On Thu, Feb 18, 2016 at 09:10:53AM -0500, Jeffrey Schwartz wrote:
> Yes, what I was trying to say is that with Jump, there's never time
> dilation due to relativistic speed.... so a lot of the causality
> breaking stuff doesn't seem to break.

They do break relativity or causality, but in a more subtle way.
Relativity says that the laws of physics are the same from the point
of view of any inertial observer.  In particular, if a ship can go
into jump, and emerge 3 parsecs forward in space and 1 week forward in
time, then it doesn't matter what speed it was moving.

The trouble is that in relativity, the location of "3 parsecs and 1
week forward" varies.  The most obvious example is that ships facing
in different directions will disagree on what direction "forward" is.

If they're facing the same direction but have different starting
velocities, it is less obvious but still not hard to see that they
will disagree on where "3 parsecs in 1 week" is.  After all, if they
hadn't jumped then they'd each consider themselves to be "at rest" in
different locations in 1 week.  If you displace 3 parsecs from there,
you get 2 different exit points.

Not at all obvious if you haven't studied relativity is that they will
disagree on *when* "3 parsecs in 1 week" is.  Velocities in relativity
behave like angles in spacetime, and a small angle over a very long
interval can result in large differences in the endpoint.  "3 parsecs
and 1 week" behaves like a length of 3600 days in spacetime, and so a
0.2% difference in "angle" will displace the endpoint by more than a
week in time.

That is, if they differ in speed by more than 0.2% of the speed of
light, they will disagree on when "3 parsecs and 1 week forward" is by
more than a week.  One of them will come out of jumpspace more than a
week before the other -- which is to say, before the other one went
into jump.

If they beam a message onto another ship just about to jump back the
other way, they can carry a message into the past of both ships.

So obviously, relativity *does not* hold in Traveller.  The laws of
physics do depend upon some absolute frame of reference, because some
ships cannot jump "3 parsecs and 1 week forward" of their current
position and time.  There is an absolute "jumpspace rest" standard
that determines when and where ships can go.

So in that sense, it is not at all like an Einstein-Rosen bridge,
since (if they can exist) they do obey relativity -- with all the
causality complications that would entail.

- Tim