Tuesday, November 15, 2016

The de Broglie-Bohm Pilot Wave Interpretation of Quantum Mechanics

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Recorded: 2016/10/15 Published: 2016/11/15

Jim talks to Randy about the pilot wave interpretation of quantum mechanics, which separates the particle and wave behavior of a non-relativistic quantum particle into that of a particle moving in and exciting a quantum mechanical medium.


(1) Related Episodes of Physics Frontiers:
(2) Also check out the related PhysicsFM episodes:
  • Quantum Paradoxes 9: Quantum Cats
    which talks about various interpretations of quantum mechanics in light of Schrodinger's Cat, and the need for an interpretation of quantum mechanics.
  • Quantum Paradoxes (Intermission): Quantum Metaphysics,
    which discusses a categorization of quantum interpretations based on the work of Anthony Sudbery's excellent text, Quantum Mechanics and the Particles of Nature. [Amazon]
  • Weekly Electronic Podcast 3: The Machine in the Ghost,
    which discusses Bohmian quantum cosmology.

(3) David Bohm's Quantum Mechanics [Amazon], published just before the papers where Bohm introduces his pilot wave interpretation of the wavefunction.

(4) A Veritasium video on pilot wave hydrodynamics which shows the dynamics of the "oil droplets" that we're talking about in the podcast, which shows the physical analogy at the heart of David Bohm's interpretation. This video came out after we recorded the episode and before I started editing it, and we discuss it in PF0005. As mentioned in the notes to PF0005, the interpretation of this experiment has subsequently been deemed to be in error.

(5) Discuss this episode in the comments, on our subreddit, or our Facebook page.

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Transcript (Rough draft; added 2020/07/02)

11:42:42 Well this week on physics frontiers we're going to be talking about the Brierley bomb pilot wave theory, most people aren't even aware that that quantum mechanics has sort of an alternative theory or an alternative interpretation, I think, I think if this 11:42:56 is an entirely different theory, but it's been looked at as in a different interpretation because experimentally it makes the same predictions is the Copenhagen interpretation of quantum mechanics, which is all of the stuff that drives people crazy about 11:43:12 quantum physics, the Copenhagen interpretation. 11:43:17 Looks to me like a collection of equations which are very functional at replicating and predicting experimental results because it was basically designed upon experimental results. 11:43:30 And it sort of hobbled together conceptually, to make that math makes sense. 11:43:35 And so you get stuck with ideas like a superposition. And you get cats that are simultaneously alive and dead, until you look inside the box. 11:43:46 You get stuck with paradoxes. 11:43:48 And I think that that's largely because there wasn't really an effort made to make a comprehensible theory. There was a almost a tyranny I think of the mathematical approach to physics at that time, led by bore and Heisenberg and Polly to, to come up 11:44:07 with a purely elegant mathematical description of microscopic nature. 11:44:15 And there was sort of a disregard for the idea that being readily comprehensible as if only pedestrian mouth breathers would need to have a model that they could visualize and into intuitively comprehend, there was kind of this can see for physical credibility. 11:44:35 And so we ended up with ideas. 11:44:47 In quantum physics which don't appear anywhere else and. In our experience. And that was kind of its boasting point was this we're so advanced now that you can't even make heads or tails out of what we've got. 11:44:49 There's a particle wave particle duality, and so even even particles themselves don't have a classical under interpretation that you can't get your mind around. 11:45:04 Now, deploy early, early on in 1927 was a brilliant physicist, and he came up with a way of describing the effects that they're observing in quantum physics, using a combination of a particle and wave that he called a pilot wave theory. 11:45:27 And it was surprisingly well received at the time, but very quickly, the, I guess the establishment, the academics, decided they didn't like it and just kind of turn their back on it for until 1952. 11:45:43 The pilot wave theory, gives you a model where the, the particle has a wave like almost like a wave like field that surrounds it, which guides its motion through space between one point in the next. 11:46:03 And whereas in Copenhagen interpretation, the electron doesn't exist in any specific point, or in any specific trajectory between point A where it's admitted, and point B where it's detected in the pilot wave theory, the electron has a definitive position 11:46:22 and momentum and trajectory from point A to point B, the whole time, whether you're looking at it or not, you don't need to make any measurements. 11:46:31 But, but its movement is is guided by this wave function. 11:46:37 And the wave function creates kind of an counterintuitive trajectory from point A to point B. 11:46:43 So, I guess, in a nutshell, that's the idea is that particles in in deploying these thinking, which was later elucidated by David boom in 1952 and onward. 11:46:56 There is a, there's a there's a particle that's constantly interacting with a field of its own creation its wave function, which is coupled to the entire universe. 11:47:08 So, It's a non local theory, because all of the other way functions of the universe, have an effect upon the, the way function of the particle and its trajectory. 11:47:19 But, um, but it is deterministic and a lot more sensible and we're going to talk about the oil. The oil drop experiments which, I think, brilliantly illustrate the idea. 11:47:30 You're saying you want to talk about the oil drop experiments that are really classical experiments people dropping little drops of oil onto a vibrating table with oil on it. 11:47:56 Researchers in Paris, who had come up with this kind of experiment, if they had had this demonstration available in 1927 to show the leading minds in physics I think that we would have seen a completely different trajectory for quantum physics, I think 11:48:05 it would be teaching pilot wave theory in schools. And I think that people would find it a lot more intuitive. 11:48:08 Like I said in my email, I'd like to do a completely separate podcast on the oil drop because I think they're interesting in their own right, and I think it's an interesting thing to try to get to. 11:48:18 I would like to at least put a link down below so that people can see the oil drop experiment, footage, and you can really see that system can particle aspect, and the wave aspect, and you can actually just see that working in a little video clips of 11:48:33 the oil drop experiments, and you can see how they're coupled. So the we're taught conventionally in quantum physics that you can only observe a wave aspect or a particle aspect, depending on what your measurement is, but there's this idea that they can't 11:48:48 both be true simultaneously and what I love about the oil drop experiment is it shows you that. No, this one system can have both aspects, it doesn't have to be mystical, you know, that's the appeal but the the broken a bone interpretation is had right 11:49:05 is that by separating out those two aspects, by having a pilot wave, and the particle itself, which is supposed to be bouncing around, now all of a sudden you have something that's a little more practical, you can look at that and you can have some sort 11:49:21 of intuition about what's going to happen and and in different situations, based on your, your experience, rather than based on a complicated series of equations, actually not that complicated the equations, but implications of those equations aren't 11:49:35 always readily apparent because they don't quite apply to things in the same in the sort of normal whale this beyond that, I think that there's a there's a philosophical position where we can get started. 11:49:49 I mean, it seems like in the late 20s and 30s when quantum mechanics was being developed. There was a rejection of the idea that we needed comprehensible model to explain and visualize what was happening in the microcosm and, and that whole idea that 11:50:10 you need something that you could, you know, really, you could get your classical intuition around that got thrown out. I think it actually got a kind of looked down upon by the academics. 11:50:22 And honestly, I think that Einstein and a lot of a lot of other physicists that I find myself relating to feel like that's a betrayal of the entire point of physics. 11:50:33 I mean there's more to physics then predicting the correct outcome of an experiment because like Carver mead said, if you're a good enough mathematician, you can mathematically describe almost any system, but doesn't have any relationship to what's really 11:51:00 It's true to that primary mission I think a physics, which is to get a deeper understanding of the processes happening in nature, and where, where the math isn't completely divert divorced from the factors that you're manipulating in the lab, but rather 11:51:19 tell you something about those factors and help you understand them. 11:51:23 So that's why I think we need an interpretation of quantum mechanics, is that ultimately physics is a very special kind of conceptual art, and there is something to be said for having a model that can be grasped by the mind, that isn't just purely a mathematical 11:51:44 object and. 11:51:47 And I think the boy he was on the right track with giving us something that we could get our head around. Are there any particular phenomena that gave this idea track you saying we have this prospect that we have this group of three people, which would 11:51:59 be bore Heisenberg and Pauly who are, you know, pretty much giants in the field at that point but, I mean, I don't think that's quite enough. If you don't have any reason why people would be attracted to that idea when you know they wouldn't do the same 11:52:17 thing and say electromagnetism or in thermodynamics, people wouldn't be attracted to that in fact people really hated that idea in those in those two cases, right, and it made it very very difficult for science to progress because people hated that idea. 11:52:28 Are there any particular experiments particular effects possibly that were so counterintuitive or so strange. People were willing to look at this interpretation of quantum mechanics, which is basically, we have a set of equations, and so whenever we do 11:52:48 an experiment we prepared as close as we can to the same state. We're going to have an electron we try to get the electron in as close to the same state as we can, by doing exactly the same things every time. 11:52:54 Then we wander over to the output of the experiment, and we just measure it, and then we sort of see it, the histogram of results matches probabilities based on this equation when this rule for calculating possibilities. 11:53:08 Right. you're talking about the double slit experiment, right, it's every equation basically but one way to look at the double slit experiment is you say okay I prepare my electrons with this electron gun I'm just going to leave it at the same voltage 11:53:18 it's going to have the same temperature it's going to eject the same number of electrons going to reject them at the same rate, and they're going to have same kinetic energy, and they're going to go through this double slit right where I got partitions 11:53:32 blocking the electrons, and then they're going to go hit some sort of screen or some sort of photographic plate or whatever it is that it's going to be when they first did the experiments that I observed these light and dark fans which make them look 11:53:44 like a wave phenomenon, and then I'm going to just calculate, where I think these bands are going to be and how bright they are based on these equations and not going to worry about whether or not this electron went through this lead or that slit. 11:53:58 I'm just going to say possibility of going through either one of those splits or possibly both at the same time or whatever it is, because I don't really want to think too much about this, all I really care about the only thing I give any ontological 11:54:11 validity to is this way function, I don't give the electron any ontological validity. The only thing I care about is I have this wave function which is a square root of a probability basically it's a complex field, which means that it's a complex number 11:54:25 of that has a different value at different points. Right, so every point in space has a complex number associated with it and when I square that every point in space has a probability of where that electrons going to be well yeah you're talking about 11:54:41 the, the Copenhagen interpretation, which is, which is popularized because basically with the double slit experiment. When scientists, put photons through the double slit. 11:54:51 And they discovered that even when you put one photon at a time. through a grading with two, two slits in it, and it's still produced an interference pattern that pretty much blew everybody's mind and everything was up for grabs at that point. 11:55:07 So the whole idea of the physicality of reality and all of it was secondary to the crisis of how do we explain this. You've got if you got something which is a particle, you're sending it through one at a time. 11:55:18 How do you get an interference pattern on the other side, because there's no clear and intuitive way to do that if you've got one object like a bullet, that's going through two holes in a screen, you should just see two regions of intensity, right on 11:55:32 the other side of those two slits on the background, where they get detected but instead you get this range. So all those these ideas came out in the Copenhagen interpretation was maybe that photon is going through both slit simultaneously and interfering 11:55:47 with itself. Before the wave function collapse is when it hits the screen on the other side and we detect it. 11:55:54 That's not a necessity of the situation. That's just one potential explanation, and the pilot wave theory gives you a way of looking at it where. Well, maybe the particle itself goes through one slit or the other, but the wave function associated with 11:56:11 the particle can go through both at the same time and interfere with itself and basically create troughs of probability or troughs of low energy where the particles more likely to go. 11:56:21 That's intriguing. The idea there's ripples, you end up with a model. 11:56:26 There was I was watching a brief interview with I think his name is basil Healy, he's a. 11:56:33 He was a physicist who worked with David Boehm. 11:56:38 I'll believe on this theory for quite a while, and I think they worked on the book, The and divided universe together. 11:56:44 And he was saying that the new model. The, we have is that particle and wave particle is really the right way of looking at it but rather, every particle is a process you there's something going on there. 11:56:59 And although it has certain properties which are constant in time. 11:57:03 It's cycling, and it's like a standing wave. And so you could have just like a photon transforms itself constantly from an electrical charge to a magnetic field to a to an opposite electrical charge to an opposite leaf oriented magnetic field, you can 11:57:21 have a particle that's doing the same thing, it's this is a point, and then it oscillates into a field condition in that field condition interacts with his environment. 11:57:31 And so you get the pilot wave, and that shapes trajectory. 11:57:35 One thing I'm interested in finding out about this is what sort of ontological status, does the broccoli, boom, interpretation give to both this particle and this way, when you look at your oil drop thing, you know your particle is just this drop of oil 11:57:52 that's bouncing on top of some vibrating oil and so the wave is the vibrating oil, as I understand it, and the oil drop is part. So the question is, is, is there any deeper meaning as far as the Berkeley, boom, and possibly Healy or whoever it is that's 11:58:13 carrying on this sort of creation right now, is there any idea about what those two things are I assume that the particle when you talk about the particle is the electron. 11:58:23 Sure. Well, that's true of I think any body of matter to boil the was, was the introduced us to the idea that really any fundamental particle has a wavelength associated with it. 11:58:34 And although these, these new wave particle duality is only easily experimentally evident in small very lightweight particles like protons and electrons that there's a wave function associated with all matter, right. 11:58:48 I think that the ontological significance is that's tricky because I don't know, can we detect the wave aspect of a particle at a distance. That seems like there was the main problem is that maybe if you can only detect the, the wave function. 11:59:05 That's the pilot wave, if the system is coupled to that particle, because the way isn't the pilot way but an aspect of the particle that moves along with it, that I'm not really sure about. 11:59:18 And you also end up with all that non linearity stuff we were talking about earlier. 11:59:23 I was wondering if the pilot wave was decoupled or not. So, you know, the way I would understand it, keen about it would be to look at the pilot way as not just being a pilot way but being something out there, interacting with something. 11:59:42 I mean I don't really know what 11:59:47 I do. I'd expect you know something, doing something with the electromagnetic field or something like that with the proton field where the electron would be bouncing around on that. 11:59:57 But you're saying that what we're looking at is really we can only detect the bouncing around a bit too little part of the lead from the, you cannot detect the pilot wave. 12:00:10 So, do we really know anything about that pilot wave. Yeah, I mean that's that's really what I want to know is what would that pilot wave be not necessarily whether or not we can detect it because obviously we can't because if we could detect a separate 12:00:23 pilot wave or even disproved there was a separate pilot way, we wouldn't have to worry about it right. 12:00:29 Because now all of a sudden, it wouldn't be an interpretation would be a theory. Right. But the question is, is, is there any relevance or is there any. 12:00:40 What I mean, what I mean by ontological relevance is, is there anything about this pilot way that the theory postulates that tells us what it is being different than its interaction with the particle interaction, being different with its and then its 12:00:56 interaction with the electron, or with a full time or with the core, or whatever it was, I mean, consider this came out came about long before corks but still. 12:01:13 Is there something that would give it some sort of meaning that we could actually grasp on yeah well in recent years you've seen the week measurement experiments, I believe that error on of came up with. 12:01:24 to. to do a sort of a statistical experiment where you could move your detection the screen on the other side of double slit experiment to various distances, and you could reconstruct the path that the particles must have taken in order to get to the 12:01:43 screen and that, in that configuration, and it fits in and matched perfectly with the predictions of pilot wave theory that showed the trajectory of the particle through the through the slits as it was influenced by the interference pattern of the wave 12:02:01 function of the particle, so you get these there's these beautiful trajectories that come out both of the slits that aren't straight lines they there are. 12:02:12 They have a ripple to them. And that's kind of weird to think of it that way. But apparently, if you use a week measurement technique you can sort of infer that the path of photons or electrons through those slits takes this really interesting kind of 12:02:30 meandering ripple trajectory from the slit down to the screen. 12:02:34 So by doing the week measurements you're saying we can take a peek at what these trajectories actually look like. That's right, which which which kind of illustrates those trajectories kind of illustrate the interference pattern of the quantum potential, 12:02:48 which is. 12:03:00 Okay, that makes it sound a little more like the field is something that the electrons or whatever, bouncing around off of. 12:03:10 Then something that's sort of falling around with electron. 12:03:14 It sounds like some the field ends up being something they're interacting with, I really have to think about that. 12:03:21 So, what we're seeing here then is that there's something that these electrons are interacting with. 12:03:28 And they have these trajectories that are based on that thing that they're that they're interacting with so a particles interacting with another thing, it looks like. 12:03:38 And that, especially, and that's what these interpretation. That's what this interpretation says, and then they go off and they do things, however this interpretation doesn't really make any different predictions than standard quantum mechanics. 12:03:57 So, this is an interpretation of non relativistic quantum mechanics, and it makes no significantly different predictions that we could actually look at than regular quantum mechanics, based on the showrunner equation. 12:04:11 And just sort of eliminating any ontological status to the particles between their the preparation of the experiment and the measurement afterwards. One question that I was really interested about and I'm not sure if you know the answer to this is what 12:04:34 would the explanation for quantum tunneling be in the, the Broadway boom theory. Do you know them well i think that there's I think that the. 12:04:39 There's a relationship between the particle and its pilot wave, which, if we look at the oil drop experiment, it could be, could show that there is almost a cycling, a level of energy, depending on its position. 12:04:56 So maybe I know that there's a, there's an experiment where they they had the, they called the walkers, these little oil droplets bouncing across the surface, and the surface is energized. 12:05:08 When the timing is just right when the when the particle bounces. 12:05:14 I guess it just the right moment when the pilot wave can give it a little more energy than usual, it'll have a probability function for getting enough height to jump over barrier. 12:05:28 And I guess it just quantum tunneling is just a matter of energy. So if the particle has enough energy to to move through with through a barrier, then it'll do so. 12:05:43 And I guess there's a time function associated with a way of equation, that, you know, if the particle arrives at the barrier at the right moment when there's the high energy contribution from the, from the, from the wave function that it can tunnel right 12:05:58 through. But um, it's hard to visualize that, because in reality and electron isn't bouncing particle on a surface on a Friday on its own plane wave right. 12:06:11 It's doing something else in a three dimensional sense. And we need sort of a better analogy to really understand what's happening, the real relationship between the wave aspect and the political aspect, I think, partially. 12:06:27 The reason why I was asking that was, because in the oil drop experiment. It was hard for me to try to get an idea of what tunneling would be other than randomly bouncing all over a wall, which is what a potential barrier would look like and obviously 12:06:42 you can't send that backwards into quantum mechanics right because quantum mechanics you know you have that wave function or in the Copenhagen interpretation at least you have that wave function. 12:06:53 And that wave function tells you that if you have a potential barrier, then something that hits that potential barrier, even if it would be classically disallowed that if that potential is too high and the and the speed of the electron is too low to jump 12:07:12 over that, or to pass over the barrier doesn't have that much energy. 12:07:35 There still is that probability that the electron or the particle of whatever, whatever particle you want will end up on the other side and so I was wondering whether whether or not that would be something really, really interesting. 12:07:34 In, such as the particle being absorbed temporarily and showing up on the other side or if it was just the sort of bouncing over the wall, sort of thing. 12:07:45 I think what we're, I think what we're seeing is that the energy of the particle is. Although the energy on it's on a time average basis, maybe insufficient to penetrate a barrier that requires a certain amount of energy to get through that, that in any 12:08:05 given instance there's a possibility that the, there will be a higher, a higher energy level than the time average energy level. And I guess if you're if if that happens if that if that peak in energy happens it just the right under the right circumstances 12:08:24 where the trajectory is correct and all everything, then a particle can move through right i mean if if you've got an electron, and it's needs to penetrate a body of matter. 12:08:36 It's going to be facing basically an upward hill of potential energy created by the electron charged field of all the matter, that it needs to get through. 12:08:47 But there are pathways, right where there's lower energy. And if the electron has enough. 12:08:54 Enough velocity momentum. It just the right moment and at the at the mouth of one of those trajectories, it can get through it right. Isn't it just a matter of having enough energy to get to get by. 12:09:05 Yeah, just has to sort of get over but you know the idea of quantum tunneling is this electron does not have the kinetic energy to the speed to get over a potential barrier classically has a probability of passing passing through it. 12:09:22 So all I see here is just one more argument that there's some extra logical status that we have to give to that pilot way that it's not really just a part of that electron, it has to be something else that has that kinetic energy or at least you know 12:09:39 has the ability to, you know, have this some sort of temporary account balancing, like taking on a negative kinetic energy which would be sort of crazy but not crazier than anything else and you know when you're dealing with some of these things. 12:09:53 Well, there is. We know, we know that the there is a vacuum fluctuation field right there's a quantum fluctuation field of virtual particles. 12:10:03 Isn't that what we're. Isn't that the, the analog for the energized oil, and that experiment. Is it that there's a background of energy, and that part in that a particle can borrow temporarily energy from that field and get excited if it gives it back 12:10:18 right within a certain time. So the balancing oil drop doesn't have to give that energy back, it ends up someplace weird right it doesn't necessarily go back into that field now I'm not sure with this silent wave, whether it's possible to not give that 12:10:34 back, right, but it wouldn't probably give that back in the tunnel and that's not the thing that I'm thinking about the thing I'm thinking about is if the particle can pull energy out of the pilot way, then it seems to me that the pilot wave and the particle 12:10:51 have to in some way he separate entities. They can't be the same entity, right, because the pilot way would have move along with the particle, and it would have similarly limited amount of energy. 12:11:04 And then you also have to think about, well, how does the pilot way to get to the other side of the potential barrier, just seems to me to look like this pilot wave has to have this separate ontological status, and in some way be one of these all encompassing 12:11:20 weird things like an electric field or something like that. 12:11:23 Well, it seems like it might be some kind of interaction. Right. I mean maybe the maybe the pilot wave is the product of the interaction between the field of the particle 10 the field of all of the other particles that it's a couple to in that face been 12:11:36 entire universe but certainly it's nearest areas, and maybe it's a it's an interesting interdependency maybe if you had one electron in alone in an empty universe, maybe then you kind of have a pilot way because it wouldn't have any other fields to interact 12:11:50 interact with. And again, that would go on to this whole idea that there's something there now. I mean you can keep adding more things right so you say there's this field that's independent. 12:12:03 And then, maybe there's this pilot wave that is sort of in that field, and then that's what's actually interacting with the electron, but that doesn't get you away from having this sort of feel from everything else. 12:12:15 So you still end up with this sort of all encompassing thing all out there, it's not too much of an issue as long as you're wandering around talking about electric fields and temperature gradients and things like that you can still talk about something 12:12:28 like that. But at some point you really do have to sort of figure out what it is. 12:12:33 And the right now I know that I don't have a really good idea about what that thing is. 12:12:38 And it sounds like probably nobody has a really good idea what it is. It's something that, as far as I recall, people say you just can't in principle measure it, which it's always a little bit scary. 12:12:50 Yeah, but you know, I think they, they would have said the same thing about the vector potential until her on off boom came up with that effect where they can detect it without, you know, a magnetic or, or an electrical field. 12:13:06 The Toronto Film effect that we talked about in quantum paradoxes right, so maybe there's some way that we haven't figured out yet to interact with the pilot way with the quantum potential that will prove that it has a physical existence right there could 12:13:22 be, but I mean just have to figure out what that would be. Right. Okay, I think we've done a reasonably good introduction at least to that. Okay. All right, James. 12:13:33 Are All right, Well, thank you very much. All right, James. 12:13:36 Thanks for your time.

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