Bohmian Mechanics

[Grayman]

https://m.youtube.com/watch?v=rbRVnC92sMs


The best alternative to quantum mechanics.

 

[Puffy]

The concept of true randomness has never made much sense to me. I always assumed, like she said in the video, that what we conceive of as random or accidental is simply down to not being able to perceive the chains of causation that led to that event occurring.

I'm very ignorant when it comes down to physics and would be interested to hear more from someone more knowledgeable on how randomness is conceived/dealt with in physics.

 

[Grayman]

http://www.physics.umd.edu/courses/Phys276/Hill/Information/Notes/ErrorAnalysis.html

Random error is randomness in classical science. See link

In quantum mechanics the randomness is probabilistic so it isn't entirely random but it isn't deterministic either. Perhaps an inherent chaos in the system that is constrained to a set of possibilities. Id argue that it is mathematical and philosophical but not something that is physical in nature. Peoples need to understand it's cause and origin has resulted in several non scientific but more philosophical concepts like the many worlds concept or the idea that consciousness can affect reality on the quantum level.

The internet can be very misleading in regards to quantum mechanics. Example: There are several videos and sites that show a human observer effecting the pattern on a double slit experiment by simply looking. This never happened. This is a false interpretation of an oversimplified interpretation of the quantum delayed eraser experiment. The pattern is altered in that experiment when the measurement is taken 'before' it is observed by any human being.

 

[Grayman]

Physics has always been something that was intuitive. You visualize it and model it. It was understandable on a physical and mathematical level.

Quantum mamechanics is so difficult to understand because it is almost all mathematical and it breaks all the rules of what we perceive to be reality when we try to conceptualize it.

The argument is that we dont need bohm mechanics because it does everything that quantum mechanics does. They both come to the same results.

So I sit here in frustration at this because...dont we want something that is understandable, physical, intuitive, visual, and doesn't break the rules of what we have always perceived to be reality?

Someone please quiet my heart and show me the experiment that disproves bohmian mechanics so that my faith in science might be restored.

 

[Turnevies]

Bohmian Mechanics is not an alternative to quantum mechanics.

It is an alternative interpretation to quantum mechanics, as opposed to the common interpretation which is the Copenhagen interpretation and other possible interpretations include the many-worlds interpretation.

There are basically two parts of quantum mechanics. The first one is calculating time-evolution of the state when no-one is looking. Actually, this is just solving the schrödinger equation, but if you like you can add some fancy mathematics such as Hilbert spaces. Without much ambiguity, you can calculate a time-evolution operator that is entirely deterministic and even unitary. I estimate at least 95% of the research on quantum mechanics nowadays is only concerned with this first part. Also quantum field theory related stuff such as the standard model or string theory only cares about this part.

The second part of quantum mechanics is performing measurements, and this is the part that is more tricky and according to the copenhagen interpretation there is this 'collapse of the wavefunction' and there seems to emerge randomness. An alternative to this randomness are the hidden variable theories that say that perfect knowledge of the system would result in a perfect prediction of the result of a measurement. Now local hidden-variable theories have been proven to be inconsistent by Bell's theorem, but non-local hidden-variable theories such as this bohmian mechanics are not ruled out. Maybe one day, the different interpretations could be distinguished from each other by experiment.

What one can also do as a way to try to get around this measurement problem is consider the measurement device as an inclusive part of the quantum system. Then there will be no collapse, the (typically big) device will arrive in a superposition as well after measuring. And because the device is typically macroscopic and coupled with the environment it will show decoherence, making it look like a classical object in the end.

 

[Ex-User (9086)]

The concept of true randomness has never made much sense to me. I always assumed, like she said in the video, that what we conceive of as random or accidental is simply down to not being able to perceive the chains of causation that led to that event occurring.

A degree of randomness is calculated.

If you monitor cosmic background radiation and compare it to a single stellar radio source you're going to get a different moving average of randomness.

You can turn a chain of values into a delta of the predictive factor. The equation isn't that difficult, usually it's some kind of average or adding values / monitoring repetitions.

There are also a few main ways to establish randomness, for example statistical or analytical.

As a result you get I'd say 3 main groups:
Non-random sources/processes (including pseudo random),

moderately random (including some whose processes are unknown) such sources have certain tendencies, they might get somewhat more random and then go to being a lot less random, usually they are time-variable in their randomness,

truly random sources with an almost perfect moving average of randomness, of course there are times they will follow short patterns, but as a whole they are very pure in their randomness and that's why they're considered as random.

Some of the patterns or randomness/non-randomness can be caused by imperfections of the measuring devices and various interferences from other sources.

 

[Grayman]

Bohmian Mechanics is not an alternative to quantum mechanics.

Have you heard of 'the old quantum theory'? I would see bohmian Mechanics replacing Quantum Mechanics in the same way Quantum Mechanics replaced the old quantum theory.
I understand that Quantum Mechanics orthodoxy has labeled Bohmian Mechanics as an interpretation but it is debatably more than just an interpretation. It has different mechanics, models, equations, adds another theory (Pilot Wave Theory) and even has an experiment that supports it. Yes it interprets many of the quantum experiments differently but that is because the mechanics are different, not the philosophy. When reading other interpretations of quantum mechanics it seems very much a philosophical difference and not a technical or scientific difference.
Furthermore I would argue that BM isn't so much a particle-wave duality concept but actually defines the wave and particle as separate entities.

What one can also do as a way to try to get around this measurement problem is consider the measurement device as an inclusive part of the quantum system. Then there will be no collapse, the (typically big) device will arrive in a superposition as well after measuring. And because the device is typically macroscopic and coupled with the environment it will show decoherence, making it look like a classical object in the end.

Bohmian Mechanics doesn't include a collapse of the wave function and is viewed from the perspective that you propose, at least that is what I understood from reading various documents. I don't know the details on how the non-locality interactions work through the waves in respect to bells and the delayed eraser experiments. Two thoughts on this are 'path memory' and 'wave entanglement'.


I think this article on an experiment in relation to 'pilot waves' talks about some of the things brought up. https://www.quantamagazine.org/20140624-fluid-tests-hint-at-concrete-quantum-reality/
I would be interested on your take on it.

Short video (in article): https://www.bing.com/videos/search?...7302BC31FEBC30E046E17302B&fsscr=0&FORM=VDQVAP

 

[Grayman]

Through the wormhole version https://www.youtube.com/watch?v=fnUBaBdl0Aw

 

[Turnevies]

Looks all very interesting indeed, and I certainly don't want to rule out this pilot-wave theory.

What you need to realize is that learning quantum mechanics from popular science can be misleading, this particle-wave duality is typical example where qm seems to be non-intuitive, but people educated in qm would never talk in terms of these things. The underlying mathematical formalism where the central object is just the state-vector(with associated density matrix, green functions...) is actually very elegant and conveys a large symmetry. A theory that would need both a seperate wave ánd particle seems much more complicated to me.

Now, I don't feel like an expert on these particular issues, because you come up with this kind of theories more often, you may consider having an account physicsforums.com, where people much more knowledgeable than I hang around.

 

[QuickTwist]

There are basically two parts of quantum mechanics. The first one is calculating time-evolution of the state when no-one is looking.

This makes no sense to me on a philosophical level. Either everyone is always watching or, what I believe, No one is ever "looking". I assume looking to mean observing, but observation is dependent on perfect senses, which we do not have.

 

[Turnevies]

This makes no sense to me on a philosophical level. Either everyone is always watching or, what I believe, No one is ever "looking". I assume looking to mean observing, but observation is dependent on perfect senses, which we do not have.

You give me an initial state and a hamiltonian (describes which forces are present) and I can tell you how the state evolves. This is in case of a closed system, and 'looking' at it would mean there is no closed system, because information needs to get transferred by something e.g. photons.

When you do perform a measurement (second part I mentioned), let's say of the position of a particle, it doesn't matter that our senses are not perfect. We could build a measurement device that is arbitrarily precise (as long as there is no combination of measurements violating heisenberg's uncertainty) and connect the device to a yuuuuuuuuge LED-screen that shows you as a human the result of the measurement.

 

[QuickTwist]

You give me an initial state and a hamiltonian (describes which forces are present) and I can tell you how the state evolves. This is in case of a closed system, and 'looking' at it would mean there is no closed system, because information needs to get transferred by something e.g. photons.

When you do perform a measurement (second part I mentioned), let's say of the position of a particle, it doesn't matter that our senses are not perfect. We could build a measurement device that is arbitrarily precise (as long as there is no combination of measurements violating heisenberg's uncertainty) and connect the device to a yuuuuuuuuge LED-screen that shows you as a human the result of the measurement.

So what you are saying is that on paper, there are no mistakes being made and in practice the assumption is so close that it doesn't really matter.

Correct me if I am wrong.

 

[Grayman]

This makes no sense to me on a philosophical level. Either everyone is always watching or, what I believe, No one is ever "looking". I assume looking to mean observing, but observation is dependent on perfect senses, which we do not have.

Looking and observing is often better stated as measuring. You cannot measure the photons path because it would 'interact' with the photon and destroy it or alter it. We are only able to measure were it ends up. The earaser experiment manages to take an extra measurement in the photons path by observing the paired photon instead of the original one. Yet, somehow measuring the paired photon affects the path of original even though it is locally isolated. Anyways, you cannot prove path because you cannot measure it. I am assuming 'time-evolution' is a reference to this path.

 

[QuickTwist]

Looking and observing is often better stated as measuring. You cannot measure the photons path because it would 'interact' with the photon and destroy it or alter it. We are only able to measure were it ends up. The earaser experiment manages to take an extra measurement in the photons path by observing the paired photon instead of the original one. Yet, somehow measuring the paired photon affects the path of original even though it is locally isolated. Anyways, you cannot prove path because you cannot measure it. I am assuming 'time-evolution' is a reference to this path.

The thing is, we are never going to fully understand. Whether there is true randomness or not is of no consequence because we are never going to get to the bottom of it. So in all practical purposes, true randomness is in fact a real thing. To understand in full would be to discover the origins (or cause) of the universe. So what you say about 'time evolution' is very true in this regard since because before the universe was caused there was no time, so there is no possibility to find out what caused it. It is the same principle here.. since we can only really measure where something (photon, sure) is now (which is actually fallible) we will never be able to predict where it ends up. It is much the same as our own lives. We can make a plan that seems infallible, but we all know that things do not always go according to plan. The reason I bring this up is because I feel that this type of observation (I joke) is really what the foundation of being human is all about. Discovering the micros and macros of the world does indeed have some benefit, but if we can't apply that to our everyday living it is somewhat in vain. Essentially I am saying to stay focussed on what is happening in your (our) everyday lives because that is of utmost importance in how we conduct our lives. After all, how differently are we really than the micros and macros of the world? We are essentially the same, so why not focus on where we are at instead of up or down?

/cheesy rant

 

[Turnevies]

So what you are saying is that on paper, there are no mistakes being made and in practice the assumption is so close that it doesn't really matter.

Correct me if I am wrong.

I mostly agree. Look at it this way: on paper, it is at least possible to measure without mistakes. So a perfect measurement is the extreme example of a measurement, even if another measurement is certainly imperfect, the overall picture doesn't change, the result of an imperfect measurement could seem like a 'weaker' manifestation of the phenomena you observe in the 'perfect' measurement. (and to make things even more complicated, any imperfect measurement of a quantity you want to measure is in fact a perfect measurement of some other unknown quantity that you didn't want to measure).

Looking and observing is often better stated as measuring. You cannot measure the photons path because it would 'interact' with the photon and destroy it or alter it. We are only able to measure were it ends up. The eraser experiment manages to take an extra measurement in the photons path by observing the paired photon instead of the original one. Yet, somehow measuring the paired photon affects the path of original even though it is locally isolated. Anyways, you cannot prove path because you cannot measure it. I am assuming 'time-evolution' is a reference to this path.

Agreed. Time evolution refers to the paths, but is more general. Even if particles are not always moving on a single physical path, the state of the whole quantum system is always well-defined and evolves in time.

 

[QuickTwist]

I mostly agree. Look at it this way: on paper, it is at least possible to measure without mistakes. So a perfect measurement is the extreme example of a measurement, even if another measurement is certainly imperfect, the overall picture doesn't change, the result of an imperfect measurement could seem like a 'weaker' manifestation of the phenomena you observe in the 'perfect' measurement. (and to make things even more complicated, any imperfect measurement of a quantity you want to measure is in fact a perfect measurement of some other unknown quantity that you didn't want to measure).

I get what you are saying. True, perfect measurements get exponentially more difficult to maintain as the process becomes more and more complex, but that would imply that we are searching above and beyond what we are capable. No one who is a competent mathematician and not crazy would say 2+2 = 3.9, but that is what we are doing with all this advance mathematics and measurements. It is heuristics. To some degree I can see the merit of it. Progress can't be made unless you go beyond your bounds, so naturally imperfections are going to occur. I guess it is not even a matter of the heuristics that is a problem for me. Its more a problem with the motivation for wanting to go beyond your bounds. That said, change is good and I think coming up with alternate ways of finding things out is good as well. But I think its the act of changing rather that the strive to accomplish that I like most of all.

I am pretty sure people are going to think I am nuts for saying what I said given this topic was originally about science, but I am not a scientist.

 

[Grayman]

I get what you are saying. True, perfect measurements get exponentially more difficult to maintain as the process becomes more and more complex, but that would imply that we are searching above and beyond what we are capable. No one who is a competent mathematician and not crazy would say 2+2 = 3.9, but that is what we are doing with all this advance mathematics and measurements. It is heuristics. To some degree I can see the merit of it. Progress can't be made unless you go beyond your bounds, so naturally imperfections are going to occur. I guess it is not even a matter of the heuristics that is a problem for me. Its more a problem with the motivation for wanting to go beyond your bounds. That said, change is good and I think coming up with alternate ways of finding things out is good as well. But I think its the act of changing rather that the strive to accomplish that I like most of all.

I am pretty sure people are going to think I am nuts for saying what I said given this topic was originally about science, but I am not a scientist.

We have always accepted a level of uncertainty and used it to gauge what is plausible. Technology is advancing drastically and our ability to measure with more accuracy is increasing.


I find PMT photomultiplier tubes to be an amazing discovery. To be able to measure a single photon is amazing. It shows real progress and has really focused new light on how these experiments are operating.

 

[QuickTwist]

We have always accepted a level of uncertainty and used it to gauge what is plausible. Technology is advancing drastically and our ability to measure with more accuracy is increasing.

I would like to hear you thought on this Grayman.

I agree that uncertainty inherently breeds knowledge. So then the question is, are we staying uncertain enough to keep looking elsewhere or simply finding new ways to do the same thing? Along with this, I cannot deny that technology is getting more developed drastically and that has an effect on our ability to measure. But are we just measuring the same thing in different ways? From what I can tell, its been about atoms and now waves. This is very scientific stuff and gives us an aptitude to learn things on a small level. But like I said before, unless that translates into the level we do things as animals what is the real benefit there?

I am sorry if I am forcing this off on a tangent, but I just want to know what the practical application of knowing all this science stuff is besides it ending up just resulting in a more comfortable living arrangement. I guess I am asking about what your thought are on what this discovery is going to do for us far into the future of our species. Are we actually getting smarter or just more knowledgeable?

 

[Grayman]

I was referring to the term 'measurement uncertainty' not skeptism or doubt.

I am working on a response in my head to your concerns in what physics should be concerned with but I am afk and working. I can only provide short responses on break from my phone.

 

[Grayman]

Holy shit: http://arxiv.org/pdf/1305.6822.pdf

I'm probably the only one excited but hell I am enjoying it for all of us or me I guess...


The waves of the liquid interact even though the particles or drops are separated. This violates bells inequality and manages entanglement. It does everything QM can do but using classical physics. This cannot just be coincidence.

In this paper we show that Bell’s inequality can be violated in a completely classical system. In fluid mechanics, non-local phenomena arise from local processes. For example, the energy and angular momentum of a vortex are delocalised in the fluid. Here we show that Euler’s equation for a compressible inviscid fluid has quasiparticle solutions that are correlated in precisely the same way as as the quantum mechanical particles discussed in Bell’s original paper. This correlation violates Bell’s inequality.

We conclude that Bell's analysis does not exclude the possibility of purely local interactions underlying and explaining quantum mechanics.

Thanks @Turnevies. I got this from phsyicsforums.com. A wealth of stuff there for sure. I set up an account.