# In what sense is QM not understood ?

1. Jun 10, 2012

### Doofy

In what sense is QM "not understood"?

This is something that I've seen repeated many times, but I'm wondering how accurate it is. I mean, we've got this mathematical framework where we deal with vector spaces, eigenstates, superpositions, mixed states etc. that works to a high degree of accuracy.

Is it just the fact that QM deals with probabilities of measuring final states rather than the 1 input --> 1 output style of classical mechanics that makes people say it's "not understood" ? Is "not understood" just another way of saying "not familiar in terms of everyday human experience" ?

What I wonder about is how the founders of QM figured out that the mathematics we use in QM (operators, bras, kets etc.) was the right thing to use. They didn't just pull it out of thin air, they must have reasoned their way to at least some of it, eg. Schrodinger didn't just get out a pen and write down $H\Psi = i\hbar \frac{d}{dt}\Psi$ out of nowhere. Why isn't that considered "understanding" it?

Last edited: Jun 10, 2012
2. Jun 10, 2012

### Staff: Mentor

Re: In what sense is QM "not understood"?

To me, "not understood" means that there is no generally accepted interpretation for what is "really happening" underneath the probabilistic mathematics of QM. See all the arguments about interpretational / metaphysical / philosophical issues surrounding QM in this forum.

3. Jun 10, 2012

### Bill_K

Re: In what sense is QM "not understood"?

I'd agree with that. Feynman was one person often quoted as saying that he didn't understand quantum mechanics (!) But his idea of "understanding" was, "can I explain it to somebody without using mathematics" and of course he found that difficult to do.
I'm not a big fan of the historical approach. The founders didn't understand what they were doing. How they came to their conclusions was often a complex process and largely irrelevant. They made many wrong guesses along the way, and some of these guesses have even become immortalized. People often allude to Dirac's hole theory, for example, without mentioning that it was shown to be false by Heisenberg only a few years later.

4. Jun 10, 2012

### Doofy

Re: In what sense is QM "not understood"?

ah I like to know these things if possible, it gives you a fuller picture. I kind of feel like I won't "understand" QM in any significant way until I know how its starting postulates were dreamt up. There must be some account out there of how they did it that cuts out all the dead-ends and mistakes.

5. Jun 10, 2012

### kith

Re: In what sense is QM "not understood"?

I agree with Bill that the historical approach is not very good if you want to understand how the postulates of QM can be motivated. A good motivation is contained in Ballentine's book on QM.

6. Jun 10, 2012

### Bill_K

Re: In what sense is QM "not understood"?

OK, I can see I haven't convinced you. But ask yourself this - do the Feynman lectures try to teach physics by recounting what Dr X and Prof Y once thought years ago, or do they take an entirely fresh and modern viewpoint?

7. Jun 10, 2012

### vanhees71

Re: In what sense is QM "not understood"?

I agree that the historical approach to learn physics is not very good. Particularly quantum theory is complex enough without all the balast of the early history. Most hindering in understandung modern quantum theory is the Bohr model and "philosophical considerations". The best thing is to first learn the formalism and then think about the socalled "interpretation". I think the best interpretation is the minimal statistical interpretation, very nicely covered in Ballentines book.

However, on the other hand it is very interesting to know about the history of science. Sometimes it indeed helps to dig deeper into the meaning of theories. That's also true for quantum theory after you have come to term with its physics content in terms of the minimal interpretation. A very concise source is the multi-volume work by Mehra and Rechenberg on the history of quantum mechanics.

8. Jun 10, 2012

### kith

Re: In what sense is QM "not understood"?

Have you read all of it?

I read some reviews and got the impression that it would be a good thing to read through when you are retired. ;-) Unfortunately, it is quite expensive.

9. Jun 10, 2012

### Ken G

Re: In what sense is QM "not understood"?

The answers you have already are excellent, but perhaps we can dig a little deeper into what is "not understood." So far you have your own idea that this can mean "not familiar with everyday experience", but I don't think that quite cuts it, because we don't necessarily expect everything we discover to be familiar to us, our everyday experience is limited. Relativity is a perfect example-- we all know it includes extra elements that break our naive concept of time, and yet you don't hear nearly as much said about "no one understands relativity." Relativity simply involves postulates that go beyond our daily experience-- they don't contradict our daily experience. It's like if our daily experience was a trunk, we're not surprised to discover, with higher precision, that the trunk has an elephant that we were not familiar with attached to it.

We also have that it could mean that there is no accepted interpretation of what is "really happening", below the surface. I think that is getting closer, but as one who never tends to regard physics as a story of "what is really happening", I don't see classical mechanics as providing that story much better than quantum does (why is action minimized? Why do forces produce acceleration? We really don't have much of a sense of what is "really happening" in classical mechanics either). Indeed, some people interpret the backstory of classical mechanics to be quantum mechanics, by invoking the "correspondence principle."

Nor can we say that what is not understood about quantum mechanics is that it admits to multiple interpretations that have no obvious connection. In classical mechanics, we can learn F=ma, or we can learn the Lagrangian, or even the Hamiltonian approaches, and on the surface, these sound about as different as night and day. So what is so special about quantum mechanics that makes people like Bohr and Feynman, who have earned Nobel prizes in that very field, say that no one understands it?

I think it comes from the measurement problem. Quantum mechanics seems to be built, from the ground up, to revolve around an inherent contradiction. Its fundamental dynamical equation is deterministic, yet it is only used to make statistical predictions. On a related note, the states in quantum mechanics always evolve unitarily (so multiple possible observed outcomes are intrinsically included in the state), but the act of measuring them, and connecting to macroscopic instruments, appears to break that unitarity (since only one outcome is perceived, from the superposition of possibilities). The "cause" of that break is not at all clear, and differs substantially in the different interpretations. That is what I think is at the source of what is "not understood"-- not just that we have multiple interpretations, but that the interpretations all have to grapple with what seems like a central contradiction.

So what is not known is whether some future theory will address this contradiction and remove it, or if the contradiction is in some sense supposed to be there-- it is something we were supposed to discover about reality as we advanced. (And in my view, understanding decoherence in no way resolves this contradiction, it merely shifts the focus of when the contradiction is encountered, since all the interpretations can account for decoherence but do so in very different ways.)

10. Jun 10, 2012

### skippy1729

Re: In what sense is QM "not understood"?

In a sense they did pull the formalism out of thin air. They tinkered with classical mechanics trying to make it fit their experiments. Relaxing the commutative law of multiplication lead them to q-numbers then to generalized matrices and then to operators and Hilbert space. The founders of QM invented Hilbert space. If you want to see how they did it try "Sources of Quantum Mechanics" edited by B. L. van der Waerden for English translations of many of the important early papers.

Perhaps the fact that there has never been a consensus as to what might be the nature of the underlying physical system could be taken as evidence that Quantum Mechanics really is a fundamental theory. Maybe reductionism stops here.

11. Jun 10, 2012

### stevendaryl

Staff Emeritus
Re: In what sense is QM "not understood"?

I'm not sure what you are alluding to here. Dirac's hole theory has been abandoned in favor of a theory of electrons&positrons (as opposed to positive and negative energy electrons), but I always thought of that as a reinterpretation of essentially the same theory. In what sense did Heisenberg prove Dirac's hole theory wrong?

12. Jun 10, 2012

### Ken G

Re: In what sense is QM "not understood"?

"Never" is a very short time when it comes to quantum mechanics! Less than one century. I'd say it's far too early to predict the ending point of reductionism, indeed I don't think there is any time limit on keeping that question open. But by way of analogy, note that there was never a consensus as to what might be the nature of the underlying physical system that holds for gravity in either general relativity, or even Newtonian mechanics! The latter has been around for 250 years, with no consensus about its underlying physical system, and now there isn't even a consensus on what theory we should be looking for an underlying physical system for in the first place! The sands of knowledge can be stable for a long time, and still undergo seismic shifts.

13. Jun 10, 2012

### nikman

Re: In what sense is QM "not understood"?

If reductionism has an ending point you'll find it via QFT, specifically by exploring the fermion minus sign problem and its distressing/disturbing ramifications. Strong fermionic interaction (nothing to do with the "strong" force) has so far proven not to be computable, not even mathematizable.

It's all about how real messy stuff becomes real messy stuff instead of remaining neat coherent wave functions (which are, of course, symbolic formulations). Anyway if a thing depends for its own definition on the definition of what it interacts with and one continuously re-defines the other you've essentially got an irreducible recursion.

14. Jun 10, 2012

### Fiziqs

Re: In what sense is QM "not understood"?

Speaking as one who happens to agree with the idea that today's physicists don't really understand what they're talking about, puts me in a unique position to answer this question. It seems to me that QM does a fantastic job of predicting what will happen, but it is for the most part clueless when it comes to explaining why it will happen. To me, if you can't explain why something happens, then you probably don't understand what's going on. I can predict with pretty darn good accuracy that the sun will rise in the east tomorrow, but predicting the outcome, and understanding the cause, are two completely different things.

The excuse that QM is simply too complicated, or too far removed from our normal daily experience to be grasped by a layman, is in my view, complete BS. If I can't understand it, it's most likely due to the fact that the person trying to explain it, doesn't understand it either, so has to rely on psuedo-scientific gibberish in an attempt to fain competence. If you can't put it in layman's terms, it's not due to the fact that it's too complex, it's due to the fact that you don't understand it well enough.

I often hear theists describe science as a religion. Such comments invariably come from people who have little understanding of the true nature of either. But in one disturbing way, they are indeed alike, in both, the elite presume themselves to be in a position of understanding, unattainable by the uneducated layman. They assume a position of intellectual superiority. In truth, if we the laymen fail to understand, the fault lies in the inadequacy of the explanation, not some lack of "divine" insight.

15. Jun 10, 2012

### robertjford80

Re: In what sense is QM "not understood"?

I think this statement is correct. The charge that no one understands QM is overblown or hyperbolic. It's just that QM is not deterministic. So long as you understand that it is not deterministic and that that is just a brute fact, then there is nothing to misunderstand.

16. Jun 11, 2012

### stevendaryl

Staff Emeritus
Re: In what sense is QM "not understood"?

I would say that it is not just because it is nondeterministic that people say they don't understand quantum mechanics. It's the combination of nondeterminism together with extremely strong correlations that is hard to understand.

In an EPR experiment, you produce a twin pair of spin-1/2 particles. Alice measures the spin of one particle along some axis, and gets +1/2 or -1/2. Bob measures the spin of the other particle along a different axis, and gets +1/2 or -1/2.

The fact that Alice's result is nondeterministic is not hard to understand. But the fact that, in the case where Alice and Bob choose the same axis, they always get opposite result, is hard to understand. If Alice knew what axis Bob was going to choose, and Alice did her measurement a second before Bob, then she would know exactly what result Bob would get. So in that situation, from her point of view, Bob's result isn't nondeterministic--it's completely predictable.

It's the combination of perfect nondeterminism and perfect correlations that is hard to understand about quantum mechanics.

17. Jun 11, 2012

### Fredrik

Staff Emeritus
Re: In what sense is QM "not understood"?

The theory is of course understood very well. What's not understood is how the things described by the theory correspond to things in reality, especially at times between state preparation and measurement. The theory tells us how to calculate the probabilities of all possible results of all possible measurements, using knowledge of how the system was prepared as input. It doesn't tell us what the system is "really doing" at times between state preparation and measurement, at least not in terms that we can easily understand. In particular, we don't even know if particles have positions or not.

What's considered "understanding" is of course highly subjective.

18. Jun 11, 2012

### stevendaryl

Staff Emeritus
Re: In what sense is QM "not understood"?

I would say that there are aspects of the theory that are not understood, either. We have the recipe for using quantum mechanics, which is:

1. Between measurements, the system evolves according to Schrodinger's equation.
2. Measurement of any observable results in an eigenvalue of that observable, with probability computed from the wavefunction.
3. After a measurement, the system is in the eigenstate of the observable corresponding to the eigenvalue measured.

What's really not understood, at a theoretical level, is what constitutes a "measurement". We have a rule of thumb answer, which is that an interaction counts as a measurement if it leaves an irreversible record, such as a photograph, or a bubble in a bubble chamber, or a click in a Geiger counter, etc. But I wouldn't say that there is a very good theoretical understanding of what a measurement is.

19. Jun 11, 2012

### harrylin

Re: In what sense is QM "not understood"?

On this I agree with Doofy: I still would feel that SR is "magic" if I had not studied and understood its historical development. Regretfully I don't know much of the historical development of QM and its motivations (and it's still like magic to me).
Exactly.

Last edited: Jun 11, 2012
20. Jun 11, 2012

### krd

Re: In what sense is QM "not understood"?

It depends on your learning style. I'm out of college a very long time. And I'm only coming back to my physics now. I find I'm learning much more, and having a much better understanding of the physics by studying the history. It's helping me re-learn my physics. And sometimes I find little tid bits that I would have missed otherwise. And it's sometimes just little tiny ideas, that link other pieces together.

Learning the formulas and learning how to execute them is not enough. I have seen a few instances where professional scientists - who can do all the fancy calculus - have had misunderstandings of the fundamental theory - or have had gaps in their understanding that shouldn't be there. It's bad science, to know all the names, know all the maths, but have misunderstandings of the underlying theory.

I don't know how much the interpretations may change - but physics and chemistry, the way those subjects are taught in schools, the teaching materials probably need to be completely gutted and rebuilt from the ground up. There might be better ways to describe physics and chemistry to make everything fit more coherently.