: : Consider the following system: A black hole with a given schwarzschild radius and a particle with positive energy with respect to the gravitational field of the black hole. If the particle is sent towards the black hole in such a direction that it will pass through the event horizon, and we presume that the particle does not interact in other ways with the black hole than gravitationally, why cant this particle escape the black holes gravitational field? : : The energy of the particle should be constant while only interacting gravitationally with the black hole, and thus still be a free particle with respect to the gravitational field from the black hole, even though it is inside of the event horizon. What mechanism makes it impossible for the particle to escape again?

: Short answer: gravitational field is too strong - escape velocity is greater than the velocity of light.

Is it possible to define an escape velocity for a black hole. What i you consider a photon, which is created inside the event horizon and traveling outwards. There will be a redshift due to the gravitational field, but the photon cannot lose speed as it travels with the speed of light. As the photon is redshifted it losses energy and the gravitation upon it will therefore decrease, and if you do the calculation it is seen that it can never lose all its energy. And the other problem with a particle, how can you explain it from energy considerations?