Hacker News Reader

Comment by mbell | Hacker News Reader

> 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.

View on HN

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

Reply on news.ycombinator.com