Is Quantum Mechanics REALLY "Unintuitive?"

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Is Quantum Mechanics REALLY "Unintuitive?"

Postby snow5379 » Fri May 31, 2013 2:47 pm UTC

I hear people say this a lot, like, on the quantum scale things act weirdly... but a lot of this "weirdness" I think comes not from the actual mechanics but the way they're poorly explained by people trying to explain it to others. For example with the double slit experiment I hear people saying that the particle is going through "both slits" and it matters if you're "watching it" or not. That is so terribly untrue and misleading since (with the Copenhagen Interpretation) the particle's location is in a superposition until you measure it. Knowing that the result of the experiment is suddenly logical, intuitive, and not "weird" at all.

I'm still learning quantum mechanics, and I find the hardest part of learning it is dealing with how people try to teach it and compare particles to other things. Why compare a particle to a ball, for example, when it acts nothing like a ball? That just adds a pointless and distracting information and confuses people. Furthermore you get people who misunderstand quantum mechanics then try to teach it and somewhere down the road you get something like this: http://www.youtube.com/watch?v=u6HWbs11zdU.

Finally it just bugs me that people say quantum objects act "differently" than macroscopic objects. Yeah a tiny, tiny cat would act different than a very, very large dog, not because one is large and one is small, but because one is a cat and one is a dog. You're comparing apples to oranges and size doesn't really have anything to do with anything... unless you've seen a very large hydrogen atom or a cat that's the size of a proton...? Given their properties everything acts exactly like it should act and I'm not seeing any weirdness.

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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby tomandlu » Fri May 31, 2013 3:00 pm UTC

snow5379 wrote:I hear people say this a lot, like, on the quantum scale things act weirdly... but a lot of this "weirdness" I think comes not from the actual mechanics but the way they're poorly explained by people trying to explain it to others. For example with the double slit experiment


I think the double-slit experiment is one that people can understand if they take even a moderate interest in QM. Where I run into trouble/doubts is when it comes to the sheer proliferation of particles.

(drifting OT here) It's an unscientific prejudice of mine, but I think QM at the present time is not just incomplete, but incomplete in a way that is 'wrong'. As an analogy, I'd say it's still at the Lamarkian* stage of rightness, and we're still waiting for Darwin...

* Lamarkism isn't a bad stab at solving the problem of evolution, so this really isn't intended as derogatory.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby Tchebu » Fri May 31, 2013 3:07 pm UTC

It's an unscientific prejudice of mine, but I think QM at the present time is not just incomplete, but incomplete in a way that is 'wrong'. As an analogy, I'd say it's still at the Lamarkian* stage of rightness, and we're still waiting for Darwin...


I'd say QM is just as complete as classical mechanics is, the difference between the two is just that the assumptions that go into setting up QM's framework seem less intuitive than those of CM, but that's to be expected (edit: and completely irrelevant). Otherwise both theories possess the full package that's expected of a theory: a space of states, a set of observables and equations of motion which govern the evolution of the system in question.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby Copper Bezel » Fri May 31, 2013 3:30 pm UTC

It's odd to compare CM and QM in that sense. I mean, to take tomandlu's comparison, it's a bit like comparing Darwin's own theories with the neo-Darwinian synthesis. Classical mechanics is "complete" because it has a defined space that it covers. Does QM really have a defined limit, where new knowledge at the edges would fall outside of the realm of QM?

snow5379 wrote:I'm still learning quantum mechanics, and I find the hardest part of learning it is dealing with how people try to teach it and compare particles to other things. Why compare a particle to a ball, for example, when it acts nothing like a ball? That just adds a pointless and distracting information and confuses people. Furthermore you get people who misunderstand quantum mechanics then try to teach it and somewhere down the road you get something like this: http://www.youtube.com/watch?v=u6HWbs11zdU.

Finally it just bugs me that people say quantum objects act "differently" than macroscopic objects. Yeah a tiny, tiny cat would act different than a very, very large dog, not because one is large and one is small, but because one is a cat and one is a dog. You're comparing apples to oranges and size doesn't really have anything to do with anything... unless you've seen a very large hydrogen atom or a cat that's the size of a proton...? Given their properties everything acts exactly like it should act and I'm not seeing any weirdness.

Well, not everyone thinks that way. I mean, that's a very abstract way of approaching the problem, and I don't think that most people could so readily remove their everyday experience with macroscopic objects from their interpretation of quantum interactions. So you don't use a video with little spheres and instead explain that quanta are distributed waves - a concept that's already an abstraction and in some ways a metaphor, since people have seen waves in a medium but not waves that don't travel through a medium - that sometimes interact with each other as "points," except that those points actually have a cross-sectional area....

People who aren't prone to running abstract and arbitrary simulations in their heads - I'd wager that's most - are just going to picture all of that through images of waves in water and little disks running on little axis lines. Or wires. = ' I guess that enough geometry homework and video games can eventually retrain people to visualize in metaphors other than the ones that we're constantly exposed to in physical reality. But I don't think you can fault people for falling back on what they can visualize.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby tomandlu » Fri May 31, 2013 4:34 pm UTC

Tchebu wrote:I'd say QM is just as complete as classical mechanics is


Really? When parts of the theory rely on theoretical particles, and with no grand unification, this seems a stretch...
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby Tchebu » Fri May 31, 2013 5:09 pm UTC

I guess I should distinguish between the formalism and the models. The formalism, i.e. the model building toolkit, of QM is just as complete as the formalism of CM. The individual models themselves may or may not be complete, but there's no indication that the formalism of QM is the problem rather than the particular choice of model. The problems that the standard model or quantum field theory in general might have are model-specific and not at all of the sort that classical mechanics faced in light of the first quantum phenomena.

Also, when people talk about QM being unintuitive i'm pretty sure they mean the formalism, not the particular models. People don't go "wtf? tau neutrinos?", instead they go "wtf, superposition of states?"
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby PolakoVoador » Fri May 31, 2013 7:05 pm UTC

Yeah, if most people find it really bizarre and going against their intuition, then it is by definition "unintuitive".

And for the "apple and oranges" the OP cited: imagine you find a remote civilization, the only fruit they've ever seen is an apple and you've got to explain them what an orange is. You can say oranges are kinda like apples, even though you know this isn't quite right, but those guys have no concept whatsoever of what a citric fruit even is, so meh, good enough.

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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby thoughtfully » Fri May 31, 2013 7:29 pm UTC

The dual slit experiment, wave-particle duality, and being is many states at once is one level of weirdness. Entanglement, delayed choice, quantum eraser, and the Elitzur–Vaidman bomb detector are another level that really challenge intuition. Of course everything I just said is pretty subjective. The point being is that if you think you understand QM (and survive the encounter without having been cognitively dissonated), then you probably don't, as Bohr famously said. I believe Feynmann had different quote that conveyed a similar idea. Ahh yes, it's there at Wikiquotes too.

I'm going to go out ona limb here and generalize. I think a lot of QM's weirdness is due to its statistical nature. CM has this too in statistical mechanics, but it's understood that there's a underlying determism there, and the statistical bit is a (in principle) shortcut.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby Dopefish » Fri May 31, 2013 7:39 pm UTC

It's certainly possible to train your intuition so things don't seem unintuitive anymore, and once you get familiar with all the relevant material it's quite possible to intuit quantum results without actually calculating them first.

Out of the gate everything is unintuitive though, even things as self-apparent as your own existence when the 'gate' is the womb. Plenty seem to find gravity counter intuitive as well at first ("Of course heavier things fall faster!"), and same goes for algebra, Newtonian mechanics in general, special relativity, quantum, general relativity and so on.

I like to think quantum isn't super counter intuitive to me and I find GR far less intuitive compared to quantum for whatever that's worth, but I suspect if I was more familiar with differential geometry and tensor analysis and such that could become intuitive too.

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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby nitePhyyre » Fri May 31, 2013 7:43 pm UTC

snow5379 wrote:I hear people say this a lot, like, on the quantum scale things act weirdly... but a lot of this "weirdness" I think comes not from the actual mechanics but the way they're poorly explained by people trying to explain it to others. For example with the double slit experiment I hear people saying that the particle is going through "both slits" and it matters if you're "watching it" or not. That is so terribly untrue and misleading since (with the Copenhagen Interpretation) the particle's location is in a superposition until you measure it. Knowing that the result of the experiment is suddenly logical, intuitive, and not "weird" at all.
"superposition" and 'going through "both slits"' mean the same thing. So does "watching it" and "measuring it". Instead of that I could write "collapse the wave function" instead and we would have all said the same thing. None of the terminology matters as to how intuitive something is. It doesn't matter whether you say that the particle is in superposition or not. The fact remains that a single particle interferes with itself. That's unintuitive! The fact remains that simply observing this particle in super position changes how it behaves. That's unintuitive!

As long a an explanation isn't obfusticated, I don't understand why you think terminology changes whether or not a phenomenon is intuitive.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby gmalivuk » Fri May 31, 2013 7:45 pm UTC

Yeah, after training yourself lots of things become intuitive, but when we describe things as counterintuitive we're obviously not talking about going against the intuition of experts in the field.

snow5379 wrote:That is so terribly untrue and misleading since (with the Copenhagen Interpretation) the particle's location is in a superposition until you measure it. Knowing that the result of the experiment is suddenly logical, intuitive, and not "weird" at all.
Sure, if you ignore the part where nothing we encounter and interact with on a daily basis appears to act even remotely like a particle sent through a pair of slits. How come an electron interferes with itself and presents a diffraction pattern, while your apples and oranges would do nothing of the sort? How come the superposition is always resolved into just one single point on the screen for each electron sent through? Intuition provides no reason to expect a "spread out" particle to behave like this.

Saying the location "is in a superposition" doesn't magically make things intuitive, unless you can also explain all the other things that happen in an intuitive way.

So, can you?
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby Dopefish » Fri May 31, 2013 8:28 pm UTC

I don't think you need a lot more exposure to quantum then to basic gravity to get the intuition for it, it's just people have a lot of exposure to gravity and other 'intuitive' things from their day to day lives so they've already become experts in some sense on those everyday macro scale phenomena.

It's not clear to me that the results of the double slit experiment are inherently any less intuitive then the idea that objects continue to exist even when we can't see them. Double slit is usually one of the first things people are taught about quantum mechanics, so such a persons experience in the field there is likely measured on the scale of hours, compared to a variety of 'intuitive' phenomena which we've already spent days/months/years experiencing.

I don't think one needs to be an 'expert' to the degree of knowing practically everything there is to know about the particular field to build intuition about the field, it's just that ones intuition will be proportional to their experience with it. IMHO, we have a lot of experience with classical phenomena so they seem intuitive is all, as opposed to classical phenomena being inherently more intuitive.

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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby gmalivuk » Fri May 31, 2013 8:49 pm UTC

No, they are inherently more intuitive, because we inherently have more experience with them.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby Tchebu » Fri May 31, 2013 9:16 pm UTC

well... not with feathers and hammers falling at the same rate...

But you're definitely right when it comes to the notion of the world being made of objects occupying a volume and being localized and following single trajectories. Those are definitely more intuitive and I think part of what the OP is complaining about is pedagogical attempts to bring things back to such notions when explaining QM, when the truth of the matter is that quantum objects are simply not like that. However, we also have a rather decent intuition for the behavior of vectors, so QM doesn't HAVE to be unintuitive, it just requires that you realize that the state vectors represent things that don't look anything like arrows IRL...
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby lgw » Fri May 31, 2013 10:02 pm UTC

I think one of the biggest shortcomings of modern quantum physics is the inability to explain/teach it in intuitive terms and metaphors. Coming up with such an explanation would be very hard work, IMO on par with a breakthrough new theory, but perhaps without the acclaim that would accompany discovering a GUT or non-annoying replacement for the Standard Model. OTOH, Feynman diagrams get a lot of respect, and were also "just" a better way to talk about what was already known.

The problem in coming up with an intuitive model is that, clearly, thinking of a photon or electron as a little ball or as a wave isn't even a good starting point, or someone would have done it by now. Starting over with a radically different basic idea of a particle seems quite challenging, because if it doesn't lead you nicely to a place where all the math works, what's the point? And it would take years to see that any arbitrary new approach actually worked out, and presumably many attempts to find one that did.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby doogly » Fri May 31, 2013 11:01 pm UTC

tomandlu wrote:
Tchebu wrote:I'd say QM is just as complete as classical mechanics is


Really? When parts of the theory rely on theoretical particles, and with no grand unification, this seems a stretch...


"Particles" are just not as important to the world as you think. Sorry?

Quantum mechanics is deeply, deeply unintuitive. It is not impossible to learn though. You want to go grab a bunch of hydrogen spectra, say, and try to intuit the rules of quantum mechanics from that, and you will get a gold star.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby idobox » Sun Jun 02, 2013 6:43 pm UTC

I had to learn a little bit of QM, especially with semiconductor stuff, and I have been able to wrap my head around the probabilistic behaviour, the wave-particle thing and tunnelling, but even though I've worked through the math, superposition, spin, entanglement and the Heisenberg principle are just alien.

Seriously, if someone can give me an intuitive understanding of why measuring impulses must destroy information on position, I'd be thankful.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby doogly » Sun Jun 02, 2013 6:59 pm UTC

It doesn't destroy the information. The information was never there to begin with, and by measuring momentum you are now not going to cause the position to become definite. It's not like there was some information or definiteness which you now destroyed or blurred.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby eternauta3k » Sun Jun 02, 2013 7:12 pm UTC

You can increase the spread of a wavefunction in p space by measuring r to high precision.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby doogly » Sun Jun 02, 2013 7:24 pm UTC

Sure, though I don't think there is a sense in which you had information that was destroyed. But this probably isn't helpful.

I think the easiest way to make sense of the position / momentum uncertainty is to just take the Fourier perspective. It is just like the purely classical uncertainty in signal analysis of frequency and pulse duration.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby idobox » Mon Jun 03, 2013 2:19 pm UTC

eternauta3k wrote:You can increase the spread of a wavefunction in p space by measuring r to high precision.

I know how it works, I know how to do the math, but I don't have an intuitive understanding of it.

doogly wrote:I think the easiest way to make sense of the position / momentum uncertainty is to just take the Fourier perspective. It is just like the purely classical uncertainty in signal analysis of frequency and pulse duration.

The analogy works for a fixed number of samples, but that's an artificial limit. With proper measurements, you can get both frequency and duration with arbitrary precision.
This is an intuitive understating of the principle, all right, but it assumes a universal limit on the number of samples, which is itself counter-intuitive.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby doogly » Mon Jun 03, 2013 2:28 pm UTC

Making a measurement is like putting a pin on a p vs x phase space chart, but the area of the pin itself must be hbar? But you can change the shape of that area?

Wait though, how do the sample numbers come into play in the signal processing analogy? I thought it was just a pure result of Fourier transforms.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby Tchebu » Mon Jun 03, 2013 3:01 pm UTC

... Yeah, what?... any finite duration signal must be composed of a whole Fourier integral worth of frequencies... how can you measure both precisely?
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby idobox » Mon Jun 03, 2013 9:13 pm UTC

The frequency precision is given by the length of the sample, and the length precision by the sampling frequency.
So if you sample your signal for a very long time with a high frequency, you get good precision for both frequency and length. I'm too lazy to work out the math, but I expect you'd get something like delta f * delta t = (nb of data points)*(some constant, probably pi or 2*pi)

What's counter intuitive is that the precision of your measure can be worse than what your device can do, just because you measured something else before. When the cops measure the speed of your car on the highway, you still know on what lane you are. And it works with macroscopic waves too.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby doogly » Mon Jun 03, 2013 9:31 pm UTC

yeah no no, this is not how fourier go go

let's say I want to make a pulse out of pure sine modes. if I want that pulse to have a delta function duration, it has to be all frequencies, and vice versa. or I can do something intermediate. this sort of thing.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby eternauta3k » Mon Jun 03, 2013 10:37 pm UTC

The bound on delta x times delta k comes straight from fourier transforms. No sampling is involved.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby tomandlu » Tue Jun 04, 2013 7:51 am UTC

So, basically, QM is intuitive provided you've spent your life working with it? This may be at odds with most standard definitions of "intuitive".

Still, remarkably little in science is 'intuitive' - chemical reactions, electro-magnets, etc. I'd suggest that the issue isn't one of intuitive/unintuitive as comprehensible/incomprehensible, and QM, for the casual reader, is rather definitely on the right-hand side of that distinction (unlike chemical reactions, etc.).
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby Dopefish » Tue Jun 04, 2013 7:58 pm UTC

The point I was raising is that it's not obvious to me that for a given amount of time exposed to <classical phenomenon> that your intuition about <classical phenomenon> would be any better than if you had the same amount of exposure to <quantum phenomenon>.

Since of course we live in a world we typically perceive on macro scales and speeds where everything is classical, of course we're going to have significantly more exposure to classical stuff, and hence our classical intuition will be better. If you instead compared the quality of intuition of a quantum oriented undergrad about quantum stuff to the quality of intuition of a newborn about classical stuff, I'm not sure that the baby would do any better (or at least not as a result of it being classical material).

There's certainly a lot more obstacles to even beginning to train ones intuition regarding quantum matters (mathematical prereqs on top of us being classical beings), and it's probably not helped by the occasional attempt at giving classical explanations so that people build an intuition that's wrong right from the start. If you try and apply classical intuition to quantum stuff it won't go well, and similarly in reverse (unless they're particularly adept at quantum anyway).

I imagine a similar argument could be made with languages. If you take someone who was a monolingual English speaker and tried to teach them, say, Mandarin, they'd probably find it unintuitive. I don't think that's necessarily because Mandarin is inherently unintuitive, it's just something different, and I think classical intuition compared with quantum intuition is similar.

It seems wrong to me to dismiss any quantum intuition because someone spent time building it up and so it's not what people mean by intuition, and yet not dismiss classical intuition when people have necessarily spent even more time (indeed, their entire life) building it up.

Sure, if you're defining 'intuition' as 'classical-like' then by definition quantum mechanics is unintuitive, but that seems a rather narrow definition.

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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby doogly » Tue Jun 04, 2013 8:31 pm UTC

People's intuition for classical physics is also pretty shitty though.
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby stianhat » Wed Jun 05, 2013 8:04 am UTC

Well, to chip in my two pieces of eight.

I have never found the deeper underlying parts of quantum mechanics especially intuitive, however the most popularized and common parts of it are quite intuitive if you are adept at visualizing stuff as waves and know how waves behave. Then all you need is accept the wave-particle duality and you are set for the introductory part. If you ask the electron a wave-like question you will get a wave-like answer. To me quantum chemistry becomes a little bit fuzzy around the intuitive edges when plowing into the amplitude / probability case and it gets thoroughly messy when trying to construct a casual relationship between stuff. It has been a while since I was this deep into it but I remember wracking my brain over it - finally coming to the conclusion that acceptance was more important than understanding at that point. Shut up and calculate =P

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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby DeGuerre » Thu Jun 06, 2013 4:28 am UTC

I think that QM is more mathematically intuitive than CM, in the sense that the laws just make more sense. Fields which are algebraically closed (e.g. complex numbers) make more sense than ones which aren't. A system of "probabilities" which obeys a 2-norm makes more sense than one which obeys a 1-norm. Most of the "weirdness" is a natural consequence of axioms which make more sense than their classical alternatives.

So in that sense, I don't believe that QM is harder to understand than CM. It's just harder to believe.

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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby SU3SU2U1 » Thu Jun 06, 2013 5:59 am UTC

I think that QM is more mathematically intuitive than CM, in the sense that the laws just make more sense. Fields which are algebraically closed (e.g. complex numbers) make more sense than ones which aren't.


I think maybe you just haven't seen the deeper formalisms of classical mechanics. Why is the Poisson bracket any less intuitive then the commutator?

Most of the "weirdness" is a natural consequence of axioms which make more sense than their classical alternatives.


I disagree, the problem with quantum "weirdness" is that the existing formalism is either circular or incomplete- classical mechanics needs to be a limit of quantum mechanics, but its also explicitly referenced in the axioms. The measurement axiom as Von Neumann formulated requires classical objects to do the measuring, which leads to circularity.

There have been attempts to do better, but no one has a 'clean' set of quantum axioms. There is a reason special relativity feels intuitive in a way that quantum mechanics doesn't.

DeGuerre
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Re: Is Quantum Mechanics REALLY "Unintuitive?"

Postby DeGuerre » Thu Jun 06, 2013 8:16 am UTC

SU3SU2U1 wrote:I think maybe you just haven't seen the deeper formalisms of classical mechanics.

Yes, I have. If it helps, you can add an "all other things being equal" qualifier. :-)

Perhaps the reason why I find QM more intuitive (mathematically speaking) is that I come from an information theory background. Classical probabilities just don't behave themselves in the way that Hilbert space does.

(Anyone who has ever tried to fit a curve knows that least-squares/RMS fitting is in a sense "easier" than least-absolute-difference/minimax!)


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