# Can light be launched outwards from an event horizon? [duplicate]

There's something which I don't quite get about black holes and event horizons (feel free to tell me if I got something completely backwards at any step along the way):

Say we have a singularity and we launch a particle from the singularity's Schwarzchild radius R at the speed of light directed outwards from the black hole - wouldn't that particle be able to escape beyond R, slowly decelerating in a parabolic path to a halt somewhere outside it (at a radius of, say, R1)?

If so - wouldn't an observer A between R and R1 be able to see that particle as it decelerates? And if so, we couldn't define the event horizon to be at R, because light from R can affect A who's outside it. We couldn't define the event horizon to be at R1 either, because light from the observer could escape R1 entirely.

So where would we define the event horizon in the case presented above? How can there be an event horizon if light can be launched slightly below it and take an parabolic path outside it, even if only for a moment?

• The thing is, once you're on the horizon, there does not exist an "outward" direction anymore. Feb 1, 2014 at 22:14
• Isn't escape velocity defined in the outward direction? Care to expand on that a little? Feb 1, 2014 at 22:21
• Escape velocity is really a Newtonian concept. The event horizon is not defined in terms of escape velocity, but rather as the boundary of a region beyond which the outside world can not be affected anymore. Another way to express this is by looking at what happens with light cones in the geometry of a black hole. As you get closer to the whole, the future light cone tips towards the black hole. On the event horizon, it lies completely inside the boundary. Feb 1, 2014 at 22:23
• Read up on lightcones a bit first, the last picture shows what happens in the black hole case. If that doesn't answer your question yet, I'll try to write a full reply. Feb 1, 2014 at 22:33
• possible duplicate of Why is a black hole black? Feb 2, 2014 at 9:49