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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 wo

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.

kowdermeister 9y ago
PBS Space Time did great episode on this subject: https://www.youtube.com/watch?v=izqaWyZsEtY https://www.youtube.com/watch?v=izqaWyZsEtY
- david927 9y ago
  It wasn't really relevant to my question, but you're right, it's a good clip. Thanks.
comboy 9y ago
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.
- david927 9y ago
  With the double-slit experiment, my understanding is that if it's done in a closed room, the wave function won't collapse and you'll get an interference pattern, but if you're in the room, it will collapse and there will be no interference pattern. Aren't you testing it by observing it?
  - kowdermeister 9y ago
    I'm pretty sure that's not the case. The interference pattern only disappears if you try to figure out which slit a single photon went through. It does not require a conscious observer to be present. What matters is if you extract information from the system or not.
    - david927 9y ago
      Sorry, you're right.
    - kgwgk 9y ago
      > only disappears if you try to figure out > What matters is if you extract information > It does not require a conscious observer to be present. But does it require a “you” trying to figure out and extracting information?
  - Strilanc 9y ago
    > if it's done in a closed room, the wave function won't collapse and you'll get an interference pattern, but if you're in the room, it will collapse That is absolutely 100% not what happens. The experiments play out the same way whether or not a human's face happens to be nearby. All the data is collected automatically by photographic plates or electronic counters or other instruments, and "observation" always refers to some instrument being present or not.
    - david927 9y ago
      I understand now. So it's, "can the information be known?" and the answer to that question, whether instrument or human or decision (such as marking the photon), is what creates the entanglement which causes the collapse. P.S. Everyone - thanks! I get it now. And it's fascinating!
      - monktastic1 9y ago
        But the entanglement doesn't "cause collapse." If you entangle states A and B, their joint state is in a superposition and will still exhibit interference (given the right experimental setup). It's just that either particle by itself will not.
        david927 9y ago
        oh shit. ok, I see. wow
meri_dian 9y ago
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.
- deleted 9y ago
  [deleted]
- lisper 9y ago
  To the contrary, it is very easy to answer: measurement does not cause the collapse of the wave function. 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 http://www.flownet.com/ron/QM.pdf or the video version: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf
  - meri_dian 9y ago
    Thanks for posting that. Can you reconcile these two parts for me though: "It accounts for the apparent contradiction between quantum theory, which says that entropy is conserved in unitary transformations, and the apparent increase in entropy that arises from the randomness in quantum measurements." And "randomness is not an essential cornerstone of quantum measurement but rather an illusion created by it." So is it random or is it not, in your view?
    - lisper 9y ago
      > So is it random or is it not, in your view? It depends on your perspective. From the perspective of a classical entity, yes, it's random. From the perspective of the quantum wave function, no, it's not.
  - 77pt77 9y ago
    > or the video version: Both links are identical!
chrischen 9y ago
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).
- Koshkin 9y ago
  > Observation means interaction in any form Strictly speaking, this is incorrect. The interaction between a photon and an electron, for instance, or an electron in an atom with the atom's nucleus are not "observations" and do not lead to the collapse of the wavefunction. Observation (a.k.a. measurement) in QM implies interaction with "classical", i.e. non-quantum mechanical object.
  - chrischen 9y ago
    A photon from the observer or outside of the system you are observing will still cause collapse. I have corrected the sentence to clarify that it's external interaction.
v_lisivka 9y ago
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. https://www.youtube.com/watch?v=nsaUX48t0w8 https://www.youtube.com/watch?v=nsaUX48t0w8
whatshisface 9y ago
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.
Koshkin 9y ago
"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).
lwhalen 9y ago
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.
- david927 9y ago
  I'm still confused: where is the interaction in the double-slit experiment? Here's a quote: In the famous double-slit experiment, single particles, such as photons, pass one at a time through a screen containing two slits. If either path is monitored, a photon seemingly passes through one slit or the other, and no interference will be seen. Conversely, if neither is checked, a photon will appear to have passed through both slits simultaneously before interfering with itself, acting like a wave. What does "monitored" mean here? The problem is that everywhere I look, I find a synonym of "monitored". My question is, "How does that break down?"
  - Strilanc 9y ago
    Well, the simplest way to "monitor" it is to just block one of the slits. Then you definitely know which slit photons that hit the screen went through! Alternatives include: - Marking the photon itself (e.g. by rotating its polarization with a waveplate). - Placing a crystal that does spontaneous parametric down-conversion after one of the slits. This splits the photon into two photons. Aim the secondary photon at a detector. Detector clicks -> went through that slit. No click -> went through other slit. But this will also mark the photon itself with a change in frequency, so really should do it to both slits. - Having a magical device that stores a bit and toggles it anytime a photon passes through a piece of glass. Place the magical toggle-glass over one of the slits. Anything that can distinguish the case where a photon goes through the left slit from the case where a photon went through right slit will work fine.
pishpash 9y ago
Observation in QM is about information, not the actual mechanics of observation.
westoncb 9y ago
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.
- Koshkin 9y ago
  The mistake in this line of thinking is in the tacit assumption that a quantum system possesses a certain property prior to or independent from a measurement. It does not. Anything we measure is essentially an artifact of the measurement process itself, and the randomness of the result comes from bringing together two incompatible worlds - quantum and classical. All we are left with at the quantum level is wave-functions; but what matters for us is, of course, what we can experience, being ourselves classical, and QM has so far provided us with a perfectly good way of finding that out.
  - westoncb 9y ago
    Would you mind either arguing for the superiority of that interpretation or showing flaws with the alternative I pointed out? I know that what you've stated is nearly cannon at the moment, but the issue isn't actually settled, which is why a discussion of it here is still potentially interesting, and why merely re-asserting the supposed truth of the most common interpretation isn't. Edit: in other words, when you say: > The mistake in this line of thinking is in the tacit assumption that a quantum system possesses a certain property prior to or independent from a measurement. It does not. You are only asserting that's the case, not defending it. The rest of your post operates on the opposite, undefended assumption that a quantum system does not possess a certain property prior to or independent from a measurement.
neel8986 9y ago
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. You can look at a better explanation here [1]https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc
Strilanc 9y ago
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.
lisper 9y ago
> how does 'observation' cause the wave function to collapse 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 http://www.flownet.com/ron/QM.pdf or the video version: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf
- david927 9y ago
  Thanks, Ron. I can imagine that measurement and entanglement are the same thing, especially that both, like the uncertainty principle, are about the limits of extractible information. The video link is the same as the paper; if you find it, please post it. Thanks again.
  - lisper 9y ago
    Oops, sorry. https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc
    - david927 9y ago
      I loved it. Thanks.
- stallmanite 9y ago
  Your paper describing measurement in terms of entanglement is interesting. Would you mind posting the link to the video you mentioned?
  - lisper 9y ago
    Sorry about that. https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc
jackson1372 9y ago
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" https://www.academia.edu/32885328/Three_measurement_problems https://www.academia.edu/32885328/Three_measurement_problems
- platz 9y ago
  > Physicists tend to like the many worlds interpretations. According to Sean Carroll, this is because if you 'buy' the mathematics of the wavefunction, even before collapse you already have to accept "many worlds" at the quantum level. "many worlds" for them simply means superposition/linear combinations. After all, if you already accept "superposition" at the level of quantum states, you don't have to invoke anything new to deal with so-called "collapse", if everything simply stays as superpositions i.e. linear combinations. Or, in other words, if you assume classical behavior first, and you need to get that out of superpositions at the quantum level, you need wavefunction collapse. but if you start with superpositions at the quantum level, you already have superpositions, and then classical behavior can simply be derived from locating yourself in one of those superpositions. Explained this way, "many worlds" doesn't seem so shocking, if you have already resigned yourself to quantum level superpositions. I think of many worlds as kind of a literal interpretation that quantum superpositions are fundamentally real, as opposed to quantum superpositions merely being a very accurate mathematical model. (Most scientists think QM represents something real instead of some lucky equations.)
deleted 9y ago
[deleted]
ssivark 9y ago
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.

kowdermeister 9y ago

PBS Space Time did great episode on this subject: https://www.youtube.com/watch?v=izqaWyZsEtY https://www.youtube.com/watch?v=izqaWyZsEtY

david927 9y ago
It wasn't really relevant to my question, but you're right, it's a good clip. Thanks.

comboy 9y ago

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.

david927 9y ago
With the double-slit experiment, my understanding is that if it's done in a closed room, the wave function won't collapse and you'll get an interference pattern, but if you're in the room, it will collapse and there will be no interference pattern. Aren't you testing it by observing it?
- kowdermeister 9y ago
  I'm pretty sure that's not the case. The interference pattern only disappears if you try to figure out which slit a single photon went through. It does not require a conscious observer to be present. What matters is if you extract information from the system or not.
  - david927 9y ago
    Sorry, you're right.
  - kgwgk 9y ago
    > only disappears if you try to figure out > What matters is if you extract information > It does not require a conscious observer to be present. But does it require a “you” trying to figure out and extracting information?
- Strilanc 9y ago
  > if it's done in a closed room, the wave function won't collapse and you'll get an interference pattern, but if you're in the room, it will collapse That is absolutely 100% not what happens. The experiments play out the same way whether or not a human's face happens to be nearby. All the data is collected automatically by photographic plates or electronic counters or other instruments, and "observation" always refers to some instrument being present or not.
  - david927 9y ago
    I understand now. So it's, "can the information be known?" and the answer to that question, whether instrument or human or decision (such as marking the photon), is what creates the entanglement which causes the collapse. P.S. Everyone - thanks! I get it now. And it's fascinating!
    - monktastic1 9y ago
      But the entanglement doesn't "cause collapse." If you entangle states A and B, their joint state is in a superposition and will still exhibit interference (given the right experimental setup). It's just that either particle by itself will not.
      - david927 9y ago
        oh shit. ok, I see. wow

meri_dian 9y ago

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.

deleted 9y ago
[deleted]
lisper 9y ago
To the contrary, it is very easy to answer: measurement does not cause the collapse of the wave function. 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 http://www.flownet.com/ron/QM.pdf or the video version: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf
- meri_dian 9y ago
  Thanks for posting that. Can you reconcile these two parts for me though: "It accounts for the apparent contradiction between quantum theory, which says that entropy is conserved in unitary transformations, and the apparent increase in entropy that arises from the randomness in quantum measurements." And "randomness is not an essential cornerstone of quantum measurement but rather an illusion created by it." So is it random or is it not, in your view?
  - lisper 9y ago
    > So is it random or is it not, in your view? It depends on your perspective. From the perspective of a classical entity, yes, it's random. From the perspective of the quantum wave function, no, it's not.
- 77pt77 9y ago
  > or the video version: Both links are identical!

chrischen 9y ago

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).

Koshkin 9y ago
> Observation means interaction in any form Strictly speaking, this is incorrect. The interaction between a photon and an electron, for instance, or an electron in an atom with the atom's nucleus are not "observations" and do not lead to the collapse of the wavefunction. Observation (a.k.a. measurement) in QM implies interaction with "classical", i.e. non-quantum mechanical object.
- chrischen 9y ago
  A photon from the observer or outside of the system you are observing will still cause collapse. I have corrected the sentence to clarify that it's external interaction.

v_lisivka 9y ago

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. https://www.youtube.com/watch?v=nsaUX48t0w8 https://www.youtube.com/watch?v=nsaUX48t0w8

whatshisface 9y ago

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.

Koshkin 9y ago

"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).

lwhalen 9y ago

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.

david927 9y ago
I'm still confused: where is the interaction in the double-slit experiment? Here's a quote: In the famous double-slit experiment, single particles, such as photons, pass one at a time through a screen containing two slits. If either path is monitored, a photon seemingly passes through one slit or the other, and no interference will be seen. Conversely, if neither is checked, a photon will appear to have passed through both slits simultaneously before interfering with itself, acting like a wave. What does "monitored" mean here? The problem is that everywhere I look, I find a synonym of "monitored". My question is, "How does that break down?"
- Strilanc 9y ago
  Well, the simplest way to "monitor" it is to just block one of the slits. Then you definitely know which slit photons that hit the screen went through! Alternatives include: - Marking the photon itself (e.g. by rotating its polarization with a waveplate). - Placing a crystal that does spontaneous parametric down-conversion after one of the slits. This splits the photon into two photons. Aim the secondary photon at a detector. Detector clicks -> went through that slit. No click -> went through other slit. But this will also mark the photon itself with a change in frequency, so really should do it to both slits. - Having a magical device that stores a bit and toggles it anytime a photon passes through a piece of glass. Place the magical toggle-glass over one of the slits. Anything that can distinguish the case where a photon goes through the left slit from the case where a photon went through right slit will work fine.

pishpash 9y ago

Observation in QM is about information, not the actual mechanics of observation.

westoncb 9y ago

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.

Koshkin 9y ago
The mistake in this line of thinking is in the tacit assumption that a quantum system possesses a certain property prior to or independent from a measurement. It does not. Anything we measure is essentially an artifact of the measurement process itself, and the randomness of the result comes from bringing together two incompatible worlds - quantum and classical. All we are left with at the quantum level is wave-functions; but what matters for us is, of course, what we can experience, being ourselves classical, and QM has so far provided us with a perfectly good way of finding that out.
- westoncb 9y ago
  Would you mind either arguing for the superiority of that interpretation or showing flaws with the alternative I pointed out? I know that what you've stated is nearly cannon at the moment, but the issue isn't actually settled, which is why a discussion of it here is still potentially interesting, and why merely re-asserting the supposed truth of the most common interpretation isn't. Edit: in other words, when you say: > The mistake in this line of thinking is in the tacit assumption that a quantum system possesses a certain property prior to or independent from a measurement. It does not. You are only asserting that's the case, not defending it. The rest of your post operates on the opposite, undefended assumption that a quantum system does not possess a certain property prior to or independent from a measurement.

neel8986 9y ago

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. You can look at a better explanation here [1]https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc

Strilanc 9y ago

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.

lisper 9y ago

> how does 'observation' cause the wave function to collapse 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 http://www.flownet.com/ron/QM.pdf or the video version: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf

david927 9y ago
Thanks, Ron. I can imagine that measurement and entanglement are the same thing, especially that both, like the uncertainty principle, are about the limits of extractible information. The video link is the same as the paper; if you find it, please post it. Thanks again.
- lisper 9y ago
  Oops, sorry. https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc
  - david927 9y ago
    I loved it. Thanks.
stallmanite 9y ago
Your paper describing measurement in terms of entanglement is interesting. Would you mind posting the link to the video you mentioned?
- lisper 9y ago
  Sorry about that. https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc

jackson1372 9y ago

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" https://www.academia.edu/32885328/Three_measurement_problems https://www.academia.edu/32885328/Three_measurement_problems

platz 9y ago
> Physicists tend to like the many worlds interpretations. According to Sean Carroll, this is because if you 'buy' the mathematics of the wavefunction, even before collapse you already have to accept "many worlds" at the quantum level. "many worlds" for them simply means superposition/linear combinations. After all, if you already accept "superposition" at the level of quantum states, you don't have to invoke anything new to deal with so-called "collapse", if everything simply stays as superpositions i.e. linear combinations. Or, in other words, if you assume classical behavior first, and you need to get that out of superpositions at the quantum level, you need wavefunction collapse. but if you start with superpositions at the quantum level, you already have superpositions, and then classical behavior can simply be derived from locating yourself in one of those superpositions. Explained this way, "many worlds" doesn't seem so shocking, if you have already resigned yourself to quantum level superpositions. I think of many worlds as kind of a literal interpretation that quantum superpositions are fundamentally real, as opposed to quantum superpositions merely being a very accurate mathematical model. (Most scientists think QM represents something real instead of some lucky equations.)

deleted 9y ago

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ssivark 9y ago

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.