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8 comments

Comments on this article are now closed.

• 1
  Vladimir Tamari Mar 6, 2013 11:19 AM Tokyo, Japan

  Not spooky at all!

  Well done Martin - an excellent succinct presentation of the 'problem'. But where is the mystery? The two particles are 'entangled' from start to finish - their physical state is determined when they are created and is unchanged as they go in different directions. Probability is just an abstract mathematical concept that happens to describe quantum waves, but it should not be used as a blanket explanation for everything.

  Edited by Tushna Commissariat on Mar 8, 2013 2:53 PM.

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• 2
  bjorne Mar 8, 2013 8:42 PM Göteborg, Sweden

  A crucial piece missing

  Unfortunately, each particle could have been created up or down from the start, and that's how your intended audience will see it. Nothing strange here. Not even something hidden.

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• 3
  jrlloyd Mar 9, 2013 1:35 PM

  Can someone please explain why bjorne is wrong? I agree, if they were entangled at the start. One can also come up with a simple argument that violates the uncertainty principle as well from this experiment.

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• 4
  vug Mar 10, 2013 4:29 AM

  Quote:

  Originally posted by jrlloyd View comment

  Can someone please explain why bjorne is wrong? I agree, if they were entangled at the start. One can also come up with a simple argument that violates the uncertainty principle as well from this experiment.

  Bjorne is right, but that itself can not lead to violation of uncertainty principle. What the speaker did not tell in the video is that, the particles are in a quantum state called the "singlet state". When a pair of particles are in the singlet state, and their spins are measured in the same direction (say both in x-direction) then %100 their spins come up in opposite directions. The spin of individual particle will be random (either +x or -x with %50 probability) but if A is +x, then for sure B is -x and if A is -x, then B is for sure +x. This holds for y-axis, z-axis or any other direction. Their correlation which is "the average of spinA measurement outcome times spinB measurement outcome for many pairs" is -1.
  
  The things start to become more interesting when one particle's spin is measured in one direction (say x) and other particle's spins is measured in other direction (say z). x-spin and z-spin are incompatible quantities in quantum mechanics. Which means that they cannot have definite values at the same time. If one prepare a spin state which is always +1 in x direction, then the same particle's spin is %50 +z and %50 -z.
  
  The idea of violating uncertainty principle comes to mind when one says "oh, then I can measure x spin on A and z spin on B. If A is x+ then I'll know that B will be x-, and if B comes out z-, then I'll know that A will be z+. Look, I know the values of incompatible quantities at the same time."
  
  So, let me make it clear that, QM is about distributions. Of course one will get single numbers when incompatible variables are measured. The common phrase "Due to the uncertainty principle one cannot measure incompatible observables at the same time" is, unfortunately, incorrect. We can measure them.
  
  Say we can create many of these pairs in singlet state, which has the property of -1 correlation for same direction spin measurements. They will not give the same answer to x spin measurement on A and z spin measurement on B. That means that x and z does not have definite values, and the correlation is not +1 or -1.
  
  Hope this helps.

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• 5
  M. Asghar Mar 13, 2013 10:24 AM

  problem of commutation

  vug, you did a good job to expalin the things. If you allow me, I would just say that in the case of spin, the eigenvalue loses its pertinance, if one uses two different coordinates, because they do not commute with each other. cheerio.
  
  Quote:

  Originally posted by vug View comment

  Quote:

  Originally posted by jrlloyd View comment

  Can someone please explain why bjorne is wrong? I agree, if they were entangled at the start. One can also come up with a simple argument that violates the uncertainty principle as well from this experiment.

  Bjorne is right, but that itself can not lead to violation of uncertainty principle. What the speaker did not tell in the video is that, the particles are in a quantum state called the "singlet state". When a pair of particles are in the singlet state, and their spins are measured in the same direction (say both in x-direction) then %100 their spins come up in opposite directions. The spin of individual particle will be random (either +x or -x with %50 probability) but if A is +x, then for sure B is -x and if A is -x, then B is for sure +x. This holds for y-axis, z-axis or any other direction. Their correlation which is "the average of spinA measurement outcome times spinB measurement outcome for many pairs" is -1.
  
  The things start to become more interesting when one particle's spin is measured in one direction (say x) and other particle's spins is measured in other direction (say z). x-spin and z-spin are incompatible quantities in quantum mechanics. Which means that they cannot have definite values at the same time. If one prepare a spin state which is always +1 in x direction, then the same particle's spin is %50 +z and %50 -z.
  
  The idea of violating uncertainty principle comes to mind when one says "oh, then I can measure x spin on A and z spin on B. If A is x+ then I'll know that B will be x-, and if B comes out z-, then I'll know that A will be z+. Look, I know the values of incompatible quantities at the same time."
  
  So, let me make it clear that, QM is about distributions. Of course one will get single numbers when incompatible variables are measured. The common phrase "Due to the uncertainty principle one cannot measure incompatible observables at the same time" is, unfortunately, incorrect. We can measure them.
  
  Say we can create many of these pairs in singlet state, which has the property of -1 correlation for same direction spin measurements. They will not give the same answer to x spin measurement on A and z spin measurement on B. That means that x and z does not have definite values, and the correlation is not +1 or -1.
  
  Hope this helps.

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• 6
  stevek28 Mar 13, 2013 6:28 PM

  Quote:

  Originally posted by bjorne View comment

  Unfortunately, each particle could have been created up or down from the start, and that's how your intended audience will see it. Nothing strange here. Not even something hidden.

  The situation is NOT like this: a pair of shoes is produced. Each show is placed in a different box and sent to different parts of the world. Someone opens one of the boxes and finds there is a right shoe in the box. She "knows" the other box contains a left shoe.
  Why isn't the particle case similar? Because the(to follow the analogy) the shoe in the first box isn't a left or right shoe UNTIL it is observed.

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• 7
  M. Asghar Mar 14, 2013 11:38 AM

  identical particle

  stevek, unlike the left and right foot shoes, the two electrons with their S= !/2 are indistinguishable till they are entangled and then observed.
  
  Quote:

  Originally posted by stevek28 View comment

  Quote:

  Originally posted by bjorne View comment

  Unfortunately, each particle could have been created up or down from the start, and that's how your intended audience will see it. Nothing strange here. Not even something hidden.

  The situation is NOT like this: a pair of shoes is produced. Each show is placed in a different box and sent to different parts of the world. Someone opens one of the boxes and finds there is a right shoe in the box. She "knows" the other box contains a left shoe.
  Why isn't the particle case similar? Because the(to follow the analogy) the shoe in the first box isn't a left or right shoe UNTIL it is observed.

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• 8
  stevek28 Mar 14, 2013 8:53 PM

  Quote:

  Originally posted by M. Asghar View comment

  stevek, unlike the left and right foot shoes, the two electrons with their S= !/2 are indistinguishable till they are entangled and then observed.
  
  Quote:

  Originally posted by stevek28 View comment

  Quote:

  Originally posted by bjorne View comment

  Unfortunately, each particle could have been created up or down from the start, and that's how your intended audience will see it. Nothing strange here. Not even something hidden.

  The situation is NOT like this: a pair of shoes is produced. Each show is placed in a different box and sent to different parts of the world. Someone opens one of the boxes and finds there is a right shoe in the box. She "knows" the other box contains a left shoe.
  Why isn't the particle case similar? Because the(to follow the analogy) the shoe in the first box isn't a left or right shoe UNTIL it is observed.

  Read my comment again. Pay close attention to the "NOT"

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