Time slows at the speed of light

DianaPrince
Hi,
I am not exactly sure how to present this question. Even thinking about it creates some confusion,so I will only try my best. My question is about special relativity. Since the first time I saw Carl Sagan's Cosmos series, I have been fascinated by this, but still don't totally get it. I can accept as fact that time slows down as you approach the speed of light, but actually understanding this presents a whole different problem.

If for example time elapses differently for a stationary observer than it does for someone traveling at the speed of light, how can we determine how much time has elapsed? Suppose I am standing with a friend in an open field. My friend takes a round trip ride from Earth to the sun at the speed of light (let's just pretend that he didn't get burnt to a crisp). Once he leaves my side in his imaginary spacecraft , I stand there waiting for him to return, which should only take about 16 minutes (round-trip). What will I notice when he returns? Is the amount of time that he was gone different for him than it was for me? For how long (stationary-earth-time) must he travel at the speed of light in order for there to be a visible difference in the aging process once we meet back up? If we as humans are accustomed to time passing at a very particular rate, what experience does the person traveling at the speed of light have? For example, he goes on a speed-of-light-journey let's say to Proxima Centauri. I sit around waiting for 8 1/2 years for him to get there and back. Does he feel as if he were only on a short journey? If so, according to his clock and his perception of elapsed time, how long did his journey last - for him? I hope I made my question clear. Any answers or additional info to help me understand this would be appreciated.
Jasmin

Homework Helper
Welcome to PF!

Hi Jasmin! Welcome to PF!
… Once he leaves my side in his imaginary spacecraft , I stand there waiting for him to return, which should only take about 16 minutes (round-trip). What will I notice when he returns?

Nothing … he can't get any younger, and you'll only be 16 minutes older!
If we as humans are accustomed to time passing at a very particular rate, what experience does the person traveling at the speed of light have? For example, he goes on a speed-of-light-journey let's say to Proxima Centauri. I sit around waiting for 8 1/2 years for him to get there and back. Does he feel as if he were only on a short journey?

If he goes on an almost-speed-of-light-journey, then everything seems normal to him (apart of course from the immense g-forces at the start the turn-round and the finish!), and yes he feels as if he were only on a short journey.
If so, according to his clock and his perception of elapsed time, how long did his journey last - for him?

Depends how close he was to the speed of light … could even be less than a nano-second …

The precise factor is √(1 - v²/c²), so the closer v is to c, the slower his time is.