# Gravitational redshift and black holes

• niin

#### niin

Questions: Gravitational redshift and black holes

I have some questions:

1. What does gravitational redshift do to light trying to escape a black hole? Is the light destroyed?

2. And what is the physical cause of this redshift? (I’m not interested in equations and math, only the physical cause).

I hope someone can help me. Thanks.

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The beginning to understand gravitational redshift is the Equivalence Principle.

"In a local inertial frame the laws of physics are given by the laws of special relativity"

Now we have to reference frames S1 and S2, S1 situated at some distance above S2.
They are in a gravitational field.
S1 will always remain still and S2 will start free falling at t=0.

At t=0 we emit a photon from the origin to S2 towards S1.

... this continues but some math are required.

you can work out the frequency that S1 will receive if you use the weak field approximation.

Kuon,
I don't see why the "equivalence principle" should solve it. If you think of the first reference frame as being accelerated, then there is no redshift, because both reference frame must be accelerated or the distance would change between them. And it's not the distance that changes in this case. Right?