In case anyone is interested in the collective wisdom of the Hacker News community, you could look at the extensive discussion of this article from just over a month ago:
I know almost nothing about Quantum Mechanics, this seems to provide an interesting model for all particle behavior. I realize it's just a model, but why isn't anyone attempting to explain the actual 'fluid' part? What is that? They just refer to it as 'space and time'.. is this some form of pure energy field? is this the Higgs Field ? (or some other field) I realize that this might be an extremely stupid question. It seems consistent that matter (or any particle that has enough energy to become matter) would exhibit similar physical behavior as the experiments shown (like the pilot wave experiments in silicon oil,etc), but I'm sure my understanding is extremely naive.
Fields in quantum mechanics really just describe the ways that the force fundamental forces (gravitational, electromagnetic, strong nuclear, and weak nuclear) travel around.
The double slit experiment, which this article focusses on, looks at photons. Photons, are carriers of electromagnetic energy – they're their own field.
The field though isn't really a fluid or medium – thinking about ripples moving across a pond won't help you. The field is just the distance over which the photon will travel. It's more about an area of opportunity for something to happen (e.g. in the event that something gets within range of the photon) rather than a big blobby mass actually hanging around in space.
There's no such thing as "pure energy" (although the fundamental forces are probably the simplest forms of energy). Energy is an emergent property of a system (it is a function of how things in one part of a system are arranged with respect to another part of the system).
Isn't that common knowledge? First, there's no consensus among physicists on any particular interpretation, so right off the bat that should tell you that we are missing something really deep. Second, the Copenhagen interpretation itself is completely nonsensical. I'm convinced it is responsible for most of the millions made by Deepak Chopra.
> Second, the Copenhagen interpretation itself is completely nonsensical.
The problem is that most of the discussions about quantum mechanics interpretation are handwaving without enough linear algebra background. My two favorite questions before starting a discussion are:
* ) The standard basis B_s = {[1,0],[0,1]} and the Hadamard basis B_h={[1,1],[1,-1]} are a mutually unbiased bases, which means any basis vector lies exactly "in between" the vectors from the other basis.
The use of mutually unbiased bases has applications in quantum crypto: if the same basis is used for encoding and decoding a bit of information the bit will be transmitted faithfully, else if a different basis is used for decoding than the basis used for encoding, the receiver will decode pure noise (i.e. a 50-50% random bit). This "uncertainty about which basis is used to encode the message" is the key idea behind the BB84 protocol.
>>>> First, there's no consensus among physicists on any particular interpretation, so right off the bat that should tell you that we are missing something really deep.
One thing it should tell you is that we have not been "interpreting quantum mechanics wrong this whole time" because we've made relatively little effort to arrive at an interpretation at all. During my time as a physics student, deeper philosophical issues weren't mentioned in class, and it seemed like the Copenhagen interpretation was grudgingly accepted as a de facto standard in order to just get on with life.
We knew there was some activity in the area of finding a more philosophically pleasing interpretation, and one or two seminar speakers talked about it, but it just wasn't a big deal. Many physicists even questioned the value or physics-ness of the pursuit.
It's an interesting macro-scale analogy. You can demonstrate interference in a double-slit experiment with oil drop.
But look what the article leaves out: You can't, as far as I can tell from the article, demonstrate the collapse effect of detecting which slit the particle passes through. In fact, you can always observe which slit the particle passes through, so measurement has no effect on the wave interference pattern.
If there is a "pilot wave" version of quantum mechanics, these experiments are only a suggestion of some of the mathematics. They appear not to be a macro-scale example.
> And just as measuring the trajectories of particles seems to “collapse” their simultaneous realities, disturbing the pilot wave in the bouncing-droplet experiment destroys the interference pattern.
Did we read the same article? Pilot wave theory has the same mathematics as quantum mechanics. It's a different interpretation of those mathematics - the probability wave is real and physical.
I read that. Note that it says "disturbing the pilot wave." That's not "measuring." The sentence is trying to draw an analogy with "collapse." But to do that you have to explicitly "disturb" the pilot wave.
In these droplet experiments, you can measure all you want without affecting the wave and the interference pattern. This might point to an analogous form of quantum mechanics. But it also could be just an interesting but incomplete analogy in classical physics.
I'm not seeing how that is so. You can measure every aspect of the oil drop at every point along the path it takes, before and after and as is passes through the slits and it won't do a darn thing to the pilot wave and the interference pattern. The ability to measure is 100% classical 100% of the time.
We can measure every aspect of the oil droplet because we're measuring it with trillions upon trillions of infinitely smaller and faster photons which give us a great insight without disturbing the structure. What would we bounce off of photons to get an equally accurate picture without disturbing them?
The analogy isn't a problem. The fact that there's nothing small enough to observe photons like we can observe the oil is a problem, but it says nothing about the integrity of the analogy.
In the analogy the water is space-time, meaning "observing" the droplet like we observe photons would require shooting another droplet past or into the first droplet.
I agree with your first two paragraphs. But, be careful about the last. The pilot wave in Bohmian mechanics is not a wave in space time, but rather a separate physical entity. Basically the dynamics of both the particle and its pilot wave are described by different portions of the wavefunction. (The wavefunction is complex, and so has enough variables to describe two physical quantities).
> In fact, you can always observe which slit the particle passes through, so measurement has no effect on the wave interference pattern.
The first half of that sentence is true, while the second half is not (unless I am misinterpreting what you are trying to say).
Empirically, nobody has (yet) managed to come up with an experiment that measures which slit the photon/electron/other quantum object passes through AND preserves the wave nature of the double-slit effect. In other words, when you set up an experiment to measure which slit is traversed, the characteristic double-slit interference pattern disappears. Again, I'm talking about this purely in practice, or what people observe when they try to perform this experiment[1].
Theoretically, this is result is completely expected. To determine which slit the photon traversed, you must interact with the photon to observe it. Any observation has two results: First, you know the state of the quantum object. Second, the quantum object is in an eigenstate of whatever operator you used to observe it. Since you have forced the photon into an eigenstate, the interference pattern disappears.
[1] Not that you can still see two superimposed single-slit interference patterns, which might confuse some people into thinking they are seeing a double-slit pattern.
There was a recent quantum doc on BBC which was a pretty good watch. I think it mentioned the term pilot wave but the message they gave was that Bell test experiments proved that hidden variable interpretations were impossible and that the probabilistic reality of particles was undeniably true so you have to suck it up.
You can access that content by installing something like Hola plugin in your browser, works quite well for watching 'Hey I've Got Bit More News For You' ;)
> the message they gave was that Bell test experiments proved that hidden variable interpretations were impossible and that the probabilistic reality of particles was undeniably true
This isn't actually the case - Bell's theorem and subsequent verification demonstrate only that a theory of local hidden variables cannot be successful - the local part means that faster than light information transfer doesn't occur. You're allowed to have a pilot wave interpretation if it's nonlocal.
I think this is actually a problem with these experiments - it's nice that they work, but we already knew pilot wave mathematics could describe quantum mechanics, with the caveat that nonlocality is necessary to explain everything. I don't think this nonlocality is (or can be) expressed in the fluid experiments, so presumably they can't actually replicate QM.
I would recommend David Bohm's "Wholeness and the implicate order." And Bohm in general: this guy was stripped from his US citizenship during the McC hysteria.
Suppose I have a particle at coordinates (0,0) with some velocity in x-direction.
Due to symmetry, the trajectory of a pilot-wave particle has to stay on the x-axis, while in QM the heisenberg uncertainty principle results in some momentum in y-direction.
Wouldn't you need to add some kind of imperfection to disturb the pilot-wave in order to reproduce the probabilistic results?
One of the main objections to the pilot-wave theory of QM is that it brings us back to the idea of a "luminiferous aether" that was famously disproved by the null result of the Michaelson-Morley experiment. What would the substrate be? The article gives a very brief hand-wavy mention to it being "space-time" and I've heard this before, but I have yet to hear even a good suggestion for how this might work.
A related problem is that both De Broglie's and Bohm's pilot-wave models leave open the possibility of "empty waves" that carry no momentum or energy and are not related to any particles. Even if we assume that it is space-time through which they propagate, this gives us another piece of math that we need to just throw away, similar to the non-observed probabilities after state collapse, which runs counter to the goal of creating a formalism where the math corresponds to something real in the universe(s).
> One of the main objections to the pilot-wave theory of QM is that it brings us back to the idea of a "luminiferous aether"
No it does not. We already have the Schrödinger field, without any notion of an aether. In fact the whole quantum field theory is all about fields and does not require it.
30 comments
[ 7.0 ms ] story [ 161 ms ] threadhttps://news.ycombinator.com/item?id=8554996
Earlier submissions, but with no discussion:
https://news.ycombinator.com/item?id=7964797
https://news.ycombinator.com/item?id=8034390
The double slit experiment, which this article focusses on, looks at photons. Photons, are carriers of electromagnetic energy – they're their own field.
The field though isn't really a fluid or medium – thinking about ripples moving across a pond won't help you. The field is just the distance over which the photon will travel. It's more about an area of opportunity for something to happen (e.g. in the event that something gets within range of the photon) rather than a big blobby mass actually hanging around in space.
There's no such thing as "pure energy" (although the fundamental forces are probably the simplest forms of energy). Energy is an emergent property of a system (it is a function of how things in one part of a system are arranged with respect to another part of the system).
The problem is that most of the discussions about quantum mechanics interpretation are handwaving without enough linear algebra background. My two favorite questions before starting a discussion are:
* ) What are the eigenvalues of the matrix
* ) Why are they important for quantum mechanics?I'm sorry I can't answer your question, I only learned about the Schroedinger's Equation
* ) The standard basis B_s = {[1,0],[0,1]} and the Hadamard basis B_h={[1,1],[1,-1]} are a mutually unbiased bases, which means any basis vector lies exactly "in between" the vectors from the other basis.
The use of mutually unbiased bases has applications in quantum crypto: if the same basis is used for encoding and decoding a bit of information the bit will be transmitted faithfully, else if a different basis is used for decoding than the basis used for encoding, the receiver will decode pure noise (i.e. a 50-50% random bit). This "uncertainty about which basis is used to encode the message" is the key idea behind the BB84 protocol.
One thing it should tell you is that we have not been "interpreting quantum mechanics wrong this whole time" because we've made relatively little effort to arrive at an interpretation at all. During my time as a physics student, deeper philosophical issues weren't mentioned in class, and it seemed like the Copenhagen interpretation was grudgingly accepted as a de facto standard in order to just get on with life.
We knew there was some activity in the area of finding a more philosophically pleasing interpretation, and one or two seminar speakers talked about it, but it just wasn't a big deal. Many physicists even questioned the value or physics-ness of the pursuit.
But look what the article leaves out: You can't, as far as I can tell from the article, demonstrate the collapse effect of detecting which slit the particle passes through. In fact, you can always observe which slit the particle passes through, so measurement has no effect on the wave interference pattern.
If there is a "pilot wave" version of quantum mechanics, these experiments are only a suggestion of some of the mathematics. They appear not to be a macro-scale example.
Did we read the same article? Pilot wave theory has the same mathematics as quantum mechanics. It's a different interpretation of those mathematics - the probability wave is real and physical.
In these droplet experiments, you can measure all you want without affecting the wave and the interference pattern. This might point to an analogous form of quantum mechanics. But it also could be just an interesting but incomplete analogy in classical physics.
http://en.wikipedia.org/wiki/Weak_measurement
The analogy isn't a problem. The fact that there's nothing small enough to observe photons like we can observe the oil is a problem, but it says nothing about the integrity of the analogy.
In the analogy the water is space-time, meaning "observing" the droplet like we observe photons would require shooting another droplet past or into the first droplet.
The first half of that sentence is true, while the second half is not (unless I am misinterpreting what you are trying to say).
Empirically, nobody has (yet) managed to come up with an experiment that measures which slit the photon/electron/other quantum object passes through AND preserves the wave nature of the double-slit effect. In other words, when you set up an experiment to measure which slit is traversed, the characteristic double-slit interference pattern disappears. Again, I'm talking about this purely in practice, or what people observe when they try to perform this experiment[1].
Theoretically, this is result is completely expected. To determine which slit the photon traversed, you must interact with the photon to observe it. Any observation has two results: First, you know the state of the quantum object. Second, the quantum object is in an eigenstate of whatever operator you used to observe it. Since you have forced the photon into an eigenstate, the interference pattern disappears.
[1] Not that you can still see two superimposed single-slit interference patterns, which might confuse some people into thinking they are seeing a double-slit pattern.
http://www.bbc.co.uk/iplayer/episode/b04tr9x9/the-secrets-of...
This isn't actually the case - Bell's theorem and subsequent verification demonstrate only that a theory of local hidden variables cannot be successful - the local part means that faster than light information transfer doesn't occur. You're allowed to have a pilot wave interpretation if it's nonlocal.
I think this is actually a problem with these experiments - it's nice that they work, but we already knew pilot wave mathematics could describe quantum mechanics, with the caveat that nonlocality is necessary to explain everything. I don't think this nonlocality is (or can be) expressed in the fluid experiments, so presumably they can't actually replicate QM.
Suppose I have a particle at coordinates (0,0) with some velocity in x-direction. Due to symmetry, the trajectory of a pilot-wave particle has to stay on the x-axis, while in QM the heisenberg uncertainty principle results in some momentum in y-direction.
Wouldn't you need to add some kind of imperfection to disturb the pilot-wave in order to reproduce the probabilistic results?
A related problem is that both De Broglie's and Bohm's pilot-wave models leave open the possibility of "empty waves" that carry no momentum or energy and are not related to any particles. Even if we assume that it is space-time through which they propagate, this gives us another piece of math that we need to just throw away, similar to the non-observed probabilities after state collapse, which runs counter to the goal of creating a formalism where the math corresponds to something real in the universe(s).
No it does not. We already have the Schrödinger field, without any notion of an aether. In fact the whole quantum field theory is all about fields and does not require it.