how long a light year on earth will seem First of all, sorry if this is not the right StackExchange to ask this question, but i don't know a better place (open to suggestion in case).
So, the question:
Suppose that for some reason, we can travel at the speed of light, and lets suppose that there is no acceleration involved, so it's something like uniform rectilinear motion.
Let's say that today someone leave with this spaceship at the speed of light, the after one year (for him), he will have travelled 9460730777119.56 km, but how much will be spent on the Earth since he has leaved?
So, in other words, I read about a special planet 31 light years from Earth, and I was wondering, if we can travel at the speed of light and someone travels for 31 years to that location, how much older would I be from when he has leaved the Earth?
You can never reach the speed of light but can get very close to it. Thus, when you travel with, say, 99.99% speed of light it will take you a little more than 31 years to reach the star 31 light years away. This is, however, the time it takes seen from earth. In the spaceship, the time taken will be dilated by
$$\gamma ={1\over \sqrt {1-{v^2\over c^2}}}$$
Which is roughly a factor of 70.7. This means in the space ship it is only 0.44 years or 160 days.