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Comment by mikeash | Hacker News Reader

That seems to be begging the question. Qubits do more than this universe is capable of, therefore there must be multiple universes. The obvious refutation is: qubits don't do more than this universe is capable of, because the universe does not run on classical limits. 10^80 atoms implying on the order of 10^80 computations that can be done implicitly assumes that these things operate on classical-ish rules. There's an implicit notion of "a computation" in there, which is assumed to be something roughly like what a standard computer is able to do in a standard machine instruction. What if, instead, the fundamental operations that the universe works on are things like Hadamard transforms?

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Explain the double slit experiment to me. The single photon somehow calculates the interference pattern with photons that don't exist, in our world at least. If there's not parallel universes, how does a single atom calculate that interference pattern? Where do those calculations occur? All other theories say it happens by magic and give all sorts of complicated non-real obfuscations to explain it.

Bohmian mechanics seems like a better explanation than "there are infinite universes out there"

"We don't understand the deeper mechanisms yet" is also a valid explanation - although one that's sure to be unpopular with scientists.

"Entia non sunt multiplicanda praeter necessitatem" and all that.

I'm sure you could confuse the heck out of me trying to explain it. Deutsch would say it's just a clever obfuscation of what's really happening. And his many-worlds explanation is so incredibly simple that my small mind uses Occam's razor to figure out what to believe.

As simple as using God to explain how the universe came to be

It's not about a simple explanation, it's about explaining using simple things.

A wave is simpler than "infinite worlds"

The wavefunction is exactly the same as "infinite worlds". The essential aspect of the Everett / many-worlds view is that the wavefunction is the complete description of reality.

The "worlds" are just approximate ways of slicing up the wavefunction which correspond to our sense of classical reality. They are clearly (and uncontroversially) identifiable if you run a simulation.

Ah that's a better explanation

The "worlds" are just points in the space that supports the wave. You can use any other name, if you prefer, but there seem to be an infinite number of them.

All quantum mechanics has notions of "points in space that support the wavefunction," but these are different from the "worlds" in MWI.

Seems like I have been misunderstanding MWI all the time. How are they different?

Yes, I find the whole "Explain this in any other way!" sort of a bizarre demand. You've got your choice of 5 or 6 decent isomorphic explanations. Preference for "many worlds" seems to boil down to "it annoys my common sense less than some of the others", which is a very personal concern rather than a scientific one.

Personally, I actually think retrocausality, once you twist your brain around it, is another perfectly sensible option that doesn't involve multiplying the universe into a near-infinity of inaccessible, unobservable parallels. The solution in my mind is that retrocausality actually isn't; what it means instead is that what "the present" is is even less like what you thought it was, and is even more of a partial order than relativity already told us it was. The answer to the "double-slit" experiment, to my mind, is that there simply isn't a "before" and "after" and some sort of "retrocausal self-interference" of the wave... the whole thing is simply one atomic event that doesn't have a before and an after, and your imposition of a temporal order on the events is much simplified from the reality. The mystery lies in your attempt to impose a global temporal order, which is appropriate enough at scale, onto a single local event where it doesn't apply in the way you expect. The results of the double-slit experiment itself is not that mysterious, merely counterintuitive, once you release the small scale from your imposed large-scale idea of how time flows.

It means that "the present" is actually quite complicated in a quantum universe with as many qubits as ours has, but, just as QM may be counterintuitive but eventually looks classical at a large enough scale, time may in fact be massively complicated but at scale it still works as we expect.

It's more of a recognition that many-worlds actually has a physical basis to it, while the others talk about an obfuscated non-physical construction.

"Many worlds has a physical basis to it" has no physics-meaning that I can discern. In order to have one, it would have to deviate from the other isomorphisms and cease to be an isomorphism, and thus somehow be testable, which it isn't. (My previous post is word salad to some extent too, since I'm jamming it into an HN post. But the core insight there is basically, let the math be the math and stop imposing your globally-based ideas on the local world.)

It would have a "physical basis" to it if you could visit the alternatives somehow, in which case they could be demonstrated to physically exist and all doubt would be removed. But so far that is not on offer.

Pretty much. Seems that the "shut up and do the math" QM interpretation is still the important one

Bohmian mechanics is just many worlds in denial, for people with prior ontological beliefs they can't give up.

The entire wave-function is there in the theory, containing all the "worlds" and it's responsible for 100% of the dynamics.

Additionally, Bohmians add a single "world particle" indicating which world we are in, and it's guided by the wavefunction. They claim only that particle is "real", but since it has no effect on the state of the universe, I would call it the only part of the theory which isn't real.

There are no photons. There are only photon-like events, with associated probabilities and scattering processes.

It's a lot easier to assume that reality is the visible output of an heroically counter-intuitive but relatively tractable probabilistic calculation process, than it is to assume there are infinite universes with poorly defined boundary conditions between them and even more poorly defined properties, and that by some magical process some, or maybe all, of those universes conspire together in certain situations only to create quantum effects.

The Shor's Algorithm argument is irrelevant. There's a huge leap between saying saying "Qubits can handle superpositions and we don't know why yet" and saying "The only possible explanation is that qubits exist in multiple universes at the same time. Because they just have to. Obviously."

The problem is simple - you're inventing entire multiple universes out of nothing just to explain the behaviour of a single qubit.

That may indeed be how reality works. but it's a very strong claim and it needs some correspondingly irrefutable evidence to support it.

That's exactly what I'm saying! All other explanations say something confusing and obfuscating like "there are no photons". Many-worlds keeps it simple and says "there are photons, and the reason they behave like that is because they're interacting with parallel universes". You can't explain why they behave like that, so you say that the photon isn't really real, and have a complicated explanation of it.

Clearly this is all subjective, but Pilot Wave theory (De Broglie & Bohm) is the simplest least "crazy" of all the explanations.

Horribly simplified: Space isn't empty, there is an "ether". Matter moving through the ether interacts with it and produces waves just like a very light boat in a lake - these crests and troughs exactly line up with the magical waveform. All of the quantum behavior we see is explained by the waves in this "ether".

Ie the waveform is no longer this crazy math thing its a real thing. And just like "air" was considered empty until we discovered a vacuum, space itself is considered empty until we fully define the "ether". It matches up with all of the results of Copenhagen, but was dismissed because physicists like "locality" and it gives up locality.

But guess what? The surface of a lake is non-local. Your boat is affected by boats far away via waves, the effect just diminishes over distance.

And the double slit experiment is so straightforward now that it makes Copenhagen and many-worlds look kinda silly.

While one can take a bit of issue with using the ether as analogy for a number of reasons, the most important difference to keep in mind is that the wave function exists in configuration space. For 10^80 particles, this is a space with 3^(10^80) dimensions. This is the source of nonlocality.

While PWT is the most sensible to me, it is strange though solely because of the wave function which is common to all theories, including MW. The novel part of PWT, that of particles having definite locations at all times being guided by the wave, is quite ordinary.

So here I may be stepping past my knowledge, but I thought that the reason for appearing to require so many would be simply due to using the wrong "basis" to calculate the wave function. Or separating the concept of how to generate the wave function from the values it produces.

Via analogy, let's imaging the perfectly still lake. In the lake at an (x,y) position is a mechanical oscillator which moves up and down and continuously produces waves. Knowing the position of the oscillator and it's phase I can calculate the position of crests and troughs of waves on the surface and the height of the water at any location.

If now I had 100 oscillators spread on the surface of the lake, I would need 100 inputs of x,y coordinates of where the oscillators are and each their phases. In this case i'd need 3100 inputs/dimensions to calculate the wave function.

At first, this seems like a lot; but I don't need 3100 dimensions to describe the resultant wave function. It still is simply the surface of a lake. It can always be described at any moment in time in 2 dimensions (x,y) with a height value for each point of the surface. If you want to capture it changing over time you need 3 dimensions (x,y,t) with height values for each.

The fact I'd need to go through all 3100 inputs to calculate the wave, doesn't mean I need 3100 values to store the results of that calculation. This it what I mean by using the wrong "basis" to describe the wave function.

In programmer terms you can describe a resultant value as either a function, and all of its required parameters. Or you can just describe the output value. 310^80 is describing all of the inputs and the "function" to call.

And one note, on the math. I believe there be 3(10^80) dimensions as inputs. Which would mean R^(3*(10^80)) possible configurations. You wrote 3^(10^80) which is significantly larger.

Error: Yes, it should be 310^80. I think my brain was half-writing the dimension and half-writing the more useful view of (R^3)^10^80. Thanks for catching that.
As for your analogy, I don't quite follow. The actual physical model is that of the oscillators, more or less. The wave is the mathematical abstraction that does allow us to replace it with a function. In terms of information, the wave has more detail, but it is abstracted into a nice form. I think we agree on this.
But the quantum wave function is serving a different role. The water wave is something that varies over the 2d space. As we look at different values, we change the x and y coordinates to see the new values. In that sense, it is 2d.
For the quantum wave function, if we keep all but one of the 10^80 particles in the same exact position, but we vary the position of that one particle, we do get different values of the wave function and it does matter to all the particles as it will affect the gradient vector defining the velocity vector of each of the particles. The change is not localized to just the one particle that is changing, but impacts all particles.
Of course, most of the time, practically speaking, changes by one particle will not make a difference, but situations such as present in Bell's Theorem and quantum computing are exceptions to this.

Fundamentally, the current state of all the particles is needed to be known in order to know what part of the wave function is needed. That is the sense in which the dimension is not 3, but 310^80.

The Pilot Wave Interpretation is one of the strangest contortions I've ever seen. You go to all the trouble to set up the wave equation acting on phase space of the entire universe, but then you remember that you think particles are fundamental elements of reality, so you drop them in and say they're guided by your wave equation.

If you take Pilot Wave Interpretation and just take out the particles, you're left with the Many Worlds Interpretation with exactly the same predictions. To paraphrase Netwon, there's no need for that hypothesis.

> If you take Pilot Wave Interpretation and just take out the particles, you're left with the Many Worlds Interpretation with exactly the same predictions. To paraphrase Netwon, there's no need for that hypothesis.

And if you take Many World and simply add a particle you have Pilot Wave hypothesis. No need for the crazy multiple universes nonsense. All 3 produce (including Copenh) the exact same predictions, so that's not gonna be the distinguishing factor.

Which is more contorted? Waves are physical and particles exist? Or we live in a countless number of simultaneously subdividing universes?

It's fascinating to hear people discuss PW vs MWs - because (in my opinion) it becomes very clear which they became comfortable with first.

If you were to stop a random person on the street and give them a summary of the two theories it's pretty clear that someone unfamiliar with both would say the PW is a ton simpler. But it's fascinating to see that a good number of people who are comfortable with many worlds will argue that it's less complex. My view is obviously their opinion is shaped by what they've been exposed to first.

Copenhagen isn't even a proper theory, because it never tells you precise criteria for when wavefunction collapse occurs. It's not a surprise that nobody noticed this at the time, because the Copenhagen Interpretation was invented prior to the foundational work on computation and formalization of what an algorithm is. But looking at it now, it should be clear that there's no formal definition of what a "measurement" is. As soon as you give precise criteria for wavefunction collapse, then you have a difference in predictions between Copenhagen and MW.

And PW is strictly more complicated than MW, because you still need to track the same wavefunction on phase space, but then you add this particle to have something to point to and say "this is reality". But all the complexity of the wavefunction is still there. If you try to reduce this complexity by collapsing the wave function, you're back to having the same problems as Copenhagen.

> All other explanations say something confusing

The problem here is that you want an explanation that is intuitive and easy for you to grasp. What if the true answer is one that is too hard for you (or maybe any human) to easily understand?

Our brains aren't evolved for understanding the true nature of the universe, they're evolved for reproducing and surviving in a very simple world. We're using re-purposed hardware for a very different task- we shouldn't be surprised that it's difficult.

> Our brains aren't evolved for understanding the true nature of the universe, they're evolved for reproducing and surviving in a very simple world.

Absolutely. For survival-to-reproduction -- and for quite a bit more than that -- Aristotle's physics are enough; but they're full of absurdities and strange edge cases when you look at them more carefully. To explain the flight of an arrow, you need Newton... and then you start noticing that Newton has edge cases too.

I don't pretend to understand quantum mechanics, but I'm fine with cheering from the sidelines on that one. Whatever the answers are at that level, they certainly seem to be shaping up to be something deeply unlike our intuitions...

Feynman said, "I think I can safely say that nobody understands quantum mechanics." This is a guy who won the Nobel Prize for work in quantum mechanics.

QM does not line up with human intuition at all. It looks like this is a fundamental issue with how the universe works, not a problem with our explanations. And really, that shouldn't be any surprise. There's no reason to expect the universe's workings to match our intuition in any situation that's substantially different from what our intuition evolved in.

Feynman said that quote in 1965, and it may have been true when he said it. The Many-Worlds Interpretation was technically invented before then (1957), but was a fringe theory and not disseminated much prior to the 1980s. I don't think you could get away with saying the same quote today.

Implying that many-worlds is the key to unlocking true understanding of QM? What's the basis for that?

Yea, that's exactly what I'm implying. As for the basis for it -- I'm not sure how to put it. Every other interpretation is deeply confused, but MWI gives a deterministic, algorithmic, and non-mysterious explanation.

MWI may or may not be correct, but an adherent definitely would not say they "don't understand" QM.

I thought the various competing interpretations of QM were all exactly equivalent when it came time to actually crank through the math in an algorithmic fashion.

Not exactly equivalent.

The Copenhagen Interpretation has a pretty big theoretical hole in that the condition for when a wavefunction collapse occurs is not defined within the theory itself. Supposedly collapse occurs during a "measurement". Well, what's a measurement? Your measuring apparatus are all made up of matter that supposed to be following the same fundamental physical laws. I think you'd be hard pressed to write a computer program that takes as input a description of all the particles and their wavefunctions and compute when a measurement occurs. In practice, scientists doing calculations under the Copenhagen Interpretation will choose the points in time to perform a collapse so that the calculations match experimental results. This is what I mean when I say that Copenhagen is non-algorithmic. That's a forgivable mistake if you invent your theory before the idea of an algorithm is formalized (as was CI), but today it's not a forgivable mistake.

If you decide to fix the problem with Copenhagen Interpretation by explicitly defining that a measurement is when X happens for some X, then you've allowed for an experiment whose results can differentiate between Copenhagen and Many Worlds. MWI says that different parts of the wave function can affect each other no matter how distant, though the effect size decreases exponentially with distance. So set up an experiment so that X occurs and then be able to measure interference from distant parts of the wavefunction that are either present (in MWI) or not present (in CI). That distinguishes the interpretations via experiment.

Thanks. I must have gotten mixed up with the equivalence of some of these interpretations, and the lack of actual experiments to distinguish the others.

The mix up is not your fault. I see it routinely written that the interpretations don't make any difference to predictions, and I never see anyone talk about or acknowledge the issues I just raised. I feel like I'm taking crazy pills.

> All other explanations say something confusing and obfuscating like "there are no photons". Many-worlds keeps it simple and says "there are photons

OK so far...

> and the reason they behave like that is because they're interacting with parallel universes.

No longer OK. You consider that the simple explanation? You don't think that's obfuscating at least as much as "there are no photons" does?

Inventing infinite universes to explain something isn't simpler than "there are no photons"; it might please you more, but infinite universes is the more complex explanation as it essentially asserts magic. It's far simpler to believe we don't fully understand quantum mechanics yet than it is to believe there are an infinite number of parallel universes.

You have an unusual definition of simple.

If you still don't understand, consider this: MWI only works if photons are kept in a coherent superposition of states.

Supposedly this means each universe defines a state.

But... the photons have to be isolated from their respective universes. Otherwise coherence is broken and you no longer have your coherent superposition.

Given that, what's the rationale for inventing an entire universe around a quantum particle if the math only works if you keep that particle isolated from everything else in that universe?

The atom is a wave, which can interfere with itself.

I think Deutsch would say there isn't actually a wave-particle duality. That's just an obfuscation that doesn't have any reality to it. Many-worlds has a much simpler explanation - there is simply a particle that interacts with particles in parallel universes that produce a wave-like interference effect.

He might say that, but so what? It's clear that he favors many-worlds, but that doesn't mean the rest of us have to. My point is that he's attempting to prove many-worlds by assuming that none of the other explanations are true, i.e. he's begging the question.

Well the other explanations say that the atom doesn't really exist, it's just an abstraction. And many-worlds says the atom does exist, and this is why you see these quantum effects. That's a meaningful difference.

I'm pretty sure that's not what the other explanations say at all.

Pilot Wave is to Many Worlds, as Many Worlds is to Copenhagen.

Pilot wave is the most "concrete". If you're moving away from abstraction, then why stop at Many Worlds?

I think you can just dispose of the concept of particle and everything works fine as waves (with discrete levels where the boundary conditions force so, and with collapse when coupled to a macroscopic system that restricts the possible states).

How is "a particle that interacts with particles in parallel universes" any simpler? That sounds like a crazy special rule. Why and how do individual photons/particles interact with parallel universes when nothing else can do so? Honestly, MWI sounds to me like just a fancy way of saying "magic".

What? So in other universes the interference pattern is wrong?

A photon isn't a computer. It doesn't compute. It acts according to some nature which we equate with computation.

Double slit experiment? It appears that some people, e.g., Feynman, believe that this experiment is profound. I don't have an explanation as good as I would like, but from what I studied in physics class, reading Feynman, etc. here's my explanation.

So, shoot a photon, or an electron, neutron, proton, alpha particle, etc., at the double slit barrier. Only one particle, please.

Well, now give up on the idea that we shot a particle. F'get about particles because they don't exist and only seem to exist from some interactions. Instead of a particle, what we shoot at the target is a wave or, in quantum mechanics, a wave function.

So, the wave function hits the barrier. Nearly all that wave function hits the barrier and disappears. Why? Because physics can't think of anything else it could do. So, what is left are the two parts of the wave function that pass through the two slits. Now that one wave function is in two pieces, but no energy has been lost bumping into the barrier and squeezing through the double slits (amazing thing a wave function) -- but maybe with some barriers some of the wave function is reflected, but let's leave that to the next semester!

The two pieces of the wave function that went through the slits continue on to our array of detectors. As wave functions tend to do, especially as they pass through a small hole or slit, they spread out. By the time the two parts of the wave function hit the detectors, they overlap. But, they are just two parts of the same wave function. So, they interfere and create a sine wave pattern at the detectors. Well one of the detectors, at random, gets a detection of the particle.

So, lesson: What travels is a wave, not a point like particle. The wave can be split into two parts and come back together, and then it interferes with itself in a way different from having waves from two independent (I hope the physics people do say independent instead of uncorrelated) particles hitting the detectors. When the wave is detected the detection is at a point as if there were a particle. Moreover, there is no more than one detection, as if there were a particle.

Still, there are no particles; it's just that when a wave function interacts, say, at a detector, it interacts at a point, just some one point, as if it were a point particle. Amazing.

Amazing experiment.

Generalize the experiment some and get the Michelson-Morley experiment, even more amazing.

> So, the wave function hits the barrier. Nearly all that wave function hits the barrier and disappears.

No, it doesn't disappear. The barrier itself is illuminated. Most of the wave function that hits the barrier is absorbed (or reflected) by the barrier. But the light that hits the barrier is uninteresting so it is ignored.

(On the other hand, if you make a reflective barrier with a lot of slits, not just two, then you have a diffraction grating and things get interesting again.)

When I let go of something, it falls to the floor and hits it at exactly the correct speed. The correct speed accounting for gravity and the air and air currents and all sorts of other things. That's a lot of calculations to make. And it was exactly correct at every instant of the fall as well! Somehow, all these calculations are being made instantly. Where are those calculations occurring?

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