(urth) Sundial with "multitudinous faces"?

[Jeff Wilson]

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? :)

>> 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.

> 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.

-- 
Jeff Wilson - jwilson at io.com
Computational Intelligence Laboratory - Texas A&M Texarkana
< http://www.tamut.edu/CIL >