>> My understanding of general relativity is limited, but I was under the
>> impression that "first" is not a meaningful concept at relativistic
>> velocities.
>
>    That would be paradoxical.

Yes. It's general relativity. It bemuses everybody.

>    Assume that intelligent life evolves in planet A, and it starts sending
> radio signals to outer space. These radio signals eventually reach planet B.
>
>    Some time later life

Your language is still assuming that "earlier" and "later" are valid terms.

A small thought experiment: There are two parallel train tracks and a 
train station in the middle of the tracks. There is a train sitting at 
one end of one line, and another train at the opposite end of the other 
line. The station and both trains synchronise their clocks, and at 
exactly noon the two trains accelerate down the track to relativistic 
speeds. Each train emits a flash of light at 1PM, according to the 
on-board clock.

Applying the Lorentz transform makes my head spin. But the general 
result is this:

- Train A emits a flash at 1PM. Since train B is travelling at 
relativistic speed with respect to train A, from train A the clock on 
train B appears to have slowed down, and so it appears that flash A 
happens /before/ flash B.

- Train B emits a flash at 1PM, and by the same argument it appears that 
flash B occurs /before/ flash A.

- Standing on the station platform, both trains' clocks appear to have 
slowed down by the same factor, and hence both flashes appear to happen 
at /the same time/.

Thus, depending on who you ask, flash A occurs either before, after, or 
at the same time as flash B. QED.

Now replace speeding trains with moving planets, add in the influence of 
gravity and perhaps the negative curvature of space, and call me back 
when you get over the searing pain in your brain...

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