Links for 2012-05-21

A collection of classic estimation problems, with answers, written by E.M. Purcell for the American Journal of Physics back in the day.

Purcell had the insight that in a cavity, the number of states available for photons is not quadratic in frequency anymore. Instead, a cavity on resonance has a photon density of states that is proportional to the "quality factor", Q, of the cavity, and inversely proportional to the size of the cavity. The better the cavity and the smaller the cavity, the higher the density of states at the cavity resonance frequency, and off-resonance the photon density of states approaches zero. This means that the spontaneous emission rate of atoms, a property that seems like it should be fundamental, can actually be tuned by the local environment of the radiating system. The Purcell factor is the ratio of the spontaneous emission rate with the cavity to that in free space. While I was doing some writing today, I decided to look up the original citation for this idea. Remarkably, the "paper" turned out to be just an abstract!

The Oatmeal is a fantastic comic that I recommend that you make a habit of reading. However, even the greatest can go astray, and I'm pained to admit that The Oatmeal has done so regarding someone I regard very highly, and that's Nikola Tesla. Alas, The Oatmeal has fallen prey to Tesla idolatry, confusing his genius for godhood and of course, setting up the now all-too-common 'Edison as Tesla's arch-villain' narrative. There are quite a few errors and misconceptions about both Tesla and Edison in this comic. But they're errors that I've seen before and are often repeated, so it's worth the time, I think, to address some of the big ones.