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>> 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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