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Imagine we live eternally and can survive tidal forces. Say, we are point-particles.

From a fixed distance someone jumps into a black hole. I wait a million years and jump from the same position.

How do we end up relative to one another in space and time? Will I be a lot older than him/her? Will our distance wrt to each other grow to infinity, due to the tidal force that accelerates him/her away from me?

It seems that from the fixed position he/she ends up at the horizon while my clock ticks on. When I jump in however, will he/she have disappeared behind the horizon through which I will fall too. His/her image will be red- shifted. So it seems we end up with a lot of space between us in radial direction as well as with different ages.

What is there to be said about this? Is this what is meant with ending up in the past relative to the universe around the hole? It seems we don't end up at the same point in spacetime.

So let's assume a classical black hole without Hawking radiation. The basic question: If two particles fall in from a hovering platform outside the horizon, a large time after one another (say a million years) how will they end up relative to one another in the frame of the particle that falls last? I can't imagine the second particle will ever meet the second. So am I right if I say they end up separated in space (in radial direction), while their clocks show a difference?

ErnieB
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  • Not knowing much about the topic, I'd hazard to guess that a million years is plenty of time for particle 1 to get past the event horizon. We don't really know what happens after that - it's like dying - nobody has come back to report. – user121330 Aug 31 '22 at 03:33
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    Time wrt. which coordinate system/observer? – Qmechanic Aug 31 '22 at 11:25
  • @Qmechanic we read my watch on the hovering platform. Then the other jumps in and uses his watch. And I use mine. So his will go slower wrt mine. Then after a million years has passed on my watch I jump in too and follow the same path. It seems we end up far away from each other. How can I ever reach him at the singularity? He always seem ahead of me. And if we compare clocks mine will show a later time. At least, so I think. – ErnieB Aug 31 '22 at 14:31
  • @safesphere I don't believe in the uncertainty principle. Every particle jumps around non-locally within the bounds of the wavefunction. But if I jump around with another particle on a hovering platform our identities get mixed up. So it could be me jumping or not. But one of us does. But say I stay behind and wait a million years. Will I ever see my falling friend again. There is no point in the hole like the surface of the earth. How can you end up in a point? As soon as there is a point it means you can't fall further. Unless the point is situated at infinity. – ErnieB Sep 01 '22 at 08:31

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From the way you word your question, I gather you are thinking of a black hole in terms of the 'frozen star' picture common in early theory. Things fall 'onto' the event horizon, but as time slows down there, they get slower and slower, the light more red-shifted, and never reach it.

This isn't what actually happens. To explain what is going on, here is a picture of a black hole, using a special coordinate system (Kruskal-Szekeres coordinates) that twists round to keep the lightcones aligned. Tilt your head $45^{\circ}$ to the right to see the usual 'spherical object with a singularity in the middle sitting in space' picture. Time experienced by a local observer goes up the page, space goes horizontally, lightspeed is shown by lines at $45^{\circ}$. (Bear in mind that time differences and distances are distorted, we've crammed a curved 4D spacetime into a flat 2D picture, and you should therefore be careful not to take it too literally.)

What any point in spacetime 'sees' directly towards or away from the black hole can be determined by looking along the two $45^{\circ}$ lines pointing down the page. These are called the past lightcone at the point.

Black hole spacetime

An observer hovering at a constant distance outside the black hole travels along the hyperbola shown on the right. They see the light of the collapsing star that fell into the hole millions of years ago, still struggling up out of the gravity well. The longer the time, the more oblique the angle and the slower (more red-shifted) the emissions seem to be, but the process never stops. No matter how far up the page you go, the hovering observer can still see the light from everything that has fallen in in the past. This continues long after the objects have hit the singularity and ceased to exist. (Although times are not really comparable as being 'before' and 'after' one another when separated by an event horizon like this.)

For a falling observer, their past lightcone rapidly approaches and crosses the event horizon, and their view of the past is no longer being redshifted as much. They see the collapsing star pass through the event horizon and head rapidly towards the singularity. But the falling observer hits the singularity themselves before they see the star's matter complete its journey.

The singularity is twisted round so instead of being timelike it is now spacelike. So as seen in the diagram, the collapsing star and the observer hit it at different now-spatial positions along it. The time experienced by either between crossing the event horizon and hitting the singularity is roughly the same. If the star fell in millions of years earlier, its clock would show the much earlier time.

This assumes an idealised theoretical Schwarzchild black hole, where the infalling mass is assumed to have no gravity itself. The true picture in a more realistic Belinsky-Khalatnikov-Lifshitz model has the spacetime stretching and oscillating chaotically as the masses approach the singularity. But the above discussion should be accurate enough to undertand what's going on outside and near the horizon.