Re: seeing black holes

From: Hal Finney (hal@finney.org)
Date: Thu Aug 28 2003 - 10:04:52 MDT

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    Damien B. asks about a comment in
    http://www.abc.net.au/rn/science/ss/stories/s917636.htm:
    > > ...this object in
    > > the middle casts a shadow, it absorbs all the light incident on
    > > it and it also bends light from behind it away from our line of
    > > sight, so light that would normally reach our eye from the
    > > gas behind it gets deflected...
    >
    > But hang on... yeah, it bends light behind it *away* from us, but equally
    > it should bend some of the light coming at it sideways *toward* us, no?
    > more or less exactly compensating for what we'd see were it not there?

    Yes, this happens, but not enough to compensate. There are a couple of
    ways to explain it. The more literal one is that the compensatory light
    rays are emitted at a narrower range of angles than the ones shifted away,
    hence there is less light.

    Another way to see what is happening is illustrated for example at
    http://www.astronomynotes.com/relativity/s4.htm, which shows how a
    gravitational field can deflect light coming from being it. The net
    effect is a spatial distortion where objects move outward from their
    actual position, and are seen as farther from the center of the nearby
    object than they are.

    Now imagine this happening to a uniform grid behind the gravitating
    object. The dots near the object will be displaced outward; those far
    from the object won't be affected; and in between there will therefore
    be a ring with a higher density of dots. If we think of the dots as
    uniform light emission, we see a darker area near the gravitating object
    and a brighter ring surrounding it.

    If you imagine this happening to a uniform light behind the nearby object
    (like the BH accretion disk, which is uniform in one dimension, anyway),
    the effect is the same, that the image is distorted and shifted outward.
    This means that more light comes from a ring around the BH and less from
    the immediate apparent vicinity. I think this is what they mean by its
    "shadow".

    Hal



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