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The Quantum Theory and Reality (1979) [pdf]
- whatshisface 9y agoIt'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.
- david927 9y agoOk, 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 agoPBS Space Time did great episode on this subject: https://www.youtube.com/watch?v=izqaWyZsEtY https://www.youtube.com/watch?v=izqaWyZsEtY
- david927 9y agoIt wasn't really relevant to my question, but you're right, it's a good clip. Thanks.
- comboy 9y agoIt 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 agoWith 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 agoI'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.
- 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 agoI 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!
- meri_dian 9y agoIt'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.
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- lisper 9y agoTo 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 agoThanks 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 agoObservation 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 agoA 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 agoInterference 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 agoIf 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 agoI 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 agoI'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 agoWell, 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 agoObservation in QM is about information, not the actual mechanics of observation.
- westoncb 9y agoThe 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 agoThe 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 agoWould 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 agoOne 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 agoIn 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 agoThanks, 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.
- stallmanite 9y agoYour paper describing measurement in terms of entanglement is interesting. Would you mind posting the link to the video you mentioned?
- lisper 9y agoSorry about that. https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc
- jackson1372 9y agoThe 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.)
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- ssivark 9y agoThe 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.
- virgil_disgr4ce 9y agoI'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.
- fiatjaf 9y agoWhat do you mean? What is the death of the Conpenhagen interpretation?
- pikchurn 9y agoThe Conpenhagen interpretation is flat out wrong by any reasonable philosophy of science. You can choose many-worlds interpretation, or pilot-wave theory and either one would give you a consistent, simpler explanation of what is going on then some mumbo-jumbo hocus pocus about consciousness and observation of cats in boxes, with fewer assumptions to boot, and no paradoxical conclusions that require religious like mysteries to explain. Yet for inane reasons the Conpenhagen interpretation is still ALL that is taught to the next generation of physicists, who in turn teach it to their students. Only the weird physics students (like me) who go "wtf?" in class and refuse to believe the teacher go out and learn pilot-wave theory (my preference) or many-worlds interpretation to re-inject some sanity into the world.
- cscurmudgeon 9y ago> The Conpenhagen interpretation is flat out wrong by any reasonable philosophy of science. Why? Is there a formal proof of that claim?
- pikchurn 9y agoRhetorical question: why are epicycles wrong? An infinite sequence of epicycles could be used to accurate model any orbital path, in a similar sense to how a Taylor series can represent any function as an infinite series of polynomials. It's not wrong in the mathematical sense, but rather the philosophical: a needlessly complex theory that is hard to work with and which provides no advantages or insight over the simpler theory is declared wrong, even if it provides, or could provide the same predictions. So it is with quantum mechanics. The standard Copenhagen interpretation of QM requires notions of observers and mysterious faster-than-light transfer of state which is really hard to reconcile with the modern scientific view of the world. The mystery surrounding it (in the religious sense) has let to disproportionally many cranks who misinterpret the theory into statements about "quantum consciousness" or other new-age nonsensical tie-ins. However these issues arise to a lesser extent with the many-worlds interpretation, and not at all with pilot wave theory. The former is nothing more than reinterpretation of the same equations, and the latter is a different formulation that is nevertheless mathematically identical as far as it has been worked out. If we had started with many-worlds, or better yet pilot-wave, then we wouldn't have a century of people growing up with 2nd-hand tales about how the world is governed by a mystical and incomprehensible theory of matter that even physicists don't understand. Which is utter bullocks.
- platz 9y agoNo, you don't need consciousness. Since 1979, the concept of decoherence has replaced consciousness in the understanding of QM.
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- nabla9 9y agoWave 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.
- akvadrako 9y agoWhat you say is true, except as long as there is no evidence for collapse and it's designed to be undetectable, accepting it in a theory is nonsense. 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.
- nabla9 9y agoIt's not my logic. I was attempting to describe the situation. If you know better how the logic in interpretations requiring wave function collapse work, be kind and explain it.
- cygx 9y agoThe 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.