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Quantum Entanglement

INRM

Philosopher
I
Joined
Jul 24, 2002
Messages
5,505
Okay, how do you "entangle" stuff?

And if you've entangled stuff, can you "Un-entangle" stuff?

-INRM
 
Okay, here's my simplistic, and likely wrong, understanding of it.

Entanglement occurs when two particles interact, and define one another's spin. They interact in such a way that if one is spinning one way, the other must be spinning in another, and vice-versa. (Spin doesn't actually refer to spinning like a ball, I'm given to believe, but something pretty indefinable using normal language.) But at this point, neither particle has been measured or observed, which means that their wave-functions have not yet collapsed, and they are actually both in both states of spin. Unentanglement occurs at the point of measurement. Once one of the particle's spin has been ascertained, the other's is also immediately defined, wherever it is in the universe.

Weird.

So, how bad is this explanation? Anybody?
 
No sir right on the button, but limited. It IS a wierd conterintutive thing but it is now approching practical application. The most exciting ( for me ) applications will be in both computational devices ( that will make a 3 GHz pentium 4 look like an abacus) and data ecriptation, which will do away with DES and RSA. Heres a link to a verbious yet understandable tract for the layman. http://plato.stanford.edu/entries/qt-entangle/
 
My brain is on the fritz lately.

I can't seem to grasp even the basic concepts.

I know that quantum entanglement is when two particles are linked.

I could use an explanation of how this happens, but the next thing I need to know is: How do you dis-entangle that data?

-INRM
 
Nucular said:
Okay, here's my simplistic, and likely wrong, understanding of it.

Entanglement occurs when two particles interact, and define one another's spin. They interact in such a way that if one is spinning one way, the other must be spinning in another, and vice-versa. (Spin doesn't actually refer to spinning like a ball, I'm given to believe, but something pretty indefinable using normal language.) But at this point, neither particle has been measured or observed, which means that their wave-functions have not yet collapsed, and they are actually both in both states of spin. Unentanglement occurs at the point of measurement. Once one of the particle's spin has been ascertained, the other's is also immediately defined, wherever it is in the universe.

Weird.

So, how bad is this explanation? Anybody?
Click to expand...

Not bad - unfortunately your explanation misses why entanglement is so weird. As you've described it, the measurement on the one particle collapsing the state of the other isnt weird, because exactly the same thing would happen if I stuck two pencils you knew were parallel into two different boxes and then sent them two far separated stations. When one person opens the box and sees what direction their pencil is pointing, they automatically know the direction of the other pencil!!

If you really want to understand quantum entanglement you have to try the following puzzle. I originally posted it in "Puzzles" years ago (my first post IIRC), but its been lost with computer changes...


Here's a logic-type puzzle phrased around the "million dollar challenge". I promise that sweating over this problem is worth the effort - the solution is as close to magic as this universe will presumably allow, and cannot fail to intrigue all except the most apathetic or unimaginative....

THE PUZZLE:

A group of three self-proclaimed psychics is captured and imprisoned by the infamous underground group PSI (Protectors of Scientific Integrity). The leader of the kidnappers, a certain Rames Jandi ("Rah-mez Yan-dee"), makes the generous offer that their lives will be spared if they can demonstrate psychic ability in ANY test of their own choosing (subject to the kidnappers' agreement that it does demonstrate psychic ability of course!).

The psychics propose the following test:

1. The three psychics will be taken to separate locations and locked in completely sealed rooms. (If picky one can add the restriction that the rooms are separated by many light years in order to avoid the "Peter Popoff effect" so to speak.)

2. Each psychic will be asked a question by a captor - the question will be either "WHAT IS X" or "WHAT IS Y".

3. The captors agree that either:
(A) All three psychics will be asked the "X" question; or
(B) One particular psychic will be asked the "X" question while the other two will be asked the "Y" question.
The captors will secretly choose uniformly at random from the four possibilities [i.e. {XXX,XYY,YXY,YYX}] on each and every trial.

4. To the (meaningless) question the psychics must reply either "+1" or "-1" as they wish (utilizing whatever telepathic powers they deem necessary).

5. The psychics are deemed to have "won" if the product of their answers is -1 when case (A) of step 3 has occurred, but +1 when case (B) occurred.

6. Steps 2-5 are repeated one thousand times, and the psychics will be released if they win every single time.

Rames Jandi is a pretty good mathematician, and after a little thought he decides that they cannot possibly win more than 75% of the time without using some form of psychic ability [a copy of his proof is given below]. He therefore makes the generous offer that they need only win on more than 900 of the trials, and that if they do he will not only release them but he will give up his nefarious ways and become a naturopath for energy unbalanced alien visitors.

Unfortunately for Jandi the psychics DO pass the test - and in fact they win every single one of the 1000 trials.

How did they do it?

[Aside:
Jandi's proof that they cannot win 100% of the time:
Label the psychics A,B,C.
Let X(A) be the value (+1 or -1) that psychic A will answer if she is asked the X question, Y(A) is what she answers if asked the Y question. Similarly define X(B), Y(B), X(C), Y(C).
To win 100% of the time would require:
X(A)*X(B)*X(C) = -1
X(A)*Y(B)*Y(C) = +1
Y(A)*X(B)*Y(C) = +1
Y(A)*Y(B)*X(C) = +1
Multiplying the 4 equations together gives:
X(A)^2 * Y(A)^2 * X(B)^2 * Y(B)^2 * X(C)^2 * Y(C)^2 = -1
Since every factor on the left is the square of +1 or -1, the left hand side must be +1, which contradicts the right hand side. Thus all four equations cannot have simultaneous solutions. It is simple to find a set of values satisfying 3 of the 4 equations however.]
Click to expand...
 
This is a tough test? Maybe I'm missing something, but why not answer every X question as -1 and every Y question as +1? X^3 gives you the correct +1 answer, and X^2*Y gives you the correct +1 answer, doesn't it?

What did I miss? How the psychics guessed the right strategy?
 
garys_2k said:
This is a tough test? Maybe I'm missing something, but why not answer every X question as -1 and every Y question as +1? X^3 gives you the correct +1 answer, and X^2*Y gives you the correct +1 answer, doesn't it?

What did I miss? How the psychics guessed the right strategy?
Click to expand...

No - its two Y questions and an X, not two X questions and a Y...
 
Thanks!

Yes, I see the impossibility of getting 100% of the answers correct. There is no causal way to get those results.
 
Yah - so the point is that if the three psychics share entangled systems (in whats called the GHZ state) and make measurements of either X or Y spin according to the question posed to them, then they'll win 100% of the time.

This is why entanglement is strange - the particles couldnt have "known" their spin along X and Y prior to the measurement (and independent of the measurements on the other particles) since if they did then the psychics wouldnt need such an elaborate cheating strategy....
 
But does it demonstrate that information travels faster than light between the two particles. And does it show that "everything is connected"?
 
RichardR said:
But does it demonstrate that information travels faster than light between the two particles. And does it show that "everything is connected"?
Click to expand...

IT shows that either some form of information travels faster than light (which does not necessarily mean signal transfer FTL) or that "everything is connected" (the ontic objects of the universe are nonlocal entities). To date those are, broadly speaking, the only possibilities that people have come up with - though I'd be very interested in hearing any ideas you or others might have.

All I can say is please make sure you do understand the above puzzle and how your explanation of it would work precisely. I get a lot of suggestions which are frustrating because the person hasnt actually understood the conundrum...
 
Or it shows that the two particles decided beforehand to switch their spins in a seemingly random, but in reality a predetermined pattern. So when you measure one's spin, of course the other will have the opposite spin at the same time because they had already set their plan in place.

This interpretation is called "hidden variables" because it assumes there is something the particles know that we don't (the pattern of future spins). Hidden variables was proven not to be the case several years ago, but I've heard recently that maybe it's not ruled out after all. It's at this point that my knowledge is depleted and I have to ask for help...
 
CurtC said:
Or it shows that the two particles decided beforehand to switch their spins in a seemingly random, but in reality a predetermined pattern. So when you measure one's spin, of course the other will have the opposite spin at the same time because they had already set their plan in place.

This interpretation is called "hidden variables" because it assumes there is something the particles know that we don't (the pattern of future spins). Hidden variables was proven not to be the case several years ago, but I've heard recently that maybe it's not ruled out after all. It's at this point that my knowledge is depleted and I have to ask for help...
Click to expand...

no - another misconception. The point is that the scheme you describe wouldnt work (otherwise the psychics in the puzzle above could use it). The only way to make it work would be to have the one spin "tell" the other spin which measurement was performed on it - some form of superluminal (and omnipresent) communication.

I beg of you - try the above puzzle and see why what you said doesnt work. You will learn something that is truly mysterious about the universe, something that 99.9% of the people around you will never know exists, and which is at least as marevelous as most claimed paranormal phenomena...
 
Tez said:
IT shows that either some form of information travels faster than light (which does not necessarily mean signal transfer FTL)
Click to expand...
That puzzles me. How can information travel faster than light, and yet there is no faster than light signal transfer?

Isn't "information" the same as "signal"?
 
RichardR said:
That puzzles me. How can information travel faster than light, and yet there is no faster than light signal transfer?

Isn't "information" the same as "signal"?
Click to expand...

well, for humans to signal basically takes a cause here to lead to an effect there which is amplifiable to human scales. There exist hidden variable theories in which the particles communicate with each other FTL - but the theories are contrived in such a way as to not allow access to the information in such a way that we humans could make use of it. (Since this is what we believe i.e. no FTL communication possible). Some of the extreme advocates of those theories do, in fact, believe that it could be used for signalling. More precisely, one of the particular contrivations (is that a word) has it that the only reason we cant signal is very similar to the only reason someone in thermal equilibrium with their environment couldnt do work - i.e. its a contingent manifestation of the equation of state of the universe.

ANyway to answer your question - its possible to construct theories which allow the appropriate FTL communication for the Bell inequalities to be violated, but which ensure that no information can be signalled. TO my mind theyre contrived and unnatural...
 
doesnt work fishbob - since when two people get asked the Y question and one person gets asked the X then the product of their answers is -1 and not +1 as required...
 
Now, here's my question

How fast does the information travel?

Does it travel infinite? Or does it travel faster than the speed of light? Is there any way to predict this?

The other question is: is there any other way quantom entanglement can "appear" to occur, without actually occuring?

-INRM
 
Tez said:
Yah - so the point is that if the three psychics share entangled systems (in whats called the GHZ state) and make measurements of either X or Y spin according to the question posed to them, then they'll win 100% of the time.
Click to expand...

Hey Tez, can you elaborate a little on this for me, or point me to a good link?

One thing I'm wondering is why it works on subsequent trials, after the first set of measurments, the entanglement is still there?
It's not effected by the measuring in some way?

I beg of you - try the above puzzle and see why what you said doesnt work. You will learn something that is truly mysterious about the universe, something that 99.9% of the people around you will never know exists, and which is at least as marevelous as most claimed paranormal phenomena...
Click to expand...
All I can say is please make sure you do understand the above puzzle and how your explanation of it would work precisely. I get a lot of suggestions which are frustrating because the person hasnt actually understood the conundrum...
Click to expand...

How would you feel about giving a long winded explanation of exactly how the solution works? :D

Seriously, I'm interested.

Adam
 
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