Courses Getting distrustful of my QM course

[CyberShot]

I'm at a UC taking upper division QM, and I get the feeling that the course is designed so as to ignore physical interpretations of equations or the true meaning behind the derivation of the wave function, to the point where I'm having doubts of even my professor's real understanding of the course itself.

Even the Griffiths textbook said the following:

"The purpose of this book is to teach you how to do quantum mechanics. Apart

from some essential background in Chapter 1, the deeper quasi-philosophical ques-

tions are saved for the end. "

I get the feeling that quantum physicists are somewhat embarrassed by the incompleteness of the theory and that's why they chose to leave out physical interpretations why certain steps seem so arbitrary and not well evinced.

But isn't physics about the physical interpretatioss of things? After all, all the math and equations really stand for something physical, so how come this just isn't emphasized enough in university classrooms?

This makes me scared to go to grad school, and study quantum field theory, where maybe the emphasis is on "how to do" so and so, rather than really understand its physical implication.

 

[fss]

Raise your hand and ask an intelligent question, then. Or take advantage of office hours. Complaining about your class on an online forum likely isn't going to resolve anything.

 

[CyberShot]

Raise your hand and ask an intelligent question, then. Or take advantage of office hours. Complaining about your class on an online forum likely isn't going to resolve anything.

My main point was in my last sentence.

 

[jtbell]

I get the feeling that quantum physicists are somewhat embarrassed by the incompleteness of the theory and that's why they chose to leave out physical interpretations why certain steps seem so arbitrary and not well evinced.

I don't think it's embarrassment, but rather a recognition of the fact that the interpretation of QM is not a settled issue, and is hampered by the fact that most experimental data does not (and cannot) address the issue, except for the experiments related to Bell's Theorem. You have to look for recent specialized monographs, or review articles, to get an idea of the current areas of research/speculation on this topic.

To learn about "settled" topics, read a textbook or take a course. To learn about "unsettled" areas, read the research literature and talk to people who are doing research in that area.

 

[Vanadium 50]

This makes me scared to go to grad school, and study quantum field theory,

Fair enough. Don't go. Problem solved.

 

[Pengwuino]

If you're asking for answers to "What is a wavefunction?" and things like that... good luck.

 

[Shackleford]

It's just a more-accurate mathematical model of real-world physical phenomena. You might not be able to go beyond that.

 

[Klockan3]

I'm at a UC taking upper division QM, and I get the feeling that the course is designed so as to ignore physical interpretations of equations or the true meaning behind the derivation of the wave function, to the point where I'm having doubts of even my professor's real understanding of the course itself.

Quantum mechanics builds on the principle that all particles are insepparable waves. That is a wave that you can't break up into smaller waves, instead if you try to break it all of it will somehow go into one of the possible containers while the others gets emptied. This leads to a heap of strange results but the exact mechanics on when and how this happens is still a mystery.

There, that is all the physical interpretation you can get from it. Quantum mechanics is a bit too strange, that is all.

 

[capandbells]

Physics is about physical interpretations to the extent that they can be used to predict and explain the results of experiments.

 

[twofish-quant]

I'm at a UC taking upper division QM, and I get the feeling that the course is designed so as to ignore physical interpretations of equations or the true meaning behind the derivation of the wave function.

That's because it is. :-) :-)

The purpose of a QM course is to teach you how to do the equations to come up with answers about what happens in a particular situation. Now *why* the equations are the way that they are is something that's is (and in my opinion should be) outside the area of the basic QM class.

Ultimately, the equations are what they are because that's what we see when we do the experiment. If you are interested in the philosophy of quantum mechanics, there are a ton of books that you can read on your own, but personally, I think it's a good thing that QM courses focus on the practical bits, because learning to "do the math" is something that's more difficult to learn on your own. Once you can "do the math" you can think about what the math means, but it doesn't work the other way.

I get the feeling that quantum physicists are somewhat embarrassed by the incompleteness of the theory and that's why they chose to leave out physical interpretations why certain steps seem so arbitrary and not well evinced.

It's not so much embarassment, than the fact that you can come up with several different interpretations of quantum mechanics, and no one has figured out an experimental way of distinguishing between the two.

Yes, certain steps seem "weird", but ultimately its a matter of "yes, this doesn't make sense but that's how the universe works."

But isn't physics about the physical interpretations of things?

No, it isn't.

After all, all the math and equations really stand for something physical, so how come this just isn't emphasized enough in university classrooms?

In part it's sociology. Learning interpretation of quantum mechanics doesn't let you build a better transistor, which generates cash, which funds the course, and the classes are geared more toward engineers that want to build things, than philosophers that want to ask deep questions.

The other problem is that if you were to ask someone experienced in physics, what quantum mechanics means, the answer is likely "I don't know." I know how to solve Schoedinger's equation, so I can teach that to you if you want. The more I know about quantum mechanics, the less sense it makes to me, so if you want me to tell you what the deep meaning of QM is, my answer is "I don't know."

This makes me scared to go to grad school, and study quantum field theory, where maybe the emphasis is on "how to do" so and so, rather than really understand its physical implication.

I don't know why this is *scary*. Ultimately you do what you can, and sometimes the answer is "I don't know." If you wanted certainty or clarity, you picked the wrong topic to study.

 

[johng23]

Since I've gotten to grad school I've realized that, while everyone around you has some extremely specialized and complicated topic that they study, it's often the most fundamental questions that even these experts can't answer (whether it's truly an unanswered question or they just haven't had the chance to explore it fully). Don't go into science because you want the answers, go in because you like the questions. You will always have more of the latter.

 

[CyberShot]

That's because it is. :-) :-)

The purpose of a QM course is to teach you how to do the equations to come up with answers about what happens in a particular situation. Now *why* the equations are the way that they are is something that's is (and in my opinion should be) outside the area of the basic QM class.

So, in essence, this "shut up and calculate" methodology is being heavily employed? Do you think it's unfair for me to complain about this? Or to at least surmise that this raises a red flag about of the potential merit of the theory, at least with respect to the highly regarded principle that you don't really know something if you can't explain it in simple terms.

No, it isn't.

What is it about then?

In part it's sociology. Learning interpretation of quantum mechanics doesn't let you build a better transistor, which generates cash, which funds the course, and the classes are geared more toward engineers that want to build things, than philosophers that want to ask deep questions.

I see where you're coming from here. Good insight. And from the point of view of practicality, and making the world go 'round, this makes a lot of sense. Perhaps some universities teach QM classes on the predication that a lot of the students are aiming to work in industry?

The other problem is that if you were to ask someone experienced in physics, what quantum mechanics means, the answer is likely "I don't know." I know how to solve Schoedinger's equation, so I can teach that to you if you want. The more I know about quantum mechanics, the less sense it makes to me, so if you want me to tell you what the deep meaning of QM is, my answer is "I don't know."

This actually makes me feel a lot better about myself. If physicists have a hard time behind what QM actually means, then maybe a layman like myself would not have a lot of pressure going into grad school to do above and beyond.

I don't know why this is *scary*. Ultimately you do what you can, and sometimes the answer is "I don't know." If you wanted certainty or clarity, you picked the wrong topic to study.

Do you think I should switch majors then? Is this really what grad school is going to be like. Finding out the hard way that some problems are just counter-intuitive and do not yield results that are beyond the grasp, at least in small part, of a smart person's intuition? This is what I'm hoping physics grad school is NOT like. I don't want to be chasing uncertainties, and leaving things to chance. I'd like to finally be able to put my "mind" at rest.

Since I've gotten to grad school I've realized that, while everyone around you has some extremely specialized and complicated topic that they study, it's often the most fundamental questions that even these experts can't answer (whether it's truly an unanswered question or they just haven't had the chance to explore it fully). Don't go into science because you want the answers, go in because you like the questions. You will always have more of the latter.

So, what would you say to someone like me; a potential grad school applicant who's more interested in the meaning behind things, as abstract as that may sound. Would I be disappointed to find out I cannot work on problems, using math only as a tool, and always making sure certain things are in agreement, at least in some part, with human reason.

 

[twofish-quant]

So, in essence, this "shut up and calculate" methodology is being heavily employed?

It's more along the lines of "if we start talking about quantum interpretation, we'll be part of a conversation that has been going on for several decades without a conclusive result." It's not that interpretation of QM is uninterested or unimportant, it's that it's just not something that works with a physics class whose goal is for you to get a result.

Also if you go into philosophy, then it gets *really* complicated. You very quickly start getting into questions like "what is physics?" "what is a theory?"

Do you think it's unfair for me to complain about this? Or to at least surmise that this raises a red flag about of the potential merit of the theory.

Hard to say. Also what do you mean by "merit of the theory"? QM works. You calculate something, you do the measurement, it matches.

It can be argued that if the universe doesn't work the way that you think it should work, then you're the one with the problem. The universe is really messy. A lot of physics has been said to be the "quest for beauty" and it's a hard quest because the universe isn't obviously "beautiful."

What is it about then?

"What is physics?" and "What is science?" is one of those obvious questions that become a lot less obvious once you start to think about it. Once definition of "science" is "science is what scientists do." Then you get into the question of "who is a scientist?" and then you can answer that by pointing to scientists.

But that's *another* course.

And from the point of view of practicality, and making the world go 'round, this makes a lot of sense.

I tend to look at social institutions in economic terms, but that comes from critical theory which ultimately comes from Karl Marx. But that's yet another course.

Perhaps some universities teach QM classes on the predication that a lot of the students are aiming to work in industry?

If you aren't working in industry then why is society paying you to go to school?

If physicists have a hard time behind what QM actually means, then maybe a layman like myself would not have a lot of pressure going into grad school to do above and beyond.

Saying "I don't know" is harder than it seems. Also what you'll be required to do in graduate school is to do something original and new. The reason that graduate students tend not to work on quantum interpretation is that it's hard to come up with something original and new on the topic, whereas there is lots of stuff in other areas were you can get original stuff done.

Do you think I should switch majors then? Is this really what grad school is going to be like.

I can tell you what graduate school is going to be like, and yes it will be like this. Whether that makes physics more or less interesting is something that you have to figure out.

Finding out the hard way that some problems are just counter-intuitive and do not yield results that are beyond the grasp, at least in small part, of a smart person's intuition?

What you end up doing is to change your intuition. After you've done enough problems in quantum mechanics long enough, the weirdness becomes "normal" and you get used to it. The reason that people use math like they do is that you often end up with really nutty results, and math is the only way of explaining things.

Also, it helps if you don't try to explain everything. Once you work on your Ph.D., you'll be thankful if you can explain *anything*.

This is what I'm hoping physics grad school is NOT like. I don't want to be chasing uncertainties, and leaving things to chance. I'd like to finally be able to put my "mind" at rest.

Physics is a *horrible* major for that.

If you want certainty, then you need to find a religion.

So, what would you say to someone like me; a potential grad school applicant who's more interested in the meaning behind things, as abstract as that may sound. Would I be disappointed to find out I cannot work on problems, using math only as a tool, and always making sure certain things are in agreement, at least in some part, with human reason.

Yes. One thing that you'll quickly find out is that human intuition can be wrong. The reason that quantum mechanics is "weird" is because you don't see it in your daily life, and it so happens that the assumptions that you make that work when you are dealing with every day objects, just don't work when it comes to electrons.

 

[Leveret]

Physics is about understanding how the universe works--note the use of "how," not "why." If you want answers to the latter question, you'd be more likely to find them in a philosophy class. In fact, there are some schools with a philosophy of science program designed to answer just those types of questions. One thing that physicists have to learn to cope with is the knowledge that we will probably not find all the answers within our lifetimes (if ever). The thrill is in the pursuit of that goal, and in the little victories every now and then that reveal another tiny piece of the puzzle.

It's certainly not unfair for you to complain about that. However, bear in mind that complaining about physics not answering philosophical questions, not giving us all the answers we seek, or just plain not making sense is kind of like complaining when your general physician refuses to treat your toothache: it's not really what he/she is there for. Physics above all else tries to explain empirical results. Making those explanations pretty is nice, but ultimately just icing on the cake. You've probably heard Einstein's saying about quantum mechanics, "God does not play dice with the universe." Somewhat less well-known is Bohr's response, "Who are you to tell God what to do?" If physics doesn't cohere to human intuition, aesthetics, or desires, all we can really do is shrug and make the equations, however messy they may appear, work.

 

[johng23]

Just to stress what was already said: doing a PhD is completely at odds with putting your mind at rest.

So, what would you say to someone like me; a potential grad school applicant who's more interested in the meaning behind things, as abstract as that may sound. Would I be disappointed to find out I cannot work on problems, using math only as a tool, and always making sure certain things are in agreement, at least in some part, with human reason.

You brought up the notion that you don't understand anything unless you can explain it in simple terms. Well, first you need to apply that to yourself. What exactly is the "meaning behind things" which you refer to? For many scientists, the definition of understanding is being able to predict behavior. To you, there is more to understanding than accurate prediction. What is that missing something?

What "human reason" are you referring to which may disagree with scientific results? To me, the ultimate manifestation of human reason is to devise a logical process for extracting information from the natural world, and to accept the results of that process as the closest thing you have to "truth" at the moment.

No one is telling you to stop asking the big questions, the existential ones. Science just can't help with those any more than being a truck driver.

 

[chiro]

In building on what other great posts above, I would recommend that if you wanted to study something that is "understandable" then choose any man-made science like computer science or mathematics and not a natural science like physics.

Mathematics is something that has been created by man (whether that's a result of evolution, divine intervention, luck, or hard work I don't know). With mathematics you are dealing with ways of representing things and these things are very clearly defined (and as time goes on, the definition gets clearer if it wasn't clear enough as it is).

With mathematics, human beings invented it, and based on that fact, people can learn it if they understand the reasoning behind the advancement. There is a lot of pure mathematics out there, but even then there was a particular reason for someone to go in that particular direction.

Also unlike sciences like physics, we don't have to figure out the rules like those guys do. We invented the representations, the models, the theorems, the transformations, basically everything.

With regard to computer science, the same kind of thing applies. If your domain is from the logical gates and upward (taking things like transistors and semiconductors and all of the physics for granted), again this is something that is man-made and easily understandable by other human beings. We don't have to guess the rules or predict random events because things are well defined and certain results are expected.

Now I know that there are problems in both computer science and mathematics that haven't yet been solved, but when it comes to explaining mechanics with respect to these sciences, we can always do it, whereas the reasons behind many natural processes are not known and are hard to know because we weren't involved in its creation.

 

[maverick280857]

I think its quite healthy for someone to get upset or worked-up over the physical interpretation of quantum mechanics. I also think you should pick up from that, and try to improve your own understanding by reading more, discussing the specifics on this forum and with other physicists. Most of all though, ask questions in class and talk to your professor about what bothers you. Imho, most learning takes place outside class. With a mathematically abstract subject such as QM, there is only so much that a person can 'teach' you in a span of 1-2 hours. Usually the idea is to teach the tools, not the way to think.

Despite some of the in-your-face comments you may get from professional physicists who can no longer remember your pain (like Landau said once about not remembering a time when he did not know calculus), don't be deterred. If anything, your own efforts will make you a better student of physics and sharpen your understanding of quantum mechanics.

But isn't physics about the physical interpretatioss of things? After all, all the math and equations really stand for something physical, so how come this just isn't emphasized enough in university classrooms?

So, in essence, this "shut up and calculate" methodology is being heavily employed? Do you think it's unfair for me to complain about this?

This depends on the perspective of the particular professor who teaches you QM. The conventional wisdom seems to be to teach the shut-up-and-calculate method till one is familiar enough with the vocabulary, to become accustomed to the dialect. A course based on the book 'Applied Quantum Mechanics' by AFJ Levi may be closer to 'reality', as may be one that's taught by an engineering professor, but it may miss out a lot on the foundations which you are grappling with presently.

But it is through the problems that one really learns (if I can use the word) the subject. Solving several problems will build a certain degree of 'quantum intuition'. QM is also the first subject you probably study in which most of the intermediate quantities you deal with are completely disconnected with experiment, or more abstractly, aren't observables. In fact, many so called observables can't be measured easily.

I think the general belief in the community is to force students to shed their understanding of CM when studying QM, and to stop asking questions like 'what is the wavefunction?' or 'what is a state?', or 'why does this work?'.

So while you are justifiably frustrated with QM, you should be a bit patient to get to the point when the math makes 'physical sense'.

This makes me scared to go to grad school, and study quantum field theory, where maybe the emphasis is on "how to do" so and so, rather than really understand its physical implication.

Quite the contrary..at least in theoretical or experimental physics (I cannot speak for mathematical physics). It depends on what you are doing with the quantum field theory though. If you are trying to compute quantities, you will obviously want to keep your bearings with reality. But if you are evolving new techniques, new theories or working at an abstract level, then "understanding its physical implication" may really not be possible and all you may do is scratch the surface. Like someone said, if you want to study settled issues, you need to read textbooks and if you want to study unsettled issues, you have to read papers...or perhaps write them :-). What frustrates you about QM bothers most people who work on and think about QM, and recent theoretical research in quantum information theory has opened up the field.

On a more serious note, quantum field theory, except when applied to the study of experiments we have some idea of (say in high energy scattering, or condensed matter maybe), is quite abstract. It has a different 'kind' of 'physical intuition' which is quite different from even QM sometimes, though deeply rooted in it. Developing it again is like flexing your muscles over and over till you build strength, and that comes with time, experience and discussion with peers. I am not an expert in either QM and QFT, but both depress me a lot less after a few years of playing around with them (for no logical purpose, I should add).

But if this is indeed your first class on QM, or the first one based on a book such as Griffiths, you should relax and try to learn more, without worrying about things such as grad school or switching majors. Well, if you end up hating QM for some reason, you may be better off switching your major. But then, it's more likely that you would hate the particular exposition you're forced to sit through, rather than the subject. Something to think about..

Perhaps some universities teach QM classes on the predication that a lot of the students are aiming to work in industry?

Haha, well, no. But then a physics major is usually not hired just for his or her QM prowess. Ironic as this may seem, a good understanding of the limits and restrictions QM places on measurement, light-matter interaction, and information communication, may well govern how future devices and equipments are built. So while we may be about 50-100 years from the first time QM gains mainstay acceptance and understanding, like CM or EM today, it may well happen that QM may dictate industry a century from now. So who knows...

 

[mal4mac]

Physics is about understanding how the universe works--note the use of "how," not "why." If you want answers to the latter question, you'd be more likely to find them in a philosophy class.

Which philosophy class would that be? The *sensible* philosophy books I've read always leave you with more questions than answers!

Which philosophy of science program is "designed to answer just those types of questions"? If it pretends to provide the definitive interpretation of QM avoid it! As Feynman said, no one understands QM...

Philosophers (good ones!), like physicists, also have to learn to cope with the knowledge that we will probably never find most of the answers. In fact, looking closely at this process is part of the philosophers trade.

QM courses, and textbooks like Griffiths, are not only (or mainly!) about creating industrial engineers who know some tough physics - they are about *creating physicists*. For instance, a particle physicists needs to master everything in Griffiths and many other, harder, textbooks. They do not have the time (or support!) needed to read deeply into "philosophy of physics".

Given the questions the OP is asking maybe he should study philosophy of physics, and become a philosopher, but if he does that he will not have the time (or backing!) to become a physicist who actually uses QM in designing new computers, or in helping to discover new particles.

 

[maverick280857]

For instance, a particle physicists needs to master everything in Griffiths and many other, harder, textbooks. They do not have the time (or support!) needed to read deeply into "philosophy of physics".

Do I sense cynicism and disapproval? :-)

Given the questions the OP is asking maybe he should study philosophy of physics, and become a philosopher, but if he does that he will not have the time (or backing!) to become a physicist who actually uses QM in designing new computers, or in helping to discover new particles.

Fortunately there are fields of physics such as quantum information theory, where one may be able to do something other than designing new computers (where many skills other than QM are used) and discovering new particles (in which the foundations of QM are not usually questioned).

Anyway, Griffiths may be a good first book to learn QM from, but it definitely doesn't explain much quantum physics.

 

[twofish-quant]

Despite some of the in-your-face comments you may get from professional physicists who can no longer remember your pain (like Landau said once about not remembering a time when he did not know calculus), don't be deterred.

Oh I remember the pain and difficulty of learning QM.

However, one thing that you should be aware of is that after working on mathematical problems for a year, I understood how to crunch the differential equations for Schoedinger's equation. As far as what the QM means, I can honestly say that I am more confused now than I was when I was an undergraduate.

That's why the courses focus on "doing the math".

I think the general belief in the community is to force students to shed their understanding of CM when studying QM, and to stop asking questions like 'what is the wavefunction?' or 'what is a state?', or 'why does this work?'.

Oh, I don't think that this is the intention at all. The intention is to teach "this is a state" and "this is how QM works." You can keep asking how and why it works. You really should keep asking how and why it works. Just realize that no one really knows, so we can't tell you.

On a more serious note, quantum field theory, except when applied to the study of experiments we have some idea of (say in high energy scattering, or condensed matter maybe), is quite abstract.

It's abstract if you want to make it abstract. Personally what I want out of QFT is for someone to go through the calculations and tell me what happens when particle A scatters off particle B. That part is pretty concrete.

Haha, well, no. But then a physics major is usually not hired just for his or her QM prowess.

Not true. The reason that QM courses are designed the way that they are is that a lot of physicists go into semiconductors where you have to use QM to do calculations.

So while we may be about 50-100 years from the first time QM gains mainstay acceptance and understanding, like CM or EM today, it may well happen that QM may dictate industry a century from now. So who knows...

QM dictates industry today. Your computers require QM to work. Anyone that designs transistors has to understand QM, or else they can't design transistors.

 

[twofish-quant]

QM courses, and textbooks like Griffiths, are not only (or mainly!) about creating industrial engineers who know some tough physics - they are about *creating physicists*. For instance, a particle physicists needs to master everything in Griffiths and many other, harder, textbooks. They do not have the time (or support!) needed to read deeply into "philosophy of physics".

Disagree strongly. Most physicists that I know really care and are interested about things like what QM means. It's just that no one knows. The most that can be taught is all of the ideas that people have come up with. Also knowing Griffiths helps you figure out why it is a tough problem. There are some obvious ways of interpreting QM, but those obvious ways are wrong because they don't fit what QM says.

"Philosophy of science" is interesting because a lot of the works on what scientists do involve observing scientists. Something that is odd is that a lot of the ideas on what science "is" are quite new. For example, the idea that science is about "falsification" came about in the *1920's*, and the idea of a science as a framework came about in the *1950's*.

Given the questions the OP is asking maybe he should study philosophy of physics, and become a philosopher, but if he does that he will not have the time (or backing!) to become a physicist who actually uses QM in designing new computers, or in helping to discover new particles.

Don't think that is a good way of doing science.

It's good to ask questions. Just realize that you may not get any answers.

 

[physics girl phd]

Most physicists that I know really care and are interested about things like what QM means. It's just that no one knows. The most that can be taught is all of the ideas that people have come up with. Also knowing Griffiths helps you figure out why it is a tough problem. There are some obvious ways of interpreting QM, but those obvious ways are wrong because they don't fit what QM says.

I like this statement. I've also found myself wondering why the OP (and others) often seek "quasi-philosophical" explanations and discussion more in QM in general than with other areas of physics... why the probability and wavefunction mathematics of QM are really any more mysterious than field theories of EM and gravitation. I suppose because it's both new in terms of historical chronology, and new in terms of educational chronology (i.e. not massively emphasized in prerequisite coursework). But quite frankly, I (and probably many others) see properties like charge and mass, and the interactions between them (like Coulomb's law, the Lorentz force, and Newton's law of Gravitational Attraction) pretty darn cool and unexplained, despite their mathematical certainty. Then there's all that strong-force/weak-force quarky stuff that goes on in the nucleus too. Wow. And that's why physicists are still working on these things too. http://www.nature.com/news/2010/100823/full/4661030a.html" (To my understanding, the measurements of G might eventually say something for or against string theory, something of which I know too little about to even start to ask any questions about, quasi-philosophical or otherwise.)

 

[twofish-quant]

I like this statement. I've also found myself wondering why the OP (and others) often seek "quasi-philosophical" explanations and discussion more in QM in general than with other areas of physics... why the probability and wavefunction mathematics of QM are really any more mysterious than field theories of EM and gravitation.

Blame Fritjof Capra and David Bohm.

He was working in Stanford in the 1960's which was both a center of physics and hippie-dippie eastern religion and so he wrote the "Tao of Physics" to try to combine the two.

Curiously John Hagelin tried to do the same thing in the 1980's, and it didn't work out. The 80's were different from the 60's.

The other curious thing about people that mix particle physics and Eastern mysticism is that no one that I know that has deep knowledge of Eastern religion (and I'm a practicing Buddhist) really takes that stuff seriously.

One other curious thing is that I like a lot of other Chinese in science come out of a philosophical tradition that is extremely anti-mystic.