@uly - Now that was cruel and unnecessary. Not everyone can be an expert at everything. I guarantee there are subjects I know more about than you do and there are subjects you know more about than I do. For instance, I doubt you play Trombone at a professional level. And whatever you do as an avocation or a vocation, you are proably way better at than I could ever dream of being (unless it is my vocation as well).

I'm at least 90% certain that no one on this site actually understands any of it. Just too bizarre to wrap our heads around. Anyone that claims to "understand" it just wants to seem intellectual.

I don't think anyone in the world understands it.

Well, Steve, you're obviously welcome to your opinion. But, I'm afraid you'd lose your bet, as there appear to be multiple people in this thread who use it on a regular basis and would lose their jobs if they didn't understand quantum mechanics, myself included.

As abgemacht pointed out, we do have machines which depend upon our understanding of quantum mechanics. Some of them, such as transistors (without which you wouldn't be using a computer) and lasers, you're probably pretty familiar with. So, it's fortunate that there are indeed some people who understand quantum mechanics.

Quantum mechanics comes down to infinite-dimensional linear algebra. It's not that hard to do, in the sense of making predictions and understanding what those will be, as CSteinhardt has ably pointed out. In the most straightforward sense, to understand quantum mechanics just means to understand its predictions and how to make them.

There are certainly conceptual problems at its foundations, though, and I think it's fair to say nobody understands how to fix those right now, or exactly what they mean. So if by "understand quantum mechanics" you mean "understand what it tells us the nature of the world is," then the answer is no. Even if you just mean, "How to precisely define the theory," the answer is still no.

@Steve

Your argument reminds me of people who are certain aliens build the Pyramids because they don't know how the Egyptians did it. Just because you don't understand something doesn't mean it's impossible for anyone else to.

I thought Egyptian pyramid building was pretty clear. Now the ancient grpund carvings only visible from decent elevatioms... How is also easy but why is the question. I don't think aliens visoted but O suspect that it was something to do with those ancient people.thinkong comets and eclipses and such were "gods" much like the Vikings drove off the wolf.god who tried to devour the sub god every so often amd hollering and threatenimg with their gestures until he left (solar eclipse).

Please just wait till you get to a PC, Draug.

@steinheart

Isn't it easier to explain HUP the way he explained it? That you can't simultaneously determine position and momentum of a particle?

To detect a particles position, you've got to hit it with something that can bounce back to a detector, say a photon of a certain energy. When it bounces back to your detector, you can determine where it was, but you've now altered its momentum so you can't figure out what it was.

I have an undergrad level of understanding of QM, early nowhere near that of steinheart but I'll tell you that as a Physics major, when we took quantum our senior year, after taking partial diff eqs, and everything under it, Newtonian mechanics ( lagrangian and hamiltonians) vector calc

Shit, accidentally hit post ... Continuing ...

We were some mathematically inclined mufukas! And we knew stat therm, and E&M and even that Einstein derived relativity from assuming that magnetism was just a relativistic effect of moving electric charge. We knew all that shit! Then we hit quantum. All of sudden our math went out the window. We had to learn about hermitian operators and " A"dagger dagger and shit! Blew us away, and we were there to learn physics! That's when I decided to be a programmer;-) actually it was when I found out how much people were willing to pay programmers, but the difficulty of a senior level QM class definitely had an impact, knowing that we had barely scratched the surface.

"When it bounces back to your detector, you can determine where it was, but you've now altered its momentum so you can't figure out what it was."

While this is true, HUP says something more. independent of what tool you use to make your measurement, it is mathematically impossible for your to know both momentum and position with absolute certainty at the same time.

ΔxΔp>= ħ/2

This has nothing to do with *how* it is measured, which is a subtle, but important point.

Philcore,

The thing is, while it's true that you can look at the uncertainty principle as being due to the fact that measuring position (or momentum) alters momentum (respectively, position), what QM actually implies is more radical than that: it's that a particle with a definite position *doesn't actually have* a well-defined momentum. It's not just that we can't measure it. It's that the concept doesn't exist in a well-defined way.

@steinhardt,

I disagree that entanglement violates relativity. It seems like it does, but it doesn't. If you're careful, what relativity says is that *we as experimenters* cannot send information faster than the speed of light. Entanglement doesn't allow us to do so, because while the particles appear to be communicating hyper-luminally, we can't actually influence the outcome of the experiment, so we can't (for example) send a bit of information that way.

What entanglement implies is isntantaneous correlation of a random outcome. It's very mysterious and confusing, and it certainly *seems* to violate SR, but it does not.

@semck

Agreed 100%. In no way does entanglement violate relativity.

Damn abge, you not only found deltas but h-bar too? Impressive.

I'm always ready to talk about ħ.

Have you studied the path integral formulation much, abge?

So of the people who obviously know what theyre talking about, who has degrees in physics and what level? I have a BS in physics, what about abge, smell and steinheart ?

The three of you seem to have a deeper understanding than I do.

I have a Master's degree in Electrical Engineering. My thesis focused on computational nanoelectronics, specifically performing electronic structure calculations on arbitrary nanodevices. Most of the work was Numerical Analysis, along with some computer science, but as we were working with a fully QM system, I picked up a lot on the way.

Sorry smeck, I blame my phone for accusing you of smelling

It's cool, philcore, it's nice to have it be non-deliberate for a change. ; )

I have a BS in physics. I've also kept studying physics (both formally and not) while pursuing graduate degrees in other fields. I got a lot better at QM during law school.

"Have you studied the path integral formulation much, abge?"

I looked at it briefly while trying to understand another grad student's works. Didn't look at it too closely, though.

Yeah. I've studied it to some extent, but studying it much more is one of my biggest current physics projects.

One of its interesting properties is that it much more explicitly shows the dependancy of quantum behavior and properties on h-bar and its size. You can see things like classical mechanics emerging in the limit h-bar -> 0.

(When I say it's one of my biggest current physics projects, I mean, one of the earliest things on my to-do list, not something I'm currently spending time on. Just to clarify).

Yeah, I hear ya.

I realize that QM is hard, but I always find it interesting how there are people that treat it like such a mystical thing. I mean, all of our computers rely on true QM principles.

True enough.

Although in fairness, Werner Heisenberg kind of treated it like a mystical thing too, and he certainly understood it well enough. For that matter, Paul Dirac (in a very different way) somewhat did.

I think there's a distinction to be made between treating QM mystically in the sense of acting like nobody can even know what it says or how to use it, which is silly; and treating it mystically in the sense of taking what it (very precisely) tells us and asking if that implies anything mystical or surprising about the nature of our universe, which may or may not be silly, but at least isn't absurd like the other is. (And is what Heisenberg, for example, as well as Wheeler and others, have done).

Well, with people like Dirac, etc I think it was more that they understood the universe really well (or so they thought) and then QM came around and changed things. But, they didn't act as though it was unlearnable, only that it was different in a way they didn't things could even be different.

Right.

Well, when I say that Dirac (for example) was mystical about it, I mean that he believed that it was very connected in a mysterious way to beauty, and that one should search for physical laws by looking for beautiful mathematics, which he considered a fairly mystical truth.

Similarly Heisenberg drew very strong conclusions about the nature of the universe and materialism and stuff from QM.

But you're right. They certainly didn't think it was unlearnable, or that our intellects couldn't grapple with it, or the like. Just odd and unexpected.

Oh, that could all very well be true. I've found that most physicists like to find inner beauty in the universe. As an engineer, I tend to only have a passing interest in physics until I can use it to make something. : )

"I realize that QM is hard, but I always find it interesting how there are people that treat it like such a mystical thing. I mean, all of our computers rely on true QM principles."

I can personally attest that I have (had? I don't really do much physics anymore, sadly) a much much harder time getting my head around relativity than QM. Perhaps it's because QM is so far from every day observations that I don't have any predisposed ideas of what's supposed to happen. With relativity, you have an idea of what should happen in the equivalent non-relativistic scenario so it's a complete mindfuck.

Unfortunately, QM seems very exotic and intimidating to many people who haven't studied it (like Steve) but, if you have the mathematical background to handle it, it's not "impossible to understand."

Also, great explanation CS.

@semck: I agree, entanglement does not violate our modern understanding of relativity; otherwise we'd no longer believe in relativity. What it "violates" is the shorthand idea that causality includes information transfer.

However, it's worth noting that the Einstein-Podolsky-Rosen paper discussing this did interpret relativity, at least at the time, as being at odds with this answer; the EPR paper thought that the solution was that relativity wins and that this would show a flaw in quantum mechanics. So, although our modern understanding is that this is compatible with relativity, a large part of our modern understanding of how the two are compatible came from trying to come to terms with the results of this experiment.

@philcore: I'm a postdoc, currently working in high-energy astrophysics, with other research interests that include high-energy physics and computer science. The work I've done that's closest to this discussion has involved using astrophysics to place limits on particle physics beyond the standard model, and particularly on the variability of coupling constants. In non-technical terms, string theory predicts that the constants which determine strength of forces, such as G for gravity, are not in fact constant, but rather change with time/space/etc. I've used everything from distant gas clouds to the stability of the solar system to nearly two-billion year old nuclear reactors to place limits on how much the laws of physics might be changing with time.