Quantum Mechanics: Myths and Facts

February 25, 2008 1:21 PM

Quantum Mechanics: Myths and Facts (pdf), a recently-updated paper on the Cornell arXiv peer-review site. By Hrvoje Nikolić of the Rudjer Bošković Institute in Croatia.

Note: the presence of a paper on arXiv does not necessarily mean it has been reviewed and is not equivalent to having been published in a journal.

posted by XMLicious (47 comments total)

I can just barely understand enough of this that it's very interesting.
posted by XMLicious at 1:22 PM on February 25 [1 favorite]

(Sorry for an immediate tangent, but my favorite arxiv post recently was Optimal boarding method for airline passengers.)
posted by Wolfdog at 1:31 PM on February 25 [2 favorites]

There's no uncertainty about the number of tags! AMIRITE?

(But i'm enjoying the article)
posted by mrnutty at 1:33 PM on February 25

Thanks Wolfdog!
posted by pjenks at 1:39 PM on February 25

It takes a tough man to make a tender chicken. I mean, to thoroughly tag a MeFi post.
posted by XMLicious at 1:45 PM on February 25

All that and no drbeckett tag... I'm very disappointed in you.
posted by vorfeed at 1:52 PM on February 25

Yeah, I mean why stop there? Why not include every significant word from the linked arcticle?
posted by BeerFilter at 2:02 PM on February 25

Whozatt? It doesn't even get another single hit on MeFi search. But if you can explain I will consult with my brethren in the Order of Extraordinarily Tagged Antelopes and perhaps add it.
posted by XMLicious at 2:03 PM on February 25

The reason to build up lots of tags is because that's how the “related words” associations are made.
posted by XMLicious at 2:04 PM on February 25

I'm not even considering assessing this article for validity, but be warned some crazy stuff gets posted to arXiv (i.e. the E8 grand unified surfer physics paper a while back).
posted by Skorgu at 2:05 PM on February 25

Definitely a good caveat Skorgu, that's why I made the note about validity in the post. But I should mention that Googling Hrvoje Nikolić turns up lots of links to other papers and conferences where he has presented. (Though this could still of course be by a doppleganger or impostor.)
posted by XMLicious at 2:11 PM on February 25

Oh boy! (re: drbeckett)
posted by mrnutty at 2:16 PM on February 25

Don't talk about Captain Archer that way! (Interesting side note, though… the phrase “quantum leap” actually isn't something in science itself as many people I've run into have thought, it's referring to the historical significance and speed of the development of QM.)
posted by XMLicious at 2:51 PM on February 25

I always privately assigned the meaning "tunnelling" to "quantum leap". It's cooler than imagining it to mean "a tiny discrete increment of leap".
posted by Jpfed at 2:57 PM on February 25

MONGO LIKE QUANTUM MECHANICS
posted by Henry C. Mabuse at 3:02 PM on February 25

Myths, or useful pedagogical tools to help understand difficult concepts that are still a matter of debate?
posted by casaubon at 3:04 PM on February 25

Yes, and no.
posted by Henry C. Mabuse at 3:06 PM on February 25 [1 favorite]

Great stuff:

Myth: You can get Quantum Mechanics from a toilet seat.
posted by puckupdate at 3:09 PM on February 25 [1 favorite]

But what about the myth that Quantum Mechanics are afraid of dogs?
posted by Astro Zombie at 3:21 PM on February 25 [3 favorites]

His main point seems to be that QM doesn't map well onto the English language. Not exactly a groundbreaking revelation.
posted by FissionChips at 3:29 PM on February 25

I don't really see much point to this paper. There certainly are lots of words, but I don't see what one can really take from it. For example, the whole section on wave/particle duality seems to argue that there really isn't a duality because "particle" states are in fact described by "wavefunctions." It's just that these wavefunctions often behave very much like particles and don't resemble anything any normal person would describe as a wave. But really, they are in fact wavefunctions.

In summary, this paper taught me the first thing I ever learned about quantum mechanics, and not much more.
posted by dsword at 3:37 PM on February 25

It's an interesting paper, but this is a typical statement:

4.1 Fundamental randomness as a myth

... Of course, if the usual form of QM is really the ultimate truth, then it is true that nature is fundamentally random. But who says that the usual form of QM really is the ultimate truth? A priori, one cannot exclude the existence of some hidden variables (not described by the usual form of QM) that provide a deterministic cause for all seemingly random quantum phenomena.

I'm no physicist, but I seem to remember that hidden variables have been ruled out. (The author puts it this way: "in QM there exist rigorous no-hidden-variable theorems. ... However, each theorem has assumptions. ... Thus, what these theorems actually prove, is that hidden variables, if [they] exist, cannot have these additional assumed properties.") In the discussion of the EPR (Einstein-Podolsky-Rosen) paradox -- the one about entangled particles separated by a huge distance ⇒ "instant" communication between particles. Einstein badly wanted hidden variables to work out ("God does not play dice with the universe") but Bell proved that no good-enough theory was compatible with hidden variables.

His views (no such thing as particle hence no wave/particle duality; no intrinsic randomness vs. many-universes vs. ...; the measurement "problem"; ...) are things that grad students have been talking about over beers for a long time. In other words, it's not "myths and facts about QM", but more like "one person's view of why the Copenhagen Interpretation is WRONG WRONG WRONG."
posted by phliar at 3:45 PM on February 25 [1 favorite]

I'm not a physicist either but to my knowledge it's only local hidden variables that have been ruled out (by the Bell's Inequalities test? is that what you're talking about philar?). Global hidden variables, which I believe are part of the Bohmian interpretation which he mentions, are not considered disproven.

My interpretation of what I've read about the Bell's Inequalities has been that the experiments were mostly concerned with proving that quantum mechanics is not on some level equivalent to classical physics. By my reading they did not prove that QM is non-deterministic, as many people seem to think they did.

If this is the same Hrvoje Nikolić who comes up in this Google search as the author of papers about fermions and gravitational lensing and chirality and was a presenter at The Second International Conference on Quantum, Nano, and Micro Technologies I don't think it's just some wacko reeling off his disagreements with the Copenhagen interpretation.
posted by XMLicious at 4:12 PM on February 25

And I'm no physicist either.


What?
posted by Smedleyman at 4:18 PM on February 25

I'm no physicist, but I seem to remember that hidden variables have been ruled out. (The author puts it this way: "in QM there exist rigorous no-hidden-variable theorems. ... However, each theorem has assumptions. ... Thus, what these theorems actually prove, is that hidden variables, if [they] exist, cannot have these additional assumed properties.")

Only local hidden variable theories have been ruled out by the Bell experiments. However, global hidden variables are significantly strange that randomness is just more likely...
posted by voltairemodern at 4:20 PM on February 25 [2 favorites]

Yes, XMLicious, that's exactly what I was thinking of.

I'm sure that this guy is not a nutter; all I'm saying is that I've spent many happy hours in bars talking to physics grad students and post-docs as they ranted about Bohr and the Copenhagen Interpretation, going over the same sorts of things in this paper.
posted by phliar at 4:24 PM on February 25

That should be "going over the same sorts of things as in this paper."
posted by phliar at 4:25 PM on February 25

Lotta exclamation points. More than I’d’ve thought. Not sure the statement that the theory of relativity is just a theory is relevant. Seems to technical to be really clear and too pedantic to be really useful.

“But what about the myth that Quantum Mechanics are afraid of dogs?”

Not true Astro Zombie. Quantum Mechanics both are and are not more afraid of you than you are and are not of them.
Plus dogs.
(Or so I assume)
posted by Smedleyman at 4:29 PM on February 25

Jpfed: I always privately assigned the meaning "tunnelling" to "quantum leap". It's cooler than imagining it to mean "a tiny discrete increment of leap".

My interpretation (and James Burke's interpretation too if I'm correctly remembering an episode of Connections) is that it's referring to the way that in the space of less than a decade all of physics was turned completely upside down by the formulation of quantum mechanics, compared to the slow plodding progress of Victorian and Renaissance stuff building on Newtonian physics, or the way we've pretty much just been farting around with innumerable different experimentally-unconfirmable supersymmetry and superstring theories and combinations of them in the 8 to 9 decades since the advent of QM, relatively.

(But all due respect to Feynman and developments in QFT and QCD and all that other stuff I can't fully understand. It's just that as far as I've read they don't represent a paradigm shift the way QM did.)

A quantum leap is like a systemic epiphany or a group epiphany.
posted by XMLicious at 4:33 PM on February 25

philar: not Bohr ― Bohm, the American who worked on the Manhattan project and sort of went low-profile in physics in his later years.
posted by XMLicious at 4:37 PM on February 25

Wolfdog,

But what about Taking a shower in Youth Hostels: risks and delights of heterogeneity ?

... which is about temperature fluctuations, not international exchanges of bodily fluids.
posted by lukemeister at 5:01 PM on February 25

Not so impressed by this. The wave-particle duality part was pretty bad. The paper gives the impression that wave-particle duality originated as a description of locality (the wave is in a little packet) when my understanding is that it originated because of the discrete nature of some interactions (e.g. in the photoelectric effect, only integer numbers of photons are absorbed, whereas we might expect that a wave could interact fractionally). There is a big problem with thinking about things as wave-particles but the author hasn't properly identified it. And the resolution is certainly not that quantum mechanics only describes waves. Quantum mechanics requires you to learn/understand the properties of quantum fields, which happen to have some particle-like properties, and some wave-like properties, but are neither waves nor particles (much like a bat is neither a rat nor a bird).

Skimming over the rest of the paper revealed similar issues. There are lots of interesting things to say/clear up about quantum mechanics. This paper doesn't appear to address them in a productive manner.
posted by Humanzee at 5:21 PM on February 25

Yeah, Humanzee, I have to agree with you about the wave / particle thing. I think the issue is that we just need a third term to describe whatever these quantum entities are, forget “wave” and “particle”.
posted by XMLicious at 6:04 PM on February 25

"A third term ... forget 'wave' and 'particle'" -- now every time I see "parve" on my food, I'm going to consider it a warning about Quantum Mechanics, not a label about its Kosher acceptablity.

IAAP (physicist -- or mostly so), and though I liked this paper, I saw one problem. You practically have to take a college-senior level course in QM to follow all the math here, and by the time I finished that course, I knew most of this stuff. It could be an interesting addition to the syllabus, though.
posted by rossmik at 7:25 PM on February 25

we just need a third term to describe whatever these quantum entities are, forget “wave” and “particle”.

wavicle.
posted by cytherea at 8:43 PM on February 25

Actually, after a quick perusal of the text, the author correctly addresses the points you've brought up here, namely locality, hidden variables, and randomness, as well as the historical origin of the concept of wave/particle duality.

Looks pretty spot on to me.
posted by cytherea at 8:51 PM on February 25

You practically have to take a college-senior level course in QM to follow all the math here, and by the time I finished that course, I knew most of this stuff.

Unless you have a background in just some higher math instead of high-level physics, like me. ;^)
posted by XMLicious at 9:08 PM on February 25

It would be cool if I could understand it. I'd like to see an article examining or debunking popular mythologies about QM (example)
posted by sswiller at 9:44 PM on February 25

Good point sswiller. I should have hunted down something more basic-level about QM, that would've made for a better overall post.

But here's a consolation prize: the Scientific American article Misconceptions About the Big Bang.
posted by XMLicious at 11:24 PM on February 25 [2 favorites]

I always privately assigned the meaning "tunnelling" to "quantum leap". It's cooler than imagining it to mean "a tiny discrete increment of leap".

It's tinyness is an affectation of some physicists. The key word here is discrete - meaning it doesn't pass through some intermediary stage during the leap - which means it's appropriate to describe electrons transferring between valence shells and, with a certain linguistic pinch of salt, epiphanies.
posted by Sparx at 5:10 AM on February 26

I'm seriously curious now about the phrase “quantum leap” now. I've never heard it used to refer to quantum tunneling or electrons shifting between energy levels on its own, not once to my recollection, and I've known about those QM effects since I was a little kid. I've only ever heard it used that way by someone claiming that it doesn't mean a sudden systemic advance.

I wish I had something more definite but here's a Google Books quote from Reading Popular Physics: Disciplinary Skirmishes and Textual Strategies by Elizabeth Leane, p.34:

The phrase “quantum leap,” meaning “a sudden large advance,” entered common usage in the 1970's (Oxford English Dictionary).

No further comment there except about the TV show. It's too bad she didn't track down whether this original popular meaning of the term came from science history or from a misapplication of someone's 1970's understanding of quantum tunneling.

…On the other hand, I just researched and found that the German version of it, “Quantensprung” was used from early on (1937) for the transitions between electron energy states. So, alas, I guess I've always been wrong about it.
posted by XMLicious at 2:28 PM on February 26

Just out of curiousity, XMLicious, did they use the bra-ket notation for vectors in your other higher math experience? I know physicists call it the Dirac notation, and use it exclusively for QM, not for other eigenvector sorts of problems. Even though I had Fourier analysis and boundary value problems as a math course, I still might have been thrown off by the notation if I didn't know what it meant.
posted by rossmik at 6:05 PM on February 26

No, I'm not aware of that notation being used in pure math. I've skimmed over explanations of it but I haven't studied it at all. As I said in the first comment, I wouldn't by any means say that I thoroughly understood the paper in entirety, but having gone pretty far in mathematics helps.
posted by XMLicious at 7:43 PM on February 26

From tonight's Colbert Report, talking about a news item:

Steven: “I've seen more provocative titles on physics papers!”
Bullet Point: “How To Get Longer-Lasting, Firmer Electrons”
posted by XMLicious at 9:35 PM on February 26

Not all physicists use bra-ket exclusively for quantum ---I know of at least one who finds it useful in many situations involving linear algebra. Of course I have to translate back afterwards (or else endure incredulous stares and/or giggles while talking about bras). :)
More relevantly, in Halmos' Finite Dimensional Vector Spaces the notation (a, b) is used to denote the bra-ket equivalent of <a|b>. It's kind of nice, because it at least distinguishes vectors from covectors, although it's much less flexible because you can't really break apart the parenthesis with that notation. My recollection is that Halmos gets around this by having separate symbols for vectors and their duals, which is kind of clumsy. I always thought of it as being halfway to bra-ket.

I honestly don't know if having a background in physics is automatically any better than a background in math for getting at "deep" understanding of QM (I have a graduate background in both). One thing I do agree with the author on is that a lot of physicists don't talk about this or study it carefully, and therefore there are myths that persist. I encountered some of this stuff only because I took a quantum computation class on a whim. There we had to really understand what measurement was, because we had to worry about the stability of quantum states as well as deliberate manipulation of transitions between states. We had to worry about exactly what entanglement was, etc. I thought that the issues raised were really interesting, so I bugged people who seemed to know their stuff, and I read what I could find. Most of the other grad students I encountered seemed unaware that "What is a wavefunction really?" or "What is a measurement?" were questions that they'd never seen addressed.
posted by Humanzee at 9:48 PM on February 26

Yeah, that's the impression I've gotten of many physicists / physics students both from talking to them and reading what they write on the web. You'll see guys answer the most intricate technical questions with confidence and aplomb and verbose detail, then fumble on a basic conceptual question. Or some rigidly refuse to talk about anything conceptual.

That reminds me of a story my high school calc teacher told about having a conversation with a student and realizing that she'd somehow made it all the way to Calc I without understanding what fractions really were; she just knew how to manipulate them algebraically.

Measurement science is crazy stuff. In studying for software engineering quality control certifications I got a fair dose of it, though it doesn't apply to software engineering directly but we were required to be cross-disciplinary.
posted by XMLicious at 10:54 PM on February 26

Even physicists shed bra-ket notation when just "too much stuff" is around, as in the case of tensors in general relativity. Then it's time for the big guns: the Einstein summation convention and index raising/lowering. That's also what differential geometers employ in hands-on calculations.
posted by pantufla_milagrosa at 11:41 PM on February 26