Looking for the name of a movie!?!? HELP?

I'm looking for the name of the movie where the earth receives a message from some aliens, telling us how to build a kind of spaceship to go visit their planet, which is located at something like 50 light years of earth. At the end of the movie they think that the trip to go visit their planet didn't work because time was in "fast forward" during the trip... Sorry my description sucks, i'm not english and I have a crappy memory. Thank you!