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Rcomian t1_iy2wge6 wrote

let's imagine you're hanging out near a black hole (but not in it).

one thing to keep in mind, you would not notice time slowing down. your time as far as you're concerned would feel exactly like it is now. you'd look at your watch and it would tick at the same rate. you'd still be breathing, your heart pumping as normal. you'd hold normal conversations with the people next to you.

but, me, hanging out over here on earth, i could watch you through a telescope. and I'd see you moving in slow motion. your watch would tick more slowly than mine. your conversations would be lower in pitch and take longer. if you looked at me through a telescope, I'd be sped up. my watch would tick faster, my voice would be sped up like a cartoon character.

the closer you get to the event horizon the more extreme this effect. the most likely impact of this would be communications. in reality I'd never have a telescope that could view you. but you could flash a really bright light towards me, and communicate in morse code (or more likely a digital computer protocol). I'd see your flashes going very slowly. if i knew how fast you were flashing your light from your perspective, i could tell exactly how close to the horizon you were. weirdly, your light would get more and more red as well, for the same reason your voice would seem lower to me.

conversely, if i flashed a light at you, you'd see it flashing faster and more blue.

this effect is actually seen in our gps satellites. time goes slightly different for them (i think faster, as they're not in our gravity well, but there's other aspects that affect this too, like speed). so we need to account for the different clocks for gps to be accurate.

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Quaytsar t1_iy3o2df wrote

> this effect is actually seen in our gps satellites. time goes slightly different for them

There's a specific orbital height where the slow down in time from traveling at orbital velocity and the speed up in time from leaving the local gravity well exactly cancel each other out so you experience time at the same rate as someone standing on the surface.

It's just above 3000 km for Earth. So GPS goes slower faster due to their speed height.

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Bensemus t1_iy4liea wrote

They don't. GPS's go faster due to being farther from Earth.

> The combination of these two relativitic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)!

https://www.astronomy.ohio-state.edu/pogge.1/Ast162/Unit5/gps.html#:~:text=As%20such%2C%20when%20viewed%20from,by%2045%20microseconds%20per%20day.

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tjrae1807 t1_iy4s5ns wrote

Great explanation, and very fascinating. I knew about time displacement, but I didn't realize that the color of light would be affected as well

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katha757 t1_iy5uze6 wrote

I’m having trouble reconciling some of these facts, you had such a great explanation maybe you can help me understand.

If time slows down the closer you are to a black hole, the light is reaching you slower (red shifted), but I remember reading that light travels the same speed regardless of the point of view. How can light redshift but still travel the same speed?

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QuantumR4ge t1_iy7fj7u wrote

Redshift changes the energy of the light but it still travels the same speed locally

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voidentityofspace t1_iy50mqf wrote

this is a terrible explanation, telescopes pick up audio now apparently, and they are powerful enough to look at a 6 inch watch face. instead of just regurgitating a textbook paragraph, try to actually explain it like they’re 5. do better.

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Rcomian t1_iy51gew wrote

thanks you for your feedback.

yes, telescopes can pick up and recover sound by registering the distortion on objects in the room. this used to be done with lasers but even that isn't necessary now.

and you might notice that i said in reality we couldn't have a telescope that did that, but we could communicate and see the effect in other ways. like flashing lights at each other.

that's kind of important. we can imagine what we would see, and describe the effects, without worrying about what the physical limitations are. visualising the situation can help with understanding.

my point was that our experience of time doesn't change, but the time we see pass for others does.

I'm sorry if you found the explanation confusing, however.

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