1. The problem statement, all variables and given/known data A traveller with mass m = 66,5 kg travels with a velocity of 0,82 c to the Andromeda galaxy which is a distance of 2,55·106 lightyears away. Light with the wavelength 459,4 nm is emitted from the galaxy. assume the galaxy doesnt move and use the exact speed of light. How long time (in million years, year=365,25 days) will the journey take for an observer on earth? 2. Relevant equations i dont really know... possibly: time dialation T = To / √( 1 - (v2/c2) 3. The attempt at a solution first of all, id just like to confess that the whole relativity concept is, unsurprisingly, doing my head in a little. i can pretty much grasp the way the speed of light is a constant and that time slows down when things approach the speed of light, and im usually ok with questions on the doppler effect, red/blue shift and relativistic energy, but im fairly certain all of the pieces have yet to fall into place. also, i know a lot of the info in the question isnt relevant (im guessing the velocity and the distance are the ones that actually matter) nonetheless, im giving the entire question just in case. Ok, so the part that's confusing me about this specific problem is the matter of what effect time dialation would have on the observer on earth. i understand that the observer on earth would observe clocks moving slowly on the spaceship, but wouldnt that not affect the observer since he is outside the travelling object? intuitively, i'd say that the answer is just 2,55·106 lightyears divided by 0.82 c, but this is obviously too simple. my next instinct is to plug the values into the time dialation equation, but i would love to understand why im supposed to do that. even then, i imagine i'm missing some aspects of the question.... obviously, its a little amateurish to try to do physics on instinct and intuition, but reading up on the subject has only made me more confused so far. would love any help on the subject, even if its just a link to yet another explanation on special relativity.