# 9: Perturbation Theory

The Schrödinger equation for realistic systems quickly becomes unwieldy, and analytical solutions are only available for very simple systems - the ones we have described as fundamental systems in this module. Numerical approaches can cope with more complex problems, but are still (and will remain for a good while) limited by the available computer power. Approximations are necessary to cope with real systems. Perturbation theory is one such approximation that is best used for small changes to a known system, whereby the Hamiltonian is modified.

- 9.1: Time-Independent Perturbation Theory
- This method, termed perturbation theory, is the single most important method of solving problems in quantum mechanics, and is widely used in atomic physics, condensed matter and particle physics.

- 9.2: The Peierls Transition - an Unexpected Insulator
- The first satisfactory theory of “ordinary” superconductivity, that of Bardeen, Cooper, and Schrieffer (BCS) had appeared a few years earlier, in 1957. The key point was that electrons became bound together in opposite spin pairs, and at sufficiently low temperatures these bound pairs, being boson like, formed a coherent condensate—all the pairs had the same total momentum, so all traveled together, a supercurrent. The locking of the electrons into this condensate effectively eliminated the usua

- 9.3: Van Der Waals Forces between Atoms
- The perfect gas equation of state PV=NkT is manifestly incapable of describing actual gases at low temperatures, since they undergo a discontinuous change of volume and become liquids. In the 1870’s, the Dutch physicist Van der Waals came up with an improvement: a gas law that recognized the molecules interacted with each other. He put in two parameters to mimic this interaction.

- 9.4: The Interaction Representation
- For perturbation theory problems with a time-dependent potential, an intermediate representation, the interaction representation, is very convenient.

- 9.5: Time-Dependent Perturbation Theory
- We look at a Hamiltonian with some time-dependent perturbation, so now the wavefunction will have perturbation-induced time dependence.

- 9.6: The Photoelectric Effect in Hydrogen
- In the photoelectric effect, incoming light causes an atom to eject an electron. We consider the simplest possible scenario: that the atom is hydrogen in its ground state. The interesting question is: for an ingoing light wave of definite frequency and amplitude, what is the probability of ionization of a hydrogen atom in a given time? In other words, assuming we can use time-dependent perturbation theory, what is the ionization rate?

- 9.7: Quantizing Radiation
- The electromagnetic field itself is quantize and made up of photons. Recall Planck’s successful analysis of radiation in a box: he considered all possible normal modes for the radiation, and asserted that a mode of energy ωω could only gain or lose energy in amounts ℏω . This led to the correct formula for blackbody radiation, then Einstein proved that the same assumption. We now understand that these modes of oscillation of radiation are just simple harmonic oscillators.

*Thumbnail: The unperturbed Hamiltonian (blue curve) of a known system is modified by adding a perturbation (red curve) with a variable control parameter λ, which governs the extent to which the system is perturbed. Image used with permission (CC BY-SA 3.0; Rudolf Winter at Aberystwyth University).*