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Re: Event Horizon And Escape Velocity

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Posted by Zephram Cochrane/">Zephram Cochrane on February 24, 2000 06:41:36 UTC

:What mechanism makes it impossible for the particle to escape again?

Though most people do, I don't like to think of it in terms of escape velocity. The actual mechanism is gravitational time dilation. Once a Scwartzchild hole is stabley formed, and as long as we take the geometry of the space-time to remain static(no mass loss due to Hawking radiation) lets imagine a probe is dropped or in your case(E > 0) thrown into the hole. The time that goes by according to the probe between the time its released and the time it reaches the event horizon is finite. However, according to a clock kept remote from the hole the falling clock is entering extreme gravitational time dilation. According to the remote clock it takes an infinite time before the falling clock reaches the event horizon. Now lets extrapolate the scenario reversed. A clock seen falling out of the hole must have been falling out for an infinite time, or more realistically ever since its creation. Therefor no object will ever be observed emerging from the hole throughout the time that the hole remains static. So it is due to this time dilation that the only times on a remote observer's clock that an object may be observed as emerging from an event horizon is when the geometry of the space-time is not static.


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