 A light signal that left the Earth at the same time as the traveller would always get to the destination before the traveller.Since one might not travel faster than light, one might conclude that a human can never travel further from the Earth than 40 light-years if the traveler is active between the age of 20 and 60.A traveler would then never be able to reach more than the very few star systems which exist within the limit of 20–40 light-years from the Earth.