[SOLVED] Find the velocity using special relativity According to observations on the Earth, the nearest star to the solar system is 4.25 light years away. A space ship which leaves the Earth and travels at uniform velocity takes 4.25 years, according to the ship-borne clocks, to reach the star. What is the speed of the space ship, expressed as a fraction of the speed of light c? Ux = (U'x + v)/(1 + v*U'x/c^2) We know that: x = 4.25 light years t' = 4.25 years c = constant. I have been stuck on this question for nearly a week, and i was hoping someone could give me a push in the right direction. Now i know about time dilation and length contraction and that they will be seen differently between the two frames. Thats why i don't suggest that the v of the ship is equal to c. I have also tried to rearrange the equation using Lorentz transformations, however in most cases i need gamma which means i need v. Any suggestions would be a great help, Thanks.