
Oil droplets guided by pilot waves do not give rise to double-slit interference - erik_landerholm
https://www.quantamagazine.org/famous-experiment-dooms-pilot-wave-alternative-to-quantum-weirdness-20181011/
======
mehwoot
Why does this matter? Does anybody seriously question whether the basic math
can work? So somebody found a nice parallel between pilot wave theory and
another physical phenomenon, but then the parallel wasn't as strong as
thought. This experiment is the equivalent of arguing that light isn't a
particle because your tennis ball doesn't bounce off the wall the way you
thought it would.

When I saw the droplet experiments, I thought "that's cool, but it doesn't
prove anything about pilot wave theory because the arguments are over much
deeper things than what this is demonstrating". Debunking it is just as
meaningless. The people who actually know about this stuff are arguing over
determinism and non-locality and what not. I thought oil droplets are just a
nice way to introduce people to the concept.

Disclaimer: I don't really know anything about any of this

~~~
stupidcar
It matters because the theory in this case wasn’t that both light and tennis
balls behave like a particle. It was that light behaves like a tennis ball. As
such, demonstrating that tennis balls succeed or fail to display certain
light-like behavior is certainly an important result.

So, the most controversial aspect of pilot wave theory is the idea that a
classical system can reproduce some phenomena considered only to arise in a
quantum one. The 2006 result was important because it appeared to definitively
prove a classical system could do just that, and for one of the most famously
“weird” quantum results. How accurately it correlated with a hypothetical
pilot wave based quantum reality wasn’t really relevant. And if the original
result was important, then the failure to reproduce it is also important.

More generally, scientists can argue about theory, mathematics and the “deep”
stuff until the cows come home. But what actually advances science is
experiment. It may not always be the most glamorous or exciting work (CERN
notwithstanding). It can be an expensive, years-log slog to add a single
negative data point against an idea a theoretician dreamt up in an afternoon,
and which they’ll dismiss as irrelevant, or avoid by tweaking a single
equation. But it’s, quite literally, the part of science that gets results.

~~~
whatshisface
One excercise that is fun to do: reduce the Schrodinger equation by splitting
the real and complex parts, the figure out how to build a damped spring system
that can support the wave. My interpretation of quantum mechanics is that
everything is springs and dashpots in the end.

~~~
mileszim
This is great, I think I'm going to explain it using your method from now on.

Edit: I love that there's a segment of people in the QM community approaching
these "very serious" thought experiments like Schrodinger's Cat (as the
classic example), instead using absurdism to produce equivalent but blatantly
ridiculous results. Like the Surrealist reactionary art movement but with the
often insane results QM produces. Sometimes I think string theorists need a
group kind of like this too.

~~~
matthewwiese
My favorite of these types of bizarro QM communities is that of the
"holographic fractal" universe. Nassim Haramein[1] being one of the popular
figures of this movement. If you're willing to sift through the mysticism, the
r/holofractal subreddit has some interesting ideas to chew on during an
otherwise dull lunch break.[2]

[1] [https://resonance.is/about-haramein/](https://resonance.is/about-
haramein/)

[2]
[https://old.reddit.com/r/holofractal/](https://old.reddit.com/r/holofractal/)

------
branksy
> _Unsustained by the particle or droplet, the wavefront disperses long before
> reaching its slit, and there’s no interference pattern. The Danish
> researchers verified these arguments with computer simulations._

Wait, what? From what I've always understood, the math behind de Broglie-Bohm
interpretation results in the same exact results as the Copenhagen
interpretation. It shouldn't be possible for any "computer simulation" to
disprove it, by definition.

This article feels 1) pointless and 2) like it has an agenda. Oil droplets are
a macro-scale approximation of something on the particle level where we
already know the math works. This doesn't disprove anything, any more than
doing experiments with rubber sheets and basketballs lets you disprove the
general theory of relativity.

> _crushing a century-old dream that there exists a single, concrete reality._

This is just sensationalist, cheap journalism. I expected far better from
Quanta Magazine, and I'm disappointed in them. I've enjoyed many of their
articles in the past, but I'm not sure I can trust them editorially any more
if they print something so obviously incorrect as this article.

~~~
andrewla
> Wait, what? From what I've always understood, the math behind de Broglie-
> Bohm interpretation results in the same exact results as the Copenhagen
> interpretation.

The de Broglie-Bohm interpretation is not the same as the pilot-wave theory,
which was never fully fleshed out. de Broglie-Bohm is valid and makes the same
predictions as the standard interpretation, but as I understood it, has never
been generalized to the relativistic versions of the standard theory. That is,
there is no de Broglie-Bohm version of quantum field theory.

This is discussed in the article; the de Broglie-Bohm interpretation relies on
a global pilot wave, which has non-locality issues that pilot wave theory
hoped to avoid.

> It shouldn't be possible for any "computer simulation" to disprove it, by
> definition.

Here I believe they are disproving the idea that the oil droplets riding the
pilot waves cannot result in the double-slit interference pattern, using
standard fluid mechanics (which appears to be Tomas Bohr's area of expertise).

~~~
potiuper
> The de Broglie-Bohm interpretation is not the same as the pilot-wave theory,
> which was never fully fleshed out.

Most contexts
([https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory](https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory))
define pilot-wave theory and the de Broglie-Bohm interpretation as the same.
de Broglie himself also proposed a specific pilot-wave theory that is not
valid (see article). When referring to a "pilot-wave" theory it is helpful to
state specifically who the author is in order to resolve any likely confusion,
which this topic usually seems to involve.

> has never been generalized to the relativistic versions of the standard
> theory

Quantum field theory generalizations(
[https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory...](https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory#Quantum_field_theory"))
do exist.

------
twtw
Bohm's pilot wave theory is interesting to me not because it is so intuitive
or tasteful or something, but because it works without sacrificing realism,
and thus demonstrates that the Cophenhagen interpretation (neither local nor
real) claims strictly more about the nature of the universe than is required
by the evidence we have. Nonlocality is established by experiment, non-realism
is not. In my experience, this tends to be underappreciated (or I'm
misunderstanding - please teach me if so).

I personally favor epistemological interpretations, in which the wave function
models our knowledge, and the universe operates via an as-yet-unknown
dynamics. Wave function collapse on measurement isn't weird anymore if the
wave function represents your knowledge of the system. Thinking about it this
way makes "shut up and calculate" work nicely in my head.

~~~
naasking
> I personally favor epistemological interpretations, in which the wave
> function models our knowledge

These have been largely ruled out via no-go theorems [1].

[1]
[https://en.wikipedia.org/wiki/PBR_theorem](https://en.wikipedia.org/wiki/PBR_theorem)

~~~
acjohnson55
Could you explain this further? The Wikipedia article doesn't go into much
depth, especially w.r.t. de Broglie-Bohm interpretation.

~~~
naasking
It has no effect on Bohmian mechanics because it posits a real wave function.
PBR rules out most "psi-epistemic" models where the wave function represents
only our ignorance of the system, and not some fundamental, real property of
the system.

------
inciampati
There is recent research on photons (rather than bouncing oil droplets) that
backs up the Bohmian interpretation. "Experimental nonlocal and surreal
Bohmian trajectories"
[http://advances.sciencemag.org/content/2/2/e1501466](http://advances.sciencemag.org/content/2/2/e1501466)

I am not sure if the different QM interpretations are only a matter of taste
in terms of all predictions. These surreal trajectories are seen as nonsense
under the Copenhagen interpretation.

------
naasking
I don't think this is all that surprising to anyone. The "pilot waves" in the
oil bath aren't superluminal, so of course they don't correspond with
perfectly QM pilot waves, ie. they will only correspond in scenarios where the
non-locality doesn't matter.

Edit: the article actually agrees with this, so it's hyperbolic all the way
through except this critical passage:

> In a quantum reality driven by local interactions between a particle and a
> pilot wave, you lose the necessary symmetry to produce double-slit
> interference and other nonlocal quantum phenomena. An ethereal, nonlocal
> wave function is needed that can travel unimpeded on both sides of any wall.

------
Lerc
So, am I right in thinking that we have.

    
    
        1. Theory X hypothesizes Q
    
        2. Phenomenon Y models theory X
    
        3. Phenomenon Y does not demonstrate Q
    
    

Surely there would be an insurmountable burden of proof on 2. Similarity does
not mean identical. For any Y claiming to be the same as X, why can't it be
rebutted with a "No it isn't" Proving that they are the same would be a
different thing altogether but if as the article suggests X is not fully
defined, it strikes me as impossible to prove. It just ends up in the not-
falsifiable bin.

~~~
dumbfoundded
That's a fair observation. Though, I think the thought experiment to about the
distance between the two slits as irrelevant to QM more concretely disproves
the pilot wave theory.

Most physicists when asked about QM will just say that this is a useful model
to predict the outcomes of experiments. In fact the most successful model,
ever. The magic that happens in the middle is currently untestable and thus
uninteresting beyond philosophy.

The problem with determinism is that it introduces more complexity to the
model without improving any accuracy. Moreover, it doesn't help us predict the
outcome of any experiments.

Even if this experiment was successful, who cares? Does it change the math or
predictions? Science works in a very simple way. We have evidence about how
the universe works. We make models to explain that evidence. The models also
make predictions about new evidence that we would expect to see. New evidence
appears and either confirms or limits the domain of the model. Basically
nothing but gravity has limited the domain of QM.

~~~
naasking
> Though, I think the thought experiment to about the distance between the two
> slits as irrelevant to QM more concretely disproves the pilot wave theory.

This sort of experiment does not disprove pilot waves.

> Science works in a very simple way. We have evidence about how the universe
> works. We make models to explain that evidence. The models also make
> predictions about new evidence that we would expect to see. New evidence
> appears and either confirms or limits the domain of the model

This is a very anemic view of the scientific process. Scientific theories have
predictive and explanatory power. You described the process covering only the
former, but the latter is _critical_ for advancing theory.

For instance, without Bohmian mechanics John Bell probably would never have
developed Bell's Theorem and all the subsequent no-go theorems which have
dramatically improved our understanding of quantum foundations.

Theories that provide a mental framework for thinking about problems guide you
in devising new experiments. Theories with no explanatory power provide no
meaningful framework within which to devise new experiments.

------
m12k
I'm a bit sad that they couldn't reproduce the double slit interference
pattern. I'm still holding out hope that we'll eventually develop a model of
reality where the apparent indeterminism of quantum mechanics can be explained
by currently unobservable interactions in higher dimensional space (such as
the space described here: [https://www.quantamagazine.org/physicists-discover-
geometry-...](https://www.quantamagazine.org/physicists-discover-geometry-
underlying-particle-physics-20130917/) )

~~~
whatshisface
What you're describing has been desired by many (including Einstein). However,
John Bell more or less proved it impossible[0] in a landmark, sweepingly
general result. He would have won a Nobel for it if he had lived long enough.

On the other hand, if you are willing to accept superluminal messages and also
unobservable mechanics, there is always the pilot wave interpretation[1]. The
fluid analogy is disappointing to see go but pilot wave theory lives on.

[1][https://en.wikipedia.org/wiki/Pilot_wave_theory](https://en.wikipedia.org/wiki/Pilot_wave_theory)
[0][https://en.wikipedia.org/wiki/Bell%27s_theorem](https://en.wikipedia.org/wiki/Bell%27s_theorem)

Edit: fixed order of references.

~~~
m12k
Thanks for the link to Bell's Theorem (I recalled there being a result like
that, but didn't remember the details of it). If the hidden variables really
are in a higher dimensional space, then maybe superluminal communication (in
our observable dimensions of space) isn't off the table either? But that comes
with its own can of worms that would have to be taken care of as well.

~~~
whatshisface
If you give me both sides of a superluminal telephone I can use it along with
special relativity to set up a time paradox. If I only have one end of the
phone, or if I can't have a phone at all (not all apparently superluminal
interactions allow signaling) then it's considered an undesirable trait but
not paradoxical.

~~~
effie
By paradox, do you mean something like reverting order of events in some
inertial frames?

~~~
whatshisface
The order of events is fair game to reverse so long as they are far enough
away from each other that one couldn't have caused the other. The order of
pairs of events like that reverse all the time. It's only a time paradox if
you can actually do something paradoxical, like killing your grandfather.

~~~
effie
> if you can actually do something paradoxical, like killing your grandfather.
> ... before certain crucial event in his life, you mean, I assume.

How exactly does having a superluminal telephone imply I could change history?
Changing history is the paradoxical part - don't assume it can happen.

~~~
whatshisface
Here is an article that goes in to the detail[0]. In so many words, you can
use the superluminal telephone to transmit information to your own past. In
essence, there is a certain amount of freedom in the simultaneity of events
that is well-known to exist (having been experimentally verified). You can
abuse it to create time paradoxes if and only if you can send messages faster
than light - without FTL it is a curiosity that seems strange to some but
otherwise doesn't upset anything.

[0]
[http://www.theculture.org/rich/sharpblue/archives/000089.htm...](http://www.theculture.org/rich/sharpblue/archives/000089.html)

~~~
effie
Thanks for the link. See also the comments below the essay which give some
valid criticisms.

------
User23
For some background on pilot waves:
[https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory](https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory).

Also from the same source, a couple years back:
[https://www.quantamagazine.org/pilot-wave-theory-gains-
exper...](https://www.quantamagazine.org/pilot-wave-theory-gains-experimental-
support-20160516/)

------
GregBuchholz
Does anyone have a link to a video of the oil drop tunneling experiment?

[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.10...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.102.240401)

(like
[https://www.youtube.com/watch?v=nmC0ygr08tE](https://www.youtube.com/watch?v=nmC0ygr08tE))

------
clydesdale
So, pilot wave theory remains a strong intuitive model, by confirming an
inability to perfectly predict the future state of a system of quantum events.

------
gpm
> droplets were seen to tunnel through barriers

If true, one wonders if you could use this to build a d-wave like annealer.

------
snarfy
I don't understand why non-locality is a problem. Fields are non-local. If
particles are point-like, fields are volume-like and permeate all of space.
Problems like spooky action at a distance aren't really problems if there is
no distance from the field's point of view.

~~~
danenania
Why isn't there distance from the field's point of view? Isn't the field
spread throughout space?

~~~
snarfy
But it's still considered one field, not many individual fields at each point.
That's why I called it volume-like. The electric field of the universe doesn't
have a position. It has a volume.

------
mncharity
"Visualizing pilot-wave phenomena: the $60 rig"
[http://math.mit.edu/~bush/?p=3230](http://math.mit.edu/~bush/?p=3230)

------
phkahler
Has anyone done the double slit experiment with the long wall before the
slits? I'm not entirely convinced it still works unless it has been
demonstrated.

~~~
eggie
That would be something to directly ask Prof. Bohr who's mentioned in the
article.

------
soveran
All the paradoxes disappear if we use Quantum Field Theory instead of Quantum
Mechanics. I would really like someone with deep knowledge on Quantum Theory
to explain if something is wrong with a theory that otherwise makes a lot of
sense to me. A good read on the topic is the paper "There are no particles,
there are only fields" —
[https://arxiv.org/pdf/1204.4616.pdf](https://arxiv.org/pdf/1204.4616.pdf)

~~~
abdullahkhalids
Surprisingly, physicists are very much aware of the field nature of all
"entities" in the world, having invented QFT themselves. Even more
surprisingly, popular journalistic explanations simplify things considerably,
making it seem that the thoughts of physicists are much less refined than they
really are. To complete the trifecta of surprises, in the refined thought
process of us physicists, using QFT, many problems and paradoxes of
interpretational nature or otherwise remain with quantum theory that need to
be resolved.

~~~
perl4ever
Regarding "in the refined thought process of us physicists...many problems and
paradoxes...remain".

It seems to me that a true paradox in physics must mean a theory provides at
least two different incompatible predictions for a given physical situation.
Given the success of the Standard Model, that surprises me. Do you have an
example?

~~~
whatshisface
The use of the word "paradox" in physics usually means any conclusion that is
considered unacceptable or unnatural for any reason, even intuitive. In logic
there are no paradoxes, only contradictions. As far as we know there aren't
any contradictions in the standard model, but there are plenty of suspicious
conclusions that we would like to see resolved either with greater
understanding or a better theory.

------
pg_bot
I was thinking about the double-slit problem the other day, and was trying to
make connections to any other phenomenon that exists in the natural world and
I think I came up with one.

In conditional probability theory, there exists something called the boy or
girl paradox. The paradox shows that the likelihood of an event can be
drastically affected by _how you know_ even seemingly innocuous information. I
think that observation of the atoms passing through the slit is affecting the
search space for the other atoms in such a way that we currently do not
understand. This observation while seemingly innocuous has a drastic affect on
the distribution which would explain why we see such confounding results. It
follows the same logic as this other naturally occurring phenomenon so I think
its an avenue at least worth exploring.

An example of the paradox would be if I told you that my neighbor has 2
children and at least one of them is a boy. If I asked you what the
probability that their other child is a girl, normally the result would be 2 /
3, but if I know one child is a boy because I spot him in front of a tree then
the odds his other child is a girl drops to 1 / 2.

[0]:
[https://en.wikipedia.org/wiki/Boy_or_Girl_paradox](https://en.wikipedia.org/wiki/Boy_or_Girl_paradox)

~~~
TeMPOraL
I'm having a fundamental problem understanding why the paradox is modeled the
way it is.

"Mr. Smith has two children. At least one of them is a boy. What is the
probability that both children are boys?"

For some reason, it's expanded as a choice from {BB, BG, GB, GG}. But why on
Earth does the order of children matter in this question? The way I see it, it
should be modeled as {both-boys, boy-and-girl, both-girls}; by telling me that
one of the kids is a boy, you eliminated one possibility out of three, leaving
us with 1/2 as an answer.

~~~
function_seven
Let's say I flipped two coins. I tell you I did this. You ask me, "Did heads
come up at least once?" I answer yes.

What are the odds that the other coin was tails? 2/3, because there are three
scenarios (HH, HT, TH) that satisfy my first answer to you.

If you flip a pair of coins a few thousand times, and search the flip pairs
for all the ones that had a heads among them, you'll find a tails in those
pairs 2/3 of the time, and a pair of heads 1/3 of the time.

Same with the girl-boy problem.

Now if you ask me if the _left_ coin came up heads—and I say yes—that doesn't
give you any information whatsoever on the right coin. And if you search
through the thousands of pair flips looking for an "H" in the left column, the
right column will 50/50 have an H or a T.

~~~
titzer
That's a good explanation.

> Now if you ask me if the left coin came up heads—and I say yes—that doesn't
> give you any information whatsoever on the right coin.

Exactly. "At least one" gives information about the _whole set_ , implicitly
giving information about the second one whereas "the first one is X" gives no
information about the second one.

