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View Full Version : Falling into a black hole, event horizons, escape velocity, etc



Nereid
2008-Oct-20, 01:30 PM
There are several threads on black holes, event horizons, etc, here in the Q&A section and in the ATM section.

Common to many are questions on the event horizon(s), escape velocity, and the differences in what various observers would see.

This thread is an attempt to get clear answers, from the perspective of textbook General Relativity (GR).

First question: what is a black hole's event horizon?
Let's also address the different kinds of event horizons a rotating black hole has.
Also, how does a GR event horizon differ from the place(s) where the escape velocity equals c for the Newtonian equivalent of a black hole?

Second question, which really should come first: in what ways, if any, does the concept of 'escape velocity' differ between GR and classical Newtonian physics?

Third question: you are equipped with a very good, modern physics lab, and you fall into a super-duper massive black hole (to avoid being shredded by tidal forces). You measure your local speed of light every picosecond; does it vary as you approach, then cross, the event horizon?
You also emit an intense burst of light towards the rest of us (who are safely a long way from the BH), with a set of highly distinct lines (so redshift can be measured unambiguously) , every picosecond.
We do the same, towards you.
How do you see the redshift of the light you receive from us changing as you fall?
How does the time between our picosecond pulses change, as measured by you?

Fourth question: What do we measure as the redshift of your lab, as you fall?
How does the time between your picosecond pulses change, as measured by us?

Note: in this thread I am not asking From march's questions, about how observers see reflected light (etc).