Quantum Spin: Forgetting, Entanglement & Change

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Discussion Overview

The discussion revolves around the concepts of quantum spin, particularly focusing on the implications of measuring an electron's spin along different axes and the effects of entanglement on these measurements. Participants explore the relationship between measurements, the notion of "forgetting" previous spin states, and the consequences for entangled particles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that measuring an electron's spin on one axis leads to the "forgetting" of its previous spin state, implying that subsequent measurements could yield different results.
  • Another participant challenges the idea that measuring one electron's spin would "change" the state of an entangled partner, arguing that the first measurement provides information about both electrons and that further measurements generally break the entanglement.
  • A third participant expresses a need to rethink the communication of concepts related to faster-than-light (FTL) implications in quantum mechanics.
  • A later reply reiterates the idea that after measuring spin along one axis, the electron will be in an eigenstate of that axis, and subsequent measurements on a different axis will not yield a deterministic result for the first axis.

Areas of Agreement / Disagreement

Participants express differing views on the implications of measuring spin in entangled systems, with no consensus reached on whether measuring one electron's spin affects the other or how the concept of "forgetting" applies in this context.

Contextual Notes

Participants note that the relationship between measurements and the resulting states is complex and dependent on the axes chosen for measurement, with unresolved aspects regarding the nature of entanglement and measurement outcomes.

Xori
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My understanding is that when you measure an electron's spin on on Axis A, and then on Axis B, the spin on Axis A is "forgotten" and can be something different next time you measure it. Is this correct?

If it is, then how does this work across entanglement? If you measure electron A's spin at Axis A, then you know electron B's spin is the opposite. But if you then measure electron's A spin again on Axis B, you can potentially "change" electron B's spin on Axis A?
 
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Who says that would "change" the other electron?

If A and B were entangled, measuring A once will give you information about both A and B. It will also generally break the entanglement, so further measurements of A tell you nothing further about B.

For example, if you made a third measurement of A (on axis A for the second time, giving a random result) and then measured B, you would find it correlated with the first (rather than third) measurement of A.
 
Damn, now I got to think of another way to communicate FTL.
 
Good luck with that.
 
Xori said:
My understanding is that when you measure an electron's spin on on Axis A, and then on Axis B, the spin on Axis A is "forgotten" and can be something different next time you measure it. Is this correct?
Have a clear answer for the first part: after the mesurement of spin along the B axis the particle will be in an eigenstate of spin pojected on B. Unless A and B are collinear, this is not an eigenstate of spin projected on A, thus if we make a third measuremnet (aling A again), the probability distribution will no longer be a Kronicker-delta
 

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