In the “uncomfortable questions” comment thread, Thony C. suggests:
You say you’re teaching “modern physics” so how about a running commentary on the stuff your teaching?
That’s a good suggestion, and I’ll start posting some sketchy reports soon. First, though, Bora asks:
What is un-modern physics?
Roughly speaking, physics gets divided into “Classical Physics” and “Modern Physics,” with the dividing line coming right around 1900.
“Classical Physics” basically covers fields that were well established before 1900: Newtonian Dynamics, Electricity and Magnetism, most of Thermodynamics, most of optics. It pretty much begins with Newton’s Laws of Motion (well, maybe Galileo), and ends with Maxwell’s Equations.
“Modern Physics” is the stuff that was developed in response to problems that cropped up right around 1900. It consists of Relativity (Special and General), and Quantum Mechanics. It starts with Max Planck’s introduction of quantum ideas to explain black-body radiation in 1900, and Einstein’s Special Relativity papers in 1905. It ends with the Standard Model of particle physics (more or less– theories of physics beyond the Standard Model are technically modern, but they’re also all entirely speculative at this point).
As a pedagogical and curricular term, “Modern Physics” refers to a course, usually at the sophomore level, that provides a first introduction to Relativity and Quantum Mechanics. In Relativity, this generally involves things like the Lorentz Transformation, addition of velocities, and relativistic momentum and energy (E=mc2), but not four-vectors. Light cones and spacetime diagrams may or may not appear.
In Quantum Mechanics, topics covered are usually the historical experiments that led to the quantum theory (the photoelectric effect, the Compton effect, electron diffraction), and really simple solutions of the Schrödinger Equation (infinite square well, simple harmonic oscillator, etc.). Such classes don’t get into perturbation theory, state-vector notation, matrix methods, or any of that stuff.
“Modern Physics” classes generally also include a quick survey of applications of these ideas. When I teach it, we generally spend a class or so on multi-electron atoms, a couple of classes on band structure (very qualitatively), a couple of classes on basic nuclear physics (Rutherford scattering, nuclear decays), and a quick mention of QED.
Returning to the field as a whole, not the classroom version, ObsessiveMathsFreak gets nasty:
Is modern physics actually getting anywhere?
Absolutely.
The idea that physics as a field is somehow stalled is a mistake that comes from identifying the entire field with the subset of people who do high energy and particle physics, particularly theoretical high energy physics. Even there, people will tell you that progress is being made all the time, though there’s some dispute about that.
Physics is much broader than just high energy physics, though, and there are exciting developments all over the place in lower-energy regimes. Bose-Einstein Condensation in dilute atomic vapors has only been achieved in the last fifteen years, and there are cool things being done with BEC all over the place. Quantum information (quantum computing, quantum cryptography, quantum teleportation, etc.) has only been an active research topic for about twenty years, and there’s a steady stream of fascinating stuff from those quarters.
Condensed matter physics generates cool stuff all the time. The hot substance-of-the-moment appears to be graphene, but within the last few years a whole new class of superconducting materials has turned up, generating a good deal of excitement in that field. And there are all sorts of cool developments in “nano” fields, using new maunfacturing techniques to make fascinating new structures.
Biophysics has really taken off, as well. The advent of tools for probing and manipulating single molecules, and doing atomic-scale imaging has opened up all sorts of cool new fields of study.
Modern theoretical high energy physics may or may not be stuck in a rut. Modern physics as a whole is thriving.