how long a light year on earth will seem

Question

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?

Sorry, your question cannot be answered in its current form. Nothing with non-zero mass can travel at the speed of light. But if the spacecraft is travelling very close to light speed, then for the passenger the journey would take much less than 31 years, (and slightly more than 31 years for the people on Earth). Please see https://astronomy.stackexchange.com/q/21022/16685 — PM 2Ring, Aug 08 '20 at 02:20
I’m voting to close this question because objects with mass can't travel at light speed. — James K, Aug 08 '20 at 07:32
@PM2Ring so in other words, traveling near the speed of light for 30years (for the traveler) won't be felt a lot longer on the earth? — , Aug 08 '20 at 10:38
@JamesK ok so suppose he is traveling with a velocity very close to the speed of light — , Aug 08 '20 at 10:40
Okay Berto99, please [edit] your question. I'm fairly sure there are similar questions here or on physics stack exchange, so perhaps have a little search around too. — James K, Aug 08 '20 at 11:33
Massive things can not go with the speed of light, but they can go with nearly the speed of the light. So I suggest to edit your question to ask from that. Furthermore, you are asking many questions here, ask only a single one. Third, ask this on the https://physics.stackexchange.com . — peterh, Aug 08 '20 at 18:31
With enough speed, in 30 years ship time you could easily cross the galaxy & back. But when you got back to Earth, many thousands of years would have passed. You might like to browse http://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html There's a fair bit of mathematics on that page, but you can skip over the hard stuff. ;) — PM 2Ring, Aug 08 '20 at 18:43

score 1 · Answer 1 · edited Aug 09 '20 at 12:04

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.

how long a light year on earth will seem

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