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Comment by ridewinter | Hacker News Reader

The phrase "wave-particle duality" obscures the central mystery in quantum mechanics. You can visualize waves quantized into particles - that's not mysterious. But how does a single particle create an interference pattern when there's no other particles for it to interfere with? And how does covering up one slit destroy the interference pattern when there is only one particle going though one of the slits in the first place?

Just saying "wave-particle duality explains it" is false. It was a shock for me to realize that everything taught in physics textbooks was so misleading. (For a good explanation see MWI)

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> But how does a single particle create an interference pattern when there's no other particles for it to interfere with?

The particles aren't interfering with each other. A wave is interfering with itself due to the two slits 'splitting' the wave. The presence of other particles has no baring on this.

> And how does covering up one slit destroy the interference pattern when there is only one particle going though one of the slits in the first place?

With one slit you don't split the wave so there is no interference. The particle doesn't go through one of the slits, it goes through both simultaneously in 'wave form', this is the mystery.

Place a detector in front of the slits. Only a single particle in a single slit is detected. There is no "splitting" of the particle that we can see.

Edit: If you try to actually define "wave function collapse" and "observer" you are back into the same mumbo-jumbo as "wave-particle duality". They are just placeholders that obscure the mystery.

Why can't a particle induce a wavelike field that interacts with slots and itself?

Because there's only one particle that we can observe going through one slit. Cover up the other slit that nothing is going through and there's no interference.

Edit: I think I understand what you're saying. The field isn't separate from the particle - the particle is the quantized field itself.

I believe "everything is fields" is indeed a common interpretation. Particles are an unnecessary component we invent because of our familiarity with "stuff"

This lecture is long but goes into the everything is fields idea https://youtu.be/gEKSpZPByD0

In this interpretation what we colloquially call a particle is essentially a (localised) quantisized interaction between two fields (or self interaction in a single field).

As strange as it sounds, it does make some sense. Think of elementary particles; we never observe them as such, we only observe them when they're interacting with something.

And also "we only observe them when they're interacting" - that process itself, the observing is also just made up of such interactions so at every step of the way there are these interactions, and those are basically the only ones we have direct access to, not the particles themselves. So it makes a lot of sense actually.

> The field isn't separate from the particle - the particle is the quantized field itself.

Yes. A particle is nothing but a very localized wave. Aka. "everything is fields."

(At least that's may layman's understanding having read quite a bit about this in recent years.)

You can model it that way if you want (it's called Bohmian mechanics), but at that point the particle is kind of an extraneous assumption - the wave alone explains most of the behaviour you observe, and you still have to explain why/how the wave is quantised.

It does. Quantum noise causes vibration of particle. Acceleration of charged particle causes EM waves. EM wave is going trough both slits, and then cause interference with itself.

(In Ukrainian) https://www.facebook.com/groups/194031297610629/permalink/10...

Congratulations, that's the first time I've ever seen anyone ask 'what about interaction with the slits?'. Nor have I ever seen the results of a triple-slit experiment!

If you place a detector in front of (or behind!) the slits the wave function collapses and there is no interference pattern. This is all part of the mystery of it, as soon as you 'observe' a particle, in any way, it stops acting like a wave. See the Delayed Choice Quantum Eraser experiment for even more weirdness, a good video on it: https://www.youtube.com/watch?v=8ORLN_KwAgs

Only the Copenhagen interpretation suffers from the mysterious collapse and time travel issues. Interpretations like Many Worlds make it significantly less 'mysterious': https://www.preposterousuniverse.com/blog/2019/09/21/the-not...

Many Worlds is just Copenhagen trying to pitch a science fiction TV series. The issues don't disappear at all, they just get obscured by story telling about other universes.

The more you try to nail down MWI on specifics - like exactly when and how universes appear, and how many there are, and how this process is supposed to map back to a probability distribution in one of the universes - the less and less plausible it becomes.

Thank you, I thought it was just me. Like in what possible state of mind is reasoning about endless universes apparently being born out of thin air all the time (and also dependent on observation and interaction of partciles, but not if there is no interaction, there is also no way to hop between them) easier than somewhat tricky but still quite logical wavefunction collapse? It only requires to stop thinking about these particles as physical balls of matter and instead of a bit different entities which can behave like a particle and like a wave, and like something in between, dependent on what they are forced to interact with each moment...

I'd recommend watching this talk by Sean Carroll: https://youtu.be/F6FR08VylO4

I think it's a much simpler interpretation than the Copenhagen interpretation.

Here's an awesome discussion/debate between Carroll and a major MWI skeptic, David Albert: https://youtu.be/AglOFx6eySE

In this discussion, they cover what they consider "silly" and "important" criticisms of MWI. Respectfully, some of what you said in your comment is placed by them in the "silly" box.

The main criticism I have is that if we have no ability to ever travel between the different worlds or traverse them or transfer information between them, if we are for all intents and purposes stuck in one of these worlds, in our own "fate" so to speak, what practical purpose does a definition serve at all? It just seems like a handwaivy way of explaning all the weirdness that goes on in QM, to say "it happened in the other worlds from which we have split". I have watched some other Carroll's explanations and it is not clear that he sees this problem...

To me it seems there can be two purposes for creating a good interpretation: 1) make it more in line with most people's intuitive thinking about the world, to make them understand the theories better and faster and 2) make it easier to produce new scientific results, or predict other things.

So far I don't see how the MWI manages either of those better than the other models.

Consider that much of the universe is expanding away from us at greater than the speed of light. That is effectively another world already (without any possible information transfer) and all it requires is that you accept relativity + the expanding universe.

Many worlds isn't the universe forking. It is more straightforward to think of the other worlds as probability spaces. The probability space simply divides as we call each division a "world".

Once you start calling physical particles "probabilities", how is that easier to understand than the standard model's probability amplitudes?

I figured probabilities are slightly more intuitive to most, but you're right that it is more correct to call them probability amplitudes described by complex numbers.

> The more you try to nail down MWI on specifics - like exactly when and how universes appear, and how many there are, and how this process is supposed to map back to a probability distribution in one of the universes - the less and less plausible it becomes.

On the contrary. When you look at the specifics it's all very obvious. Look at the wavefunction, not the TV-show verbal explanation, and the answer to your first two questions are very obvious, as is the ill-formedness of your final question.

Your assumption is that wave function collapse creates new universes but MWI is the extreme opposite of that, it arises when you ask yourself "What if the wave function never collapses and every single potential outcome is equally real?"

It's a pretty simple concept. If you create a random hypothetical universe that follows all the laws of physics what reason is there for it to not exist? What constraints beyond the laws of physics ensure that our universe is the only one that can possibly exist?

MWI replaces the spookiness of wave function collapse with the spookiness of quantum decoherence and adds in an equally spooky universal wave function.

Whether this is simpler than wave collapse or hidden variable interpretations is an almost purely subjective opinion.

There are no time travel issues, it's a relativistic problem.

From the perspective of the photon the time stands still, it arrives instantly at the destination.

You can see this in that the quantum eraser experiment only works with photons and not particles with mass like electrons.

"If time travel is possible, then there is no such thing as time."—Kurt Gödel

The quantum eraser does work with particles with mass. Here is a paper that performs the experiment with electrons (specifically a transmon qubit): https://advances.sciencemag.org/content/3/5/e1603159

It works with even larger particles as well such as molecular ions or buckyballs.

That was my idle thought btw (mentioned in another comment), that this "either it's here or it's there" sounds a lot like the way two entangled particles behave in a Bell test. If Alice measures up, Bob measures down... if the particle does through the left slit, it's not going through the right slit.

So my thought was maybe the particle in the two-slit experiment was entangled with itself? Like I said, idle thoughts of a layman so not sure how right, wrong or not-even-wrong this is but hey, always fun to think about QM :)

IANAP. My (bad) understanding is based on trying to grok Scott Aaron's book on quantumn computing, which AFAIK does a really excellent job educating on, among other things, WTF quantumn physics. I am open to being horribly wrong, as I don't even know how to properly tell if I know what I'm talking about.

That out of the way -

The "thing" is neither a particle or a wave, it's a propagating probability amplitude.

With one slit, you get a simple probability distribution. With two slits, you get a complex probability distribution, where the amplitudes are summed (and since they can be negative, sometimes sum to zero)

The detector adds constraints to the probability amplitudes, zeroing part of the complex probability of the double slit, thus reducing it to a non-complex distribution.

Now do you think that propagating probability amplitude is a real physical thing?

Yes, actually! I just went looking and can't find it, but there was a science paper years ago about dropping.... hydrogen ions into liquid helium?

The expectation was that the electrostatic forces would create "bubbles" of empty around the ions, which (the bubbles, not the ions) would then be detected when they reached the bottom.

They found those bubbles. They also found many differently sized bubbles, all smaller, in a size distribution correlating to the probability that the ion would pierce the surface tension of the liquid.

The interpretation was that the smaller bubbles were the "pieces" of the waveform that traversed the surface tension. This was taken as evidence the waveform is an actual thing that actually exists.

I really wish I could find it...

“Can the wave function of an electron be divided and trapped?”

https://news.brown.edu/articles/2014/10/electron

It doesn't make much sense to argue which parts of the equations are real physical. Having said that for me the Aharonov-Bohm effect proves the "reality" of the wave function.

https://en.m.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect

Yes, for mysterious values of "physical". Clearly there is a consistent ordering principle involved, just as there is in entanglement and related phenomena.

But you can't look at a single photon/electron/whatever and get this contextual information out of what we know about the physical structure of the object itself. There is no physical marker we can read "inside" a photon that says it's entangled, or that it's passed through two slits by "interfering with itself".

So the information - and probably the process itself - is contextual. We don't know where this information lives. But it has to exist somewhere, because we can see the effects of its operation very easily.

So it's physical in the sense that we can see physical effects. Photons etc know what they're doing, and we can make excellent predictions about that behaviour. But the exact nature of the entities and relationships that makes this process work, and the way it maps to observable events and properties, remains a mystery.

The obvious implication is that what we see isn't really fundamental. Neither field excitations nor the fields of QFT are the ground truth, and there's another layer of reality that generates them - and probably spacetime too. But that layer has some very unusual properties from our POV, and we're going to need some new metaphors to understand it.

It’s interesting how classical knowledge offsets what we (me too) feel physical and what we don’t. If you think of particles like they’re hard repulsive spheres, does that make them more “real physical”? Why should matter be fundamental and not be emergent from phenomena that are unlike any of our experience?

Disclosure: I got my B.Sc (Physics) in 1986 and I barely scraped through (and have been working in IT since), so fair warning.

If you were modelling the universe and everything in it, say via a simulation, fancy probability distributions would be a good way of calculating behaviour. What if the "stuff of reality" we're looking for (and trying to map the math to), doesn't actually exist? What if the math is all there is? What if behaviour is all that matters?

In primitive cultures, spirits were hypothesised as the animating force behind behaviour of animals, stars, weird people etc. For a long time, it proved a reasonable model until it was supplanted by our more scientific notions of the observable universe. But what if quarks, leptons, photons, are just "spirits" i.e. a good enough approximation given what we know? What if it's "spirits" all the way down? i.e. the only fundamental is the math and everything else is a visualisation (of sorts)?

BTW, that kind of speculative question (and an awesome high school physics teacher), are what made me study physics at university. Shame (in one way) I was more interested in rocking out and drinking beer.

EDITED TO ADD: don't take anything I post too seriously. If you find yourself rolling your eyes, just downvote me and move on.

> What if behaviour is all that matters?

Behavior, observation, accurate prediction and empirical evidence is all that matters in science, yes. What difference does it make if you call it "particle" or "just math"? What practical difference does it make?

If none then that is not a question of science. It might be an interesting question of philosophy or spirituality, but not for science. Science deals with falsifiable hypothesis and accurate predictions and making functional machines.

We know it arrives as a particle. We don’t know it travels as a particle. What we know is that it acts very wave-like when it travels. If I were forced to bet, I’d say it travels like a wave and arrives like a particle. That doesn’t mean “duality”. It means the condition required to arrive somewhere require a set of values that are packet-like. But we insist that it’s a packet all the way through, so the discussion remains confusing to lay people for 100 years.

If you measure before/at the slit you affect the experiment. You affect how it travels through the slits, removing the interference pattern. So yes it’s not split but that has no bearing on when the detector is not at the slit. They’re different situations and have different outcomes.

So you are saying that light is always a wave but when it is being absorbed by an atom it somehow consumes the "entire" wave, no matter how spread out?

I was reading now that photons when created are already at light speed, they don't accelerate. That's because they are in this instance a wave, and a wave is a distortion of its medium; the whole wave can instantly be created at a certain speed (for example throw a rock into a pond, the waves will immediately radiate at speed).

So in reverse then, when absorbed the wave could be cancelled out and it is forced into particle behaviour?

>There is no "splitting" of the particle that we can see.

"that we can see". That's exactly the difference. The observation (measurement) affects the state of the system. That's like quantum mechanics 101.

"Nobody really understands quantum mechanics."—Richard Feynman

This just increases the mystery. The particle only goes through one of the slits only when you're able to detect through which one it's going. When you can't detect that, it goes through both slits. So observation influences whether or not the particle is going through both slits or only through one.

This is to me the most baffling aspect of QM.

TIRF is going to be very hard to explain away here without modern QM.

With a TIRF microscope you use the 'Evanescent Wave' to image fluorescent molecules in macro objects like neurons or diamonds.

Like, you can see, even more clearly, the yeast that makes your beer.

https://en.wikipedia.org/wiki/Total_internal_reflection_fluo...

https://lightmicroscopy.ucdenver.edu/img/seminars/June17.png

Earlier this week I had an idle thought around the measurement problem, did some searching when I came home and stumbled over this[1] paper where they show that the degree of "waveiness" V and degree of "particleness" P of a particle is related to the degree of self-entanglement C:

    V^2 + P^2 + C^2 = 1

They've since done a follow-up experiment[2] to demonstrate it in a quantum setup as well (first paper did classical experiment).

I'm just a layman so have no idea how profound this is, but I found it interesting.

[1]: https://www.osapublishing.org/optica/abstract.cfm?uri=optica... (open access)

[2]: https://arxiv.org/abs/2001.01749

What. So cool. Thank you!

Particles don't really exist. Nor do waves for that matter either. Generally these days, it's considered more correct to think in terms of fields and excitations. Even then though it's important to realize that what we're working with are models. Very powerful and extremely accurate models, but still models nonetheless at the end of the day.

Yes, true. The particle is a quantized excitation of the field that travels in a wave. The original point remains.

Unfortunately in my experience people aren't fond of the more correct explanation, which is that entities on the atomic scale don't behave in ways directly analogous to anything humans are intuitively familiar with.

People aren't fond of that explanation because it isn't an explanation.

"Why do some stars make loop-de-loops as they traverse the sky?"

"Because entities on the celestial scale don't behave in ways directly analogous to anything humans are intuitively familiar with."

"Oh..."

Well... they don't, unless those humans are differential geometers.

....Or play enough KSP.

KSP doesn't simulate GR

:crosses fingers for KSP 2:

Sure. But, AFAIK, newtonian physics covers most of the behavior of the planets; that GR affects the timing rather than the paths. Or am I substantially wrong?

Does it not strike you as a rather peculiar viewpoint that everything that exists should be easily understandable by untrained humans? That's quite a thing to expect from the universe.

It's not just "not an explanation", it's declaring that there _is no_ correct and easily understandable mathfree explanation or analogy. The only way you can properly understand things like this is within the context of the experimental measurements we've made and the mathematical models we've developed as a result. There's no way to explain these to a five year old in a way that's not significantly incorrect (unless perhaps they happen to be uncommonly good at linear algebra) .

The current layperson explanation is so significantly incorrect that I think it's actually detrimental. Photons aren't "particles" like marbles, and they're not also "waves" like you see on the ocean. They're sure as hell not both at the same time in some sort of religiously inspired "don't question it" union.

What they are are entities, abstract things that we can create and play with and measure. We can make somewhat complicated mathematical models to predict certain aspects of their (and similar entity's) behavior. They at times exhibit behavior one would expect from a particle. In other cases they exhibit behavior one would expect from a wave. They're neither though, they're their own thing that behave by their own rules. That's it.

Don't forget, kiddos! You're made of star stuff ...and that star stuff is made of math... and that math is not just any math, it's probability math.

But we're not intuitively familiar with what happens when those stars get really big and start being black holes and exhibiting relativity. Fortunately, like waves, we can manage the maths.

> People aren't fond of that explanation because it isn't an explanation.

It absolutely is an explanation. The explanation is not just "they don't behave as neutonian macro objects" - that's the a beginning of it. The explanation itself comes after when physicists describe in exact ways how these quantum objects behave, explained so specifically that it can be calculated upon with specific equations.

The fact that those explanations don't fit in your current set of intuitions about world and objects in it is more of a "you-problem", not "physics-problem". The universe has no obligation to make sense to you, it is more of your job to create those new intuitions in your mind and those new models of the world. While the physicists have provided you with all the needed tools, it is still up to you to build them into your mind.

Quantum physics can be explained in single picture. I did it (in Ukrainian): https://www.facebook.com/groups/194031297610629/permalink/10... .

It's easy to explain.

Any way to see that from a non-FB URL?

It awfully simple. I took screenshot from this video: https://www.youtube.com/watch?v=nsaUX48t0w8 , then label things: particle, wave, interference, and quanta. Nothing special at all.

Pilot wave theory is simplest and most obvious interpretation IMO. We already accept the Higgs field that permeates all space-time (and predictions of that theory were verified by discovery of Higgs Boson) so why can't there be waves in that or another omnipresent medium that these particles are "riding" on? These double slit, wave-particle dualities, and all the other bizarre notions (apart from non-locality which both predict) vanish with the pilot wave theory. Never understood why Copenhagen interpretation has so much mindshare except it's more sensational, controversial nature and the personalities that advocated for it.

Isn't a better and "simpler" explanation, Pilot wave theory?

Particles are like boats on a lake - they both cause waves when they move and are moved by relfections of their own and other boats' wakes.

Interference is simply waves generated by a boat interfering with other waves from the boat.

Unless your boat can simultaneously pass through two different arches of the same bridge, I don't really think that helps.

The wave a boat makes can certainly travel through two arches in a bridge.

Boat cannot, but wave can.

https://www.youtube.com/watch?v=nsaUX48t0w8

Pilot wave theory isn't actually simpler because the pilot wave must travel throughout the entire universe instantaneously. According to the theory, classical information cannot be sent along the wave so it does not violate no-communication, however the prospect of having a wave that travels instantaneously and permeates the entire universe is still hard for many to accept.

Does it require that? Can you elaborate? I haven't heard of this issue with Pilot Wave Theory before.

It's not just an issue with Pilot Wave, many QM interpretations are non-local. This table shows which are vs. are not (Local Dynamics column): https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...

Nope, it doesn't require that. In hydrodynamic quantum analogs, waves are moving slowly, but they still cause quantum effects.

>Pilot wave theory isn't actually simpler because the pilot wave must travel throughout the entire universe instantaneously

Given that quantum mechanic is non local (quantum teleportation), I don't think that this is the big issue. Why QM is non-local yet cannot send data faster than light, that's much more unexplained.

Quantum field theory is local (which explains why we cannot send data faster than light), and also fully explains QM.

Relativity showed us that our assumptions about space and time being constant were wrong and instead light is a constant.

Similarly, QFT shows us that our measurements do not have single outcomes and never did. Instead reality is made up of quantized waves whose amplitudes are complex numbers and can interact constructive or destructively as long as they remain coherent.

Interestingly, there is a (possibly still incomplete?) model of QM that explains this mystery and can even be replicated in human scale experiments: The de Broglie-Bohm pilot wave model.

In this model, both the wave and the particle are simultaneously real. the particle is being moved around by the wave. When we get near the double slit, the wave naturally travels through both slits and forms an interference pattern.The particle randomly moves through one of the slits, and is them carried further by the interference pattern, the highest probability being that it will be carried to one of the local maxima.

The nicest part is that this exact behavior can be seen by bouncing a droplet of oil or silicone on a vibrating bath of the same material: https://www.youtube.com/watch?v=nsaUX48t0w8 .

Of course, this being a consistent model for QM, it doesn't get away from the Bell inequalities, it doesn't have local realism. It does preserve realism, but it it is a nonlocal theory (some effects travel instantaneously).

Moreover, we can label things in this experiment, because we see cause and effect.

We can clearly label particle (droplet), wave (around droplet), interference caused by wave with itself (when it goes trough both slits), and quanta (single wave, distance between two valleys).

This reminds me of something I read about the time invariance in QED. I forgets the details, but I think it was Feynman who said that when you learn QM, you learn about these equations which are time invariant, they produce symeytical equations where the left side is a particle is travelling backwards in time, and the right side is travelling forwards in time. And your professor waves his hands and says we'll deal with that later. Then eventually at the end of your studies you just wipe the left side off the board and ignore it.

I highly recommend PBS Digital's Space Time for a great overview (and deeper mathy dives) into modern physics: https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g

There are lots of videos about the various interpretations of quantum weirdness through a robust scientific lens, while still being entertaining enough to watch even if you don't grasp all the details.

I’m fond of the deterministic Bohmian interpretation myself: https://en.wikipedia.org/wiki/Pilot_wave_theory. And there’s nothing particularly obscure about it...

Bohmian mechanics is very obscure. It is explicitly non-relativistic (unlike the universe we live in), and sometimes predics very weird trajectories: https://www.degruyter.com/view/j/zna.1992.47.issue-12/zna-19...

There is a reason why almost nobody is taking it seriously.

You link to a paper (ESSW) that walked through a theoretical experiment that they claimed would show it as being non-relativistic, the paper actually argues it was "surrealistic", not "non-relativistic". That said they don't actually perform the experiment, they just claim it would falsify things. They don't get to claim something is false because they think it would happen if they did the experiment. Actually do the experiment and see what happens.

> A real experiment along these lines appears feasible, as all components of the apparatus have been realized separately; for details consult Appendix A. The purpose of the proposed experiment is to verify the predicted quantum theoretical correlations: Whenever an atom hits the screen in the lower region A, the upper detector says "yes" - and likewise for the upper region B and the lower detector. Since we know that (some of) the retrodicted Böhm trajectories pass through the other detector, this verification suffices to demonstrate the asserted metaphysical character of the Böhm trajectories.

Now all of that said, a group actually did the experiment and showed that they only way someone would incorrectly conclude the trajectories were surrealistic would be if they mistakenly ignored the non-local nature of their behavior. They actually performed an experiment and demonstrated the results.

https://advances.sciencemag.org/content/2/2/e1501466.full

> We have verified the effect pointed out by ESSW that for a WWM with a delayed readout, Bohmian trajectories originating at the lower slit may be accompanied by WWM results associated with either the upper or the lower slit. However, this surreal behavior is merely the flip side of the nonlocality we also demonstrated. In Fig. 3, we showed that the trajectory of photon 1 depends on the choice of measurement (polarization basis) for photon 2. In Fig. 4, we see that the polarization of photon 2 depends on the choice of when (that is, at what point along the trajectory) to measure the position of photon 1. This nonlocality is due to the entanglement of the two photons, which, in Bohmian mechanics, makes their evolution inseparable even when the photons themselves are separated. Because entanglement is necessary for the delayed measurement scenario of ESSW, this nonlocal behavior is to be expected and is the reason for the surreal behavior they identify. Indeed, our observation of the change in polarization of a free space photon, as a function of the time of measurement of a distant photon (along one reconstructed trajectory), is an exceptionally compelling visualization of the nonlocality inherent in any realistic interpretation of quantum mechanics.

Essentially the claim in the paper you linked was incorrect, and experimentally demonstrated to be so.

Of course there is no proven interpretation, but I quite like MWI. From my understanding of the theory, the particle does not interfere with itself: the particle follows all possible trajectories in each different 'world', and we observe but one of those trajectories. Once multiple particles are fired, we are _overwhelmingly_ likely to observe a wave-like detection pattern.

One poor universe, however, sees no evidence of wave-like behaviour at all!

IMHO it's weirder than that. Here's my bad explanation of a theory, and uneducated interpretation

There's a paper out there about positing a series of non-quantum universes (particles that aren't also waves), except where each universe exerts a force on all other universes to not be identical. Apparently, under this framework, for N such universes you get an N-step approximation of the wave form (reaching the wave form as n->infinity).

Which is pretty cool in an of itself, _and then_...

We observe the wave form. Does that then mean we're observing across all of those universes?

that a particle behaves at the same time as a wave and a particle is the mystery

MWI doesn't explain much. Why should this electron be able to cross universes and talk to another one and interfere, while nothing else can cross universes?

Can I setup my experiment so that quantum decay decides if I emit an electron or not - and then wait for no electron, but I somehow detect the electron that was released in a another universe?

MWI also doesn't explain why the individual electrons would take different paths and interfere with each other - the universes are identical, so why are the electrons taking different paths?

Why should this electron be able to cross universes and talk to another one and interfere, while nothing else can cross universes?

See "decoherence": https://en.m.wikipedia.org/wiki/Quantum_decoherence

> cross universes?

MWI is a local theory. Changes spread out at the speed of light, they just spread out (and back from) adjacent universes as well. https://www.hedweb.com/manworld.htm#local

> the universes are identical...

Think of a 3D volume of space with a plane wave travelling through it, such as a beam of coherent laser light. An alternative way of thinking about this is that each point emits a spherical wave and the plane wave is just constructive interference of a continuum of spherical waves.

In many worlds theories, you can think of each point as also spreading out into non-spatial dimensions that are directions towards other universes. Similarly, aggregate behaviour is the constructive interference of not just the points in "one" universe, but also the points in a neighbouring "volume" of universes.

If the neighbouring universes are identical, or nearly so, it's like the coherent laser scenario. If they're all different, it's more like white light -- it's unpredictable and "random", but otherwise it's still just ordinary light.

Very slight deviations would result in various interference effects, such as the double-slit scenario.

These are exactly the sort of thing MWI precisely explains. It take time to go through, but this is the best explanation I've ever seen https://www.lesswrong.com/posts/apbcLXz5zB7PXfgg2/an-intuiti...

+1, this series of posts is my favorite explanation of it. I love the focus on it from the philosophy of science angle.

Because "world" doesn't actually mean anything - it's an analogy that is intuitive and snaps into place with the math for some people but not others. The intuition that works for others is "collapse", for me it's massive informational entanglement (information is physical).

I don’t think MWI would explain why particles from different universes should be able to interfere with one another. I’m not sure what that even means. But quantum mechanics and MWI certainly does explain precisely in what ways they do interfere.

The universes aren’t identical, of course. There’s a universe for every path of the electron that is physically possible.

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