14 ms·

The Trouble with Quantum Mechanics

125 points by themgt 10y ago

coldcode 10y ago
I had three semesters of QM in grad school and I still have trouble with it.
- jessriedel 10y ago
  Well, the OP author is arguably the greatest living physicist, so it's not surprising that 3 semesters isn't enough :)
- walrus1066 10y ago
  "Anyone who says that they understand Quantum Mechanics does not understand Quantum Mechanics" - Richard feynmann (might be paraphrased)
wolfgke 10y ago
Relevant for those who are interested in the topic: Two rather unusual interpretations if quantum mechanics: - De Brogle-Bohm theory: https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory - Gerard 't Hooft - The Cellular Automaton Interpretation of Quantum Mechanics: https://arxiv.org/abs/1405.1548 https://arxiv.org/abs/1405.1548 (see also https://en.wikipedia.org/wiki/Superdeterminism https://en.wikipedia.org/wiki/Superdeterminism)
- demonshalo 10y ago
  the Automaton interpretation sounds like something I would enjoy reading. But considering that I do not have the time this week nor enough knowledge in the subject, could you perhaps provide me with a quick summary as to what it is about?
  - tps5 10y ago
    My understanding is this: Hooft is an "instrumentalist" as described in this article. He does not believe that quantum mechanics describes "what's really going on" and is interested in classical models that explain why QM makes such good predictions. "Superdeterminism" is a very interesting perspective that Hooft feels could resolve this dilemma. Others feel that superdeterminism is not falsifiable, so there's no point in discussing it. I came across this interesting article on stackexchange from a few years ago: http://physics.stackexchange.com/questions/34217/why-do-people-categorically-dismiss-some-simple-quantum-models http://physics.stackexchange.com/questions/34217/why-do-peop... The first response is from Peter Shor, presumably the discoverer of Shor's algoithm. Kinda cool that such intelligent people with the highest academic credentials use a public forum.
    - jayajay 10y ago
      I think most people are "realists" as Weinberg mentions in his article. A lot of people (including myself) have bashed on so-called "instrumentalists" like Hooft, but after reading this Weinberg article, I am grateful for people like Hooft who are not here to "give up" and accept that there are necessary unknowns, and that there might be something going on underneath it all -- the Einstein way.
    - naasking 10y ago
      't Hooft's approach is the only superdeterministic theory I've heard of, so it's worthwhile for the novelty alone. I've never been pursuaded by arguments that science is impossible in a superdeterministic world though. The fact that all current observations are consistent with a superdeterministic world and yet we still came up with a viable theory already proves that science is possible in such a world.
    - evanb 10y ago
      The physics stack exchange has basically two populations: people learning physics and asking introductory questions, and experts gathering to discuss detailed questions. It's really an example of a great egalitarian online community.
    - 10y ago
- lisivka 10y ago
  - Loop quantum gravity: https://en.wikipedia.org/wiki/Loop_quantum_gravity https://en.wikipedia.org/wiki/Loop_quantum_gravity
  - wolfgke 10y ago
    As far as I know Loop quantum gravity is an alternative to string theory and says nothing about how to interprete quantum mechanics (though it better should reproduce its predictions). But convince me to be wrong.
- mathoff 10y ago
  In philosophy, this is what we call four dimensionalism: https://en.wikipedia.org/wiki/Four-dimensionalism https://en.wikipedia.org/wiki/Four-dimensionalism https://plato.stanford.edu/entries/temporal-parts/ https://plato.stanford.edu/entries/temporal-parts/
MichailP 10y ago
Let me give example why physics is hard, using most simple example. Ever encountered term observer (like observer in inertial frame)? Well, it turns out, observer in physics means whole laboratory with established procedures of measuring time and length, not observer as you would mean in everyday life. And established procedure, what is that? Say, how do you measure time when certain event happened? It turns out, you can measure time only locally, and for this you need to have synchronized clocks spread out in your lab in points of interest. Some more philosophically inclined physicist would say that time means synchronized clocks. And how do you synchronize clocks? Well, it has to do with speed of light but I will stop here. And this is for the most simple term. Imagine something more complicated. Now imagine hundreds of people writing papers and books with only partial understanding, and signal to noise ratio :-) End of rant :-)
jayajay 10y ago
> In this musical analogy, the act of measuring the spin somehow shifts all the intensity of the chord to one of the notes, which we then hear on its own. People can accept that measurements and observables are operators which act on the wave function (which can be expressed as a linear combination of eigenstates of the operator in question). People can even accept that some observables cannot be measured simultaneously, because they do not commute, and cannot share eigenstates. People cannot accept that particles can only be found in eigenstates of the observable in question, without some mechanism to explain why this happens. If a particle's measured value can only be one of the eigenvalues of the operator, it begs the question "which eigenvalue is it going to show us?". Alternatively, if nature let us measure the expectation value instead of "one of the eigenvalues", then quantum mechanics would not be "weird" at all. Too bad, nature isn't like that, or maybe, be thankful that it is. > But if the corrections to quantum mechanics represented by the new terms in the Lindblad equations (expressed as energies) were as large as one part in a hundred million billion of the energy difference of the atomic states used in the clock, this precision would have been quite lost. The new terms must therefore be even smaller than this. I always see people (physicists) complaining about String Theory, etc. because they play around in regimes which are too small for us to actually work with. We have observed macroscopic effects which are due to a build-up of quantum mechanical effects (superconductivity, solid state, etc.). It would be expected that a correction to quantum mechanics would also make macroscopic predictions...
EGreg 10y ago
What is wrong with the Pilot Wave Theory intepretation? I'm ok with FTL information transfer, and so Bell's inequalities are satisfied. And anyway the other interpretations DO NOT rule out FTL information transfer. For example if one entangled electron flies into a black hole then we would be able to know its spin by measuring the other one even if light from it won't reach us. Also there is this: https://en.m.wikipedia.org/wiki/Wigner's_friend https://en.m.wikipedia.org/wiki/Wigner's_friend
- baytrailcat 10y ago
  >I'm ok with FTL information transfer Which will lead to the conclusion that Special Relativity (which was extensively validated) is either wrong or incomplete. I have seen a revival of Pilot-wave theory here on HN, but the real conclusion (a real particle pushed around by the enigmatic pilot wave or quantum potential or whatever) is as bizarre or as satisfying as saying particle is in a superposition. Also, I haven't seen anyone satisfyingly explain, in a classical-deterministic sense, the Delayed Choice Quantum Eraser Experiment (also, see the the Wheeler Thought Experiment)
  - goatlover 10y ago
    There is the silicone oil droplet phenomena which is a classical version of the pilot wave, and it does reproduce some quantum behavior, including the double slit. That a pilot wave has been discovered (granted at macro scale) does make one reconsider. https://youtu.be/WIyTZDHuarQ https://youtu.be/WIyTZDHuarQ
    - krastanov 10y ago
      It does not, really. The macroscopic realisation is not particularly surprising (although it is quite awesome and original). If you put a ball floating on top of a wave you will observe the predictions from a mathematical model of that system, which is exactly what the pilot wave theory is, and there is nothing surprising here. Moreover, the macroscopic model simulates something which by definition is an unobservable construction in the quantum model. It does not simulate any inherently quantum behavior (classical waves is a thing we already knew exists).
nickpeterson 10y ago
They overcharge you if your license is from two simultaneous states.
runn1ng 10y ago
I know that Eliezer Yudkowsky is very controversial and some of his statements cannot be taken seriously, but I really liked his sequence of articles on QM. It made it a little bit more accessible to me, who knows very little about it. I cannot judge how accurate it is; but from googling around, it doesn't seem he does any strong errors. Also he makes some very strong statements in the end which I found preposterous (mainly, that Bayes reasoning is better than scientific reasoning, and that Bayes reasoning must lead to Many Worlds theory). The sequence is here http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ http://lesswrong.com/lw/r5/the_quantum_physics_sequence/
- danharaj 10y ago
  One of the issues with engaging with Yudkowsky's posts is how verbose his arguments are. Fortunately, for something as well formalized as quantum mechanics, there's a succinct counter-example: A formalism of quantum mechanics that makes perfect sense with neither the idea of wavefunction collapse nor some abundance of ontologically dubious and extravagant entities (alternate universes you can never observe). That formalism is relational quantum mechanics. Accessible overview: https://plato.stanford.edu/entries/qm-relational/ https://plato.stanford.edu/entries/qm-relational/ In particular, this overview addresses the fundamental difference between Everett's theory and Rovelli's theory. Original paper: https://arxiv.org/abs/quant-ph/9609002 https://arxiv.org/abs/quant-ph/9609002 A fluffy paper by the same author about QFT and its relational nature: https://arxiv.org/abs/hep-th/9910131 https://arxiv.org/abs/hep-th/9910131 In short: The way you can evade the difficulties MWI addresses completely by changing one premise of the problem: that physical systems have states independent of their observers. In relational quantum mechanics, there is no universal wavefunction because there is no external observer of the universe. Call it the zero worlds interpretation. How parsimonious! There is an element of Yudkowsky's sequence that deals with relative configuration spaces, but this is a subtly different concept and is really just a discussion from a clever guy unequipped with the right concepts about the difference between, say, affine spaces and vector spaces or torsors and groups. The idea that there is a configuration space independent of the observer is the assumption he misses. In general I find Yudkowsky's extreme certainty in his own arguments to range from amusing to obnoxious. It's not hard to find places where uncertainty creeps in. It doesn't come from mistakes in his reasoning because what he considers he is usually meticulous about considering, but from what he doesn't consider.
brianberns 10y ago
It seems that the scientific method has led us to a theory that is beautiful, accurate, and yet utterly incomprehensible. Until we can explain exactly what happens during a wave function collapse, I think QM will remain that way.
- evanb 10y ago
  Wave functions do not collapse in the realist view, as explained in the article. The cost, however, is a giant explosion of alternate possibilities all having equal reality, summed in superposition. But, the point is, there is at least one interpretation of QM where the wavefunction never collapses: it just evolves unitarily according to the Schrodinger equation, forever.
- akvadrako 10y ago
  It doesn't seem that way to me. Decoherence is understood pretty well; although not every detail is completely worked out it explains our experience and the subjective appearance of collapse. It's just approximate though and hence we get quantum interference. The reason you think it's incomprehensible is that a lot of smart people have trouble accepting what we called reality is only one of many classical "worlds". It's a philosophical concern; technically there are no major issues and it's not that hard to understand. The reality of QM won't be commonly accepted until we find a way for people to accept that the universe isn't what they expected or we change their expectations.
bike4beer 10y ago
RP Feynman long ago said "Anybody that say's he understands QM is a liar & a fool" Said, by the master, and that was 50 years ago, not much has changed.
thisrod 10y ago
This is a really good description of the measurement problem. It takes all the sides of the debate seriously, which is hard for most physicists to do. If you think about quantum mechanics all day every day, you're almost bound to settle on one interpretation as your working model, and it becomes hard to take the others seriously.
gpsx 10y ago
Weinberg mentions the quote by Eugene Wigner regarding the importance of consciousness in the section on instrumentalists. However, I see it is being fundamental to realist's interpretation also. I subscribe the the realist approach, which I view as meaning nothing special happens when a human makes an observation - it is just quantum mechanics as usual. This has interesting implications. The key is that the observer is not outside the system, he is a part of it. His wave function becomes entangled with the system when the measurement is made. But what makes this difficult to analyze is that we don't know what it means for the observer to say "I see the result was *". What physical process allows the observer to be aware of the measurement? We can make a simplifying and unsatisfying assumption - the observer has a register in his head which records the result of the measurement. Here, it is easy to imagine this register is part of the wave function just like the object being measured. As mentioned above, the wave function for this register is entangled with the object being measured so the measurement and register value are correlated. In our model this corresponds to the person independently thinking each of the possible results, which some people call multiple universes. But this simple register is not what goes on inside the brain. Not knowing this makes it difficult to accurately model the measurement process. EDIT: for clarity
- evanb 10y ago
  In the realist interpretation, even without people, you wind up in entangled states after measurement of a spin (where by "measurement" I mean: allow a giant classical system to interact with a spin and tilt a pointer on some gauge to / wind up with some register holding + or -). It's perfectly possible to formulate the realist ideas without any consciousness or insistence that the measurements register in a brain or observer.
  - gpsx 10y ago
    Yes, I agree you can formulate the realist ideas, because the observer is nothing special at all. And I also think this is how the world works. But from an experimental verification point of view I think you really need a model for what the brain does to say you understand how a measurement is made. What is a theory without verification? Granted his quote used the word formulate, but I interpret the comment as referring to not just writing some rules but also having some justification for them.
lisper 10y ago
This article presents a very inaccurate view of the realist approach. The universe does not "split" when you make a measurement. The measurement problem is a solved problem. The solution is that measurement and entanglement are the same physical phenomenon. Measurement is just entanglement extended to a macroscopic system through a process called "decoherence". The net result is that, when you do the math, you recover classical behavior by taking "slices" of the wave function (the mathematical operation is called a "trace"). This has been known for decades now, and provides a coherent and easy-to-understand picture of what is "really" going on. It is astonishing to me that the physics community still bifurcates into two camps: those who think this is common knowledge, and those who are completely unaware of it (or think it's a crazy idea). References: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc (The same content in a video) http://blog.rongarret.info/2014/09/are-parallel-universes-real.html http://blog.rongarret.info/2014/09/are-parallel-universes-re... http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
- iheartmemcache 10y ago
  Well the Copenhagen interpretation traditionally requires the act of an 'observer'. I.e., you are not allowed to perform that act of summation on the set of orthonormal vectors in N dimensional Hilbert spaces until an "observer" observes. (That's where that stupid cat came into play and the 'uncertainty' of it being both dead and alive simultaneously until it's 'observed'.) Until this discrete event occurs, the wave function doesn't "collapse" and we're just living in statistical la-la land. The disagreement lies in "I can mathematically apply the trace function, I'm just not quite sure when". Alternatively, according to the Everett interpretation (to which I subscribe, and to which you seem to as well) quite a few reputed physicists do believe that the universe "splits" (though not in an actual literal sense, there's nothing ripping the universe physically in two) every time an "observation" occurs and there are lots and lots of occurring simultaneously taken from various frames of reference. This actually makes a lot of sense. Let Sean Carroll https://youtu.be/ZacggH9wB7Y https://youtu.be/ZacggH9wB7Y convince (the plural) you.

125 points by themgt 10y ago

coldcode 10y ago

I had three semesters of QM in grad school and I still have trouble with it.

jessriedel 10y ago
Well, the OP author is arguably the greatest living physicist, so it's not surprising that 3 semesters isn't enough :)
walrus1066 10y ago
"Anyone who says that they understand Quantum Mechanics does not understand Quantum Mechanics" - Richard feynmann (might be paraphrased)

wolfgke 10y ago

Relevant for those who are interested in the topic: Two rather unusual interpretations if quantum mechanics: - De Brogle-Bohm theory: https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory - Gerard 't Hooft - The Cellular Automaton Interpretation of Quantum Mechanics: https://arxiv.org/abs/1405.1548 https://arxiv.org/abs/1405.1548 (see also https://en.wikipedia.org/wiki/Superdeterminism https://en.wikipedia.org/wiki/Superdeterminism)

demonshalo 10y ago
the Automaton interpretation sounds like something I would enjoy reading. But considering that I do not have the time this week nor enough knowledge in the subject, could you perhaps provide me with a quick summary as to what it is about?
- tps5 10y ago
  My understanding is this: Hooft is an "instrumentalist" as described in this article. He does not believe that quantum mechanics describes "what's really going on" and is interested in classical models that explain why QM makes such good predictions. "Superdeterminism" is a very interesting perspective that Hooft feels could resolve this dilemma. Others feel that superdeterminism is not falsifiable, so there's no point in discussing it. I came across this interesting article on stackexchange from a few years ago: http://physics.stackexchange.com/questions/34217/why-do-people-categorically-dismiss-some-simple-quantum-models http://physics.stackexchange.com/questions/34217/why-do-peop... The first response is from Peter Shor, presumably the discoverer of Shor's algoithm. Kinda cool that such intelligent people with the highest academic credentials use a public forum.
  - jayajay 10y ago
    I think most people are "realists" as Weinberg mentions in his article. A lot of people (including myself) have bashed on so-called "instrumentalists" like Hooft, but after reading this Weinberg article, I am grateful for people like Hooft who are not here to "give up" and accept that there are necessary unknowns, and that there might be something going on underneath it all -- the Einstein way.
  - naasking 10y ago
    't Hooft's approach is the only superdeterministic theory I've heard of, so it's worthwhile for the novelty alone. I've never been pursuaded by arguments that science is impossible in a superdeterministic world though. The fact that all current observations are consistent with a superdeterministic world and yet we still came up with a viable theory already proves that science is possible in such a world.
  - evanb 10y ago
    The physics stack exchange has basically two populations: people learning physics and asking introductory questions, and experts gathering to discuss detailed questions. It's really an example of a great egalitarian online community.
  - 10y ago
lisivka 10y ago
- Loop quantum gravity: https://en.wikipedia.org/wiki/Loop_quantum_gravity https://en.wikipedia.org/wiki/Loop_quantum_gravity
- wolfgke 10y ago
  As far as I know Loop quantum gravity is an alternative to string theory and says nothing about how to interprete quantum mechanics (though it better should reproduce its predictions). But convince me to be wrong.
mathoff 10y ago
In philosophy, this is what we call four dimensionalism: https://en.wikipedia.org/wiki/Four-dimensionalism https://en.wikipedia.org/wiki/Four-dimensionalism https://plato.stanford.edu/entries/temporal-parts/ https://plato.stanford.edu/entries/temporal-parts/

MichailP 10y ago

Let me give example why physics is hard, using most simple example. Ever encountered term observer (like observer in inertial frame)? Well, it turns out, observer in physics means whole laboratory with established procedures of measuring time and length, not observer as you would mean in everyday life. And established procedure, what is that? Say, how do you measure time when certain event happened? It turns out, you can measure time only locally, and for this you need to have synchronized clocks spread out in your lab in points of interest. Some more philosophically inclined physicist would say that time means synchronized clocks. And how do you synchronize clocks? Well, it has to do with speed of light but I will stop here. And this is for the most simple term. Imagine something more complicated. Now imagine hundreds of people writing papers and books with only partial understanding, and signal to noise ratio :-) End of rant :-)

jayajay 10y ago

> In this musical analogy, the act of measuring the spin somehow shifts all the intensity of the chord to one of the notes, which we then hear on its own. People can accept that measurements and observables are operators which act on the wave function (which can be expressed as a linear combination of eigenstates of the operator in question). People can even accept that some observables cannot be measured simultaneously, because they do not commute, and cannot share eigenstates. People cannot accept that particles can only be found in eigenstates of the observable in question, without some mechanism to explain why this happens. If a particle's measured value can only be one of the eigenvalues of the operator, it begs the question "which eigenvalue is it going to show us?". Alternatively, if nature let us measure the expectation value instead of "one of the eigenvalues", then quantum mechanics would not be "weird" at all. Too bad, nature isn't like that, or maybe, be thankful that it is. > But if the corrections to quantum mechanics represented by the new terms in the Lindblad equations (expressed as energies) were as large as one part in a hundred million billion of the energy difference of the atomic states used in the clock, this precision would have been quite lost. The new terms must therefore be even smaller than this. I always see people (physicists) complaining about String Theory, etc. because they play around in regimes which are too small for us to actually work with. We have observed macroscopic effects which are due to a build-up of quantum mechanical effects (superconductivity, solid state, etc.). It would be expected that a correction to quantum mechanics would also make macroscopic predictions...

EGreg 10y ago

What is wrong with the Pilot Wave Theory intepretation? I'm ok with FTL information transfer, and so Bell's inequalities are satisfied. And anyway the other interpretations DO NOT rule out FTL information transfer. For example if one entangled electron flies into a black hole then we would be able to know its spin by measuring the other one even if light from it won't reach us. Also there is this: https://en.m.wikipedia.org/wiki/Wigner's_friend https://en.m.wikipedia.org/wiki/Wigner's_friend

baytrailcat 10y ago
>I'm ok with FTL information transfer Which will lead to the conclusion that Special Relativity (which was extensively validated) is either wrong or incomplete. I have seen a revival of Pilot-wave theory here on HN, but the real conclusion (a real particle pushed around by the enigmatic pilot wave or quantum potential or whatever) is as bizarre or as satisfying as saying particle is in a superposition. Also, I haven't seen anyone satisfyingly explain, in a classical-deterministic sense, the Delayed Choice Quantum Eraser Experiment (also, see the the Wheeler Thought Experiment)
- goatlover 10y ago
  There is the silicone oil droplet phenomena which is a classical version of the pilot wave, and it does reproduce some quantum behavior, including the double slit. That a pilot wave has been discovered (granted at macro scale) does make one reconsider. https://youtu.be/WIyTZDHuarQ https://youtu.be/WIyTZDHuarQ
  - krastanov 10y ago
    It does not, really. The macroscopic realisation is not particularly surprising (although it is quite awesome and original). If you put a ball floating on top of a wave you will observe the predictions from a mathematical model of that system, which is exactly what the pilot wave theory is, and there is nothing surprising here. Moreover, the macroscopic model simulates something which by definition is an unobservable construction in the quantum model. It does not simulate any inherently quantum behavior (classical waves is a thing we already knew exists).

nickpeterson 10y ago

They overcharge you if your license is from two simultaneous states.

runn1ng 10y ago

I know that Eliezer Yudkowsky is very controversial and some of his statements cannot be taken seriously, but I really liked his sequence of articles on QM. It made it a little bit more accessible to me, who knows very little about it. I cannot judge how accurate it is; but from googling around, it doesn't seem he does any strong errors. Also he makes some very strong statements in the end which I found preposterous (mainly, that Bayes reasoning is better than scientific reasoning, and that Bayes reasoning must lead to Many Worlds theory). The sequence is here http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ http://lesswrong.com/lw/r5/the_quantum_physics_sequence/

danharaj 10y ago
One of the issues with engaging with Yudkowsky's posts is how verbose his arguments are. Fortunately, for something as well formalized as quantum mechanics, there's a succinct counter-example: A formalism of quantum mechanics that makes perfect sense with neither the idea of wavefunction collapse nor some abundance of ontologically dubious and extravagant entities (alternate universes you can never observe). That formalism is relational quantum mechanics. Accessible overview: https://plato.stanford.edu/entries/qm-relational/ https://plato.stanford.edu/entries/qm-relational/ In particular, this overview addresses the fundamental difference between Everett's theory and Rovelli's theory. Original paper: https://arxiv.org/abs/quant-ph/9609002 https://arxiv.org/abs/quant-ph/9609002 A fluffy paper by the same author about QFT and its relational nature: https://arxiv.org/abs/hep-th/9910131 https://arxiv.org/abs/hep-th/9910131 In short: The way you can evade the difficulties MWI addresses completely by changing one premise of the problem: that physical systems have states independent of their observers. In relational quantum mechanics, there is no universal wavefunction because there is no external observer of the universe. Call it the zero worlds interpretation. How parsimonious! There is an element of Yudkowsky's sequence that deals with relative configuration spaces, but this is a subtly different concept and is really just a discussion from a clever guy unequipped with the right concepts about the difference between, say, affine spaces and vector spaces or torsors and groups. The idea that there is a configuration space independent of the observer is the assumption he misses. In general I find Yudkowsky's extreme certainty in his own arguments to range from amusing to obnoxious. It's not hard to find places where uncertainty creeps in. It doesn't come from mistakes in his reasoning because what he considers he is usually meticulous about considering, but from what he doesn't consider.

brianberns 10y ago

It seems that the scientific method has led us to a theory that is beautiful, accurate, and yet utterly incomprehensible. Until we can explain exactly what happens during a wave function collapse, I think QM will remain that way.

evanb 10y ago
Wave functions do not collapse in the realist view, as explained in the article. The cost, however, is a giant explosion of alternate possibilities all having equal reality, summed in superposition. But, the point is, there is at least one interpretation of QM where the wavefunction never collapses: it just evolves unitarily according to the Schrodinger equation, forever.
akvadrako 10y ago
It doesn't seem that way to me. Decoherence is understood pretty well; although not every detail is completely worked out it explains our experience and the subjective appearance of collapse. It's just approximate though and hence we get quantum interference. The reason you think it's incomprehensible is that a lot of smart people have trouble accepting what we called reality is only one of many classical "worlds". It's a philosophical concern; technically there are no major issues and it's not that hard to understand. The reality of QM won't be commonly accepted until we find a way for people to accept that the universe isn't what they expected or we change their expectations.

bike4beer 10y ago

RP Feynman long ago said "Anybody that say's he understands QM is a liar & a fool" Said, by the master, and that was 50 years ago, not much has changed.

thisrod 10y ago

This is a really good description of the measurement problem. It takes all the sides of the debate seriously, which is hard for most physicists to do. If you think about quantum mechanics all day every day, you're almost bound to settle on one interpretation as your working model, and it becomes hard to take the others seriously.

gpsx 10y ago

Weinberg mentions the quote by Eugene Wigner regarding the importance of consciousness in the section on instrumentalists. However, I see it is being fundamental to realist's interpretation also. I subscribe the the realist approach, which I view as meaning nothing special happens when a human makes an observation - it is just quantum mechanics as usual. This has interesting implications. The key is that the observer is not outside the system, he is a part of it. His wave function becomes entangled with the system when the measurement is made. But what makes this difficult to analyze is that we don't know what it means for the observer to say "I see the result was *". What physical process allows the observer to be aware of the measurement? We can make a simplifying and unsatisfying assumption - the observer has a register in his head which records the result of the measurement. Here, it is easy to imagine this register is part of the wave function just like the object being measured. As mentioned above, the wave function for this register is entangled with the object being measured so the measurement and register value are correlated. In our model this corresponds to the person independently thinking each of the possible results, which some people call multiple universes. But this simple register is not what goes on inside the brain. Not knowing this makes it difficult to accurately model the measurement process. EDIT: for clarity

evanb 10y ago
In the realist interpretation, even without people, you wind up in entangled states after measurement of a spin (where by "measurement" I mean: allow a giant classical system to interact with a spin and tilt a pointer on some gauge to / wind up with some register holding + or -). It's perfectly possible to formulate the realist ideas without any consciousness or insistence that the measurements register in a brain or observer.
- gpsx 10y ago
  Yes, I agree you can formulate the realist ideas, because the observer is nothing special at all. And I also think this is how the world works. But from an experimental verification point of view I think you really need a model for what the brain does to say you understand how a measurement is made. What is a theory without verification? Granted his quote used the word formulate, but I interpret the comment as referring to not just writing some rules but also having some justification for them.

lisper 10y ago

This article presents a very inaccurate view of the realist approach. The universe does not "split" when you make a measurement. The measurement problem is a solved problem. The solution is that measurement and entanglement are the same physical phenomenon. Measurement is just entanglement extended to a macroscopic system through a process called "decoherence". The net result is that, when you do the math, you recover classical behavior by taking "slices" of the wave function (the mathematical operation is called a "trace"). This has been known for decades now, and provides a coherent and easy-to-understand picture of what is "really" going on. It is astonishing to me that the physics community still bifurcates into two camps: those who think this is common knowledge, and those who are completely unaware of it (or think it's a crazy idea). References: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc (The same content in a video) http://blog.rongarret.info/2014/09/are-parallel-universes-real.html http://blog.rongarret.info/2014/09/are-parallel-universes-re... http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...

iheartmemcache 10y ago
Well the Copenhagen interpretation traditionally requires the act of an 'observer'. I.e., you are not allowed to perform that act of summation on the set of orthonormal vectors in N dimensional Hilbert spaces until an "observer" observes. (That's where that stupid cat came into play and the 'uncertainty' of it being both dead and alive simultaneously until it's 'observed'.) Until this discrete event occurs, the wave function doesn't "collapse" and we're just living in statistical la-la land. The disagreement lies in "I can mathematically apply the trace function, I'm just not quite sure when". Alternatively, according to the Everett interpretation (to which I subscribe, and to which you seem to as well) quite a few reputed physicists do believe that the universe "splits" (though not in an actual literal sense, there's nothing ripping the universe physically in two) every time an "observation" occurs and there are lots and lots of occurring simultaneously taken from various frames of reference. This actually makes a lot of sense. Let Sean Carroll https://youtu.be/ZacggH9wB7Y https://youtu.be/ZacggH9wB7Y convince (the plural) you.