(urth) Sundial with "multitudinous faces"?

[Jerry Friedman]

> From: Jeff Wilson <jwilson at io.com>
> On 5/13/2011 12:47 PM, Jerry Friedman wrote:
> >> From: James Wynn crushtv at gmail.com
> 
> > Let me try to explain the same thing as Dan'l and Sergei.  When the sun is at 
>a
> > certain place in the sky (I mean when Urth is aiming the Citadel at a 
certain
> > angle to the sun), the sundial will show a certain time.  A day later, the
> > sundial will show  a similar time, and 365 days later [*], an even more 
>similar
> > time.  But if it was morning on Mongo during the first reading, there's no
> > reason for it to be morning on Mongo at the later two readings, since 
Mongo's
> > day presumably isn't the same length as Urth's.
> 
> Is it the same time everywhere on Mongo at once? :)

SF lore:

http://books.google.com/books?id=92TqYdj56TcC&pg=PA146

I did make this mistake when talking about time on the Whorl in an earlier post.

> >> Of course, I might have been taken in by metaphor: They could have been
> >> actually saying that if you travel instantaneously to a distant star, you 
>are
> >> moving FORWARD in Time since the star we can see from Earth is some number 
>years
> >> ago. Or something along those lines.
> 
> Since the rate of time's progression is relative, you can't say travel is 
>instantaneous without saying in which frame of reference it is measured as 
>instantaneous. Since other, equally valid frames of reference measure the time 
>between your departure and your arrival as having finite, you can switch over to 
>a frame where it was negative, and do the instantaneous thing in that one, then 
>find one where that was negative, and do it again, etc., and that's how you end 
>up  returning before you left.

That's what I was thinking of.

> > Well, if there's FTL travel, then something is wrong with relativity (as I 
>think
> > Dan'l said), so we don't have a basis to say what would happen.
> 
> There are numerous relativity-compatible physics models that seem to allow for 
>time travel, but explain the lack of of observed time travel by positing it 
>requires improbable arrangements of nigh-unobtainable resources. Robert L 
>Forward's negative mass as dramatized in his novel TIMEMASTER is once example: 
>matter whose density is negative causes a negative curvature of space-time, 
>opposing the positive curvature from positive mass, among other weird things, 
>but complies with all known physical laws.

Except the law that says there's no negative mass.

I just made that law up, but sometimes the way these things work is that they 
have no known conflict with physical laws for a while, and then somebody looks 
harder and finds one.  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.

Jerry Friedman