I have had my fair share of confusion and straightening out over the years, mostly something I accept as a challenge (life wouldn't be fun without challenges...) But it struck me as strange when I finally found out that two things they taught me when I took the undergrad 'intro to modern physics' course are not true. And I had spent a long time horribly confused by these two myths before I finally came to an understanding.

The first 'fact' that just ain't so is that the photoelectric effect requires that electromagnetic radiation be quantized for its explanation. The photoelectric effect occurs when light above a certain frequency hits a material and causes the material to emit electrons. They teach everyone taking the 'modern physics' course (and it is in both Kenneth Krane's textbook and in Tipler's book) that the observations contradict the classical model of electromagnetic waves. Specifically the experiments of the early 20th century showed that: 1. No detectable time delay between when light shines on the material and when electrons come shooting out (i.e. it takes less than 1 microsecond) even when the light is very dim. Supposedly, the classical model can't account for this because the electrons should need to take time to absorb enough energy from electromagnetic waves to pop off the atoms. At low intensities, this should take a long time. 2. Light below a certain cutoff frequency does not cause electrons to be emitted. 3. The energy of the electrons is capped by a maximum dependent on the frequency of the light, not the intensity. Cranking up the intensity only causes more electrons to pop off, not the ones that do so to have more energy. This seems to contradict the classical theory, since high intensity can energize the electrons more. But I am here to tell you...these observations on the photoelectric effect does not require the field be quantized, as long as the matter is treated quantum mechanically. This was put forth quite elequently in the 1960s by Lamb and Scully in their paper somewhat provocatively titled "the Photoelectric effect without photons" (See that paper for further discussion of this issue, I digress).

The second 'fact' that just ain't so in modern physics is that the Bohr model shows that angular momentum is quantized in units of hbar. Students are taught (and for a time I believed) that the Bohr model of the atom shows that angular momentum is quantized, since by positing circular electron orbits and assuming angular momentum must be L = n hbar, Bohr successfully predicted the energy levels of the hydrogen atom confirmed by experiment. The problem is, the Bohr model is completely wrong . As soon as you look to atoms or ions with more than one electron, the model's predictions do not agree with the experimentally observed energy levels of the atoms. And the model introduces the assumption that orbits are circular (rather than oblique or elliptical) in a totally ad hoc manner. The model is completely wrong about the orbital angular momentum of the ground state of the hydrogen atom: It predicts hbar, the observed value is zero. In other words, the very quantity championed by the model as all-important, angular momentum, the model can't even get right! To be fair, the 'modern physics' textbooks do eventually confess that the model was later superseded by the 'full' quantum-mechanical model of the atom. But they completely fail to mention that the model gets the hydrogen spectrum correct purely by accident due to a quirk of the Coulomb potential, and that the model is grotesquely wrong about every atom on the periodic table besides Hydrogen! It's not just an approximation to reality in the way classical mechanics is, nor is it a limiting case. It's just wrong and the textbooks fail to explicitly explain this very important issue, leaving students with confusing concepts that can haunt them for years to come (or did in my case at any rate).

On both of these matters, photoelectric effect and Bohr model, the textbooks present things the way they do for historical reasons, allowing students to see it from the persepective of knowledge available in approximately 1920. But everything we have learned since then is left out of the discussion. Why is this acceptable? After all, no good textbook of electromagnetic theory starts with three chapters on the dynamics of the luminiferous aether, just to let students understand how physicists viewed matters in 1890!

Quantum mechanics is discussed later, but what has been learned since 1930 is still left out even when the 1920 barrier is finally broken (Schroedinger and Heisenberg developed nonrelativistic QM in the 1920s) , apart from brief mentions of some concepts in particle physics in later chapters that are often not covered.

If 'Modern physics' goes to the 1920s, I guess everything from the 1930s on is "Postmodern Physics". We need a course for that.