# Did anyone else strongly dislike QM?

1. Oct 4, 2014

### jbrussell93

I'm a senior physics major in the first semester of a two semester QM sequence using Griffiths. All of the physics majors talk about how great quantum mechanics is and how it is the pinnacle of the undergrad physics degree. The upper level physics courses seem to build up the foundation that one needs in order to make it through quantum mechanics, etc. Well, I am extremely disappointed with the material so far...

We had our first exam and it essentially felt like a math test. I did very well but felt that I hardly understood anything. I have gained no intuition about what is physically going on so far in the course. Sure, there is quantization of energy states for a particle. Sure, a particle's state can be described by a wave function and its observables by an operator. But what does this REALLY mean? It's extremely frustrating to not understand what is going on and just chugging through the math. I decided to major in physics because I wanted to understand how things work, and so far classes like classical mechanics, optics, E&M, and thermo/stat mech have been extremely satisfying. I just can't figure out what I'm missing with quantum mechanics that other physics majors seem to get, or at least what they find satisfying/interesting about it. Is it simply the mathematical gymnastics that people like?

For the first time, I feel that I'm losing motivation in a physics course... Did anyone else feel this way with upper division quantum mechanics?

2. Oct 4, 2014

### atyy

In Copenhagen-style QM, the wave function does not necessarily represent "reality". We subjectively divide the universe into a commonsense real part, in which we, our measuring apparatus, and the results of our measurements live. Then the wave function is just a tool to calculate the probabilities of experimental results. This divide of the universe into "real" and "quantum" parts is called the Heisenberg cut. Because the cut is subjective, different people will place it in different places. I recommend Landau and Lifshitz's or Weinberg's QM books for describing this philosophy. In a sense, it is not so different from statistical mechanics - do we really think "canonical ensembles" exist?

Since the wave function is not necessarily real, does this mean if we use a wave function to represent the moon, that the moon does not exist when we are not looking at it? Quantum mechanics is silent about this question. However, we can believe that quantum mechanics is not the final theory, and yes, the moon is a coarse grained concept that represents objects that do exist when we are not looking.

QM is one of the most exciting subjects, because it describes everything we have observed to date. Also, it shows that the classical relativistic concept of locality is not the only concept of relativistic causality that is possible, because QM is not classically local, but it is consistent with relativity even though it has nonlocal things like the collapse of the wave function. Although it is revolutionary in terms of classical mechanics, it is surprisingly consistent with classical thermodynamics.

It is also conceptually exciting, because some people used to believe that the hidden variables cannot exist, and QM means the moon is not there when we are not looking. This erroneous view was due to an wrong proof by von Neumann. In this sense, QM suggests new physics, even though we have not yet observed it yet (unless some sort of Many-Worlds interpretation works). It is not so much different from quantum electrodynamics and quantum Einstein gravity which successfully describe what we see, yet also signal their own incompleteness. These subjects are different from classical electrodynamics without point charges, which are internally complete, and require observations to falsify them.

Last edited: Oct 4, 2014
3. Oct 4, 2014

### ShayanJ

That's a situation a physics student can easily fall into. To not get the concepts behind the math of QM.
In my idea, the best starting point is studying about some quantum mechanical experiments. There are a lot but two important ones which can help you a lot are Double-slit experiment and Stern–Gerlach experiment. Analyse these experiments carefully and soon you will be on the track of getting the idea behind all those math.
I should say that the moment you get the idea, is the moment you are most satisfied with your decision of becoming a physics student!

4. Oct 4, 2014

### thegreenlaser

You might just need to be patient. I find (and I think most people would agree) that quantum mechanics is an extremely tough subject to get a good intuition for. The problem is, if you don't have a solid foundation in the (reasonably difficult) mathematical formalism first, your intuition is almost certainly going to lead you astray. You kind of have to spend a while building up your knowledge of the formalism before you can start solving more useful problems and getting a more intuitive feeling for things.

By the way, I found Griffith's QM book was not nearly as good as his EM book. Unlike EM, there's no real standard approach to teaching QM, and it's possible that the approach Griffiths and your teacher are taking just isn't working for you. As far as other textbooks go, my personal favourite is "Quantum mechanics for scientists and engineers" by David Miller. It's got a slightly more applied focus, and you might find the later, more applied chapters interesting as "inspiration."

5. Oct 4, 2014

### ShayanJ

I don't think its a good idea to expose beginners to interpretational issues.
Also it seems your post is balanced towards a special interpretation which is of your own appeal (Or maybe I'm ignorant of some new researches that take such things into the realm of established facts about QM), but students in QM, only after mastering the fundamentals, should be given equal amounts of each interpretation, if given any, so that they can choose which is the one they prefer.
But one thing is, its not surprising that QM is consistent with classical thermodynamics(CT), because CT is formulated with no assumption about the microscopic world and is based on only macroscopic observations, which don't depend on our assumptions about macroscopic world and so is not affected by the "classical spirit" of classical physics, unlike statistical mechanics.

Last edited: Oct 4, 2014
6. Oct 4, 2014

### homeomorphic

I avoided that class, partly because of my bad experience with classical mechanics (which I did reasonably well in, but hated), such that I became suspicious of most physics classes. I was a math major, but strongly considered trying to get a PhD in physics at the time, and was trying to gear up for it. So, I ended up studying math in grad school, instead. I taught myself a little quantum mechanics along the way, using Sudbery's book, Quantum Mechanics and the Particles of Nature: An Outline for Mathematicians, and Penrose's The Road to Reality. Oddly enough, the book gives pretty good physical intuition, and the math, I'm guessing, is done more elegantly than in Griffiths (I have thumbed through Griffiths, but not actively studied from it). The first few chapters are particularly good.

I do think math is one of the things that makes quantum mechanics cool, but more conceptual math, rather than thickets of nasty equations. Penrose shows some of that side of things in his book. There are much more interesting ways of looking at it than Griffiths. Not just in terms of math, but also physical intuition. There are zillions of books about quantum mechanics and between all of them, you should be able to find something more interesting, both mathematically and physically speaking.

7. Oct 4, 2014

### jbrussell93

I had not considered this -- that there isn't a standard approach to teaching QM. Maybe I do need to check out some other resources.

I have read some of your posts in the past and really like your outlook on understanding the physics and not just the mathematical tricks. Again, it sounds like I really just need to find a book that "works" better for me.

Thanks for the input.

8. Oct 4, 2014

### Arsenic&Lace

I did well in my quantum courses but wound up loathing the subject, especially as I learned more about field theory, which only became more esoteric. However, now that I am taking solid state physics, which applies quantum mechanics to practical problems, I am finding that my enjoyment is strangely increasing. That isn't to say that I don't find it baffling conceptually, but the fact that it can be related ultimately to something tangible (i.e. why aluminium is a conductor while other materials are not) has made it more palatable for me.

There are plenty of applied physics problems which are more tractable to the human imagination and don't necessarily involve much quantum mechanics; I'm currently interested in polymer folding problems, which are qualitatively simple and practically quite rich.

9. Oct 5, 2014

### Hertz

They mysteriousness of quantum mechanics is the guise of secrets to be discovered :)

10. Oct 7, 2014

### WannabeNewton

You aren't alone. Almost everyone I know, myself included, feels this way. QM is just an extremely boring and rather easy subject at the undergraduate level but that's really because of the way it's taught, not really because of the subject itself. If you want to prevent yourself from becoming demotivated then try reading quantum stat mech/solid state material on the side.