Tufts/CfA/MIT Cosmology Seminar, at Tufts:

Matt Visser, Washington University

Title:  Hawking's chronology protection conjecture: 
        current status and prospects

Abstract:  I will discuss the current status of Hawking's chronology
protection conjecture. This conjecture states that no ``reasonable
and realistic'' model for the universe based on the principles of
(semi-classical) general relativity can possess or develop closed
time-like curves. [This is a fancy way of saying that you can't
build a time machine.] I shall discuss the conjecture both form
the point of view of mathematics (proving theorems) and from the
point of view of physics (to what extent do the theorems reflect
physical reality).