Re: the ultimate refrigerator

Hara Ra (
Sun, 21 Dec 1997 01:05:15 -0800

John K Clark wrote:
> >>I have thought of a problem with the Black Hole refrigerator idea
> >>however.

> >Hara Ra <>
> >There's an easy fix to this. Use a telescope.

> That won't work. A lens (or curved mirror) can only bring light to a focus if
> the wave front is parallel or nearly so, and that can only happen if the lens
> is small and far from the light source. The wave front of the blackbody
> radiation near a heat radiator would be almost spherical and so will not
> focus for the same reason a lage magnifying glass placed right next to a
> light bulb will not focus. To work you'd have to place your telescope at a
> large distance from the radiator where the wave front has become almost
> parallel, and then only a very small percentage of the heat radiation would
> intersect with the lens.

At the focus of the telescope, with a really low f-ratio, the sky will
be seen to be mostly the black hole. Imagine a an ellipsoid, with the
radiaitor at one focus and the black hole at the other. The radiator
will see the full sky as the black hole. All of the radiation from the
radiator will go to the black hole. If you limit the reflector to the
anterior half, you still have half the sky being the seen as the black
hole, exactly as if you were near the event horizon. It still works if
the radiator is a sphere which is smaller than the closest distance from
the radiator to the mirror. (I agree that if the radiator is as large as
the black hole that not all of the light will be absorbed, but someone
with an optics program can resolve this. It's still a large fraction)

Regarding your light bulb example, imagine the reflector to be a big
umbrella. We don't need to focus to a point, just to the diameter of the
event horizon as seen from a large distance.

| Hara Ra <> |
| Box 8334 Santa Cruz, CA 95061 |