Paul Schechter

MIT

Title: Fermat's principle, gravitational mirages, and Hubble's constant.

The 0th, 1st and 2nd derivatives of a "Fermat potential" are used to
derive the "3Ds" of gravitational lensing: delay, deflection and
distortion.  These explain the "classic" image configurations
associated with strong lensing: crosses, beartracks, and Mickey Mice.
The difficulties in using time delays to estimate Hubble's constant
are described and transcended.