# [OS X TeX] hebrew?

David Derbes loki at uchicago.edu
Fri Jun 11 14:27:42 CEST 2010

On Jun 11, 2010, at 7:20 AM, Peter Dyballa wrote:

>
> Am 11.06.2010 um 12:37 schrieb David Derbes:
>
>> Imagine an astronomical object so massive and so compact that even \emph{light} is too slow to leave the object's surface; for this object, $v_{\text{esc}} \ge c$. Presumably it would be \emph{invisible}.
>
> Is this really correct? Most artificial objects that left our Earth started with zero velocity (relative to the surface where they stood before ignition of the rocket engines). Isn't the curvature of space and time responsible for the effect that a black hole's supposed surface would look kind of dark? (Actually no-one would see it, because light from back or the sides is redirected into the beholder's eyes. And at the event horizon there's the effect of spontaneous pair production of particles which could involve some delusive gleam of light.)

This is always the danger of excerpting. Black holes were dreamt up in the eighteenth century assuming that light was a Newtonian object. It isn't, nor is gravity as Newton supposed it. Curiously, treating light as a Newtonian object and gravity according to Newton leads to the right expression for the Schwarzschild radius (the event horizon.) I was describing light according to Newtonian physics and then go on to do it more correctly, but not yet going into the full curved spacetime stuff.

It is well known that black holes radiate, according to the discoveries of Hawking and Bekenstein as you describe.

David Derbes

>
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> Greetings
>
>  Pete
>
> A blizzard is when it snows sideways.
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