[TML] White Dwarfs, Black Holes & 100 Diameters

[shadow at shadowgard.com]

On 5 Nov 2007 at 10:38, Jeff Zeitlin wrote:

> On Mon, 5 Nov 2007 15:30:25 +1100, Timothy Little wrote:
> 
> >The only way to avoid this problem is to define the operation of jump
> >in terms of some external reference frame.  It is probably not a
> >solution to say that the reference frame is fixed by the origin and/or
> >destination stars, since stars also have different velocities.
> 
> >One potential solution could involve some property of the galaxy (or
> >even universe) as a whole defining the reference frame of jump.  This
> >could enable retaining both relativity and FTL in the setting without
> >time travel.
> 
> Ummm... No.  This is specifically giving up relativity; the whole point
> of relativity was that there IS no preferred frame of reference.  You're
> trying to establish one here.
> 
> As Leonard is so fond of saying (or words to that effect), "Causality,
> Relativity, FTL - pick any two.".
> 
> My problem with the entire relativity-causality-ftl problem has been
> that C, the speed of light in vacuum, has been treated as privileged,
> without an adequate explanation as to WHY.  If (assume FTL) I can
> achieve effective speeds greater than C, why am I doing my observations
> with plain old C-capped methods?  I'll get my information on the state
> of the universe via the same FTL method, and if I do, all of the
> paradoxes appear to fall away.

Nope. The problemds are due to the structure of spacetime. Space and 
time (as co-ordinate sets) are linked in such a way that c is a 
conversion factor. 

You are familiar with the formula for ddetermining the distsance 
between two points given their XYZ co-ordinates? 

d = sqrt((x1-x2)^2 + (y1-y2)^2 + (z1-z2)^2)

So two observers with different co-ordinate sets (different origin 
point, axes rotated with respect to each other or a combo of both) 
will disagree on what the coordinates of the two points are, but will 
agree on the seperation between them.


There's a similar formula for determining the "interval" between two 
events in spacetime. C appears as a conversion factor in it. 

s = sqrt((x1-x2)^2 + (y1-y2)^2 + (z1-z2)^2 - (t1-t2)^2)

Everything else in relativity pretty much follows from this. The 
length and time effects, the "twin paradox" and a host of other 
things.

Observers at different velocities will disagree about the XYZT 
coordinates of events, but they will agree on the interval.

Note that different velocities is equivalent to rotating the 
coordinate axes. Some of my space is your time and vice versa.


Do note that it *is* possible to have FTL and relativity and retain 
causality. But only *global* casuality. That is, effects will still 
have causes, but for some observers the effect will precede the 
cause.

This places strict limits on FTL/time travel (the two are the same 
thing under relativity).

either you won't be able to get into your past (but the past of 
others you don't know about is fair game!) or you will not be able to 
cause any effects that would change your past as you know it.

That means that if you get a visit from your future self warning you 
about something, your future *will* include making that visit and 
saying those things. doesn't mean that you were telling the truth 
though.

If you try to go back and talk to yourself without it "already having 
happened" things *will* go wrong to prevent it. Probability goes out 
the window. 

But if you go back in time and affect the outcome of a sporting event 
on Deneb 7 there won't be a problem as long as that outcome had *no* 
effect on your personal timeline.

Which means that if you know the "original" outcome, either you won't 
be able to change it or your source of info was wrong.

This makes for "interesting" situations during the interval bewteen 
players figuring out that time travel is possible and when they 
realize that they can't change things.

Trying to work it into a game is likely to be a major headache 
though.




--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com