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.Scott

Homework Helper

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## Main Question or Discussion Point

I was just reading a review http://physics.aps.org/articles/v9/62 of a recent paper authored by Physics luminaries (http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.231301). The paper itself is behind a paywall - and I suspect the math is more than I have the energy for.

The gist of the article is that a QM mechanism is shown for keeping at least some of the EM information on the surface of the black hole - and that a corresponding QM mechanism may be found for gravity.

But I am wondering how much can be said about the mass and gravity information of material falling into a black hole without getting into QM. So I'll start by saying as much as I think I can, and let the more knowledgeable Physicists here correct and continue the story.

My thoughts can be described by considering these scenarios:

In our practice round, Alice will fall through an opening in a Dyson sphere and ultimately collide with the star at the center. Meanwhile, Bob will be on the outside of the sphere keeping close track of gravity. He will notice continuous change as Alice accelerates towards the star. Upon reaching the star, Bob will note that the mass of the star and Alice have combined to form a shared trajectory. Moments later, Bob will notice rockets firing on the Dyson sphere keeping the sphere centered around the new stellar trajectory.

In our second round, Alice will dust herself off and, as is often the case, she will dive into a black hole while Bob looks on. Now, of course, from Alice's point of view she cruises past that horizon on the way to oblivion. Or perhaps you prefer a wall of fire - it doesn't matter because her voyage beyond the horizon is not part of our universe - it is pragmatically a fiction.

On the other hand, the event horizon has a significant affect on what Bob sees. The entire trajectory of the black hole will change, just as the star did. But there will be no evidence of a collision with anything at the center of the black hole because Alice's real-world (Bob's world) trajectory never crosses the event horizon.

So, standing next to Bob and his array of gravity-measuring devices, what do we see? As Alice makes her final approach to the event horizon, her affects are limited by the speed of light to a small region of the event horizon. And it is further limited by the fact that information about her cannot travel through the non-existent interior of the event horizon. The new mass and trajectory of the black hole must be communicated across or above its surface at the speed of light. And since it is not possible to communicate this mass (or anything) across the event horizon surface, it must be communicated well above the surface - at or in the vicinity of the photon sphere.

So the mass is not absorbed by the black hole in an instant, or anything close to an instant and much of the affect will not be seen as emanating directly from the event horizon crossing point. Instead, information about the mass profile of Alice will be briefly sustained in a region surrounding the black hole and well above the event horizon.

Wouldn't Bob see Alice's gravitational trajectory start to change as she approached the photon sphere?

I would think that from that point on her mass would move across that sphere forming a hemisphere, then a full sphere, then a more and more evenly distributed sphere until her mass contribution was indistinguishable from the overall mass of the black hole.

And if that is the path for adding mass to the black hole, I would think that black hole evaporation would follow the reverse path.

The gist of the article is that a QM mechanism is shown for keeping at least some of the EM information on the surface of the black hole - and that a corresponding QM mechanism may be found for gravity.

But I am wondering how much can be said about the mass and gravity information of material falling into a black hole without getting into QM. So I'll start by saying as much as I think I can, and let the more knowledgeable Physicists here correct and continue the story.

My thoughts can be described by considering these scenarios:

In our practice round, Alice will fall through an opening in a Dyson sphere and ultimately collide with the star at the center. Meanwhile, Bob will be on the outside of the sphere keeping close track of gravity. He will notice continuous change as Alice accelerates towards the star. Upon reaching the star, Bob will note that the mass of the star and Alice have combined to form a shared trajectory. Moments later, Bob will notice rockets firing on the Dyson sphere keeping the sphere centered around the new stellar trajectory.

In our second round, Alice will dust herself off and, as is often the case, she will dive into a black hole while Bob looks on. Now, of course, from Alice's point of view she cruises past that horizon on the way to oblivion. Or perhaps you prefer a wall of fire - it doesn't matter because her voyage beyond the horizon is not part of our universe - it is pragmatically a fiction.

On the other hand, the event horizon has a significant affect on what Bob sees. The entire trajectory of the black hole will change, just as the star did. But there will be no evidence of a collision with anything at the center of the black hole because Alice's real-world (Bob's world) trajectory never crosses the event horizon.

So, standing next to Bob and his array of gravity-measuring devices, what do we see? As Alice makes her final approach to the event horizon, her affects are limited by the speed of light to a small region of the event horizon. And it is further limited by the fact that information about her cannot travel through the non-existent interior of the event horizon. The new mass and trajectory of the black hole must be communicated across or above its surface at the speed of light. And since it is not possible to communicate this mass (or anything) across the event horizon surface, it must be communicated well above the surface - at or in the vicinity of the photon sphere.

So the mass is not absorbed by the black hole in an instant, or anything close to an instant and much of the affect will not be seen as emanating directly from the event horizon crossing point. Instead, information about the mass profile of Alice will be briefly sustained in a region surrounding the black hole and well above the event horizon.

Wouldn't Bob see Alice's gravitational trajectory start to change as she approached the photon sphere?

I would think that from that point on her mass would move across that sphere forming a hemisphere, then a full sphere, then a more and more evenly distributed sphere until her mass contribution was indistinguishable from the overall mass of the black hole.

And if that is the path for adding mass to the black hole, I would think that black hole evaporation would follow the reverse path.