jack: (Default)
[personal profile] jack

Imagine Aaliyah and Bruce lived somewhere no-one had ever seen a lion. One day Aaliyah travels somewhere there are lions and comes home and tries to describe it to Bruce. She probably says things like, "it's like a domestic cat, but the size of a horse".

Now, that's not a perfect description. But it's not bad. I think most people in Bruce's position will get the idea. There's some new sort of animal, one he hasn't seen before. Which is like a cat in many ways (pounces, plays, body shape, etc), and like a horse in some other ways (bigness, mane). And a few ways it isn't really like either (earth-shaking roar). He knows there's a lot about lions he doesn't understand. But he's not confused that there *is* some new creature he doesn't know a lot about, that sometimes looks like a horse and sometimes like a cat.

Specifically, he doesn't stand around saying, "Wow! Isn't it so strange and mind-bending that there is some mysterious animal that is both a cat and a horse AT THE SAME TIME? No-one on earth could ever understand lions".


Now, I'm not sure, because I don't really understand quantum mechanics. But as far as I've been able to tell, this is basically the case for electrons too.

I don't know what electrons are. But whatever it is, it's some physics thing which really, really doesn't behave how our intuition for macroscopic objects says objects should behave. And in particular, sometimes it acts really, really like a small solid object ("a particle"). For instance, it bounces off things, it exists at a particular place (sort of), etc. And sometimes it acts really, really like a wave. For instance, when it goes throw a narrow gap or round a corner, it diffracts and creates interference bands.

As far as I can tell, this is all "wave particle duality" means. The thing that's really there is... quite weird. But if you try to shoehorn it into "specifically as a physical object" category, you get all sorts of further confusion[1]. It's not sometimes one, and sometimes the other. Nor both at the same time. It's *like* a particle, sometimes a lot, sometimes a little. And *like* a wave, sometimes a lot, sometimes a little. And occasionally not a lot like either.

What actually *is* it? There's a lot I don't understand, but I was coming to that.

Footnote [1]

Part of the reason this is so confusing is that it doesn't act like a *single* object. Rather it acts like an object where you have some smooth probability function describing where it might be, but as if that distribution of probability was a physical thing that things could happen to. See following posts.

Date: 2017-04-20 07:32 pm (UTC)
hilarita: trefoil carving (Default)
From: [personal profile] hilarita
An unrepresentative sample of physicists (i.e. ones I talk to at coffee time) really don't think of electrons as *being* waves and particles at the same time. Electrons are a thing that can be *modelled* as waves and particles, and can exhibit both *behaviours* at the same time. Or not, depending on the parameter space. Asking what an electron actually is isn't quite relevant - it's a concept that adheres to certain mathematical properties that allows us to model and predict how the universe works.

Date: 2017-04-21 05:43 am (UTC)
simont: (Default)
From: [personal profile] simont
I think the way in which your lions metaphor is most unlike the real wave/particle business is that it's not just a question of Aaliyah telling Bruce that somewhere out there is a new kind of thing, separate from all the things he thought he already knew about, and not changing his view of his existing world.

People already thought they knew what electrons were: particles. And they already thought they knew what light was: oscillations in electromagnetic fields, propagating through space in a wave motion. And QM said, no, actually these things you thought were particles, they also have wave-like behaviour, and those things you thought were waves, they also have particle-like behaviour, and everything you thought you already knew now needs revising.

So it would be more as if Aaliyah had to tell Bruce that there aren't actually any such things as cats or horses, in spite of the fact that he can see perfectly well that he's living in a country filled to the brim with both. Actually, says Aaliyah, they're all lions, and they've all been lions all along, and you haven't ever noticed before.

Specifically, he doesn't stand around saying, "Wow! Isn't it so strange and mind-bending that there is some mysterious animal that is both a cat and a horse AT THE SAME TIME?"

I think in my revised scenario he might very well consider Aaliyah's revelation to be strange and mind-bending! (Though, in a world so completely filled with lions, he might consider that standing around saying that might not be the best plan...)

Date: 2017-04-21 08:26 am (UTC)
ptc24: (Default)
From: [personal profile] ptc24
Wikipedia has an interesting collection of views.

I worry that your "lion" theory maps onto "wavicle" which doesn't seem to be terribly popular these days.

Part of the problem is "interpretational issues" - the available observations can be explained by several different models, there's no possible evidence that can tell the models apart, and some people are stubbornly saying "no no no, you can't possibly speculate on what things are really like, you just have to say what the observations are and give the associated regularities".

So under shut-up-and-calculate, you're pretty much forced to go on about wave-particle dualities. Many-worlds is basically a wave-only theory (and quantisation causes particle-like behaviour), Bohmian mechanics posits that there are both particles and waves that guide them, thus causing the particles to have wave-like behaviour, etc.

The undergrad chemist's view is that if we talk about things in overall steady states, like atoms, at one level we can talk about particles - we can say, "here's a list of the positions of the particles, they're basically point mass/charges, and they act on each other by ordinary boring electrostatics, you can calculate the potential energy without too much fuss" - and then you define something that's a bit like a probability distribution over those lists - except the "probabilities" are complex numbers - and call it the wavefunction, and then the wavefunction relates to things you can actually observe, "electron density is the square of the wavefunction". This works reasonably well for atoms and molecules, but starts to get mindbending when cats enter the picture. Chemists don't generally worry about how QM relates to things outside their area, not professionally anyway, and so can dodge a lot of the complicated issues that physicists have to grapple with.

You'll notice I haven't said "Copenhagen" yet; AFAICT "Copenhagen" seems to be used for two different things, one is for something like shut-up-and-calculate and one is for some weird thing involving "objective collapse".

Date: 2017-04-21 08:38 am (UTC)
ptc24: (Default)
From: [personal profile] ptc24
A video that has a neat macroscopic model of what a wave+particle system might be like.