Wave function collapse is not observed physical phenomenon. Some interpretations of QM require it to exist, but there is no empirical evidence of that happening.
Apparent wave function collapse happens when a wave function in a superposition of several eigenstates appears to reduce to a single eigenstate. The apparent wave function collapse collapse is mathematically equivalent of quantum decoherence where the wave function never really collapses but the states gets entangled with the observer.
If somebody were able to formulate and experience that would show the difference between decoherence and collapse, that would be new physics and we would be able to rule out some interpretations of quantum mechanics. Until that happens, 'shut up and caluclate' seems to be valid course of action.
As far I understand, the philosophical difference between apparent and actual wave function collapse is that in the apparent collapse probabilities of other states get so close to zero that they don't matter, in actual collapse they are exactly zero.
The assumption that human consciousness has something to do with setting all other states to zero is weird one and can't completely understand the assumptions behind it. I guess the idea is that we would not experience the world as we experience it now if there is just continuing decoherenće.
The philosophical difference is that without collapse, all of the eigenstates contributing to the intial state will get entangled with their own 'copy' of the environment, which is where the moniker many worlds interpretation comes from.
> Apparent wave function collapse happens when a wave function in a superposition of several eigenstates appears to reduce to a single eigenstate. The apparent wave function collapse collapse is mathematically equivalent of quantum decoherence where the wave function never really collapses but the states gets entangled with the observer.
The problem with this interpretation is the fact that density matrices work: that certain probability-weighted ensembles of states are completely, provably equivalent to other probability-weighted ensembles of states. If you don't interpret collapse as physically real, then it remains to be explained why this should be the case.
Ok, I'm going to hijack this thread to get an answer to something I've always struggled with: how does 'observation' cause the wave function to collapse.
It won't happen in a closed box, but what about an open box in a closed room? What about a closed box with a live video camera? What about a live video camera whose display no one is watching?
I'm sure this is a basic question but for the life of me, I've never really understood it. Thanks.
The standard story about measurement/observation and quantum collapse is not the only interpretation of the available data.
There are, broadly, three kinds of interpretations of what's 'really' going on behind the scenes:
1. Collapse - The world exists in an indeterminate state until observation occurs, when the world collapses into one a single determinate state. The probabilities of quantum mechanics map onto the the different parts of the unobserved indeterminate state.
2. Many Worlds - Quantum phenomena cause the world the branch into multiple worlds. The probabilities of quantum mechanics represent the 'share' of reality that branches in each direction.
3. Hidden variables - The probabilities of quantum mechanics are artifacts of our inability to know all the relevant variables. Measurement necessarily involves causal contact, and causal contact will always disturb some of the relevant variables in unpredictable ways. It would be cool if we could observe what goes on when measurement occurs, but to do that would require measurement! So we're stuck with hidden variables.
The public tends to hear the collapse interpretation most often. Physicists tend to like the many worlds interpretations. Philosophers of science tend to like the hidden variables interpretation, because the other options require an incoherent metaphysics. (I'm a Philosophy PhD student.)
People say that the hidden variables interpretation is ruled out on experimental grounds, but this is demonstrably false. Experimental data shows that hidden variables, if they exist, violate locality:
Locality - Causal interaction is a local phenomena. No action at a distance.
So long as you're willing to abandon locality, hidden variables can work. Given that the other two interpretations posit equally weird things, abandoning locality won't seem so weird.
For more on this, see Tim Maudlin's excellent paper, "Three Measurement Problems"
I think I can approach a layman's explanation to this (feel free to correct me if this is offensively inaccurate). My understanding is that 'observation', at a quantum level, isn't like "watching something with your eyes". A better description would be "taking a measurement" instead of "observation". It's impossible/difficult to take a measurement at the quantum level by passively observing, you have to de-facto interact with it in order to measure it - not unlike searching for a house of cards in a dark room with a blindfold on. Once you've 'found' the house of cards, you've 'interacted' with it, and in the process of interacting it is no longer a house of cards any more.
Observation means interaction in any form with the observer (edited as per Koshkin's comment). Most things in the universe in practice have interacted or will interact. Interaction could be as subtle as a single photon reflecting off it and hitting your eye, or it could be you smashing your face into the object.
When they present the hypothetical Schrödinger's Cat thought experiment, it's a thought experiment assuming somehow the box prevents all interaction between the cat inside and the outside world. In the real world however things like sound waves, light, x-rays, etc, will inevitably penetrate such a box. And even if the cat moves, it will cause some amount of detectable vibrations on the outside of the box. It would be impossible to theoretically prevent any energy from the cat inside the box to emanate outside as that would violate the law of preservation of information (even a black hole—the closest thing to an absolutely sealed box—releases information about its contents and expels energy).
There is no explanation for why the wave function collapses due to interaction other than "god rolling dice" and picking a collapsed outcome once such an outcome becomes relevant (through interaction/observation). Other interpretations of wave function collapse don't even suppose the collapse happens. For example in the many worlds interpretation, instead of having a single observer (the universe and its participants), there are multiple universes and multiple observers. In this interpretation there is no collapse of the wave function. Instead of many possibilities and one getting picked, there are many observers each independently observing one of the possible outcomes. Here the dice rolling still happens, but happens for every observer constantly (when deciding what outcome is observed next).
In a quantum logic circuit, every "observation" can be thought of as ultimately compiling down into a controlled-Z gate between the qubit-you-want-to-measure A and a qubit-to-hold-the-measurement-result B.
The interesting thing about the controlled-Z is that it is actually a symmetric operation. You could think of it as "if A is ON then apply a Z to B", but it is exactly equivalent to say it is performing an "if B is ON then apply a Z to A" effect. The truly symmetric description is "apply a -1 factor to the weight of cases where A and B are both ON".
Because the controlled-Z is symmetric, and every measurement ultimately involves a controlled-Z, you can't move details about A into B without also kicking back details about B into A. When B is a big well-approximated-by-classical-physics system, the back-effect plays out in a way that we call collapse.
Said another way, B (you and/or instruments) can't get information about A (the quantum system) without collapsing it.
One explanation[1] i heard is observation is a actually equivalent to entanglement. Now we know that two elements entangled will have to maintain the state corresponding to its entangled peer. Now if you observing (or taking measurement) the whole system gets entangled to the particle. you are observing and hence the wave function collapses.
The most sensible explanation I've heard came from sir Arthur Eddington. His claim (roughly) is that probabilities in quantum theory model our knowledge of the quantum system, rather than indicating anything intrinsically probabilistic in the system itself.
In that case, our knowledge allows for more possibilities before making a measurement (observation), and collapses into something more definite after the measurement.
That's probably not the best description, but I think the basic idea about modeling knowledge makes a lot of sense. Eddington was primarily a physicist, but also a philosopher. I read this account of his in 'The Philosophy of Physical Science.'
If I'm not mistaken, it's pretty much in line with the 'Pilot Wave' interpretation which is getting more attention these days.
It doesn't matter if anybody's watching. It's just that to get any information out of the system you need to interact with it somehow. This interaction is what's causing the wave function collapse.
It's almost impossible to answer that question. Which is one motivation behind hidden variables theories of Quantum Theory that don't elevate observation to having a role in quantum phenomena.
The only fully honest answer to this question: it is not understood.
A good chunk of physicists (not all) would agree that the current understanding is unsatisfactory, but it's unclear whether now is the "right time" to research that question. It is unclear whether we have the experimental tools to probe the consequences of the different models, and whether different approaches produce different predictions in some testable scenario.
Of course, there is an active field of research probing this and related questions. Different physicists in there have different ideas, depending on how they interpret quantum mechanics. (See other responses in the thread for a sampling of those). But there is no satisfactory agreed-upon textbook answer.
Personally, I find the some aspects of the decoherence perspective appealing.
"Collapse of a wavefunction" is a (mathematical) artifact of the currently used theoretical framework in which the quantum system is being brought into contact with a classical, i.e. non-quantum system (which is what observation, or measurement, is).
Interference is very fragile. It used to create the most sensitive scientific instruments. It is very easy to disturb self-interference in double slit experiment.
If it helps, in the multiverse interpretation wavefunctions never collapse. Don't think too carefully about observation in QM it's just an artifact of getting the right answers.
It's worth mentioning that "quantum theory" has no implications for consciousness, especially given the headline proclaiming the opposite. There are plenty of interpretations that don't involve consciousness at all - and hopefully my use of the word interpretation clues you in to the fact that this is isn't really a physics question.
Weird philosophy aside, this does look like a good explanation of the Bell inequality.
I'm deeply sick of the absurdly drawn-out living death of the Copenhagen interpretation, and its even more absurd "consciousness is made out of magic" descendants. This is bordering on straight-up irresponsible misinformation.
Maybe so, but please don't post unsubstantive comments to Hacker News.
The only information here is that you're annoyed and disapprove. That's not enough to make a post good. If you'd be willing to transmute annoyance into information others can learn from, that'd be great. If you don't want to, that's fine, but then please just don't post anything. When commenters use this forum merely as a venting outlet, quality suffers.
[+] [-] nabla9|8 years ago|reply
Apparent wave function collapse happens when a wave function in a superposition of several eigenstates appears to reduce to a single eigenstate. The apparent wave function collapse collapse is mathematically equivalent of quantum decoherence where the wave function never really collapses but the states gets entangled with the observer.
If somebody were able to formulate and experience that would show the difference between decoherence and collapse, that would be new physics and we would be able to rule out some interpretations of quantum mechanics. Until that happens, 'shut up and caluclate' seems to be valid course of action.
As far I understand, the philosophical difference between apparent and actual wave function collapse is that in the apparent collapse probabilities of other states get so close to zero that they don't matter, in actual collapse they are exactly zero.
The assumption that human consciousness has something to do with setting all other states to zero is weird one and can't completely understand the assumptions behind it. I guess the idea is that we would not experience the world as we experience it now if there is just continuing decoherenće.
[+] [-] cygx|8 years ago|reply
[+] [-] lmm|8 years ago|reply
The problem with this interpretation is the fact that density matrices work: that certain probability-weighted ensembles of states are completely, provably equivalent to other probability-weighted ensembles of states. If you don't interpret collapse as physically real, then it remains to be explained why this should be the case.
[+] [-] akvadrako|8 years ago|reply
You might as well postulate there are magical pink unicorns dancing on every particle. By your logic that theory also can't be ruled out.
[+] [-] david927|8 years ago|reply
It won't happen in a closed box, but what about an open box in a closed room? What about a closed box with a live video camera? What about a live video camera whose display no one is watching?
I'm sure this is a basic question but for the life of me, I've never really understood it. Thanks.
[+] [-] jackson1372|8 years ago|reply
There are, broadly, three kinds of interpretations of what's 'really' going on behind the scenes:
1. Collapse - The world exists in an indeterminate state until observation occurs, when the world collapses into one a single determinate state. The probabilities of quantum mechanics map onto the the different parts of the unobserved indeterminate state.
2. Many Worlds - Quantum phenomena cause the world the branch into multiple worlds. The probabilities of quantum mechanics represent the 'share' of reality that branches in each direction.
3. Hidden variables - The probabilities of quantum mechanics are artifacts of our inability to know all the relevant variables. Measurement necessarily involves causal contact, and causal contact will always disturb some of the relevant variables in unpredictable ways. It would be cool if we could observe what goes on when measurement occurs, but to do that would require measurement! So we're stuck with hidden variables.
The public tends to hear the collapse interpretation most often. Physicists tend to like the many worlds interpretations. Philosophers of science tend to like the hidden variables interpretation, because the other options require an incoherent metaphysics. (I'm a Philosophy PhD student.)
People say that the hidden variables interpretation is ruled out on experimental grounds, but this is demonstrably false. Experimental data shows that hidden variables, if they exist, violate locality:
Locality - Causal interaction is a local phenomena. No action at a distance.
So long as you're willing to abandon locality, hidden variables can work. Given that the other two interpretations posit equally weird things, abandoning locality won't seem so weird.
For more on this, see Tim Maudlin's excellent paper, "Three Measurement Problems"
https://www.academia.edu/32885328/Three_measurement_problems
[+] [-] lwhalen|8 years ago|reply
[+] [-] lisper|8 years ago|reply
It doesn't. This is a reasonable approximation to the truth in many common situations, but it is not the truth.
The truth is that measurement and entanglement are the same physical phenomenon. See:
http://www.flownet.com/ron/QM.pdf
or the video version:
http://www.flownet.com/ron/QM.pdf
[+] [-] chrischen|8 years ago|reply
When they present the hypothetical Schrödinger's Cat thought experiment, it's a thought experiment assuming somehow the box prevents all interaction between the cat inside and the outside world. In the real world however things like sound waves, light, x-rays, etc, will inevitably penetrate such a box. And even if the cat moves, it will cause some amount of detectable vibrations on the outside of the box. It would be impossible to theoretically prevent any energy from the cat inside the box to emanate outside as that would violate the law of preservation of information (even a black hole—the closest thing to an absolutely sealed box—releases information about its contents and expels energy).
There is no explanation for why the wave function collapses due to interaction other than "god rolling dice" and picking a collapsed outcome once such an outcome becomes relevant (through interaction/observation). Other interpretations of wave function collapse don't even suppose the collapse happens. For example in the many worlds interpretation, instead of having a single observer (the universe and its participants), there are multiple universes and multiple observers. In this interpretation there is no collapse of the wave function. Instead of many possibilities and one getting picked, there are many observers each independently observing one of the possible outcomes. Here the dice rolling still happens, but happens for every observer constantly (when deciding what outcome is observed next).
[+] [-] Strilanc|8 years ago|reply
The interesting thing about the controlled-Z is that it is actually a symmetric operation. You could think of it as "if A is ON then apply a Z to B", but it is exactly equivalent to say it is performing an "if B is ON then apply a Z to A" effect. The truly symmetric description is "apply a -1 factor to the weight of cases where A and B are both ON".
Because the controlled-Z is symmetric, and every measurement ultimately involves a controlled-Z, you can't move details about A into B without also kicking back details about B into A. When B is a big well-approximated-by-classical-physics system, the back-effect plays out in a way that we call collapse.
Said another way, B (you and/or instruments) can't get information about A (the quantum system) without collapsing it.
[+] [-] neel8986|8 years ago|reply
You can look at a better explanation here [1]https://www.youtube.com/watch?v=dEaecUuEqfc
[+] [-] westoncb|8 years ago|reply
In that case, our knowledge allows for more possibilities before making a measurement (observation), and collapses into something more definite after the measurement.
That's probably not the best description, but I think the basic idea about modeling knowledge makes a lot of sense. Eddington was primarily a physicist, but also a philosopher. I read this account of his in 'The Philosophy of Physical Science.'
If I'm not mistaken, it's pretty much in line with the 'Pilot Wave' interpretation which is getting more attention these days.
[+] [-] comboy|8 years ago|reply
[+] [-] meri_dian|8 years ago|reply
[+] [-] ssivark|8 years ago|reply
A good chunk of physicists (not all) would agree that the current understanding is unsatisfactory, but it's unclear whether now is the "right time" to research that question. It is unclear whether we have the experimental tools to probe the consequences of the different models, and whether different approaches produce different predictions in some testable scenario.
Of course, there is an active field of research probing this and related questions. Different physicists in there have different ideas, depending on how they interpret quantum mechanics. (See other responses in the thread for a sampling of those). But there is no satisfactory agreed-upon textbook answer.
Personally, I find the some aspects of the decoherence perspective appealing.
[+] [-] Koshkin|8 years ago|reply
[+] [-] unknown|8 years ago|reply
[deleted]
[+] [-] v_lisivka|8 years ago|reply
https://www.youtube.com/watch?v=nsaUX48t0w8
[+] [-] whatshisface|8 years ago|reply
[+] [-] kowdermeister|8 years ago|reply
https://www.youtube.com/watch?v=izqaWyZsEtY
[+] [-] pishpash|8 years ago|reply
[+] [-] whatshisface|8 years ago|reply
Weird philosophy aside, this does look like a good explanation of the Bell inequality.
[+] [-] platz|8 years ago|reply
[+] [-] unknown|8 years ago|reply
[deleted]
[+] [-] unknown|8 years ago|reply
[deleted]
[+] [-] virgil_disgr4ce|8 years ago|reply
[+] [-] dang|8 years ago|reply
The only information here is that you're annoyed and disapprove. That's not enough to make a post good. If you'd be willing to transmute annoyance into information others can learn from, that'd be great. If you don't want to, that's fine, but then please just don't post anything. When commenters use this forum merely as a venting outlet, quality suffers.
[+] [-] fiatjaf|8 years ago|reply