> Actually, I may have been a little poor with my calculations. If we take only significant digits, we get a time dilation of nil (gamma works out to 1.0). Here I'm assuming what I was taught is actually true, that being that you can only be as certain as your least certain measurement plus one decimal place. Therefore, .1c and 80 years being our "data", we can only be significant to the tenths place. That gives us the value of gamma being equal to 1.0 and the time dilation being nil.



Although what you say about significant digits is correct,
we are speaking theoretically and can say, for
the sake of argument, that we know our figure of
80 years to be exact up to whatever decimal place
we wish. The same can be true for the 0.1c figure.
So your answer, that the ship would be
behind by six months, is valid in the theoretical sense.

However, it is doubtful that the top speed of this
potential nuclear-powered spaceship would be
*exactly* 0.1c, and it is also doubtful that if
it were, it would take *exactly* 80 years to make
a round trip to Alpha Centauri. So if this ship
actually existed, your measurements could only be
as precise as what figures were observed to be the case.

Talking about this just made me realize something, though.
Your calculations assumed 80 earth years and
derived a figure for spaceship years. Shouldn't
you have assumed 80 spaceship years and then
calculated earth years?