must come before effect. My question is, if the first photon is rigged in an apparatus that signals to prevent the observation of either photon, but only if the first photon behaves as though it is observed, what will happen when trying to observe the second photon entangled with it?

 

[PeterDonis]

I have read

Where? Please give a reference.

 

[cgsanderson]

Where? Please give a reference.

https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser

 

[PeterDonis]

https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser

Thanks, that makes it much clearer what experiment you're talking about. Unfortunately, your OP doesn't really give a good description of either the experiment or what your question is about it, so I'm not sure how to respond. But briefly:

seemingly defying conventional wisdom that cause must come before effect.

As the Wikipedia page notes, there is no actual "retrocausality" going on.

This post in a previous PF thread gives a good brief description of how to interpret what is going on in this experiment:

https://www.physicsforums.com/threa...quantum-eraser-experiment.880236/post-5530490

I would suggest reading that post, and re-reading the Wikipedia page, and then trying to formulate a question based on actual quotes from those places.

 

[cgsanderson]

Thanks, that makes it much clearer what experiment you're talking about. Unfortunately, your OP doesn't really give a good description of either the experiment or what your question is about it, so I'm not sure how to respond. But briefly:


As the Wikipedia page notes, there is no actual "retrocausality" going on.

This post in a previous PF thread gives a good brief description of how to interpret what is going on in this experiment:

https://www.physicsforums.com/threa...quantum-eraser-experiment.880236/post-5530490

I would suggest reading that post, and re-reading the Wikipedia page, and then trying to formulate a question based on actual quotes from those places.

Thank you!

 

[PeroK]

Thank you!

Here is an explanantion of why the quantum eraser does not imply retrocausality:

 

[cgsanderson]

Thank you so much! This is extremely helpful.