Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: DaveRedTrousers on 25/06/2019 10:02:00
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I understand Dr Karl to have said that black holes have no shape or size, but cosmologists have published a photograph in recent months that proposes a circular shape and suggests a considerable size. Which is the more likely reality?
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I think that the confusion here is about 3 different aspects of a black hole:
1. The Singularity: Theory suggests that everything entering a black hole is on a 1-way journey to a "singularity" at the center - a point of zero size (and if it has no size, it has no shape - it is a "point").
2. The Event Horizon: But nobody can actually see this singularity, because it is hidden behind an "event horizon", which has a definite size and shape - if you could have a black hole the mass of the Sun, it would be basically a sphere 6 km across - although slightly squashed due to rotation. Distant observers would never see infalling objects pass behind this event horizon, as not even light can escape.
3. The Shadow: Black holes are often surrounded by an accretion disk of infalling matter, glowing at incredibly high temperatures. The light from the accretion disk is bent by the strong gravity of the black hole, which acts like a distorting lens. You can see the accretion disk on the far side of the black hole, as the light is bent to appear "above" and "below" the black circle in the center. This leaves a dark "shadow" of the black hole, which is 2.6 times the diameter of the event horizon.
See: https://en.wikipedia.org/wiki/Black_hole
a considerable size
The black hole image published this year is the largest known black hole, at the center of galaxy M87.
The diameter of this black hole is 120 times the width of Earth's orbit around the Sun - about 4 times wider than the orbit of Pluto.
See: https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole
PS: Last week I saw Brian Cox's "Universal world tour", and he gave the clearest description of black holes I have ever heard for a general audience - no equations, but lots of great graphics!