They Said It Couldn’t be Done

Michelson’s measurement of the diameter of Betelgeuse was a case where a new technology had to be invented to tackle the problem. The measurement of about 7 mas for the diameter of Sirius by Hanbury Brown and Twiss was another (called intensity interferometry). There are some famous cases, like the first measurement of solar neutrinos, of gravitational waves, and of X-rays from neutron stars and black holes. Others are less well known, for instance photographing light scattered by the dust clouds at L4 and L5 of the earth-moon system (Kordylewski clouds) and the confirmation of the prediction of polarization of the light from the Crab Nebula. At least a few of these, for instance observation of the cosmological neutrinos, are still ahead of us (and no, I don’t know how to do this either). But Sir William McCrea could think of no way to measure lambda independent of the other cosmological parameters, and you can read about it now as the Alcock-Paczynski test. The presentation will highlight 5 or 6 examples, not all necessarily mentioned here.