0

I just thought about what time realy is but I can't put my head around this so I was thinking that if time dilation happens at the edge of a black hole that just everything gets slower. But does gravity also get slower/weaker?

For example, if I drop something on earth it falls to the ground at speeds I know of. But what if we move earth now at a very close orbit of a super massive black hole. (lets just assume that a stable orbit close to the event horizon is possible and spagetthification is negated by the super massive aspect of the black hole or just doesn't exist in this example)

At this earth near the black hole, I am very slow from an outside observer because my time frame is so slow. When I drop now something, is the speed of falling still the same for me on this earth? Would an outside observer also see the falling object in slow motion?

If gravity is affected by time dilation, would not this mean that it would be impossible to see merging black holes as an outside observer because the time dilation is infinite at the moment when the black holes horizons are touching so you would forever see two nearly touching black holes? This would also affect gravitation waves because the time dilation is so big that the waves are frozen in time.

If gravity is not affected by time dilation, would that mean that gravity is something absolute. Meaning that I know I being affected by time dilation by dropping things at 1g and observe how fast it is falling (Because I am slower but the object is still falling at 'normal' speed)?

somedude324334
  • 23
  • 2
  • 1
    Kind of related: https://astronomy.stackexchange.com/q/44647/16685 – PM 2Ring Sep 17 '21 at 10:57
  • 1
    Does this answer your question? What would I experience on a planet orbiting close to a black hole? – WarpPrime Sep 17 '21 at 13:42
  • @fasterthanlight not at all. It's a whole different question. The question you linked asks what one would feel if the planet is in a close orbit. Answere: Not much if its a super massive bh. But I don't want to know if you would feel the gravitational pull from the bh but rather what the time dialtion do on the gravity. Thats why I wrote the two cases if the time dilation affects the gravity or not. – somedude324334 Sep 17 '21 at 13:54
  • 1
    "so I was thinking that if time dilation happens at the edge of a black hole that just everything gets slower. But does gravity also get slower/weaker?" Time dilation occurs between two frames of reference, not at a single location. Time is a coordinate, just like position. There is time dilation due to relative motion (special relativistic) and there is time dilation due to gravitation (general relativistic). Your question does not make sense as it is currently posed. Also, the tidal acceleration near the event horizon of a supermassive BH is actually quite weak... – Daddy Kropotkin Sep 17 '21 at 14:26
  • What do you mean "is gravity effected by time dilation?" Time dilation is an observer-dependent effect of the gravitational field. – Daddy Kropotkin Sep 17 '21 at 14:28
  • @DaddyKropotkin If I would be an outside observer and look at the planet near the black hole and see a person that drops something, that object looks for me as if falls in slow motion, right? – somedude324334 Sep 17 '21 at 14:30
  • 1
    No, I think that is an incredibly crude description of what happens. – Daddy Kropotkin Sep 17 '21 at 14:53
  • 1
    I suggest that you greatly simplify your question to only be about time dilation in principle, and remove the extra stuff that is uncessesary, i.e. black holes, planets, slow motion... Like, "If there is time dilation between two frames of reference due to nonuniform gravitational field, then is the gravitational field itself effected?" Otherwise, it's hard to know where to begin answering your many misconceptions about black holes, etc... After you get a better grip around the simple question, then you could ask more question later about black holes and other specific things. hope this helps. – Daddy Kropotkin Sep 17 '21 at 15:13
  • According to general relativity, objects following curved spacetime is what we see as gravity; curved spacetime causes a stationary observer far away from the curvature to get light back in such a way that creates the effect we see as time dilation. To ask if time dilation effects gravity is to ask if spacetime curvature as observed from far away effects spacetime curvature. Your question could use some refining. – Justin T Sep 17 '21 at 22:40
  • Gravity isn't a force in General Relativity. Time dilation is a General Relativistic effect. – ProfRob Sep 19 '21 at 13:12

2 Answers2

3

Gravitational effects affect everything equally, even the gravitational field itself.

Time dilation isn't an absolute thing; it's just a way of saying that the elapsed time on one clock is different from the elapsed time on another clock, and the details vary from experiment to experiment. But if you want to say that time dilation is real, then gravity is just as affected by it as anything else.

When I drop now something, is the speed of falling still the same for me on this earth? Would an outside observer also see the falling object in slow motion?

Yes, it will appear to you to fall at the usual rate, but someone who is stationary far from the hole, or in a more distant orbit, will see it fall slower. (At least, when you average over a full orbit. There is also a "special relativistic" blue/redshift that depends on your orbital position.)

If gravity is affected by time dilation, would not this mean that it would be impossible to see merging black holes [...]?

Yes, we can never see event horizons, whether the holes are merging or not, because by the definition of event horizon, no light or gravitational wave from them can reach us. When LIGO observes a merging pair of black holes, it's really observing the merging of the exterior fields of the holes, not the event horizons as such.

benrg
  • 3,822
  • 11
  • 19
-2

If you place Earth near a black hole, freely falling, you would still see a ball falling to its surface like you do on Earth. Supposing the hole is big enough to leave out non-local tidal forces. To a faraway onserver you seem to freeze at the horizon at infinite time. You will reach the center though in a small time! So for a faraway observer it takes an infinite time for a hole to form. The formation anSich is quite complicated. Will the whole singularity form at once, after all stuff has passed the horizon? How gets the center of the collapsing mass distribution gets the information to actually form an infinitesimal hole at infinity? Complicated.

I'm not sure what you mean exactly by asking if gravity is affected by time dilation. You mean that objects fall slower? No that's not the case if you place a planet in a time dilated curved spacetime. Your own time gets dilated too and everything falls the same as usual. Only for a faraway observer your ball falls slower. The falling of the ball by itself is caused by time going slower the closer you get to Earth. All these times get dilated the same when the Earth is placed near a black hole (of sufficient mass to evade tidal forces.

Deschele Schilder
  • 1
  • 11
  • 26
  • You probably got downvotes on your previous answers because the voters thought they weren't useful, either because they were unclear or contained errors. This answer is a bit better than your previous ones (which I didn't downvote), but it still has some problems. What does "anSich" mean? BTW, when a post gets downvoted, you should edit it to improve it. Deleted posts are still counted by the ban algorithms. – PM 2Ring Sep 19 '21 at 23:17
  • An Sich is German for on its own. Let them ban me! My book takes care of them. You are OKAY! – Deschele Schilder Sep 19 '21 at 23:19
  • @PM2Rimg And I wonder still ? WHAT is WRONG with this question? Why is it downvoted? If a shell of mass collapses, or a sphere of massive particles, does one actually KNOW how this collapses to a black hole? How particles become entangled (as is suggested to account for the unitarity of QM)? Does entanglement really take place? Is there a firewall? Is there after all a violation of unitarity as there is when a wavefunction (superposition) collapses? What did I not describe well here? – Deschele Schilder Sep 20 '21 at 04:47