Due to gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the horizon, never actually reaching it. If it were to somehow enter the event horizon, time dilation would dictate it traveling back in time toward the event horizon from the inside. Does this theory actually have merit or am I just grasping at straws?

  • $\begingroup$ From physics.stackexchange.com/a/3204/123208 "Therefore, rather than gravity having a special property that enables it to cross the horizon, in a certain sense gravity can't cross the horizon, and it is that very property that forces gravity outside of it to remain the same". Also, physics.stackexchange.com/questions/937/… $\endgroup$
    – PM 2Ring
    Commented Apr 27 at 16:02
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    $\begingroup$ This is grossly the "membrane paradigm": en.wikipedia.org/wiki/Membrane_paradigm. From the point of view of the falling particle, the horizon isn't a special place and the particle falls down the singularity without noticing there was an horizon. $\endgroup$
    – Cham
    Commented Apr 27 at 17:01
  • $\begingroup$ Cham is right. The object will fall just like you'd fall to Earth after jumping from a plane. $\endgroup$
    – Wookie
    Commented Apr 27 at 19:19
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    $\begingroup$ Voting to reopen. There are no unpublished personal theories here, the question can be answered within the context of mainstream physics, and there is a good answer below. $\endgroup$
    – gandalf61
    Commented Apr 28 at 9:07
  • $\begingroup$ If it were to somehow enter the event horizon, time dilation would dictate it traveling back in time toward the event horizon from the inside.” - This part makes no sense. (1) Nothing can cross the horizon as we observe from outside. (2) There is no concept of time dilation inside, as observed from outside. There is no relation between the time inside and the time outside. (3) Nothing inside causes the falling matter to move towards the horizon or to move back in time. $\endgroup$
    – safesphere
    Commented Apr 29 at 4:46

1 Answer 1


Is it possible that the mass of a black hole is located at the event horizon?

Yes, but not in the sense you mean it.

Objects fall into the black hole without slowing. But the light from them has a hard job escaping the gravity well, and the closer to the event horizon the 'slower' its escape (not because it's moving slowly, but because spacetime is twisted so that its outward path is skimming along close to the event horizon for ages). This is why to a distant observer it appears as if the object slows and stops at the event horizon.

(The following is a intended purely as an aid to the layman's imagination - don't take it too literally.) Imagine you are on a river that runs faster and faster until it tumbles over a waterfall. If you signal to somebody on shore by splashing in the water, so they can pick up the ripples, there comes a point when the water is moving forwards downriver at the same speed the ripples are spreading back upriver, and the ripple is frozen at that point. It takes forever to reach the shore. But the person on the boat who sent it is long gone.

However, the same thing also happens with gravity. Changes to the gravitational field we experience from distant objects (gravitational waves) travel at the speed of light. If we are one light-second from a large mass, the gravity we feel depends on the position, velocity, and acceleration of the mass one second ago, not now. So the curvature of spacetime around a black hole means that the gravitational influence of every object that has ever fallen into it, all the way back to its formation, is frozen on the event horizon like that wave. And the influence from the moments just before reaching the horizon are slowly, slowly leaking out.

Just as we can (in theory) see every object that has ever fallen towards the black hole (infinitely red-shifted/slowed down into ultra-long wavelength radio, so it appears black), so we feel the gravity of every such object's distant past.

We cannot feel the gravity of the singularity into which everything collapses, because it is in the future of every observer outside the event horizon (as is the event horizon itself), and the future cannot affect the past. (Or we could detect the future by picking up the gravitational forces from future events.) You cannot see it, and you cannot sense it. Everything we see and sense has to be in the past. Spacetime is twisted round so that the arrow of time points inwards. You cannot escape an event horizon for the same reason you cannot travel back to yesterday. The river of time carries you inexorably onwards and inwards.

  • $\begingroup$ Enough of the collapsed object (star, gas cloud, etc.) has coalesced to form a horizon and create dynamics which happen at a rate much slower than on good old Earth. $\endgroup$
    – Wookie
    Commented Apr 27 at 19:20
  • $\begingroup$ @Wookie Your comment is unclear. Note that all the original star matter that forms the horizon is still outside the horizon. A peculiar property of the horizon is that it is formed by the matter located outside of it and is impenetrable from outside in any external coordinates. This property is exactly what creates the Information Paradox. When a black hole evaporates, all matter outside the horizon should remain intact, because it is not a part of the black hole and yet has to disappear per the energy conservation. The paradox is that a black hole is made only of what is not a part of it. $\endgroup$
    – safesphere
    Commented Apr 29 at 4:19
  • $\begingroup$ @Wookie Also note that this user probably won’t respond to your comment. He’s been around for 7-8 years, not 3 days. He always creates the same ID to answer a question and then deletes the ID. His actual reputation would have been in tens of thousands, not 21. His answers generally are very good a deeply insightful, but he tends to make some statements that can be easy to misinterpret. For example, in this case: “But the person on the boat who sent it is long gone.” - Well, conceptually any fallen object can bounce off some “mirror” on its way and come back no matter how long ago it fell. $\endgroup$
    – safesphere
    Commented Apr 29 at 4:34
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    $\begingroup$ @safesphere I see. So the person who sent the answer is also long gone : ) $\endgroup$
    – Wookie
    Commented Apr 29 at 8:42

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