quantum entanglement
Jun. 2nd, 2012 01:08 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
miriam_eI've encountered a few examples recently of people who think that quantum entanglement is something that enables faster-than-light spooky action at a distance. It may be that it does, though I don't see any reason to believe that. As far as I can see, these people don't have any reason either, other than desperately wanting to find mystical stuff in our knowledge gaps.
The example that I generally hear about is the one where researchers entangled two particles, separated them by some distance, then they looked at one -- and here is the "woo" -- it instantaneously made the other take on the paired state.
I can best show why I think this is shonky thinking with a simple illustration.
A pair of cards is given to Alice and Bob. One card is black and the other is red, but they don't know who has which color. They go into separate rooms and Alice looks at her card and calls out its color. Miraculously, when Bob looks at his card it has turned into the exact complementary card!
See the error? Bob's card didn't change.
Those who want mysticism like to say that the entangled particles were in an indeterminate state and that the act of observing one "collapsed the wave-state" of both.
As far as I can work out this idea comes from some of the early experiments in wave-particle duality.
The double-slit experiment showed that light acts like a wave if you let it shine through a couple of slits side by side because the light emerging from the slits produces interference patterns. This doesn't happen if there is just a single slit. Good enough. This makes good sense.
Next came a series of very puzzling experiments where the rate of photons was slowed so that experimenters were convinced that only one photon at a time traversed the apparatus. They found to their great surprise that it still created interference patterns. But how could a single photon travelling through one slit interfere with itself? It seems they had underestimated its wave nature and it was somehow travelling through both slits.
So then many experiments were designed to try to observe the photon travelling through one or the other slit. But whenever they managed to detect it going through one or other slit the interference pattern vanished!
Now, there are two ways this can be read. We can see the apparatus as having inadvertently changed the path of the light. Or we can make mystical statements like "the light has no state until it is observed".
A lot of people have drawn the latter conclusion. Unfortunately it is wrong, as was shown by an experiment this year in which experimenters actually used a pair of entangled photons to detect the path of a photon without affecting the interference pattern. Read about it here:
http://arstechnica.com/science/2012/05/disentangling-the-wave-particle-duality-in-the-double-slit-experiment/
or in the original paper, freely available as a pdf here:
http://dx.doi.org/10.1073/pnas.1201271109
It stands to reason that it would be wrong. The observer is nothing special. A person is just matter like the material the slits are made from. If a person observing the photon can "collapse its wave function" then so should the wall with the slit, and the air the light travels through, and the screen that stops the light, displaying the interference pattern. All that was needed was a cunning enough way to detect the light.
But now we have another problem. If the light is detected to travel through just one slit at a time, how can an interference pattern possibly arise? Somehow I think another mistake has been made.
The example that I generally hear about is the one where researchers entangled two particles, separated them by some distance, then they looked at one -- and here is the "woo" -- it instantaneously made the other take on the paired state.
I can best show why I think this is shonky thinking with a simple illustration.
A pair of cards is given to Alice and Bob. One card is black and the other is red, but they don't know who has which color. They go into separate rooms and Alice looks at her card and calls out its color. Miraculously, when Bob looks at his card it has turned into the exact complementary card!
See the error? Bob's card didn't change.
Those who want mysticism like to say that the entangled particles were in an indeterminate state and that the act of observing one "collapsed the wave-state" of both.
As far as I can work out this idea comes from some of the early experiments in wave-particle duality.
The double-slit experiment showed that light acts like a wave if you let it shine through a couple of slits side by side because the light emerging from the slits produces interference patterns. This doesn't happen if there is just a single slit. Good enough. This makes good sense.
Next came a series of very puzzling experiments where the rate of photons was slowed so that experimenters were convinced that only one photon at a time traversed the apparatus. They found to their great surprise that it still created interference patterns. But how could a single photon travelling through one slit interfere with itself? It seems they had underestimated its wave nature and it was somehow travelling through both slits.
So then many experiments were designed to try to observe the photon travelling through one or the other slit. But whenever they managed to detect it going through one or other slit the interference pattern vanished!
Now, there are two ways this can be read. We can see the apparatus as having inadvertently changed the path of the light. Or we can make mystical statements like "the light has no state until it is observed".
A lot of people have drawn the latter conclusion. Unfortunately it is wrong, as was shown by an experiment this year in which experimenters actually used a pair of entangled photons to detect the path of a photon without affecting the interference pattern. Read about it here:
http://arstechnica.com/science/2012/05/disentangling-the-wave-particle-duality-in-the-double-slit-experiment/
or in the original paper, freely available as a pdf here:
http://dx.doi.org/10.1073/pnas.1201271109
It stands to reason that it would be wrong. The observer is nothing special. A person is just matter like the material the slits are made from. If a person observing the photon can "collapse its wave function" then so should the wall with the slit, and the air the light travels through, and the screen that stops the light, displaying the interference pattern. All that was needed was a cunning enough way to detect the light.
But now we have another problem. If the light is detected to travel through just one slit at a time, how can an interference pattern possibly arise? Somehow I think another mistake has been made.
no subject
Date: 2012-06-06 06:23 am (UTC)But the interpretation that the entangled pair "collapse" by halves in some way... I'll have to think on it. My immediate reaction is that I don't buy it. I think the whole "collapse" thing is a mistake borne of our not understanding all the ramifications of dealing with these waves. And I'll have to read up more about the old experiments that led to the whole "collapsing waveform" explanation.
Thanks for your thoughtful replies.