(urth) Sundial with "multitudinous faces"?

[Dan'l Danehy-Oakes]

On Fri, May 13, 2011 at 12:26 PM, Jerry Friedman
<jerry_friedman at yahoo.com> wrote:
> For instance, Tipler's rotating cylinders seemed to work
> till somebody found that the "closed time-like loops" were entirely within the
> event horizon, as I recall.

If I recall correctly the spoiler for Tipler cylinders was Hawking's
proof that it would require negative energy in some areas of the local
spacetime.

-- 
Dan'l Danehy-Oakes