
Why don't more physicists subscribe to pilot wave theory? - rfreytag
https://www.quora.com/Why-dont-more-physicists-subscribe-to-pilot-wave-theory?share=1
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lisper
This is the bottom line:

"It's been demonstrated experimentally that you can't have a local,
deterministic, real, definite theory (i.e. that would lead to demonstrably
false conclusions). People intuitively expect all of those things to be true,
but we can prove that it isn't the case. The various interpretations all try
to give up one of "local", real", "definite", or "deterministic" in order to
preserve the others, thus preserving at least a modicum of their intuition.

The pilot wave idea gives up on locality"

And I will add:

Multiple worlds gives up on definiteness.

"Zero worlds" (my personal favorite) gives up on realism.

And, of course Copenhagen gives up on determinism.

Take your pick. Or go with decoherence, which kind of lets you turn a knob to
give up a little bit of all four and dial in whatever setting you like.

~~~
phkahler
But if someone makes a conscious choice to choose one that preserves locality,
shouldn't they change their mind when "spooky action at a distance" starts
happening? Non-locality is staring them in the face and they refuse to adopt a
formalism that makes it explicit. They seem to like things that appear strange
and non-intuitive.

~~~
combatentropy
I agree. I thought that the scientific world already gave up on the idea of
locality, since entanglement has been experimentally demonstrated.

Furthermore, I don't have a problem with accepting non-locality from a
theoretical perspective either, now that I know about the Holographic
Principle. If the universe is a hologram, then the apparatus that is
projecting the hologram can be connected in ways that aren't obvious in the
hologram.

~~~
otakucode
It's not that kind of a hologram... all the hologram idea says is that all of
the information in our universe can be represented on a 2 dimensional surface.
It doesn't claim there is literally a hologram projector somewhere projecting
a hologram of our universe. Such a thing would be nonsensical. If the
projector were outside our universe it would, by definition, be causally
disconnected from our universe and thereby never capable of influencing it in
any way. If it were capable, it would, again by definition, be part of our
holographic universe.

~~~
combatentropy
Just because something is outside something else does not mean it is causally
disconnected.

The computer program runs inside the computer. The computer is outside the
program. But nevertheless the computer's hardware is causally connected to the
program. Further still, the programmer is outside the program, but he very
much can influence it. Indeed, he brought it into being.

~~~
Retra
The idea is that to be outside the universe is to be causally disconnected
from it, not to be outside of some arbitrary system. A "universe" is not an
arbitrary system -- it is the whole of reality.

A computer program is not a universe independent of the programmer. They both
exist in the same universe _because_ they are causally connected. If they were
in different universes, they must be causally disconnected. You're whole
argument rests on the assumption that the program and the programmer reside in
different universes, which is clearly not the case.

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tzs
Would this be kind of the physics equivalent of nonstandard analysis in
mathematics? Nonstandard analysis (NSA) is an approach to analysis that
essentially makes rigorous the vague notions of infinitesimals that Newton and
Leibniz and the other pioneers of calculus based their work on.

Newton's fluxions and Leibniz's differentials provided great guidance, and in
a lot of cases they even worked and gave correct results, but they were not
really rigorously defined, and later mathematicians started running into
serious problems with that approach. Those were resolved with the approach
that we all now known and love (loath?): limits and the epsilon/delta approach
to them.

NSA extends the real field to contain a new kind of number that is positive
but is less than 1/N for any integer N, giving a new field called the
hyperreals. You can then do things like define the derivative of f at x to be
(f(x+h)-f(x))/h where h is an infinitesimal, work out that using ordinary
algebraic manipulations, and then just drop any terms that are infinitesimal.

NSA goes far beyond just calculus. You can use these ideas all through
analysis.

Seems pretty cool, and a lot clearer than a limits approach. So why has it not
taken over?

I believe that a big part of it is that it has been shown that NSA and
standard analysis are equivalent. Anything that you can prove in one can be
proven in the other. Perhaps more important, one doesn't seem any better than
the other at leading to deep insights. If you can't solve a problem in
standard analysis, you probably aren't going to figure out the solution if you
use NSA, and vice versa.

So while it might be overall nicer if we were starting from scratch to use NSA
instead of standard analysis everywhere, so much is already done in standard
analysis that switching now would be too painful.

~~~
millstone
Interesting analogy but I'm not sure if it holds. Infinitesimals are an
alternative to the limit-based formalism typically seen. It changes the way
you do calculus.

However Bohmian Mechanics is a different interpretation, not a different
formalism. You can accept the interpretation, but you still end up solving the
Schrödinger equation in the same way, using the same mathematical methods.

The analogy works better for matrix mechanics as compared to Schrödinger's
wave mechanics. These are mathematically quite different even though they give
the same results.

~~~
naasking
You can use the same formalism for most things, but Bohmian mechanics is a
different formalism in some cases. For instance, you can derive the Born rule
for systems in equilibrium, but that leaves open the possibility of non-
equilibrium regimes, which simply don't exist in orthodox QM.

~~~
qubex
Very astute, I missed the implication that Pilot Wave theory has transient
non-equilibrium states. Thanks for mentioning that.

~~~
naasking
An introduction for the interested [1]. It would be neat if someone could come
up with an experimental test. The very existence of non-equilibrium would
immediately falsify Copenhagen.

[1] [https://en.wikipedia.org/wiki/Quantum_non-
equilibrium](https://en.wikipedia.org/wiki/Quantum_non-equilibrium)

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JshWright
If you're interested in this sort of thing as a lay person (as I am), I would
recommend the PBS Space Time YouTube channel. Be sure to stick around for the
end of each video, as he often calls out comments on earlier videos that
contain corrections or clarifications (in many cases, from _very_ highly
qualified physicists and other subject matter experts).

Here's their recent episode on pilot wave theory:

[https://www.youtube.com/watch?v=RlXdsyctD50](https://www.youtube.com/watch?v=RlXdsyctD50)

~~~
mblaney
Thanks for the video. He mentioned that the pilot wave is made up of "some
stuff", but never got back to that being one of the unknowns of pilot wave
theory. Seems like that questioned will need to be answered before it is taken
more seriously.

~~~
combatentropy
> He mentioned that the pilot wave is made up of "some stuff"

Could the stuff be dark matter or dark energy?

~~~
naasking
Pilot waves aren't made of anything, they are a description of configuration
space. Think of it like this: to describe particle motion, take the
ordinary/classical equations of motion, and now add another term to account
for quantum behaviour. The force exerted by this term at time t is a function
of the position of every other particle in the universe at time t, no matter
the distance.

That's pilot wave theory, where this term is sometimes called the "quantum
potential" and is governed by Schroedinger's equation. As you can see, this
extra term goes to zero when quantum effects become negligible and we recover
classical mechanics straight away. The difficulty is the obvious nonlocality
and what that means for special relativity.

~~~
bmh100
Could this connection to the rest of the universe be considered conceptually
similar to a form of entanglement?

~~~
naasking
Sort of. There is no real entanglement in this formalism, not in the way it's
typically portrayed. I suppose you could say entanglement is there all the
time. Macroscopic observation of entanglement via non-local correlations is
really just a context in which we make the non-local connection observable.

------
rwallace
The impression I have of pilot wave theories is that when you look closely at
them, the pilot waves are doing basically all the work with the particles just
along for the ride. At that point it's tempting to delete the particles and
see what happens, and then when you give the result a fresh paint job it turns
out to be isomorphic to the many worlds interpretation (or the Copenhagen
interpretation if you take the computational step of not following branches
that can no longer be significant to currently observed results). Am I missing
something?

~~~
Avshalom
It's important to remember that the math of quantum mechanics, the math people
use to model and predict phenomena, the math thats given us transistors,
lasers, quantum computing etc...

That's all very accurate and extremely successful so anything that tries to
explain why these models are the way they are is going to be pretty much the
same as any other explanation if you tilt your head to the side and squint
because they all have to produce the same very well specified set of
mathematical models.

------
scythe
I have a fond memory of taking an advanced (graduate) seminar class with
taught by one of the oldest professors at my university -- and attended by
another. The lecturer was describing something to do with manifolds and
various obscure formulations of classical field theories and he mentions that
he was at a lecture once where someone had related this theory to Bohmian
mechanics -- the other professor cut him off

"It's _wrong_ "

A student held up his hand and asked what Bohmian mechanics was. Before the
lecturer could answer

"Don't worry, it's wrong"

It was difficult to contain myself.

~~~
dmichulke
Related:
[http://www.phdcomics.com/comics.php?f=1912](http://www.phdcomics.com/comics.php?f=1912)

------
lamontcg
The biggest problem with pilot wave theory is that extending it to field
theory hasn't been done (or at least there's several published formalisms with
no approach that everyone agrees upon). Right now its useless for practical
physicists at the LHC to compute anything about collisions. Its mathematically
equivalent to the other mathematical formulations of QM, but the only utility
it offers so far is entirely philosophical. And at some point physicists don't
spend their entire lives noodling on the underpinnings of QM and need to move
on and really try to compute the g-factor of the electron, or analyze the
collision data coming out of the LHC. There needs to be a Lorentz invariant
field theory model of pilot waves that can answer some question easier than
the other mathematical formalisms for it to catch on.

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contravariant
I would have though the fact that the pilot wave theory uses the (multi-
particle) Schroedinger equation, while most modern physics is based on field
theory, would be more important. So far I've seen no formulation of the pilot
wave theory in a way that incorporates fields, or provides some other
mechanism to account for things like the annihilation of particles.

~~~
dnautics
If I'm not mistaken, It's just formalism. Mathematically a continuous wave
function is equivalent to a countably infinite vector space.

It can be hard to switch between the two (as a chemist we are taught Schroeder
and I have a hard time wrapping my head around Heisenberg)

~~~
chestervonwinch
> a continuous wave function is equivalent to a countably infinite vector
> space

I think you mean that the function _is a point_ in the vector space. More
precisely, it is a point in a particular separable Hilbert space, i.e., a
complete vector space equipped with an inner product that has countable, dense
basis.

~~~
dnautics
right, sorry. The space of continuous wave functions are equivalent to a
countably infinite vector space.

------
jostylr
I think the most problematic issues is that physicists reject the wave
function. I have never understood this since if you reject the wave function
in standard quantum mechanics, you have nothing left.

But the pilot wave is just the wave function of quantum mechanics, albeit on a
universal scale. It is in all the theories. And just because one has a
"collapse", which is always approximate anyway (no delta functions for a
position measurement, for example), one still has a wave function as the only
object in standard QM and that is an expanding object defined on 3*number of
particles in universe. But we do not experience that. We experience 3
dimensional space with rather point-like objects moving about.

Pilot wave theory starts with "We have particles moving about. How do they
move?" That's the question it starts with and it answers it in the simplest
way possible given the wave function: the wave function tells the particles
how to move using Bohm's equation which is a very easy thing to derive.
Indeed, you can derive it even more quickly than Schrodinger's equation from
the same basic facts of Einstein's light quanta hypothesis and de Broglie's
hypothesis.

It was a choice to inject mysticism into quantum mechanics. If you stick with
trying to describe the evolution of particles, Bohm is natural. If you want to
describe something else whose evolving configuration would give us our
experience, then that is fine, but you have to say what that is. The standard
interpretation does not as it is solely concerned with experiments and
measurements with no definition of them in a fundamental way. It is more like
they were trying to do regression fitting on experimental data without any
understanding of what the underlying stuff was nor any interest in such a
question.

The other difficulty is spin. Spin is trivial if you put the spin degrees of
freedom in the value space of the wave function. But many think of spin as
real as position. It is not. It matters what the experiment is; the spin value
for a particle is not defined independent of the experiment.

Bohmian mechanics gives a theory while standard quantum mechanics gives a
computational formalism. Depending on your goals, the latter can be
sufficient. The former tells us why stuff happens and also allows us to derive
that computational formalism.

------
MichaelApproved
Here's another great video from PBS Spacetime on this topic
[https://youtube.com/watch?v=RlXdsyctD50](https://youtube.com/watch?v=RlXdsyctD50)

This channel and Sixty Symbols are awesome for insight into physics/quantum
mechanics without the math.

------
snarfy
Here is my theory. Someone smarter than me please prove me wrong.

A particle's 'Zitterbewegung' motion generates wake fields in the vacuum
energy, causing the particle to generate its own pilot wave as energy
fluctuations in the vacuum.

[https://en.wikipedia.org/wiki/Zitterbewegung](https://en.wikipedia.org/wiki/Zitterbewegung)

[https://en.wikipedia.org/wiki/Vacuum_energy](https://en.wikipedia.org/wiki/Vacuum_energy)

[http://phys.org/news/2013-10-strange-behavior-pilot-wave-
dyn...](http://phys.org/news/2013-10-strange-behavior-pilot-wave-dynamics-
action.html)

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lcvella
For me, what is most important on the subject is that, even if we can't
differentiate from one interpretation to another experimentally (yet), it
doesn't mean they are all equally correct. Specifically in connection with
general relativity, which is an open issue, the exploration of alternative
interpretations of QM may lead to actual experiments differentiating one
interpretation from another, effectively tuning it into a theory.

------
bicubic
There's one major shortcoming that the answer doesn't seem to touch on: pilot
wave theory doesn't explain relativity whatsoever, while quantum field theory
does. We've experimentally confirmed relativity, so at best, pilot wave theory
is incomplete.

~~~
naasking
You're confusing quantum mechanics with quantum field theory. Pilot wave
theory is just QM, although some field theory extensions have been discussed.
There's no particular reason such an extension would be impossible.

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vorotato
I think the best answer would be, "If true it doesn't add anything new." At
least given our current understanding. If some new discovery showed some
measurable difference between the two interpretations then we'd have to
actually decide.

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phkahler
Since the pilot wave theory is deterministic, wouldn't someone be able to
simulate a quantum computer with a classical one in reasonable time and
storage?

~~~
oceanofsolaris
It is deterministic, but you still need to keep track of a field that covers
your whole configuration space (basically you need all the information
contained in a function R^n->C for n degrees of freedom). Keeping track this
high-dimensional field is what makes a quantum computer hard to simulate in
the first place, not the non-determinism. So no, it doesn't make anything
easier to simulate.

This makes a lot of sense if you think of it that way: Pilot wave theory gives
exactly the same results as any other decent interpretation of quantum
mechanics. So the work necessary to simulate it will also be exactly the same.
Simulating quantum mechanics is not hard because we don't understand it. It is
hard because it is an inherently hard problem (otherwise, how could quantum
computers be faster than classical computers?).

~~~
Ar-Curunir
Well we don't really know so far whether BQP is bigger than P.

~~~
oceanofsolaris
Okay, that is of course true :)

So replace "it is an inherently hard problem" by "it is most likely an
inherently hard problem".

------
ClayFerguson
There is one other interpretation of Pilot Wave theory that is sort of the
"inverse" of Pilot Wave. and to me makes more sense. The PW may be correct but
the particles are not following a wave that is moving around them. What is
happening is that each particle is 'emitting' (spherically and radially
outward from itself) a wave of its own. It is the interaction of all other
waves that cause the particle to move. Just like a particle with mass distorts
the spacetime around it, in a 'static way', perhaps charged particles distort
spacetime in a way that is directly tied to the speed of light, and thus
distort spacetime in a 'dynamic way'. But the fact that the slit experiment
"works" even when we know only a single particle at a time is being sent thru,
it just makes more intuitive sense that the particle is neither interacting
with itself, nor with other universes (copies of itself). So what i'm saying
is PW theory is FAR more intuitive than Copenhagen many-worlds to me. I
believe there is genuinely one particle going thru the slit experiment when we
think there is, and it follows the path thru in a way that is statistically
determines much more by a PW-type theory than any other theory.

