# Regaining Causal Contact by passing Event Horizons

• craigi
In summary: Chalnoth is right, but is talking about the apparent horizon which is the surface from which nothing can reach the observer. This is distinct from the absolute horizon which is the surface from which nothing can escape. It is the apparent horizon that is relevant to causality, but the absolute horizon has the relations to other properties of a black hole.
craigi
When an object approaches a particular event horizon, for a distant observer, in its own reference frame it is not destroyed. The distant observer sees the effect of the object's destruction at the event horizon. We resolve this paradox by the fact that the object and observer no longer have causal contact, at least from the perspective of the outside observer.

How do we resolve the case where the observer follows the object across the event horizon at a much higher speed?

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craigi said:
When an object approaches a particular event horizon, for a distant observer, in its own reference frame it is not destroyed. The distant observer sees the effect of the object's destruction at the event horizon. We resolve this paradox by the fact that the object and observer no longer have causal contact, at least from the perspective of the outside observer.

How do we resolve the case where the observer follows the object across the event horizon at a much higher speed?
Horizons are observer-dependent. We can never again interact with any object that has passed our horizon. But an object much closer to the object that crossed our horizon will have a different horizon, and may be able to interact with it.

Chalnoth said:
Horizons are observer-dependent. We can never again interact with any object that has passed our horizon. But an object much closer to the object that crossed our horizon will have a different horizon, and may be able to interact with it.

Are you sure? My understanding is the event horizon is determined by the mass of the black hole and is not observer dependent.

Thr main difference as seen by the distant observer is that the object falling into the black hole never quite makes it (time dilation).

mathman said:
Are you sure? My understanding is the event horizon is determined by the mass of the black hole and is not observer dependent.

Thr main difference as seen by the distant observer is that the object falling into the black hole never quite makes it (time dilation).

I think I worked this out while I was waiting for a response.

Chalnoth is right, but is talking about the apparent horizon which is the surface from which nothing can reach the observer. This is distinct from the absolute horizon which is the surface from which nothing can escape. It is the apparent horizon that is relevant to causality, but the absolute horizon has the relations to other properties of a black hole.

If I'm right, this does answer the original question, but a second observer could observe the effect of the destruction of the object while the first observer follows the object to maintain causal contact, but doesn't cross the absolute event horizon. The two observers would still be in causal contact and would differ about the fate of the object.

I suspect that the answer to this variant of the original problem is that "by the time" the two observers have conferred then the object must have been destroyed by the singularity anyway. So that all they differ on is when and where and we end with up a 'one man's singularity is another man's event horizon' situation. Proving this isn't going to be easy though.

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mathman said:
Are you sure? My understanding is the event horizon is determined by the mass of the black hole and is not observer dependent.

Thr main difference as seen by the distant observer is that the object falling into the black hole never quite makes it (time dilation).
You're right, I was sloppy. It does depend upon which type of horizon you're talking about. There are apparent horizons and absolute horizons. The event horizon of a black hole is an absolute horizon (an invariant property of the space-time). The de Sitter horizon for a universe dominated by a cosmological constant is an apparent horizon. For some discussion of apparent horizons and absolute horizons, see here:
http://en.wikipedia.org/wiki/Apparent_horizon

Note that "event horizon" is not always synonymous with "absolute horizon", as some call the de Sitter horizon an event horizon as well.

1 person
mathman said:
Thr main difference as seen by the distant observer is that the object falling into the black hole never quite makes it (time dilation).

I don't think that's the full picture because it would preclude the growth of a black hole.

craigi said:
I don't think that's the full picture because it would preclude the growth of a black hole.

Cf. Hawking's latest perspective.

## What is "Regaining Causal Contact by passing Event Horizons"?

"Regaining Causal Contact by passing Event Horizons" refers to the concept in physics where an object or signal that has crossed over an event horizon, the point of no return in a black hole, can still be observed or have an influence on the outside world.

## How is it possible to regain causal contact after crossing an event horizon?

This concept is based on the theory of relativity, which suggests that time and space are interconnected. Therefore, an object that has crossed an event horizon may still have a presence in the outside world, as its influence on space-time can still be observed.

## What are some examples of objects that can potentially regain causal contact?

Objects such as photons, which have no mass, can potentially regain causal contact as they are not bound by the same laws of gravity as massive objects. Additionally, objects with extremely strong electromagnetic fields, such as pulsars, may also be able to regain causal contact.

## What implications does this concept have for the study of black holes?

This concept challenges our current understanding of black holes and the laws of physics that govern them. It also opens up new possibilities for studying and observing black holes, as it suggests that we may be able to gather information from objects that have crossed their event horizons.

## What are some potential uses of regaining causal contact in practical applications?

This concept has been explored in science fiction, but its practical applications are still largely unknown. However, it could potentially lead to advancements in communication and information retrieval, as well as a deeper understanding of the universe and its mysteries.

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