B Question about quantum entanglement

[KingGambit]

TL;DR

Spin

Dear PF Forum,

I've been having this question for a long time.
I want to know how or what is measurement.

Supposed there are two observers. Alice and Bob,
They are separated 10 light minutes away. They are in the same frame of reference, meaning that their distance is the same all the time.
Not accelerating frame. So their proper time are the same, clocks are synchronized, and they have the same concept of "NOW"
They make an arrangement.

At 00:10:00, Alice measures her electron. It spins down.

I have two question here.

A. What if Bob measures his electron at 00:10:00? I think it spins up

B. What if Bob measures his electron at 00:11:00? Alice world line hasn't reached Bob's yet.
Considering in their agreement that after Alice measures her electron at 00:10:00,
- she does nothing.
- Bob does nothing either until 00:11:00

will it spins
B1. Up
B2. Random?

Thank you very much for your enlightment.

 

[PeroK]

At 00:10:00, Alice measures her electron. It spins down.

I have two question here.

A. What if Bob measures his electron at 00:10:00? I think it spins up

If they measure spin about the same axis, then they will always get opposite values. So, yes, if Alice gets spin up, then Bob must get spin down.

B. What if Bob measures his electron at 00:11:00? Alice world line hasn't reached Bob's yet.
Considering in their agreement that after Alice measures her electron at 00:10:00,
- she does nothing.
- Bob does nothing either until 00:11:00

will it spins
B1. Up
B2. Random?

This doesn't change anything. The spins (about the same axis) are always opposite.

Note that some of your ideas and statements about reference frames and worldlines are not quite right, but that doesn't affect the main issue of perfectly anti-correlated spin measurements.

 

[PeroK]

Supposed there are two observers. Alice and Bob,
They are separated 10 light minutes away. They are in the same frame of reference, meaning that their distance is the same all the time.
Not accelerating frame. So their proper time are the same, clocks are synchronized, and they have the same concept of "NOW"

A better way to describe this is that they share an inertial rest frame.

Alice world line hasn't reached Bob's yet.

I think you mean that a light signal from the event of Alice's measurement cannot have reached Bob by the time he makes his measurement. Technically this means that the measurement events are spacelike separated.

 

[KingGambit]

Thank you, thank you very much Perok

 

[KingGambit]

I think I have one other question (many actually, but it's the only one that I can think of).
When Alice measure her electron it spins up.
What happens when Alice measure her electron again some times later?
Considering that Bob does not do anything, and nobody tampering with Alice electron.
Will it
- Always spins up?
- Always spins down?
- Random

 

[PeroK]

I think I have one other question (many actually, but it's the only one that I can think of).
When Alice measure her electron it spins up.
What happens when Alice measure her electron again some times later?
Considering that Bob does not do anything, and nobody tampering with Alice electron.
Will it
- Always spins up?
- Always spins down?
- Random

In general, if you repeat a measurement of spin you get the same result. This has nothing to do with what Bob does.