Register to reply

Quant.Entaglement ifference between superposition of spins and not knowing the spin?

by Swimmingly!
Tags: knowing, spin, spins, superposition
Share this thread:
Swimmingly!
#1
Mar8-12, 11:07 AM
P: 45
Let two electrons' (A and B) spin be entangled. They'll be in a superposition of up and down spin.
When the spin of A is measured it settles randomly to up and instantly B is set to down.
What makes it unreasonable to say from the very beginning A was up and B was down?
Would we draw any different conclusions if we assumed it had an unknown but set spin from the very beginning?



My problem with the answer yes:
If there's a difference and if we then propose that "superpositioned-electron" behaves differently from a "non-superpositioned" electron. Then we can probably tell whether an electron is in superposition or not. And we can then use this to send information faster than light.
Therefore either the idea or my arguments are wrong. (possibly reading or changing them messes everything up)

So I assume it may be just a matter of how QM describes the world. Like if we knew that Angels pushed planets around. But we decided to say that it's gravity instead that does it, even though it's all done by the angels.

Also it reminds me a bit of a Schrodinger's cat. Is it that different to say we don't know from saying it's dead and alive?
Phys.Org News Partner Physics news on Phys.org
Vibrational motion of a single molecule measured in real time
Researchers demonstrate ultra low-field nuclear magnetic resonance using Earth's magnetic field
Bubbling down: Discovery suggests surprising uses for common bubbles
StevieTNZ
#2
Mar8-12, 12:36 PM
PF Gold
StevieTNZ's Avatar
P: 800
I guess whether a superposition of up and down spins for each particle would result in a different result if the spin is measured along @ axis (@ being whatever you wish) - than if you had one up and one down and you measured along @ axis.

I'm sure someone else will clarify, but that's what I suspect.
StevieTNZ
#3
Mar8-12, 12:43 PM
PF Gold
StevieTNZ's Avatar
P: 800
From "The Quantum Challenge"

A superposition of up and down spins, the particle measured along the x-axis, is it found in the position direction.
Rather if the particles were a mixture of up and down spins, then we will see 1/2 positive direction and 1/2 negative direction - when measured along the x-axis.

Swimmingly!
#4
Mar8-12, 05:15 PM
P: 45
Quant.Entaglement ifference between superposition of spins and not knowing the spin?

Quote Quote by StevieTNZ View Post
From "The Quantum Challenge"

A superposition of up and down spins, the particle measured along the x-axis, is it found in the position direction.
Rather if the particles were a mixture of up and down spins, then we will see 1/2 positive direction and 1/2 negative direction - when measured along the x-axis.

Then how does it not allow faster than light communication? Does the reading of the electrons mess everything up?
I don't understand that well the first phrase though, odd construction. I think I kind of get it but I'm not sure.
Khashishi
#5
Mar8-12, 05:21 PM
P: 887
Quantum mechanics tells you what the result of this entanglement experiment will be, but it doesn't really force you to accept a certain view of reality. You are suggesting some kind of hidden variable theory, which can be made fully consistent with QM. But, any hidden variable theory needs to have faster than light communication to explain the effects.
Swimmingly!
#6
Mar8-12, 05:32 PM
P: 45
Quote Quote by Khashishi View Post
Quantum mechanics tells you what the result of this entanglement experiment will be, but it doesn't really force you to accept a certain view of reality. You are suggesting some kind of hidden variable theory, which can be made fully consistent with QM. But, any hidden variable theory needs to have faster than light communication to explain the effects.
I don't know what the results will be. Is it just a question of an irrelevant view of reality?
I think this is weird because if it can be differentiated then it implies faster-than-light communication but if it can't then it seems irrelevant.
I want someone to tell me what's wrong here.
StevieTNZ
#7
Mar8-12, 08:08 PM
PF Gold
StevieTNZ's Avatar
P: 800
I was merely answering the first part of the question.

In which scenerio are you wanting to send information (and what sort of information) "faster than light"? The superposition or mixed states - or both?

I've sure all this has been ruled out with the no-signalling theorem and the like.
Swimmingly!
#8
Mar9-12, 08:06 AM
P: 45
Quote Quote by StevieTNZ View Post
I was merely answering the first part of the question.

In which scenerio are you wanting to send information (and what sort of information) "faster than light"? The superposition or mixed states - or both?

I've sure all this has been ruled out with the no-signalling theorem and the like.
The information would be sent in the case where electrons in superposition of spins are different from a defined value of spin. (possibly the act of reading disrupts the system too much for this but):

Get a pair of entangled electrons and send them to place A and B. We'll send a bit from A to B, which are really far apart. Let's assume electrons in superposition of spins are different from electrons with a defined spin. Let's say superposition stands for 0 and defined spin stands for 1.
  • We'll send 1 by measuring the spin the electron. Setting it to some defined value.
  • We'll send 0 by not doing anything at all.
Some time later the people in B put to test the behavior of the electron. If it behaves like it is in superposition, then it's 1. If it behaves like it's spin is defined then it's 0.
Khashishi
#9
Mar9-12, 02:05 PM
P: 887
Quote Quote by Swimmingly! View Post
Some time later the people in B put to test the behavior of the electron. If it behaves like it is in superposition, then it's 1. If it behaves like it's spin is defined then it's 0.
How do you propose to do that? The problem is, you can only acertain that B behaves like a superposition by testing many copies of B, in order to get some statistics.
DrChinese
#10
Mar9-12, 03:38 PM
Sci Advisor
PF Gold
DrChinese's Avatar
P: 5,323
Quote Quote by Swimmingly! View Post
Let's assume electrons in superposition of spins are different from electrons with a defined spin...
Yes, this is true in a sense, but... the ONLY way you can see how is to do a test on BOTH and compare them.

And that comparison can only be done classically and at light speed. So you are back to where you started.
Swimmingly!
#11
Mar9-12, 03:55 PM
P: 45
Quote Quote by Khashishi View Post
How do you propose to do that? The problem is, you can only acertain that B behaves like a superposition by testing many copies of B, in order to get some statistics.
Then I'd get lots of B's and do the same to all, I guess.

Quote Quote by DrChinese
Yes, this is true in a sense, but... the ONLY way you can see how is to do a test on BOTH and compare them.

And that comparison can only be done classically and at light speed. So you are back to where you started.
I don't understand why you can't compare them with electrons from outside of the experiment, but I understand part of what's wrong now. I still have to study Quantum Physics.
Thank you all for the help.
DrChinese
#12
Mar9-12, 04:46 PM
Sci Advisor
PF Gold
DrChinese's Avatar
P: 5,323
Quote Quote by Swimmingly! View Post
I don't understand why you can't compare them with electrons from outside of the experiment...

Thank you all for the help.
If you look at 100 electrons that are entangled (from the A stream), and compare them to 100 random electrons that are not, you will not notice any particular difference.

However, there are entangled state statistics for a stream of entangled pairs, but as mentioned you need to bring the results together using classical technique.
StevieTNZ
#13
Mar9-12, 09:42 PM
PF Gold
StevieTNZ's Avatar
P: 800
Btw, which spin state each electron takes is random. You can't do much if you wanted to send a specific message FTL.


Register to reply

Related Discussions
Difference In Number of Nuclei with Spin Up and Spin Down Atomic, Solid State, Comp. Physics 0
Whats the difference between nuclear spin and isospin? High Energy, Nuclear, Particle Physics 2
Difference between Mixed State and Superposition Quantum Physics 6
Isospin vs. flavor vs. spin - Difference and similarity? High Energy, Nuclear, Particle Physics 6