3 ms·

But you need some sort of propulsion system if you wish to escape while moving slower than local escape velocity. Rockets can do it because they have such a pr

But you need some sort of propulsion system if you wish to escape while moving slower than local escape velocity. Rockets can do it because they have such a propulsion system. (Although in practice any ordinary rocket will normally exceed the escape velocity from the surface of the Earth. Due to Oberth you want to do your burn as close to the planet as possible and escape velocity from low orbit is only slightly less than from the surface. Since when you head out into interplanetary space you're normally going somewhere you're carrying extra velocity.) Light has no propulsion system, it doesn't make it out if the local escape velocity exceeds lightspeed even if there was no event horizon or other space warping effects involved.

Maxatar 2y ago
Make it out of what though? If all there is to consider is escape velocity, than light, or any object whatsoever, can absolutely escape the event horizon if it's moving at anything remotely close to the speed of light. Without propulsion it won't be able to travel to an arbitrarily far distance, but it can certainly escape the event horizon in any scenario other than if its initial velocity came from when the the object was at the center of mass. You wouldn't even need a very significant velocity to escape the event horizon of a black hole if you had that velocity within a region close to the event horizon. As I said, escape velocity is a good way to motivate the idea of a black hole, it serves a pedagogical purpose, but the story doesn't stop there and you quickly realize that in fact, the escape velocity itself has little to do with black holes. It just happens to share the same underlying principle that the calculation for the event horizon has.
- LorenPechtel 2y ago
  And note that he was looking at that--light might come up but not get away. The model is different but the results would look the same to an outside observer.

Maxatar 2y ago

Make it out of what though? If all there is to consider is escape velocity, than light, or any object whatsoever, can absolutely escape the event horizon if it's moving at anything remotely close to the speed of light. Without propulsion it won't be able to travel to an arbitrarily far distance, but it can certainly escape the event horizon in any scenario other than if its initial velocity came from when the the object was at the center of mass. You wouldn't even need a very significant velocity to escape the event horizon of a black hole if you had that velocity within a region close to the event horizon. As I said, escape velocity is a good way to motivate the idea of a black hole, it serves a pedagogical purpose, but the story doesn't stop there and you quickly realize that in fact, the escape velocity itself has little to do with black holes. It just happens to share the same underlying principle that the calculation for the event horizon has.

LorenPechtel 2y ago
And note that he was looking at that--light might come up but not get away. The model is different but the results would look the same to an outside observer.