
Pilot-Wave Theory Gains Experimental Support - elorant
https://www.quantamagazine.org/20160517-pilot-wave-theory-gains-experimental-support/
======
sampo
As far as I understand – and Wikipedia seems to support this – pilot wave
theory, or actually any hidden variable theory, or actually any interpretation
of Quantum Mechanics, they are different mathematical models that produce
exactly the same predictions for experimental outcomes.

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

So how can, even in theory, any experiment support one interpretation any more
or less than all the others?

~~~
ewjordan
You're 100% correct - this is merely a direct presentation of the weirdness
that results if you choose to interpret this experiment in the language of
pilot wave theory. The physical results are the same as you would predict
using normal methods, and as such it's a real stretch to consider this
experimental support for Bohm's interpretation.

~~~
dandelany
One thing I've never been clear on is whether this is _empirically_ the case,
or _theoretically_ the case. That is to say, is it theoretically possible that
someday we may find evidence that supports Bohm's interpretation and falsifies
the others? Or are they truly mathematically equivalent theories?

~~~
ewjordan
If memory serves, as far as classical quantum mechanics goes, Bohm's
interpretation is 100% mathematically equivalent to the standard
interpretation, making it observationally impossible to distinguish the two
(much like many worlds vs. Copenhagen). It would be in extensions to the
theory where things might differ, though I don't know much about attempts to
extend pilot wave theory to QFT.

~~~
SEMW
> Bohm's interpretation is 100% mathematically equivalent to the standard
> interpretation, making it observationally impossible to distinguish the two

This is not quite correct.

It is mathematically equivalent to Copenhagen only at times of 'quantum
equilibrium', which is when ρ = φ² (probability density is the square of the
wavefunction). Copenhagenists call this the Born rule and treat it as
axiomatic. Bohmians don't since they don't need to, since in Bohmian mechanics
a system which is not in quantum equilibrium will evolve to be in quantum
equilibrium in a very very short time.

Which gives a way to empirically distinguish pilot-wave theory if the universe
began in quantum non-equilibrium. (We have no reason to think it did, but we
have no reason to think it didn't, so, shrug). If it did, the early universe
would have evolved differently in the first femtosecond after the big bang,
which should theoretically be detectable to us by analysis of CMB
anisotropies.

This has been worked on: a friend of mine did a masters project which among
other things involved porting some simulation code on very-early-universe
numeric simulations under quantum non-equilibrium (IIRC from fortran 77 to
fortran 95, because physicists), under Mike Towler.

------
btilly
The title is misleading.

Here is what the experiments actually showed. Quantum mechanics continues to
predict things correctly in an experiment that has a fun interpretation within
pilot-wave theory.

We also received the standard lecture on why pilot-wave theory makes more
sense than the standard Copenhagen interpretation. And a passing
acknowledgement of the many-worlds interpretation.

There is no explanation of the fact that many-worlds also explains the result,
also is mathematically simple, and lacks the explicit non-locality that is
troubling in pilot-wave theory. There is also no note of the fact that all
interpretations of quantum mechanics can explain all possible experiments that
fit the predictions of quantum mechanics. Therefore no experiment can support
one interpretation over another UNLESS the result is in contradiction to
quantum mechanics. (In this case the result was emphatically not in
contradiction to prediction.)

~~~
xlm1717
Replace non-locality with infinite universes, which is more troubling? The
article does touch on this question.

Additionally, the article's aim was to demonstrate that the ESSW does not
contradict pilot-wave theory, not to show that pilot-wave theory explains the
results better than alternative theories, and it demonstrated this by showing
that there is no contradiction if you take in account nonlocality.

The title is not misleading because the experiment described in the article
does provide experimental support to the theory, support which ESSW aimed to
take away. Importantly, it is not an experiment designed to show pilot-wave
theory describes the results better than other theories.

~~~
btilly
If the experiment does nothing to advance this theory over any competitor, or
vice versa, how can you say that it provides experimental support to the
theory?

ESSW demonstrated that pilot-wave theory requires a weird form of non-
locality. These experiments confirm that experiment requires the exact form of
non-locality that ESSW required. Whether non-locality bothers you depends on
your philosophical position, and is not affected by this experiment.

~~~
leeoniya
> If the experiment does nothing to advance this theory over any competitor,
> or vice versa, how can you say that it provides experimental support to the
> theory?

Non-contradiction of existing theories can be seen as a prerequisite for
support. As a restricted affirmation of non-wrongness perhaps.

~~~
jocro
>restricted affirmation of non-wrongness

I think this is a really good way to put it, and an important criteria given
the timescale over which these theories develop.

------
andybak
> According to Englert [...] the Bohm trajectories exist as mathematical
> objects but “lack physical meaning.”

That sounds strange coming from a defender of orthodoxy. By my (layman's)
understanding the rallying cry of the Copenhagen school could be paraphrased
as "Just do the maths. Everything else is just metaphysics" or to put it
another way - questions about what the equations 'mean' are unscientific and
Occam's Razor supports Copenhagen because it is the simplest interpretation
that doesn't contradict observation.

~~~
GregBuchholz
For another take on "Just do the math" see, "Clearing Up Mysteries - The
Original Goal" by E.T. Jaynes:

[http://bayes.wustl.edu/etj/articles/cmystery.pdf](http://bayes.wustl.edu/etj/articles/cmystery.pdf)

>While it is easy to understand and agree with this on the epistemological
level, the answer that I and many others would give is that we expect a
physical theory to do more than merely predict experimental results in the
manner of an empirical equation; we want to come down to Einstein's
ontological level and understand what is happening when an atom emits light,
when a spin enters a Stern-Gerlach magnet, etc. The Copenhagen theory, having
no answer to any question of the form: What is really happening when - - - ?",
forbids us to ask such questions and tries to persuade us that it is
philosophically naive to want to know what is happening. But I do want to
know, and I do not think this is naive; and so for me QM is not a physical
theory at all, only an empty mathematical shell in which a future theory may,
perhaps, be built.

~~~
int_19h
Of course, all such objections rest on the implicitly defined concept of "what
is REALLY happening". But trying to strictly define this - which is necessary
before we can meaningfully talk about it - is very tricky, and is firmly in
the metaphysical / philosophical realm, and not science.

~~~
naasking
I disagree. You seem to be asserting that ontologies don't drive empirical or
theoretical science, but this is obviously false. Einstein himself devised
relativity via thought experiments surrounding ontological properties he
believed made sense (such as the light speed barrier). Formulating theories
and testing them is exactly what science is all about.

~~~
int_19h
I'm not disputing that intuition can be a driver for empirical science, but
that's different from asserting that intuition overrides observable results.
If we do see multiple experiments showing that the world is non-deterministic,
trying to shoehorn it into a deterministic box because it "makes sense" to us
(which is really just a roundabout way to say that our ape brains are wired to
think that way because the portion of the physical world that we deal with has
that nature), and rejecting theories and interpretations on the grounds that
they are "troublesome", is not really science.

~~~
naasking
On the contrary, immediately accepting non-determinism simply because that's
what you seem to have measured is also unscientific. Or do you believe dipping
pencils in water actually breaks them, and then reconstitutes them when you
pull them back out? Don't mistake an illusion for reality.

We shouldn't simply naively accept what our experiment seem to be telling us,
we should always seek alternative avenues to explain the evidence. It can
yield compelling insights.

~~~
int_19h
Sure. But if we stick to the scientific method, then alternative avenues are
more experiments. With the pencil, for example, assuming that water actually
breaks them is a reasonable premise of the initial experiment, but then you
see that it's not actually broken - that's the second observation, disproving
the theory.

But if all your experiments say that it is non-deterministic, again and again
and again, Occam's razor approach is to say "yes, it is non-deterministic".
Creating an elaborate framework which basically lets you say "it's ACTUALLY
deterministic, except for all these other things that make it LOOK non-
deterministic for all practical purposes" is not really science, it's just
placating a preconceived (and possibly hardwired) notion.

------
GregBuchholz
Not necessarily directly correlated, but I couldn't resist posting about the
Couder/Fort oil drop experiments:

[https://www.youtube.com/watch?v=W9yWv5dqSKk](https://www.youtube.com/watch?v=W9yWv5dqSKk)

[https://arxiv.org/abs/1401.4356](https://arxiv.org/abs/1401.4356)

~~~
anotheryou
even prettier replication:
[https://youtu.be/YF5iHQMjcsM](https://youtu.be/YF5iHQMjcsM)

edit: and some double-slit
[https://youtu.be/nsaUX48t0w8?t=4](https://youtu.be/nsaUX48t0w8?t=4)

~~~
Frompo
This must be one of the most amazing fluid dynamics videos I've seem :)

------
joe_the_user
I know "Many Worlds" and Copenhagen are different _interpretations_ of Quantum
Mechanics. If Bohmian theory is one more interpretation, then we're talking
about explanations that give the same experimental results and thus we aren't
talking about a situation where an experimental result could prove one against
the other.

And it seems the experimental result described isn't a proof or disproof but a
"demonstration how this could make sense" \- a sort of "this analogy is still
valid here" claim of the type that isn't a proof or a disproof.

~~~
tremon
As I understand it, it is a "proof" that the ESSW paper does not disprove
Bohmiam theory. As to whether ESSW really did disprove it in the first place,
I agree with your point.

------
nhaehnle
_Some physicists are drawn instead to the Many Worlds interpretation of
quantum mechanics, in which observers in some universes see the electron go
through the left slit, while those in other universes see it go through the
right slit_

This phrasing is so unfortunate - a much better understanding of MWI comes
from looking at its original name, the "theory of the universal wave
function": there is only one universe, it just happens to be described by a
single wave function that describes the superposition of many different
possible states that the universe could be in.

The nice aspect of this theory is that it is mostly identical to the
Copenhagen interpretation, but resolves the problem of wave function collapse
in a perfectly clean way: there is no collapse, it's just that the act of
observation causes the observer to become entangled with whatever it is that
they're observing.

If you really want to talk about multiple universes, you should at least
clarify that it is the act of observation that causes the split between
universes. So it's not like there are multiple pre-existing universes, and in
some of them the electron goes left while in others it goes right. Rather, you
as the observer start it one pre-existing universe, and by the observation the
universe splits into a "left version" and a "right version". It's a subtle but
important difference.

~~~
rdtsc
> and by the observation the universe splits into a "left version" and a
> "right version"

Isn't observation just a particle interaction? Or do you mean a human brain
should be present in the loop to register the value.

~~~
TeMPOraL
From what I understand, the answer to your question is this part of GP's
comment:

> _the act of observation causes the observer to become entangled with
> whatever it is that they 're observing._

AKA particles interacting => particles get entangled. So they do not factor
out cleanly as independent from each other anymore.

------
exratione
Here's another alternative interpretation that's been around for a couple of
decades:

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

Though at a much earlier stage in the development of such things.

------
Aelinsaar
The only keyword in that article is the "If," before, "Results stand up to
scrutiny." That, if you're familiar with the nearly religious proponents of
deBB QM, is a BIG "if".

------
namuol
There sure are an awful lot of interpretations of quantum mechanics:
[https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...](https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#Comparison_of_interpretations)

:)

------
_kst_
From the article:

"In Bohmian mechanics, every electron always has a definite position, even if
observers are ignorant of what it is. An electron is pushed around by a
guiding "pilot wave" that influences the electron's location. While each
electron travels through one slit or the other, the pilot wave passes through
both slits simultaneously. Interference in the pilot wave leads to the
observed interference pattern."

In the unlikely event that I understand this correctly, isn't what Bohmian
mechanics calls "the pilot wave" just what the Copenhagen interpretation calls
"the electron"? (Or perhaps "the electron's probability distribution"?)

~~~
effie
In Copenhagen, electron is "mysterious quantum object", unlike wave, unlike
particle, just showing off properties of both in different experiments. It is
not meant to be the same thing as \psi function, which is regarded as mere
calculational device that describes this mysterious quantum object. The pilot
wave in de Broglie-Bohm theory is just the \psi function with the addition
that this function is "real", that is exists objectively and not merely on
paper.

~~~
_kst_
I have to admit that I'm unclear on the distinction between "exists
objectively" and "merely on paper" in this context.

~~~
effie
My bad, I didn't say it very well. The point is, in de Broglie-Bohm theory, it
is assumed that particles are material points that exist, have trajectories.
The motion of the particles is influenced by \psi, kind of like motion of
charged particle is influenced by electric field in EM theory. These particles
and the \psi function are the subject of the theory, so they "really exist" in
this theory.

In Copenhagen theory, \psi is understood to be mere calculational device the
main purpose of which is to get probabilities of results of measurements.
There is no idea that particles actually are there moving along trajectories;
the subject of the theory are results of measurements and \psi is only a
theoretician's tool to get their possible values and probabilities.

------
smilliken
Some nit-picks:

> Until a particle is observed, an act that causes the wave function to
> “collapse,”

The "collapse" is a overly-simplistic way to interpret QM experiments. It's
not well-defined because no one's ever been able to define the "observer".

> Quantum mechanics, in contrast, is inherently nonlocal.

This is false, and a common misinterpretation of QM. We do not have any
evidence of information travelling faster than the speed of light.

~~~
ewjordan
That's not fair - plenty of people have defined "observer" in plenty of
different ways (including defining it away, like decoherence supporters would
prefer), it's just that there's no consensus because it's mostly a matter of
philosophy without any experimental consequences.

As for nonlocality, entanglement is a fundamentally nonlocal feature of
quantum mechanics. There's no way to excise that from the theory, even if you
can't exploit it to transmit information faster than light. I'd say calling it
inherently nonlocal is entirely fair, that's a very different claim than that
we can transmit information faster than light.

...which, as an aside, would not even in and of itself necessarily be
problematic, closed timelike loops (for instance) are perfectly fine and good
as long as you account for the fact that quantum interference will have very
strong effects even at a macro scale in such geometries (the grandfather
paradox gets interfered away, more or less, even if macroscopically unlikely
things need to happen to end up with a reinforced consistent timeline).

------
justifier
i hate when talks of qm use vague standin words like measure or observe stead
describing the process

especially when those things are shown in diagrams with a little image of an
eye like in this article

observation of quantum events is very intrusive, and i would argue has an
obvious affect on the event's original outcome

it is different from having someone throw a ball in a well lit room, and
either having your eyes open or closed

------
spottedquoll
> the standard view of quantum mechanics, often called the Copenhagen
> interpretation

No. The Copenhagen interpretation is not the "standard view."

~~~
effie
That is a surprising claim. What is your basis for saying so? Perhaps some
polls?

~~~
spottedquoll
The many worlds interpretation is far more common amongst the physics
community (well, the parts that care about this sort of thing.)

We don't do things in physics with polls, really. We tend to prefer maths.
And, much of the in-your-face weirdness of QM simply goes away with MW. You
only have to accept that all paths are taken, but only one is observable to
you. Not surprising, since you're an entity that obeys quantum mechanical
laws.

Since the actual predictions and experimental evidence for Copenhagen and MW
are identical, it doesn't matter which you prefer. If you're not careful you
run into philosophy pretty fast here - fun, but not useful.

Copenhagen (kinda, sorta, not really when you do the maths) leads people to
think (and I have been guilty of this) that there could be some sort of
magical non-quantum 'observer' that causes the wave function to collapse.
Which is bollocks.

Many worlds forces you to consider the environment of the system (including
the mythical 'observer') as fully quantum systems. Which is much closer to
what the maths actually says.

Copenhagen and MW give the same answers, but Copenhagen gives sloppy thinkers
a silly avenue to get lost in. "I am a privileged, magical Observer, who is
not at any point affected by the measurement I make."

And, hidden variables are so strongly constrained by experimental results that
you are out there in the wilds if you propose it. Not actually a crank, but
carrying the burden of evidence.

We are an epiphenomenon of quantum objects interacting under the laws of
quantum mechanics. And the Many Worlds interpretation allows us to move past
the philosophy and make actual, real, testable prediction.

------
anotheryou
So how does it explain the 2 bands of the observed doubles slit experiment?

~~~
jsprogrammer
The particle goes through one slit, but the wave goes through both (all) and
continues to influence the particle.

~~~
anotheryou
that explains the interference pattern, but not the 2 clear bands you see when
you measure it one slit (or am I wrong here and that never happens?)

~~~
Natanael_L
The measurement would affect the wave which would affect the particle.

------
tomp
How does the Pilot-Wave theory explain the GHZ experiment?

------
gaze
What is with hacker news' obsession with pilot wave theory? Many in quantum
information consider uncertainty to be a feature, not a bug.

~~~
effie
The pilot wave theory is attractive partly because it is more easily
understandable theory of microscopic particles, atoms and molecules than the
alternatives (better visualizations and less bad philosophy talk). Also I
suspect it is quite popular in some circles because the minions of orthodoxy
could not convincingly refute it for decades now. Still a minority theory that
has to fight for its place under the sun, it somehow reminds of the story
about David against the Goliath...

------
kordless
AKA, India's Net.

------
kordless
> In the Bohmian view, nonlocality is even more conspicuous. The trajectory of
> any one particle depends on what all the other particles described by the
> same wave function are doing.

AKA India's Net.

~~~
milcron
What is India's Net? Do you mean Indra's Net?

