Kare wrote:Richard Feynman: "If you think you understand quantum mechanics, you don't understand quantum mechanics."

I remember having a chat with a lady who worked at the cafeteria at uni once. My friends and I were talking about some physics topic (I can't remember what, but we were all at uni studying physics remember) and this cafeteria lady overheard us and remarked "if it's so difficult, why bother?"

I don't understand why mass factors in inertia, but I use the fact that it does to maintain safe stopping distances when I drive. There is as yet no theory relating inertial mass to gravitational mass, yet the two are taken as being the same. Quite simply, no-one understands mass, but we can use this concept to make the maths work.

Quantum mechanics is, again, a tool. The quantum numbers are abstract human creations to make the maths work... but the maths DOES work. I don't understand why the quantum numbers should be as they are, but the maths works. When Feynman made this famous quote I think this is what he meant. He's not saying "be like the cafeteria lady and avoid difficult subjects", he's saying "No-one knows why, but the maths works".

This is why, every few months when I see quantum mechanics discussed in a religious forum, I despair. In general, few people know what a Hamiltonian operator is, so how can I explain quantum mechanics without maths? There is a reason quantum mechanics is barely mentioned at school, it's hard! Only at university level do we get into the guts of it, when we have all the relevant background in mathematics and classical physics.

So when I see small mistakes in a forum of friends about a topic I know, what should I do? Confuse everyone with maths? Tell everyone to be like the cafeteria lady? Try to correct mistakes as well as I can using hand-waving arguements? I go for the latter but provide topics the reader can go away and research further if they are interested. I'm a crap teacher but what else can I do?

If you prefer, Kare, I can avoid correcting mistakes. I can say "no-one knows" and leave people believing physicists are like alchemists or magicians, but that's not true. People DO understand quantum mechanics: they understand it's purpose, it's use and it's limitations. Bottom line is this, the transistors in your computer rely on knowledge of quantum mechanics, and they weren't designed and fabricated by chance.

"If you think you understand quantum mechanics, you don't understand quantum mechanics." Huh? I would change that to "If you think you understand interpretations of quantum mechanics, you don't understand quantum mechanics."