I Participatory Anthropic Principle

[Jim Kata]

TL;DR Summary

Can spooky action at a distance change the past?

Consider the following thought experiment, two electrons are in an entangled state with a total angular momentum of zero. They split apart from each other some billion or so years ago and I observe one of the electrons right now. Before the observation, there is some probability that it will be spin up and some probability that it will be spin down. If I observe the electron to be spin up, then I know that the other electron is spin down. But has it been spin down all along? What if along its way traveling to me some alien measured it say a couple million years ago. Would they necessarily have to of measured the electron being spin up too? I have no knowledge as to whether this electron has been previously measured by someone else or not. Could their past be different than my past? That is could they of measured the electron to be spin down forcing the other entangled electron to be spin up while I measured the opposite result?

 

[PeroK]

Summary:: Can spooky action at a distance change the past?

No. Retrocausality is not part of QM. Although, if you want to look at hypothetical mechanisms for entangement, the it is one of the options. See, for example:

https://www.physicsforums.com/threads/question-about-an-entanglement-paper.966466/#post-6135121

Just to emphasise that these hypothetical mechanisms are not part of QM.

They split apart from each other some billion or so years ago and I observe one of the electrons right now. Before the observation, there is some probability that it will be spin up and some probability that it will be spin down. If I observe the electron to be spin up, then I know that the other electron is spin down. But has it been spin down all along?

First, you have a misconception that each particle in an entangled pair has its own (pure) state. The entangled pair has a two-particle state and cannot be seen as two separate particles - until the system is measured, of course. After the measurement (of either particle), then you have two separate particles, each with its own state. This aspect of entanglement has been demonstrated by tests of Bell's Theorem.

Moreover, asking whether the other particle has been spin down all along assumes that the particle had a single particle state. In terms of QM, that question makes no sense, therefore.

What if along its way traveling to me some alien measured it say a couple million years ago. Would they necessarily have to of measured the electron being spin up too? I have no knowledge as to whether this electron has been previously measured by someone else or not. Could their past be different than my past? That is could they of measured the electron to be spin down forcing the other entangled electron to be spin up while I measured the opposite result?

It doesn't matter which measurement takes place first, the spins will always be opposite. If you get in touch with the alien, you will always find that they got the opposite measurement value from you.