Dear Steve Wilner:

On Friday, October 28, 2016 at 10:03:38 AM UTC-7, Steve Willner wrote:
In article ,
dlzc writes:

Quite old, actually. I first heard of it in "Spacetime
Physics" by Taylor and Wheeler.

I don't have that book on my shelf, but I can probably get
it from the library. What page are you looking at (and in
what edition)?

Second Edition, Section 8.4, Sample Problem 8-2. Quote: "A system consisting entirely of zero-mass photons can itself have nonzero mass!"

David A. Smith