# Someone moving near c to an observer?

I think i understand that if my friend, let's call him bob - were to travel to the moon and back at near the speed of light, when he got back, it would seem like only seconds to him but centuries could easily have passed on earth once he comes back to a stop.

my question is - if bob launched in his near-light-speed ship to the moon, after he waved goodbye out the window and pressed the 'launch' button, what would 'I' see him doing? since his time has slowed down, would he look like he was in suspended animation to me? if so, would i be able to anything to his ship to alter it's course?

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ghwellsjr
Gold Member
Since it only takes seconds for light to get to the moon and back, the most that bob could be younger than the rest of us on earth would be just seconds, not centuries. He'd have to go to a place that was light-centuries away in order to return to an earth that was centuries older than when he left.

When you watch an object moving rapidly away from you, you would "see" it in slow motion and you would continue to "see" it moving away from you even after it turned around and started back home. When you finally "see" it turn around, it will be almost all the way back home at which point you will "see" it in fast motion for a very short time. Since you "see" it in slow motion for the fast majority of the trip, its age has progressed very little while yours has progressed a lot.

NOTE: the reason why I put "see" in quotes is because the wavelengths of light for fast moving ojects will shift them to invisible colors, so you would need some special telescopes that could see these wavelengths and render them in a region that you could see, kind of light night-vision goggles that are sensitive to infra-red.

thanks - i guess i'm still missing something then? i thought that if bob travels at really close to the speed of light, say .9999995c then his time could slow-down by close to 1000x (according to the calculator at http://www.1728.org/reltivty.htm) . so maybe we're not talking years, but something close to an hour(?)

phinds
Gold Member
2019 Award
thanks - i guess i'm still missing something then? i thought that if bob travels at really close to the speed of light, say .9999995c then his time could slow-down by close to 1000x (according to the calculator at http://www.1728.org/reltivty.htm) . so maybe we're not talking years, but something close to an hour(?)
And how is that not consistent with what ghwellsjr said? If you choose a speed such that the dilation factor is 1000, then for you it will seem like the trip took several seconds and for him it will seem like the trip took several seconds divided by 1000.

ah ok. i guess i had it backwards then, as i thought his perspective would be several seconds multiplied, not divided, by 1000. thanks

phinds