I Entanglement + relativity

  • Thread starter snuz2001
  • Start date

DrChinese

Science Advisor
Gold Member
7,181
999
PeroK thanks, I would still like your reply to the questions I asked:

1. Was Alice’s measurement affected by Bob’s measurement an hour later?

2. Was Bob’s Measurement affected by Alice’s measurement an hour earlier?
This is a subject of great debate, and no one knows the answer definitively. That is because the mechanism by which an entangled system of 2 particles evolves into 2 independent systems (no longer entangled) is unknown. The quantum prediction is probabilistic, and depends primarily what is called the "context". The context then consists of 3 main factors:

a. The type of entanglement the system is cast into (for example: correlated or anti-correlated).
b. Alice's choice of measurement (for example: spin angle).
c. Bob's choice of measurement (for example: spin angle).

The order of the above does not matter to the quantum mechanical prediction. And each measurement outcome (when viewed alone) is completely random. So you can see why the answer is elusive. Of course, you can make assumptions - but those assumptions cannot be verified objectively.
 
16
0
DrChinese thanks for your reply. But still, elusive as it is, do you see any way Alice’s early measurement’s result could be affected by Bob’s late measurement after it was recorded and transmitted?
 

N88

219
12
N88, what are your answers to those questions?
1. Was Alice’s measurement affected by Bob’s measurement an hour later? No; how could it be?

2. Was Bob’s Measurement affected by Alice’s measurement an hour earlier? No; how could it be?

Remember that we are talking about idealised experiments; we are not concerned with the gravitational effects of Alice's movements affecting Bob ever-so-slightly an hour later; nor vice-versa.

IMHO, in plain language, which should not lead us too far astray: Entanglement is a valid colloquial expression relating to particles that are pairwise correlated at birth. Relativity is a modern and valid physical theory. There is no contradiction between these terms.

HTH. Remember: KISS = keep it simple students! Alice, Bob, each with a detector, each detector interacting with a particle that is pairwise correlated with its twin: each detector recording the number of the test and the result (±1) on its own long-life tape (and reporting both results to your location if you wish).
 
16
0
So what is the answer? Can Bob’s late measurement affected Alice’s earlier?
 

N88

219
12
Your argument is the same as the original EPR paradox (1935). Bell's Theorem (1964) shows us that does not work. The argument seems fine as long as Alice and Bob measure in the same direction. But it falls apart at most other angle settings (for example 60 degrees). Unfortunately, it takes a bit of math and analysis to understand why.

If you are not familiar with Bell (and I am pretty sure you know it): read up a bit... my own website is not a bad place to start: Bell's Theorem: An Overview with Lotsa Links
Thank you, will take a look

Before that, to help me know where best to look: I'd welcome you comment on these:

1. Note that my example does not fail at any common angle.

2. If the angle is 60º, then we can tell Bob the probability of his result: probability here, it seems to me, because our knowledge is incomplete. That is, we do not yet fully understand the dynamics of particle-detector interactions.

3. However, since the correlation is ALWAYS law-like, and not random in any way: we do know something of the dynamics.

4. Importantly: that law (as I'm sure you know) can be found via common sense analysis.
 
26,260
6,864
So what is the answer? Can Bob’s late measurement affected Alice’s earlier?
Nobody knows the answer. At least one QM interpretation, retrocausality, allows later measurements to affect earlier ones. Nobody has found a way to experimentally test one QM interpretation vs. another, so as far as anyone knows retrocausality is still a possibility, at least in principle.

I know you would like there to be a definitive answer, but as has already been pointed out several times, this is one of those cases where we don't have one at our current state of knowledge.
 
26,260
6,864
The OP question has been addressed as well as it can be. Thread closed.
 

Related Threads for: Entanglement + relativity

  • Posted
Replies
13
Views
3K
Replies
4
Views
1K
Replies
9
Views
535
Replies
2
Views
619
Replies
17
Views
3K
Replies
52
Views
6K
Replies
3
Views
1K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top