Oil droplets guided by pilot waves do not give rise to double-slit interference
quantamagazine.org
quantamagazine.org
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
The subhead seems a tad sensationalist. If interference had been shown, it would have suggested strongly that this can happen at the smaller scale as well, and that it didn't is probably disappointing for those that hoped to show it in the analogy. Not sure how it "crushes" those theories.
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.
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.
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.
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).
Most contexts (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...) do exist.
Isn't it the other way round? Pilot-Wave theory having issues with non-locality that de Broglie-Bohm hoped to avoid?
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.
Nicely put!
> Wave function collapse on measurement isn't weird anymore if the wave function represents your knowledge of the system.
To me it is intuitive if you consider that the wave function of the small entity is interacting with the much larger (and more specific) wave function of the observing apparatus. I never understood the mystical aspect with which this was treated in the typical explanations of the Copenhagen interpretation. And I would also appreciate the guidance of someone with a better understanding of the physical theories, if my stated perspective is confused.
These have been largely ruled out via no-go theorems [1].
> their result does not rule out any position that is seriously held by any researchers in quantum foundations.
http://mattleifer.info/2011/11/20/can-the-quantum-state-be-i...
Would this not be a 'hidden variable theory' of the type that is ruled out by Bell's theorem?
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.
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.
1. Theory X hypothesizes Q
2. Phenomenon Y models theory X
3. Phenomenon Y does not demonstrate Q
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.
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.
Firstly, it's primary value is that it's an ontological interpretation, meaning it preserves realism where most other interpretations sacrifice realism. The motion of its beables has nothing to do with this.
Secondly, pilot wave theory still requires the fewest number of assumptions, roughly on par with Many-Worlds, which means it's axiomatically simpler than other interpretations.
Whatever you mean by "simpler", its not a form of simplicity that actually matters when gauging plausibility of a theory.
(There's an argument of avoiding nonlocality/spooky-action-at-a-distance which I find specious: the particles are indeed localised but they don't actually play any significant role in determining behaviour. The pilot waves are doing all the work in the theory, and they have all the usual entanglement/interference/... behaviour (as they must, in order to be a correct theory of QM)).
This "Bohmian mechanics is Many-Worlds in disguise" is simply false [1]. The axiomatic basis is completely different, and this is apparent because with Bohmian mechanics you can easily derive the Born rule, and as a result, it also makes a new prediction that isn't allowed in typical QM, quantum non-equilibrium [2]. Deriving the Born rule with MWI is a hotly debated open problem.
Saying "clearly" a lot does not convince me, and seems to be all that paper does.
> The axiomatic basis is completely different, and this is apparent because with Bohmian mechanics you can easily derive the Born rule, and as a result, it also makes a new prediction that isn't allowed in typical QM, quantum non-equilibrium [2].
You've contradicted yourself: a quantum non-equilibrium condition would be one that did not follow the Born rule. Excluding quantum non-equilibrium requires an additional rule and looks to be much the same problem as deriving the Born rule under MWI. Were quantum non-equilibrium to be observed, that would be major evidence that our current understanding of QM is wrong; however, by the same token, our non-observation of it must count as evidence against a theory that predicts it.
Isn't the most common complaint about pilot-wave that the wave corresponds to normal QM and the pilot is a superfluous detail? I would not pick minimalism as one of its strengths.
Minimalism is one of its strengths. The type of objection you raise is a fundamental misunderstanding of both pilot wave theories, and what "minimalism" actually means when it comes to selecting among competing theories.
It is mostly popular among people who want their physics to be visual and understandable to our scale intuition.
With this it basically drops only thing which made it attractive.
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 [0]https://en.wikipedia.org/wiki/Bell%27s_theorem
Edit: fixed order of references.
How exactly does having a superluminal telephone imply I could change history? Changing history is the paradoxical part - don't assume it can happen.
[0] http://www.theculture.org/rich/sharpblue/archives/000089.htm...
All quantum interpretations have superluminal characteristics, but that's not the same as superluminal messaging/signaling.
Don't be fooled by "local" in there; all non-locality results in 4D spacetime would remain true, because of course they have to. It would be "local" in an isomorphism that will be extraordinarily counterintuitive and probably never really be something anyone can "visualize".
But, if this did pan out, there would be a reasonable sense in which the Copenhagen interpretation would have something physics-al about it (as in, grounded in physics, not necessarily "physical" in the traditional sense), where it wasn't in fact "action at a distance". I for one have no problem with the universe creating probability distributions and then randomly picking a result, as some seem to. (The Universe May Do As It Damned Well Pleases. It's true that a good sense of aesthetics has given us a lot of power up to this point, but "I don't think that's very aesthetic" is not a very strong argument. I'll give it more than 0, but not much.)
Also from the same source, a couple years back: https://www.quantamagazine.org/pilot-wave-theory-gains-exper...
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.10...
If true, one wonders if you could use this to build a d-wave like annealer.
Interactions in a field propagate at the speed of light or force-carrying particles.
This is one of Eintein's major contributions to physics, that nothing travels faster than light.
When two particles are entangled, maybe instead the field is 'twisted/warped' at those particles. Entangling particles alters the field, not the particles.
The paper claims "it's neutral on the interpretations" - it is not.
You have the most senior research professor of physics at caltech admitting there are problems still - there are of course physicists that insist they have the correct answer or interpretation - the hard part is convincing enough physicists to agree with them, which they universally do not agree.
What is the ranking of seniority of research professors at Caltech? Are you implying that a research professor is somehow better than a “regular” professor?
Probably when it comes to understanding and forming opinions on cutting edge research. . .
it is saying that the only thing that exists is the quantum wave function
the only theory with a hope of credibility/coherence that says the only thing that exists is the quantum wave function is many worlds.
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?
Notice that he is talking about teaching/talking/explaining. It is a for-teachers paper, not a for-researchers-working at-the-cutting-edge-of-fundamental-physics paper. And its nothing new. I taught an introductory level course on quantum mechanics recently and I refused to use the words "particle" or "wave" at any point in the course because I think they are confusing. I talked instead in terms of wavefunctions which is just an easier word for fields - at least as far as undergraduates are concerned.
I did condensed matter Physics, where QFT isn't that essential, but from the QFT I did study the "it's all fields" way of looking at it came out quite naturally (well, how natural something as profoundly weird as QM can be, can be discussed..). So I fail to see how this can be considered particularly controversial.
I mean, if you want something controversial and non-mainstream, take the subject of this article, pilot wave theory.
PS: Do you have suggestions on a good introductory QM study materials (textbooks, online stuff,...) that emphasize the fields viewpoint?
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.
There are 4 possibilities: GG GB BG BB
GG is impossible so you have GB, BG, & BB. One of them is a B, what's the other. Well that's clearly 2 / 3 G.
The other one situation is completely. It's basically just the chance of a child being a girl.
Given one of the children is a boy. The only combinations possible are: GB, BG, & BB. Where G is Girl and B is Boy. The question is about the probability of the other child being a Girl.
When we eliminate one of the B from each combination, we get:
GB, BG, BB -> G, G, B
We can see that 2/3 are G and 1/3 are B.
Does this help?
Here's another example: If I flipped 100 coins, and told you there were 99 heads, what's the likelihood the other 1 was tails? The answer is 100/101, because there are 100 ways the one tail could happen, while only one way zero tails can happen. But if you're only told that the first 99 are heads, the answer is 1/2. You know the last flip has exactly the same odds as all the other ones -- the trials are independent. The difference is in one all the flips were included in the results (and the question isn't about any one specific flip), while in the other you had no information about the last flip (which the question was about).
Another way of looking at it: In the first example, the question was asking about the sum of 100 trials and you were given information about 99 of those 100 trials. You were given 99/100s of the information you need to answer the question. In the second example, the question was asking about the last trial and you were given information about 99 unrelated trials beforehand. You were given none of the information you need to answer the question.
Imagine I flip a fair coin. The odds of you guessing it correctly are 50%. In your mind, you know it's probabilistic. Simultaneously, you imagine that if you had more information you could get it right more than 50% of the time. Maybe if you new the force or the initial velocity, that'd help. The thing with QM, there's no more information to get. You can't get better than 50%, ever. It's basically magic. The problem with any deterministic model is that it adds more complexity than just assuming magic and we can't test any of the predictions of current deterministic models so why bother.
QM effectively seems to be non-causal - or at least, much less causal.
Tidying this away under the heading of "It's probabilistic, let's just do the math and not worry about why" seems to be ignoring some very basic questions.
I can't seriously imagine Newton saying "Well, the Moon stays in the sky instead of falling down - and that's frankly a bit weird, but I'm not going to wonder why because it's just a mystery and anyway, I can predict its orbit with these epicycles."
But he did. He called it Gravity.
He didn't let figuring out "how gravity works" hold him back. He just figured out the equations that predict its effects based on observation. The unknowns happened to conveniently coalesce around a single constant G, but that doesn't make them "known".
[IANAP - do we have any convincing popularly accessible models for how gravity works even today?]
Anyway, that's kind of similar to going "oh I don't understand the subatomic mechanism that is causing this interference pattern, but the pattern itself is consistent so lets call that P!".
Unless I completely sped past the point you were trying to make - which is completely possible at this subatomic level of reasoning (-:
Newton was looking for methods to manipulate (push, pull) objects at a distance without touching them. He had strong ideas about the occult and what we today call black magic, in addition to being an alchemist. He devised that the Earth was manipulating the untouched Moon (capitalized, proper noun) and sought to explain the influence of the Earth on the Moon.
Many science-related articles on Wikipedia suffer from good-meaning but over-zealous gatekeeping of religious influences on science. So I'll just recommend to you to read a biography or three on Newton. His entire story is fascinating and we can see how tiny events in his childhood (death of father, influence of priest uncle) have affected the history of physics.
[1] https://en.wikipedia.org/wiki/Isaac_Newton [2] https://en.wikipedia.org/wiki/History_of_gravitational_theor... [3] https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gr...
The Wikipedia article does a great job of being explicit in the differences between the 1:2 and 2:3 cases.
Unfortunately articles pop up every now and then claiming 2:3 is the correct answer (since it's counterintuitive and therefore interesting), while losing the conditions necessary for 2:3 in the translation of the problem description from mathematical to plain English.
They are different sets of families. That is the point of this paradox.
This has nothing to do with quantum mechanics.
Given what I know about the current situation, the probability that their other child is a girl is 66.666%
If I look out the window to that neighbor's yard and see a boy playing outside, the probability that their other child is a girl drops to 50%.
How I know the information about on data point one affected the likelihood of an unrelated one. What seems like identical knowledge on the outside, changes outcomes based on how I accrued that knowledge. If that doesn't mimic the double-slit experiment, I don't know what does.
Somehow, you're saying that seeing the boy means you have ruled out the possibility of "a girl and a boy". But the ordering is in no way related to some arbitrary observation. It's just lexical.
For the first case here are the different combinations for two children based on sex (let's say left denotes older and right denotes younger to distinguish the difference between girl and a boy and boy and a girl.)
B B
B G
G B
G G
These outcomes are each equally likely, but since we know that one has to be a boy we can eliminate the (G G) case leaving us with two chances out of three that the other child is a girl.
The other case can be viewed as follows (S) denotes seen.
B(S) B
B B(S)
B(S) G
B G(S)
G(S) B
G B(S)
G(S) G
G G(S)
I can eliminate all cases where I see a girl out the window, leaving me with the following equally likely cases:
The neighbor has two boys and I see the older boy. The neighbor has two boys and I see the younger boy. The neighbor has an older boy whom I saw and younger girl. The neighbor has an older girl and a younger boy whom I saw. The neighbor has a girl in 50% of the remaining possible cases.
One of the twins is still considered to be born first and therefore older :-)
While you may argue that one twin is always born first, this is an idealized mathematical problem, so it can't depend on particular details of physical reality in that way. There is nothing logically preventing them from being the same age, so it can't be pivotal to solving the problem.
And in fact, I experimentally got the right result without any reference to ages.
People like to write the outcome space as ordered pairs BB, BG, GB, GG (ordered by age or whatever else), because this has the advantage that all four outcomes are equiprobable, so you can calculate probabilities just by counting.
If you prefer to regard the outcome space as unordered pairs {B,B}, {B,G}, {G,G} this is absolutely fine: you just have to bear in mind that the prior probability of {B,G} is 1/2, whereas it's 1/4 for the other pairs, so a tiny bit more calculation might be required.
P({B,G}| not {G, G}) = P({B,G})/(P{B,B} + P{B,G}) = (1/2)/((1/2) + (1/4)) = 2/3 as with the other method.
If differentiation is not possible, then they are the exact same person [1], so the parents have one child and it must be boy then ;)
[1] https://en.wikipedia.org/wiki/Identity_of_indiscernibles
Probability means this: "in X tries, how likely is it for Y to happen". The ambiguity here is this: what is the set of X? Is the set of all families with two children at least one boy, or the set of all families with 2 children of which you have determined one is a boy.
Those are different sets. It doesn't matter how you discovered the information. The apparent paradox is all grammar and set description.
Think of it this way - you want to test this out and actually run this experiment? Great! What's the experiment? Are you starting with all families with 2 children, or all families which 2 children at least one boy.
>> How I know the information about on data point one affected the likelihood of an unrelated one
No, it doesn't.
"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.
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.
> 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.
Try it with 2 coins flipped at the same time and you'll see it's the case. You'll get 2 unmatching coins about twice as often as you get either HH or TT.
If you know one of the kids is a boy then there is a 2/3 chance of there being a girl kid. I will assume you are happy with the reasoning behind this (3 options BG GB BB, two of which yield a girl).
Now why does spotting one of the boys change the odds? You already knew they had a boy right?
Well the boy being spotted conveys some information!
In particular you DIDN'T see a girl. You could have seen one, it would be possible right... but it didn't happen.
That fact means it is a bit more likely that they have two boys. How much is that bit, well 1/6th to be precise. So 1/3+1/6 = 1/2 chance they have two boys. And 2/3-1/6 = 1/2 chance they have two girls.
That gives me the intuition and makes this no longer a paradox intuitively. Although it was never a paradox logically.
CAREFUL:
The conditions under which the boy was 'revealed' to you matter.
If it was random 50-50 chance of seeing the girl or the boy then the above applies.
If the boy was picked, i.e. you said "ah you have at least 1 boy, show him to me". Then no information has been conveyed.
If something else situational (e.g. girls birthday party and all the girls are inside in the room, and boys are playing soccer outside [sorry for stereotyping but my imagination isn't so good] and so you see him) there may be a non 50-50 and non 100-0 probability then there are some more calculations to do :-).
This would be like saying "When I buy a lottery ticket, the only outcomes are {lose lottery, win lottery}; therefore, I have a 50% chance of winning the lottery."
The difference is that boy-girl is twice as likely to occur of those three choices.
> 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
Observation is the key word here. Actually, you don't need to observe the atoms to affect the pattern on the screen - though its the easiest way to do so in the double-slit experiment. All you need is the possibility of observing or distinguishing between two events to destroy the interference between them. Note that possibility of observing is a much weaker condition than observation itself. Physicists have, over the years, done many experiments to clarify these issues. Essentially, if you believe that causality only flows forward through time, then what you are saying is not correct.
Are these not different questions?
You have red and black pebbles otherwise identical. Bag has 2 pebbles, you draw a pebble, it's red. You put it back. What's the probability of bag having black and red pebble? Draw again - red again. What's the probability of bag having black and red pebble? You acquire more data, therefore your estimate changes.
That being said, I have complained about specific papers and articles making this exact mistake. For example, explanations of delayed choice eraser experiments are almost always confusing correlation or post-selection for causation ( http://algassert.com/post/1720 ). But Bell tests are not of that form, which is why the first thing you should ask of any proposed interpretation is "How do you deal with Bell tests?".