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Comment by btilly | Hacker News Reader

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.)

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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.

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.

> 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.

>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.

You seem to suggest that support for a theory is meaningless unless it is absolutely conclusive. If that was the case, I'm pretty sure most of quantum physics would have been abandoned long ago.

Until we know everything, is anything in theoretical physics really a zero-sum game?

I think the problem is in the thinking quantum physicists must possess in order to believe in such a theory in the first place. I mean, the idea of, "if we don't know where it came from, it couldn't possibly be hidden information; it must therefor be 'random'" is absurdly egotistic in the first place.

I have no idea how you get from my words to, "...support for a theory is meaningless unless it is absolutely conclusive."

What I said is that if the outcome of an experiment is predicted by two different theories, getting the expected result does nothing to help you decide between them. It happens to be the case that ANY experiment that is in accord with quantum mechanics is equally well explained by EVERY interpretation of quantum mechanics. The result is that most working physicists so not consider wondering about the right interpretation to be a useful scientific endeavor. Indeed most will gladly affirm that they belong to the "shut up and calculate" school of physics in practice.

There are plenty of detailed questions within quantum mechanics which can be asked and answered by experiment. That is what scientists tend to do instead of debating interpretations.

>> Replace non-locality with infinite universes, which is more troubling

It's a question to the philosophers, not the scientists. There is no objective measurement for what "troubling" is.

And if we don't start with any preconceived notions of how things _should_ work, then discovering that they work differently is not particularly troubling.

> It's a question to the philosophers, not the scientists.

As discussed upthread, this is all philosophy. The popular interpretations of quantum mechanics are all experimentally equivalent, so we need to use elegance, intuitiveness, troubling-ness, etc to choose between them (and these can be highly personal).

EDIT: After re-reading your comment I think I'm agreeing with you here.

You can also use the `operational non-interpretation', and just use the math to calculate without worrying about what it `means'.

(That's pretty close to the spirit of the Copenhagen interpretation, though.)

> Replace non-locality with infinite universes, which is more troubling?

The nature of the multiple universes is deply misunderstood. If you just think of the universe as a very large N-bit quantum computer, then the "multiple universes" are simply the superpositioned state of the computer itself. Nothing magical or non-physical about it.

And since we are almost certainly living in a simulation [1]... ;-)

[1] http://www.simulation-argument.com/simulation.html

The problem is that you wouldn't just need a superposition over a finite number of state, but for the theory to make sense the universe should be some superposition over all possible physical states.

There are simply just too many possible states to meaningfully describe something like a distribution over them, which is what a superposition would be. Mathematics simply gives up when you try to integrate over an infinite number of dimensions.

I don't know where you are seeing a relevant infinity. At any given time, there are a finite number of particles each with a finite number of observables. It's a large number, but finite.

How do you know? All we can see is the visible universe, and the horizon is receding faster than the speed of light. Where do you get that there are finitely many particles?

There was (to the best of our knowledge) a single big bang, since when energy is conserved. That gives us a total energy for the universe, which is large but finite, which puts an upper bound on the number of particles at any given time.

The big bang is just a theory that is obtained by extrapolating the current movement backwards. For all you know, the multiverse is infinite. And in fact so could the universe be -- the big bang could have happened much earlier and the universe could be much bigger. And when you say that there was only one big bang and there was a finite amount of energy, you are making too many asssumptions.

> it demonstrated this by showing that there is no contradiction if you take in account nonlocality

The article suggests pilot-wave theory "apparently" contradicts special relativity. We know the other interpretations "only" have a problem uniting with general relativity.

Perhaps the core issue is Steinberg's quote in the last paragraph: “The universe seems to like talking to itself faster than the speed of light. I could understand a universe where nothing can go faster than light, but a universe where the internal workings operate faster than light, and yet we’re forbidden from ever making use of that at the macroscopic level — it’s very hard to understand.”

The problem with general relativity is that general relativity can't be quantized. We have produced quite a few theories of gravity that can be quantized...then subjected them to experiment and they all failed.

These days theorists are trying to create theories out of which both QM and GR could arise in the limit. An unbelievable amount of energy has gone into things like string theory and loop quantum gravity. True believers believe that a theory will be found..but they haven't succeeded yet.

Back to pilot-wave theory. In special relativity, no inertial reference frame is privileged over the others. In pilot-wave theory one must be..but no experiment can reveal which one is. This is aesthetically ugly, but not actually a contradiction.

> In pilot-wave theory one must be..

Would this be the reference frame of the center of gravity of the universe? If so, and if some parts of the universe are unreachable due to inflation, would that be the center of gravity of everything within the light cone of a system? Sorry if I'm being imprecise, I only dabble in this stuff.

> Would this be the reference frame of the center of gravity of the universe?

This is a little confused, I'm afraid. The centre of gravity of any system doesn't define a reference frame, it defines a location. For the universe, it doesn't even do that, since 'the observable universe' is a function of the observer: anyone anywhere can look around and observe some universe for 50 odd light years in each direction. Assuming homogeneity, the 'centre of gravity of the observable universe' is just a clumsy way of saying 'my current position'.

The universe does sortof define a reference frame, and it's the frame in which the CMB (cosmic microwave background) is isotropic. In other words: if the CMB looks warmer to your left than your right, that means you're travelling left relative to the CMB frame.

But it would be very surprising if the laws of physics depended on that. (Indeed, special relativity assumes the laws of physics are the same in every frame, and so far we have no evidence to contradict that).

> In pilot-wave theory one must be [preferred]..but no experiment can reveal which one is.

Interestingly has this in common with tangherlini relativity... which, iirc, also has nonlocal effects (haven't done the math; am not sure)

No one has combined bohm + tangherlini

Many-worlds isn't mathematically simple if you demand world-splitting occurs locally.

http://arxiv.org/abs/quant-ph/0103079

Interpreting quantum mechanics is often like this: you can preserve locality or convenience, but usually not both. This is why I prefer the minimalist relational interpretation:

http://en.wikipedia.org/wiki/Relational_quantum_mechanics

However, I don't recommend this as a preferred interpretation because it does not motivate discoveries (the other interpretations all fail here too). So far, none of the interpretations has led to a new physical theory, and in particular none has really helped the study of quantum gravity, which if anything is the whole point of metaphysics: to aid in learning.

> Interpreting quantum mechanics is often like this: you can preserve locality or convenience, but usually not both.

Indeed, John Bell of Bell's theorem said that non-locality was THE big problem of quantum mechanics. He was a fan of Bohmian mechanics/pilot waves because it places what he considered the central problem front-and-center.

> There is no explanation of the fact that many-worlds also explains

They do mention many worlds albeit not in too many words ;-) it just says it is rather a convoluted explanation just like the other ones. Infinitely many universes being generated during every particle interaction is just as silly as a funny path or Copenhagen.

> 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.

So what did you mean by "many worlds" being "mathematically simple" isn't it the same mathematics, just different interpretation of it?

Not sure what you man

According to quantum mechanics, if a simple quantum mechanical system interacts with a complex one, the complex one is thrown into a superposition of states that can have no meaningful interaction from then on.

Many worlds says that quantum mechanics is all that there is, and all observed consequences can be predicted from the assumption that we, ourselves, are complex quantum mechanical systems. The prediction is that we coexist with superpositions of ourselves doing different things and cannot meaningfully interact with them. This seems weird to us, but is exactly what the theory predicts.

Consider Schroedinger's cat. We ask whether the cat is both alive and dead before we open the box. This puzzles us because that is what quantum mechanics predicts and that seems weird. Multiple worlds says that quantum mechanics holds, it remains both alive and dead AFTER we open the box, and while it seems weird, there is no actual contradiction anywhere.

So mathematically simple. Weird consequences. In perfect accord with our best theories and best experiments.

By contrast the Copenhagen interpretation says that there is a collapse somewhere even though there is no experimental evidence for it, and we have no theory about how it might happen. And the pilot-wave interpretation says that our best theories notwithstanding, the universe is something different and weird in a different way.

In any interpretation, it seems the same question remains:

Copenhagen: Why did the wave collapse into state A and not B,C,...

MSI: Why am "I" in multiverse branch A, and not B,C,...? (and who am "I" anyway?)

Pilot wave: Why is the pseudo-random seed = A', which I can only see indirectly by observing experiment result state A?

Brilliant - this is perhaps one of the clearest explanations of the disagreement between each of these interpretations I've seen, and it makes crystal clear why we can't hope to observe any differences: the questions are all fundamentally "why" questions, which we can almost never make direct observations to resolve.

At the root, we're always asking why some random variable ended up in a certain state. Our "interpretation" just changes where we ram that annoying random variable into our theory, which largely depends on where we're most philosophically comfortable with uncertainty showing up.

You are in A wondering why you are in A. You are also in B wondering why you are in B, and so on. It's all perfectly consistent.

I am in A, though. Maybe another me is in B, but why is this me in A?

'this me' in your sentence just refers to the one that is in A. Thus you seem to be asking "why is the version of me, that is in A, in A?". It just is by the definition of the object ('this me') your asking about.

Pop-science articles about quantum mechanics make this fallacy (affirming the consequent) all the time.

Them: "PilotWave implies E. E was observed. Therefore PilotWave!"

Everyone: "AAAAAaaaaaaaaaaargh! That is not how science or logic works! You're missing the 'not-PilotWave implies not-E' part!"

This is how common sense works, though, and is covered formally by probability theory. I'd also argue science works by probability theory more so than deductive logic, so you're only right that this isn't how logic works.

Actually, this issue also applies to probability theory. It does get more subtle, though, because theories might predict observations with differing probabilities and what matters is the difference in probabilities. But this this case theory A and theory B both predict observation C with the same probability p. So you gain 0 decibels of evidence (w.r.t A-vs-B) whether or not you observe C.

When I said "You're missing the 'not-PilotWave implies not-E' part!", I was making a simplified reference to the fact that the probabilities predicted by not-PilotWave (e.g. Copenhagen) need to be different from the probabilities predicted by PilotWave in order for evidence/inference to pry them apart.

See [1] if that all sounded confusing.

1: http://www.yudkowsky.net/rational/technical/

Probability theory doesn't work that way either. If A and B both imply C, then seeing C doesn't raise the probability of A compared to B.

>Quantum mechanics continues to predict things correctly in an experiment that has a fun interpretation within pilot-wave theory.

Both of them predict correctly because they both actually use quantization of particle position. The issue with both of them is that both interpret that solution to equations as a real object - QM takes it for particle itself while pilot-wave theory assigns some real world existence to that "Pilot Wave". It would be like in case of shooting a bullet from a rifle QM would say that the trajectory is the bullet, where is pilot-wave would say that there is real trajectory object guiding the bullet.

I always thought the "multiple universes" theory was just a convenient way to explain particle interactions that occur outside of our observable universe. They may very well be in the this universe but because they're not observable it's presumed that they occur in another universe or many universes simultaneously.

Not exactly.

The "multiple universes" theory is an explanation that if quantum mechanics describes both the quantum mechanical system under study and the observer, then after the interaction of "observation", the system looks like a superposition of independent systems that each look like the observer observed something different. The amplitude of each system corresponds directly to the likelihood that we will think we observed one thing versus another, and quantum mechanics + thermodynamics shows that these copies will never be able to have any meaningful interaction.

In short, in this interpretation reality, is quantum mechanics. There is never a "collapse" as there is in the Copenhagen interpretation. "Multiple universes" is merely a description of what quantum mechanics predicts the outcome will act like.

Or another way of thinking about it:

If you fire a particle at two slits with a screen on the other side you get an interference pattern on the screen implying the particle in a sense went through both slits and the results interfered. This happens even if you use quite complex particles such as C60 buckyballs. So the universe in some sense runs multiple versions some with the molecules going one way and some going the other. All the "multiple universe" explanation implies really is that this is a general phenomena and the universe runs all possible versions even when we can't observe the results.

I'm not sure what is more far fetched - many worlds or non-locality.

Why is many worlds far fetched? We observe it in a smaller scale in experiments when you pass light through a half silvered mirror. It's just the logical extrapolation of that observation to the macro scale.

You say that observation shows many worlds. I say that the photon interferes with itself. I find the latter far more plausible than invoking multiple universes for the photon to interact with.

That's fine until you start talking about a magical collapse process that happens conveniently at sufficiently large scale. IMO postulating additional non-local, non-unitary, ftl, etc. collapse process is strictly more complicated.

Most modern interpretations have "collapse" as only a change in the description level, not a physical process.

You're right that postulates such as the projection postulate complicate the theory. But it's not clear how to explain experimental results, like the stochastic nature of measurements or the Born rule, without it. I can make a theory as simple as you like if you don't require it to explain experimental results.

Don't forget that the Copenhagen people also haven't really answered the Born problem, they just lump it into the black box of "collapse".

The many-worlds interpretation has no collapse at all. :-)

In QM random outcomes occur. By what mechanism are the random values chosen? Many Worlds has a plausible mechanism for this. I'm unaware of any other interpretation claiming the same.

Indeed, I've never found the many worlds (or any other) interpretation to be more compelling than admitting that quantum mechanics is just plain weird.

Infinite universes isn't too far fetched. Any interpretation of QM is going to require exponential computing power to simulate all possible states. MW just asks, why assume these other states stop existing when you observe them?

I may be totally confused, but I don't think the CM has states "stop existing" on collapse. It is not in a state until it collapses. And the "you observe" isn't really "you". Essentially, it doesn't have a state until the state is referenced.

Again, I might be wrong, but I think the problem most people have with QM is that they think something must either exist or not exist. From that perspective it's easier to imagine that something exists in all possible states and that we are just observing one of them. However, even logically things can be in an indeterminate state.

For example, if I draw a box and then I draw another one that is twice as tall, the first box is short and the second box is tall. If I draw a third box that is 3 times as tall as the second one, then the first 2 boxes are short and the third one is tall. However, if I never drew the second or third box, then the first box is neither tall nor short. The concept doesn't actually make sense without the other boxes.

Similarly, imagine a universe with a single particle. What is its position? What is its momentum? Obviously these concepts have no meaning. As soon as I add another particle to the universe, then suddenly position and momentum have meaning. Now imagine that space is not a discrete map of particles with positions, but rather a set of particles which each have a probability of interacting with each other. This probability indicates their position.

In such a universe, if a particle does not interact with another particle, then it does not have a position. There is no meaning for position with respect to that particle. As soon as it interacts with another particle, then suddenly position has meaning. It is "created" in the same way that we "created" tall and short by drawing different sized boxes.

That is my, very possibly naive, way of thinking about it. Yes... I've read too much Taoist literature... ;-)

This is very similar to time travel hypotheticals. If you travel back in time, and totally change the past, did the future you originally observed actually happen? People existed, they lived their lives, etc. And then suddenly you "reset" the universe to a time where that never happened.

I would argue they did exist, and that the time travel event actually murdered them. I think of the universe as a computer, and any computation it does actually happens, actually exists in some sense. If the universe does a computation of my entire life, then deletes it, that life still happened. It "exists".

I think the same is true of the famous cat experiment. The computation of both possible states of the cat has to be done by the universe. It needs to compute how the cat reacts to the poison gas, it needs to compute the pain it experiences, etc. That's a lot different than a single particle like your example.

When you open the box, the universe may "collapse" to the second possibility, where the cat is still alive. But the first possibility still happened, and it may still be out there somewhere in an unobservable branch. In fact that model seems a lot more mathematically elegant, being completely deterministic and not having to deal with the weirdness of "collapse" events.

But in the pilot wave theory, there is only one actual state of the universe, though that state propogates with explicitly non-local interactions. That state includes hidden variables, namely particle positions, velocities, and IIRC quantum phases.

It may take exponential time to simulate the distribution of measured results given only the measurable inputs (which is what we do when we model experiments), but the universe itself knows the hidden variables and so can calculate the next state quite efficiently. So ontologically, a non-local hidden variables theory is much simpler than many worlds.

Copenhagen is unworkable ontollogically; it assumes some distinction between normal interactions and "measurements" which are poorly defined.

I'm not arguing against pilot wave theory, since I don't really understand it. In fact I hope it's true, I'm uncomfortable with the idea of multiple universes. I'd go for whatever model is the simplest. Whatever has the lowest kolmogorov complexity. A more formal Occam's razor, whatever has the least assumptions.

>MW just asks, why assume these other states stop existing when you observe them?

Do the others really assume that? Don't they still account for quantum erasure?

Can you explain why the more simple explanation is not favored by most scientists than?

When you have to choose between hidden variables, nonlocality, and many worlds, the opinions differ wildly which one of these is more simple.

Everyone claims their version is the most simple and logical, but then others disagree.

Surely there are people who see the equivalence of the interpretations, and don't paint themselves for color war?

Yes. I should not have said 'everyone'. Those who have one favorite interptetation.

If you are talking about Pilot Wave Theory - it definitely is not simpler mathematically. It might not require complex numbers, but it still requires the same number of parameters. It is actually more complicated to do multi-particle physics in this setup. But at the end they are just equivalent descriptions (same mathematical object, different ways to describe it).

Many-words is pleasant because it requires (arguably) less "mental gymnastics", but again, it is just an interpretation of what the equation means. It is still the same equation, the same mathematical object.

The claim is that Quantum Mechanics (or QFT more generally) is not a complete theory (and we do know that to be a fact, it does not work when space-time curvature is important). The only reason I personally find Many-words interesting is because it might be easier to generalize to a complete theory. But again, if we are constraining ourselves to flat space-time, all those interpretations are completely equivalent.

The problem isn't to get the right answer. When you have a recipe that works reliably for a known set of conditions, you're not doing science any more - you're doing technology.

The problem is to work out which questions to ask next. The different interpretations should eventually diverge into edge cases - of curvature, of observable other universe, or of some other difference - where they no longer predict identical results.

An incomplete theory suggests at least some further distinctions are possible. They may or may not be possible with current interpretations. They may need a completely fresh interpretation. But the distinctions matter, and it's important to consider the different directions they point in.

"...QFT...does not work when space-time curvature is important". Forgive the probable confusion of an ignoramus, but is there significant space-time curvature in the vicinity of a gold atom nucleus? I ask because I remember these claims about special relativity and the gold atom: https://www.fourmilab.ch/documents/golden_glow/ - of course, I could be dreadfully confusing special relativity effects with general relativity effects.

You are indeed confusing special relativity with general relativity.

Special relativity says that when things begin moving fast, they weigh more. Gold's properties are due in part to the fact that the electrons move so fast that relativistic effects alter the shape and energy of the orbitals that they stay in. That's special relativity.

In order to get significant curvature you would need the amount of gravity to be created to be significant. But electromagnetism is about 10^39 times as powerful as gravity. We only notice gravity on a macro scale because positive and negative charges almost perfectly balance. However on an atomic scale, the theoretical effects of Newtonian gravity are many orders of magnitude smaller than any possible measurement error. Let alone the size of general relativistic corrections to that theory!

You can use the Schrödinger equation in a classical, non-relativistic, space-time, do calculations and get a lot of the chemistry explained more or less correctly. There is no speed of light, and electric fields have immediate effects over long distances.

But to explain the electron transitions that make gold yellow, you need to use the Schrödinger equation in a relativistic space-time. (But just special relativity suffices here, no need to go to general relativity.)

The Everett FAQ makes even stronger claims about the status of Many Worlds. See http://www.hedweb.com/manworld.htm#interpretation

Ah, why do these kind of Physics FAQ always claim to be objective and then obviously are not? This fact commits all the sins of the typical physics FAQ. For example it claim sthat only many-world is scientific, that other interpretation requires extra assumptions. It even labels Copenhagen as being "vitalist". It also does not address some very valid cristicisms:

1. Conservation of energy. 2. Relativistic observers seeing multiple decoherence events in different orders. 3. Experiments that affect whether a particule pair did decohere inthe past. (i.e. quantum state recombination.)

Basically, MW assumes that the probability wave exists and that all outcomes exist, which is a very big assumption, which that FAQ gloss over rapidly. (And then explictly call other interpretations' assumptions as a negative...)

> 1. Conservation of energy.

This is not a problem for MWI.

> 2. Relativistic observers seeing multiple decoherence events in different orders

There is no decoherence in MWI.

There is no decoherence in MWI.

Yes there is. Decoherence provides the theoretical framework to explain why the superposition of parallel observers can never meaningfully interact with themselves again, even though a superposition of simple particles can.

See https://en.wikipedia.org/wiki/Quantum_decoherence#In_interpr... for more.

That said, the order in which an observer notices decoherence events has no particular physical importance.

Why would a MW faq address criticisms of the Copenhagen interpretation?

That is just plain bombastic. It says "All the other theories fail for logical reasons." No reasons given. It has a link, but the very first section in the link says "Copenhagen Interpretation... not fatal, but unpleasant". Well OK, you don't like non-locality. That's not a real argument! And a bunch of the questions about linearity ignore the fact that gravity is clearly non-linear.

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