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: 2012-06-04 03:07 am (UTC)D1 is being applied to the "idle" photon, which collapses its waveform and makes it a particle, and therefore you know which slit it went through and it fails to generate an interference pattern.
D2 on the "signal" photon, once it's been moved back far enough that it cannot guarantee which slit the "signal" photon went through, no longer collapses the "signal" photon - and apparently, collapsing an entangled photon does *not* collapse the pair photon (in this particular case for some reason, which is the part everyone is weirded out by), so the "signal" photon (being a wave at the time it passes through the slits), can happily interfere with itself.
no subject
Date: 2012-06-04 06:02 am (UTC):) Fair enough, though I still think it is mistaken. Does anybody really think that there is some kind of mysterious difference between the matter of the slit, the air, the screen, and the observer? Why would the observer "collapse the waveform" and not any of those other things?
It seems more likely that most experiments unwittingly contaminated the results in some way the experimenters were not allowing for. I'm not fully acquainted with all the details of the experiments, so can't say what that was, as it seems neither can anyone else (hence the mystical interpretations). However I don't think the answer is to say that observers have the spooky property of changing the wave into a particle. It seems pretty clear to me that light is a wave that acts like a particle -- not one or the other.
This latest experiment used a cunning way to avoid contaminating the experimental area and showed, I think, that the loss of the interference pattern had nothing to do with the observer being special, just that previous experiments were too clumsy.
When physicists say "collapsed the waveform" it sounds a little like when doctors call some illness "ideopathic" -- it is a way of saying they don't know what it is, but sounding like they do. But it is worse. At least when you scrutinise the doctor's term you can see it means that they don't know, whereas it seems to me that "collapsed the waveform" uses mystical wording to jump to a conclusion based on a lack of information. A conclusion which it seems to me has now been shown to be wrong.
no subject
Date: 2012-06-04 07:33 am (UTC)The screen itself, the point is that it is designed so it doesn't collapse the waveform of a single photon, because a single photon does not provide enough "information" to generate a signal. If you use a detector which does provide information at the single photon level, then the photon no longer behaves like a wave, and you lose the interference pattern.
The slits exist, but it's a question of whether the photon is behaving like a wave or a particle, whether it passes through one or both slits. And you can generate interference patterns that aren't 50/50 interference whilst still firing single photons at a time...
no subject
Date: 2012-06-06 06:35 am (UTC)Have you seen this?
http://en.wikipedia.org/wiki/File:Double-slit_experiment_results_Tanamura_2.jpg
It is a picture of electrons fired through a double slit and allowed to build up over time. Each electron is recorded, but they still generate an interference pattern. The exciting possibility that I could be totally wrong is a tantalising tingle on the tip of my tongue.
I don't know how to incorporate this into my understanding... but it seems I'm not alone there. :) Vague hand-wavy statements about collapsed waveforms aside. :)
no subject
Date: 2012-06-04 06:15 am (UTC)The experiment is interesting because it is on the one hand saying the light went through one slit, because the entangled photon lets them identify the energy level and thus the slit it was aimed at, but on the other hand it produces an interference pattern, which it could only do if it goes through both slits.
My guess is that we are underestimating how waves work. Time will tell.
no subject
Date: 2012-06-04 07:29 am (UTC)I think this experiment is interesting because it shows that quantum entanglement is a little bit funny about what happens to the other photon - it's not so clear that everything about the entangled photon pair collapses.
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.