> quantum-mech > fonda-interpret > pilot-wave-theory-explained-looking-glass-universe

Do we have to accept Quantum weirdness? De Broglie Bohm Pilot Wave Theory explained

Looking Glass Universe - 2017-08-16

An explanation and the pros and cons of Pilot Wave Theory aka Bohmian mechanics.

Links to videos referenced:
Veritasium's video: https://youtu.be/WIyTZDHuarQ
My old Bohmian mechanics video: https://youtu.be/rbRVnC92sMs
Contextuality: https://youtu.be/Qz4CHI_W-TA
Entanglement and the EPR paradox: https://youtu.be/Xzmp7byh77E

Also see: 
PBS spacetime's excellent video: https://youtu.be/RlXdsyctD50
This amazing video about 'surreal paths' in Bohmian mechanics (this channel is also very worth checking out): https://youtu.be/CCW93koLNYY

Seán O'Nilbud - 2017-08-20

Bohemian mechanics.
Is this the real life? Is this just fantasy?
Caught in a landslide, no escape from reality
Open your eyes, look up to the skies and see

I see a little silhouetto of a man
Scaramouche, Scaramouche, will you do the Fandango
Thunderbolt and lightning, very, very fright'ning me

Bohemian Gravity
Easy come, easy go, will you let me go?
Bismillah! No, we will not let you go
(Let him go) Bismillah! We will not let you go
(Let him go) Bismillah! We will not let you go
(Let me go) Will not let you go
(Let me go) Will not let you go
(Let me go) Ah, no, no, no, no, no, no, no
(Oh mamma mia, mamma mia) Mama mia, let me go

Bohemian Uncertainty Principle
Nothing really matters, anyone can see
Nothing really matters
Nothing really matters to me
Any way the wind blows

Looking Glass Universe - 2017-08-21

HAHA! Thank you!

Kamil Kashaf - 2019-12-29

Did you smoke shrooms?

Gros Pénis - 2017-08-16

"They collectively scratched their heads"
Why am I imagining monkey scientists sitting in a circle, picking each other ?

Feynstein 100 - 2017-08-17

You have an active imagination

Looking Glass Universe - 2017-08-17

So bummed I didn't draw this now.

Kraflyn - 2017-08-17

+Looking Glass Universe :D :D :D <3 <3 <3

Juan Jose - 2019-06-17

2:05 "they scratched their collective heads" lol

Mark Lee - 2019-10-22

that is Gros

fluffymcdeath - 2019-03-06

I've long felt that superpositions were just a way of pretending there are no hidden variables. Instead of admitting there will be things we don't know, throw in all possibilities, then cancel them out after the fact when we know what actually happened. It feels like a statistical trick like ideal gas law. Works fine so no prob, but one builds up from the details to get something practical and the other starts pragmatically and papers over details.

Ruby Gupta - 2017-09-13

Hey can you do a collab with 3Blue1Brown

Vacuum Diagrams - 2017-08-19

A few points are worth mentioning on this issue:

1. While some laypeople or beginning physicists complain about Bell's theorem (or even the Kochen-Specker theorem) in regards to Bohmian mechanics, most serious critics choose other lines of attack. For example, Chen and Kleinert have argued that the results of Bohmian mechanics do not agree with those of standard quantum mechanics even for the two slit experiment! (In other words, the picture at 6-24 does not agree with experiment). Neumaier has also pointed out observables made up of time correlations of, say, position operators, do not in general have the same expectation values in Bohmian mechanics as they do in standard quantum mechanics.

2. It is important to note that not even Bohmists try to argue that Bohmian mechanics is fully experimentally equivalent to standard quantum mechanics. The claimed equivalence, even if we ignore the criticisms above, rests on an assumption known as "quantum equilibrium". This assumption states that, in order for Bohmian mechanics to reproduce the Born-rule probabilities of quantum mechanics, the initial state must be distributed in this way as well. This is a highly nontrivial assumption: there is nothing a priori that states that the particles in the beginning of the universe must be distributed according to the absolute-value squared of the wavefunction. Outside of quantum equilibrium Bohmian mechanics has pathological features; in particular, it allows transmission of faster than light signals, which is a problem for any relativistic extension of the theory. The relativistic version isn't just ugly: it simply can't exist!

3. Bohmian mechanics can only describe systems with a fixed number of particles. Processes where particles are created or absorbed, such as when a photon excites the atoms on a phosphorescent screen, are not allowed. The formalism simply isn't equipped to deal with such processes. Since all consistent relativistic theories must allow creation and destruction of particles (for causality preserving reasons), Bohmian mechanics is a dead end of theoretical development. It cannot describe anything happening in a particle accelerator, not just because it's not written in a relativistic form, but because by nature it cannot be relativistic.

4. It's important to note that the "oil droplet theory" and Bohmian mechanics are extremely different. The oil droplet model is really more analogous to a plate attached to a speaker. By playing a tone you can see grains of sand arrange themselves on a plate in a "standing wave" pattern. The difference is that in the oil droplet theory a single drop eventually traverses all of the available surface, and the wavy pattern is seen in the amount of time spent in each spot. Bohmian mechanics does not work like this at all: because the evolution equation for the particle depends on the imaginary part of the wavefunction, if you place the particle in an energy eigenstate (such as the ground state of a hydrogen atom), whose wavefunction is wholly real, it will just sit there. It won't do anything. It won't move at all! The "probability distribution" of particle positions in such a state is given entirely by the initial distribution, which I hope provides some "teeth" to point 2.

All in all, I don't think it's premature to discard Bohmian mechanics as a possible theory. Draw inspiration from it if you must, but ultimately, the theory has to agree with experiment... and we've been doing experiments that prove that nature is relativistic for a solid 100 years now.

Vacuum Diagrams - 2017-08-30

Fallen Star Features You might be interested in this: physicsforums(dot)com/insights/stopped-worrying-learned-love-orthodox-quantum-mechanics/

That's a text written by Hrvoje Nikolic, the author of one of the papers you linked. Here he essentially admits that his attempts to find a relativistic generalization of Bohmian mechanics have failed. He now reconciles relativity with Bohmian mechanics by postulating that the world really isn't relativistic at its most fundamental scales, and that the apparent relativity is an emergent feature of this nonrelativistic system. This is possible, but there's no evidence for it. We have tested Lorentz invariance at extremely high energies (surprisingly, even above the Planck scale) and found no violations. This disfavors the model in favor of ordinary quantum mechanics.

Voidisyinyang Voidisyinyang - 2017-11-19

An electron flow crossing a sufficiently small aperture
would exhibit diffraction phenomena. This is the direction which we
should perhaps look for an experimental confirmation of our ideas. Louis de Broglie, 1923.

Simon Gaggl - 2017-12-31

kind of have a problem with many worlds theory tho it would be a kind of cool expinaitioin too all that stuff it seems a little bit contra-intuitive that any time when some of that quantum stuff happens , a new "world " got createt, because you know the universe is really big and basically everythig has some quantum character so would happen like 10000............... times every second. I know i should be used to infinity and stuff like that in physics but yeah ...
ps sorry for bad english

Voidisyinyang Voidisyinyang - 2017-12-31

multiverse originates from time-frequency uncertainty as origin of Big Bang - and time-frequency uncertainty originates from noncommutative time-frequency energy as nonlocality.

Iago Silva - 2018-08-11

Happy b-day this thread, basically; well anyway, I dropped by and, since I recently finished reading Bohm's 1952 papers, I decided to throw in my thoughts here, in "defense" of BM (bear in mind I'll stick to Bohm's work). Here's my takeaway from that:

a. The wavefunction is exactly the same one from QM
b. We introduce the "hidden" particle in terms of the wavefunction: you get the momentum from the phase, and the position from the usual Hamilton-Jacobi formula "corrected" by a quantum potential U
c. Quantum observables arise from an inseparable interaction apparatus-system, while "hidden" observables do not, and depend on the system alone; however, experimental outcomes refer to the former only (this is stressed several times, specially on the 2nd paper)

1. "the results of Bohmian mechanics do not agree with those of standard quantum mechanics" I dunno the details of the Chen-Kleinert-Neumaier criticisms, but c. pretty much automatically invalidates the notion QM and BM disagree experimentally by definition

2. "The claimed equivalence, even if we ignore the criticisms above, rests on an assumption known as "quantum equilibrium"." Bohm certainly suggests that such an equilibrium is so, due basically to the 'unruliness' introduced by the U-term (which we may perhaps approximate by a de facto random variable, à la Boltzmann?) on the hidden position; however, such an equivalence of statistics need not be postulated - we may simply note that, whenever the approximation of quantum equilibrium is valid, we may use quantum and hidden statistics interchangeably (an analogy might be the quasiclassical WKB technique)

3. "Bohmian mechanics can only describe systems with a fixed number of particles" This implies Bohm's treatment of Compton scattering and photoelectricity in Appendix A of paper 2 is wrong at some level, then; not obvious to me, but that's what it entails

3.1. "by nature it cannot be relativistic" This is not really the BM's fault, but rather of our usual QM formalism: sure, QM/QFT's end results are written in a relativistic form, but some objects manifestly aren't (e.g., equal-time (anti-)commutation relations still need to be invoked - AFAIK!); worst still, people couldn't figure the relativistic 1st quantization of the many-body problem, so they merely ignored it and went straight to the 2nd, claiming the former doesn't exist - which is really just a lazy copout that happens to work with most of the problems particle physicists are interested on. However, the 1st quantized description of bound states is still sorely missed by some researchers (from the top of my mind, people like Ball and Fanchi), who point to this as an open problem

4. "because the evolution equation for the particle depends on the imaginary part of the wavefunction, if you place the particle in an energy eigenstate (such as the ground state of a hydrogen atom), whose wavefunction is wholly real, it will just sit there" Just a lil' 'errata': more specifically, the HJ eq is written on terms of S and R, Psi = exp(iS/h_bar)*R - and while it's true that in that example we have p = 0 with real eigensolution psi, the eigen state Psi is not: Psi = exp(-iEt/h_bar)*psi

To conclude: even though I build this case, I'm not really a fan of BM - it introduces ad hoc objects that are not even guaranteed to substitute for the quantum statistics. But it should be born in mind: this is an interpretation - it's not a theory ; we use it to gain 'intuition' on the quantum dynamics, not to go beyond it - and, in this case, we can appreciate the hidden variables as "markers", as "visual aids" of sorts. That is all I believe them to be in BM, and that is in fact the reason why I take BM to 'bypass' the BE and the KS theo; IMO :)

shimshimma - 2017-08-17

my question is -- how is it that they came to the conclusion of superposition in the first place. it makes so much more sense that a guiding wave might be directing particles.

Jon Do - 2018-05-18

Freaky scientists who wanted to impress the people. Also the idea of multiple universe gave them the idea that they could do every shit and in one parallel universe they would still deserve heaven.

LightSource - 2019-01-19

Detect those guiding waves then, interfere with them, predict how strong or far they are. Prove it!

The freaky thing about entanglement is that it doesn't care about Light Speed.

Sander - 2019-03-22

Perhaps it has to do with the double-slit experiment being accidental in the first place?

"Hey look at this. What the hell is going on? It almost seems like the particle is interfering with itself. That's odd man, like it seems like it's doing both at the same time."
"... What if it actually did that though?"

And then further experimentation only seemed to prove that idea.

(Perhaps because it is true)

Nikipedia xyz - 2019-05-16

LightSource Yes it doesn’t care about lightspeed but there is no additional information gained or transmitted once you collapse the wave function into that singular state.

It makes a lot of sense if you consider that it is impossible to put say a negative charge In the exact same place as another negative charge, They would just repel, something has got to give. And what “gives” When you force Two Electrons to be in the exact same place is that they have to occupy a new shared state (you have forced them to be one thing after all - When they are simply two things). When you do this they occupy a realm of existence where that particular property is undefined the property is called spin, And it’s an analog of angular momentum and clockwise and anticlockwise but off axis with an eccentricity each of them random, but each of them opposing. And this has nothing to do with gravity therefore space is a irrelevant.

Trying to apply the property of lightspeed to an electron spin is a meaningless statement to attempt to impose. Spin isn’t the same as angular momentum it is something entirely different it’s just the fourth quantum state. Along with quantum number which indicates energy. And the other two which you can look up. I think we should be teaching a periodic table that organises atoms into electronic spin states now because it is “more better” at grouping.

nefaristo - 2019-06-16

@Jon Do actually multiple universes and Copenaghen are two different interpretations.

Juan T - 2017-08-16

A (new) video! Finally!

Andrew Blackney - 2017-08-17

no need to be sorry for these quality of videos I'd wait years between

Pyry Kontio - 2017-08-18

(and de Broglie!)

Chas R - 2017-08-20

Yeay :D

Abhishek Bhattacharjee - 2017-09-04

seriously..ur videos are worth waiting...

Francis D'Souza - 2018-04-12


Junior - 2017-08-16

Oh, I missed so much this almost-laugthing-cute voice that brings so much knowledge!

Looking Glass Universe - 2017-08-17

:) :)

Lepus Lunaris - 2017-08-17

11:22 "tradgectories"? Someone has clearly been to tired when editing that part of the video! :D

But hey! Great video! It reminds me to continue reading about mathematical physics in my free time.

Looking Glass Universe - 2017-08-18

Hahaha, I'm also dyslexic so I usually spell check everything I write- but I forgot to do this one clearly :P

Yay! I'm really happy to hear that :)

Navak - 2018-08-13

6:45 lol the particle regrets its decision
every time

Johan 't Hart - 2017-08-16

Yesss a video from LGU!! I learned so much from them, you are a great explainer of difficult stuff!

Looking Glass Universe - 2017-08-17

Thank you so much, I'm really glad you're back despite the fact that I upload so irregularly.

Shubham Shinde - 2017-08-16

I had to pinch myself to make sure i wasn't dreaming

Looking Glass Universe - 2017-08-17

Oh, me too...

blueckaym - 2019-03-11

Great video! Very logical and clear to understand - just like a Pilot Wave Theory ;)
I find just once small detail missing - the actual pilot-wave wasn't shown in the diagrams like at 3:59.
For newcomers to quantum mechanics (and wave theories) it can help clarify the main concept.

ohyouresilly - 2017-08-17

Perfect timing, I'm just about caught up on my rewatch of LGU. Love the videos, keep making great content!

Looking Glass Universe - 2017-08-18

Oh thank you so much! Wow, you want to watch my videos more than once..

ohyouresilly - 2017-08-20

Absolutely. Re-watching them helps me understand the concepts better, too, since I've never studied physics at the university level... I just find it fascinating.

Daniel Lassander - 2017-08-16

So glad to see more videos from you! If this is as well done as all your other videos its going to be fantastic!

Thank you for your videos.

Looking Glass Universe - 2017-08-16

Aw, that is far too kind- thank you so much!

DC Perry - 2019-05-26

I call it "quantum gaslighting!"

Charlie Bayliss - 2019-11-02

Oh BLIMEY thank you - 'hidden variable' I never really understood what was meant by that before

Roger Nevez - 2020-02-27

9:59 Looks like if there were only one slit, there will still be an interference pattern.

John Sorial - 2018-04-27

I had recommended Veritasiums video to Looking Glass universe
And now it's mentioned,
What an amazing feeling.

m n - 2019-09-22

It upsets me how long it's taken me to be aware of your videos. Great work, truly. And thank you for taking the time out so lay folk like myself could learn what we were never taught in school... but really should've.

chiepah2 - 2017-08-17

Welcome back, we missed you.
Personally I'm glad that there are valid competing theories on how the universe works, because as soon as we "figure it out" we'll choose to stop learning.

Looking Glass Universe - 2017-08-18

Thank you so much :)
You're right- but luckily there is so so much we don't know that we'll need to keep learning for a long time yet!

David Ralphsky - 2017-08-19

Ehhh, not a big fan of the "science must have two sides" thing. It either fits reality or it doesn't. It's like when people pit evolution and creationism together -- it's not really a debate.

chiepah2 - 2017-08-19

David Ralphsky
The problem with the 'evolution vs creationism' argument is that they aren't mutually exclusive. But the thing about 'Quantum vs Bohmian' is that they both have valid ideas about how the universe works and are exclusive. Only one can be right, but having multiple sides keeps us looking at the problem rather than just assuming that we understand it all.

Daryn Miller - 2018-03-22

I find your voice super relaxing. Love the content too. It's like educational asmr to me.

גלעד סופר - 2017-08-18

Great video as always!
Could you please make some videos on the main ideas in quantum information theory? Would love to know more about it :)

Looking Glass Universe - 2017-08-18

I'm doing my PhD in quantum info stuff now, so I really should! I will soon :)

CuriosusMentis - 2019-12-01

Lovely Looking Glass Universe thank you for all the great content on quantum mechanics! I am sure you have your reasons on why pause on making more of these amazing videos but if I may ask: a video on decoherence would be most appreciated :)

Green Ghouls - 2017-08-27

I don't even know why I watch these videos, I'm a biochemistry student :L

T Roger - 2017-08-16

Thanks for the new video! I love your stuff.

Looking Glass Universe - 2017-08-17

Thank you very much!

Ben Smith - 2017-08-23

Liked, subbed & commented off of this one video. GREAT WORK! :D

Looking Glass Universe - 2017-08-23

Thank you so much :)!!

Michael Bishop - 2017-08-20

"Do we have to accept Quantum weirdness? "
Unless one is psychotic, the answer is "yes".

Total Triss - 2017-08-18

I love your videos <3

Looking Glass Universe - 2017-08-20

Thank you!

Benbedra Abdssamad - 2017-08-16

Thanks for the video, it was really informative. I wonder about the significance of the wavefunction in Bohmian Mechanics?

Looking Glass Universe - 2017-08-17

I really didn't make that clear in this video because frankly this video got too long. But my old video on this went into it a lot more: https://youtu.be/rbRVnC92sMs

Common Pike - 2017-08-16

Glad to see you continue, and happy to see you do something on brogly/bohm ! +1 for a vid on decoherence !

Looking Glass Universe - 2017-08-17

Ahaha thank you! It has been a while since a bohmian mech video! Thank you, I'll get onto the decoherence video soon :)

lloydgush - 2017-08-16

Yeah, there's also some weird stuff when it comes to quantum mechanics.
Either you have non-locality, wave collapse or some other weird stuff.

Looking Glass Universe - 2017-08-17


xavierinthetube - 2017-08-21

Thanks for the video. I love seeing the interpretations that challenge QM being represented so well, and in general I love your channel and found your videos very instructive.

I have noticed something when thinking hard about the interpretations of the double slit experiment, something that I can't shake off my head but that I haven't been able to find out if someone already talked about, or, if it doesn't make sense, why it doesn't.

The Copenhagen interpretation of QM says that the wavefunction is not a "real thing". That the fact that the particle goes through all the possible paths at the same time is something that only happens "virtually", and that until measurement occurs, there is no interaction.

This has always struck me as an incomplete explanation.

1. Given that the double slit experiment can be reproduced even with a device that sends one photon at a time, that means that the wavefunction of that photon needs to be aware of the slits to interfere with itself and change the likelihood of the photon hitting one particular spot. That awareness means that, on some level, the wavefunction and the particles on the barrier interact. Not only that: if we are proposing that the wavefunction's shape is influenced by the particles in the surroundings, and we are also saying that the wavefunction is a description of all the possible paths the particle could take, then it implies that, at some level, there is interaction between the wavefunction and the entire universe, only with weaker and weaker force in bigger distances. Does it makes sense so far?

2. If 1 is correct, then that means that even QM implies that the wavefunction has reality, and this "subliminal" (not-measurable) interactions exist. Again, if they wouldn't, the wavefunction would not be affected by the particles in space. It follows that the wavefunction could potentially be interacting with other wavefunctions that are also happening in the same space, since those also have reality. Why wouldn't it? If we know some "invisible aspects" of the photon are affected by the shape of the surrounding objects, both in QM and Bohmian M, then what is stopping us from thinking that invisible aspects of other particles are also interacting with the wavefunction?

3. If 2 makes sense, and the wavefunctions can potentially interact, wouldn't that interaction be a potential source of hidden variables that explain the apparent randomness? Isn't it reasonable to think that the same aspects of the photon that we can't see directly but that are clearly affected by the shape of the slits are also interacting with other things that we can't see, because their energy levels are not enough to be measured directly?

4. And finally: isn't it possible that a measurement just happens to be the point where a continuous wave surpasses a certain threshold that makes it change the state of the other particles in a degree that we can actually measure? Isn't it possible that there is no such thing as an individual photon, but rather what we called a "photon" being emitted is the electromagnetic field being perturbed to the point that we can measure it? Do we need to assume that the electromagnetic field is, when we are not emitting a photon, at rest? What if the electromagnetic (and other quantum fields) are more like an ocean which always have waves, and a photon is more like a tsunami? The waves would affect the tsunami in a way that would seem random to us, and ripples of the tsunami that we can't measure would affect other tsunamis in ways that would, again, seem random to us. Could the apparent randomness of QM be a side effect of such ripples coming from everywhere, a kind of "white noise" of the field?

This to me seems to be neither Bohmian Mechanics (which if I understand correctly, postulates that the particle is being driven by a wave, but really exists as something being carried by the wave, and not just as an artifact of measurement) and certainly different than QM, since QM (at least Copenhagen) dismisses the physical reality of the wavefunction. Is there a theory around Quantum Mechanics that approaches it in this way? It seems to be hidden variables approach, but I don't know which one.

Thanks a lot for reading :)

Drew Dycus - 2019-08-25

10:27 is it jesus because it walks on water? lol

geo froid - 2018-08-07

Thank's so much for this video! I was wondering how does look the relativist version of orthodox QM? is it in some points better? i heard that this theory wasn't satisfying because of some paradoxs, so the quantum field theory is used. Is there a bohmian equivalent?

Farhan Tanvir - 2018-02-01

Really enjoy your videos! Am curious if you had any exposure to Representation Theory? Would love to see some videos on the topic!

TheTwyker - 2019-11-20

I love this so much. Science storytelling through the roof.

BobWidlefish - 2017-08-16

It's classical plus just the minimum of quantum weirdness / more localized weirdness. When the math is the same, the rest is just decoration.

OnTheArchipelago - 2017-08-23

This was awesome, thanks.

SiZarkX - 2017-08-16

Great video and I'm glad this interpretation is alive and coming into mainstream.
P.S. Veritasium did a great video on this interpretation a few months back, too!

Juan T - 2017-08-16

Do you mean the video that she mentions at 9:35?

Looking Glass Universe - 2017-08-17

Yeah, and PBS spacetime did one as well- very exciting that people care suddenly!

Guilherme Carvalho - 2017-08-18

chef hand kiss gesture mmmwah! your videos are a treat to watch, thank you so much! :)

Looking Glass Universe - 2017-08-20

AHAHA! Yay thank you!!

Joe Ruter - 2018-01-05

You have a pretty voice :)

jsohn18436572 - 2017-08-16

I've missed these videos. Welcome back.

Looking Glass Universe - 2017-08-17

Thank you so much. I've genuinely really missed it too.

1337 h4x0r - 2017-09-10

I've never paused a vid so many times to enjoy your subtle humor

Sean Thrasher - 2017-08-18

Hey, I love your videos, and thank you so much for teaching me so much and making it super clear! Your explanations are like feynman :)
I was wondering about the non-locality in particular. I read about the firewall paradox, and that it requires at least one of the following concepts to be false: no-drama (equivalence principle), information conservation, and locality. if the latter is false, would this have any relevance to Bohmian mechanics? I am aware that Bohmian mechanics has no trouble with Bell's theorem as Bell only applies to local systems, but if such a concept of locality is false anyway, what would this mean in terms of Bell's theorem? Thank you so much again for these videos!

Looking Glass Universe - 2017-08-20

Really great question! And one I don't think I can answer... But if I had to guess, the sort of locality they're thinking of is about signalling. But of course, no-signalling holds in QM and BM so it wouldn't violate that. But I'm not sure! How do they define that criteria?

mikki kiki - 2017-11-18

Where r your new videos ...we cant just keep on going through older ones to hear your exotic voice...Comeon

ytrew - 2017-10-09

"jesus" appearing in 10:28

Aubrey Black - 2017-10-22

Yeah, what the hell?

Dinh Khoa - 2017-08-16

This channel is just awesome! I wish you can produce more great videos for physics enthusiasts like myself.

Looking Glass Universe - 2017-08-17

Thank you so much! I will try my best :)

Anukul Das - 2018-01-23

Looking Glass Universe U r a genius..

Mark Sudduth - 2018-06-05

The great thing about De Broglie Bohm Pilot Wave interpretation is that it accepts outside reality, agrees with predictions of quantum formalism, and is not logically inconsistent (Copenhagen) and is not utter madness (many worlds)

A. I. L - 2017-08-17

I'm really glad you made a new video! Clearly there's a market for science explanations, clearly you have a niche people like, keep it up if you have the time!

Looking Glass Universe - 2017-08-18

Thank you very much- I will try :)