Submitted by [deleted] t3_z6raz6 in explainlikeimfive
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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.
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)!
Quaytsar t1_iy4mh21 wrote
Yeah, I misread my graph.
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
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?
QuantumR4ge t1_iy7fj7u wrote
Redshift changes the energy of the light but it still travels the same speed locally
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.
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.
Windexhammer t1_iy2u1pc wrote
The more gravity you are experiencing, the more that time slows down for you. Black holes have a lot of gravity so time would slow down a lot if you went near one.
Aurinaux3 t1_iy2xi5r wrote
Note that time dilation is not an experience. It is only an observation that is measured by someone else.
If you approached the event horizon of a black hole, then you would just cross it normally. You wouldn't slow down.
If I tried to observe someone crossing the event horizon of a black hole from far away then they would literally never cross it from my frame of reference. And yet, they still do.
japanb t1_iy4q8b4 wrote
the image coming back would be slower i guess
QuantumR4ge t1_iy2v72e wrote
It means as an outside observer watches you fall, they see each tick on your clock taking longer and longer to happen the closer and closer you get to this magical point
breckenridgeback t1_iy2x6ze wrote
If you were aboard the falling ship, you wouldn't notice anything unusual, though, because all the processes you'd use to observe clocks would also be slowed down. Time dilation is always something you observe in others, not yourself.
QuantumR4ge t1_iy2z05q wrote
That isn’t the reason, the reason you don’t observe time dilation in your own frame is because as an in-falling observer you are not accelerating or in motion (you always see yourself at rest), so for both the in-falling and outside observer to agree on the local speed of light, they must disagree on space and time. (Although a specific combination of these is conserved)
The in falling observer actually can’t even define where the event horizon is
[deleted] OP t1_iy31dy4 wrote
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QuantumR4ge t1_iy328gn wrote
An in falling observer is a non accelerating observer by definition. An in falling observer is inertial. You are viewing the world from a newtonian perspective. The proper acceleration of an inertial observer, is 0, an observer acting under only gravity is inertial, they are following a geodesic, that’s literally why they are falling. This is essentially what the equivalence principle is telling you. There is no “gravitational force “ pulling you.
In Kruskal-szekeres coordinates you can clearly see the event horizon is not defined for the in falling observer. The event horizon is not defined for observers following a geodesic
You are getting confused here, for reference my specialism is in general relativity, this is the field of research i do.
_CZakalwe_ t1_iy399v2 wrote
While clocks on falling ship will (technically) remain the same, tidal forces would draw you , clocks and the ship into spaghetti. That would be highly unusual for casual ship obsever.
Czl2 t1_iy2w7oe wrote
Yes that and they see the light emanating from you and your clock become ever more ‘red shifted’.
> It means as an outside observer watches you fall, they see each tick on your clock taking longer and longer to happen the closer and closer you get to this magical point
QuantumR4ge t1_iy2z3m8 wrote
Yeah, the redshift of light happens for the same reasons but rather than dilating the time, its about contracting lengths.
japanb t1_iy4q3sp wrote
They always say relative to such and such a location. So I think it just means "appears to be going slow" just like a plane appears slow when it's at 30,000 feet
Latter-Summer-5286 t1_iy9k2n5 wrote
Not quite- time literally slows down in high levels of gravity. You cannot actually experience the slower time, though, because you are slowed down as well.
You can, however, see that other things are moving faster/slower than you relative to how fast you're going in time. A common explanation is that you take two twins and stick one on a spaceship going near the speed of light for a few years. Once the ship returned, the twin from the ship would be significantly younger than the twin who stayed on Earth.
This effect is why satellites need to resynchronize their clocks, by the way. They use extraordinarily precise mechanisms to keep time extremely accurately, but because they're much further from the center of Earth's gravity well (and thus far less effected by time dilation from Earth's gravity) their clocks run ever so slightly faster than clocks on Earth. As a result, they need to repeatedly adjust their clocks backwards to prevent their clocks rapidly desynchronizing from those on Earth.
usrevenge t1_iy2ucaq wrote
The event horizon is the edge of a black hole.
Black holes exert gravity.
The move gravity in an area the slower time travels.
Technically this means if you could chill out next to a black hole for a few days you could come back to earth but it will be years ahead here.
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.