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I have read this question:

When matter is added, carefully and radially to make sure it does not add angular momentum, the horizon radius increases proportionately to the amount of mass added (but see below you may also be adding 'kinetic and potential energy' in, so it must be accounted as part of the toal mass, or energy, added). Simple from the equation, thehorizon grows.

How do Black Hole event horizons grow?

What exactly makes a black hole STAY a black hole?

And this:

How can black hole increase its mass?

How can anything ever fall into a black hole as seen from an outside observer?

Now if all the mass-energy is stored in the singularity, and the black hole is just empty vacuum, then if it acquires new mass-energy, then it has to be "stored" in the singularity, and how does this information "travel" to the outermost region, and adjust the event horizon?

If my humble understanding is correct:

  1. new mass-energy enters the black hole (black hole swallows particles etc.)

  2. this mass-energy has to "travel" to the singularity where it is "stored"

  3. information about the fact that the black hole's energy content has increased needs to travel from the singularity to the whole surface of the event horizon

  4. the event horizon is not an object, but a boundary, and the whole boundary of this event horizon needs to adjust simultaneously

Now naively thinking this could happen two ways:

Version A:

As a new particle enters the black hole, information about this propagates through the whole surface of the event horizon at the speed of light. Considering that certain black holes can be ginormous, this process could take a while.

Version B:

As the new particle enters the black hole, it needs to reach the singularity first, since black holes are just empty vacuum of space, the singularity must contain all the mass-energy. Consecutively, information needs to travel from the singularity to the whole surface of the event horizon simultaneously to give it "instruction" to expand adequately to the mass-energy increase.

In my understanding Version A is possible but this process could take time as some black holes are giants so it cannot be simultaneous throughout the surface. Now Version B is to say the least doubtful, because nothing can travel outwards from/leave the singularity.

Question:

  1. If information can't move outwards the singularity then how does the event horizon "know" it has to expand simultaneously along the whole surface?
Qmechanic
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Árpád Szendrei
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1 Answers1

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If information can't move outwards the singularity then how does the event horizon "know" it has to expand simultaneously along the whole surface?

The horizon doesn’t expand simultaneously along the whole surface. Meaning that the horizon does not typically remain spherical as it expands. It will bulge outwards towards infalling matter. It does so before the matter reaches the horizon.

Remember that the spherical symmetry of the Schwarzschild spacetime is a simplifying assumption made in order to make the math easier. When your spacetime of interest violates this symmetry then the resulting rules from the Schwarzschild geometry no longer apply.

information about the fact that the black hole's energy content has increased needs to travel from the singularity to the whole surface of the event horizon

The singularity is not a position in space, it is a moment in time. It lies to the future of the event horizon for a non-evaporating black hole. The event horizon is not caused by the singularity, and changes to the event horizon happen before the singularity.

Dale
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