Keep the speeds in units of c (e. g., v = 0.850c), distances in ly (light-years), time in yr (years), and never substitute a numerical value for c. keep in mind that the speed of light, in these units, is c = 1 ly/yr (one light-year per year). for constant speeds within any reference frame, speed = distance/time. imagine a star 76 ly from earth, and a probe traveling there at a speed of 0.930c a) how long will the journey from the earth to the star take, in earth's reference frame and in the probe's reference frame? b) using a faster probe would obviously cut down the journey time in both reference frames. however it isn't possible to cut the journey time in half in earth's reference frame. why not? words and numbers c) calculate the time the journey will take in the probe's reference frame if its speed is increased to 0.981c, and verify that this would cut its journey's time in half.