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Thread: Could a black hole become visible?

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    Could a black hole become visible?

    If a black hole swallowed a giant star, could its escape velocity fall below the speed of light?

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    Quote Originally Posted by Robert Tulip View Post
    If a black hole swallowed a giant star, could its escape velocity fall below the speed of light?
    The escape velocity at the event horizon is always the speed of light - if it swallowed a star, the size of the event horizon would increase so that it remained true.

    But, more importantly, it is not the escape velocity that stops light escaping. If it were, then light could escape a little way before falling back in (like when you throw a stone up in the air at less than the Earth's escape velocity). The reason the event horizon exists is because there are no paths that lead away from the centre of the black hole.

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    Quote Originally Posted by Robert Tulip View Post
    If a black hole swallowed a giant star, could its escape velocity fall below the speed of light?
    A stellar mass black hole ( 3 to 100 times the mass of the Sun) is very small in diameter compared to a giant star, however, if the black hole "swallowed" that giant star, it would do so in a way that was very disruptive to the star while it was happening, and in a way that highly compressed the matter from the star that it consumed. As noted above, the result would be an increase in the diameter of the event horizon (a few km per solar mass consumed). The result would be a very visible short-lived event and a nebula around the black hole. The black hole itself would never have been visible, but there would be detectable light from the area immediately around it for a while.
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    Thanks very much Strange and Antoniseb. Sounds like once a black hole, always a black hole. I was prompted to ask this after hearing an astronomer interviewed on the radio today (link). This event horizon concept seems quite spooky.

    A further science fiction question: if an atom-sized black hole were created on the earth, and it escaped from the lab, would it eat the earth, then send the moon crashing out of orbit, then eventually eat the whole solar system? And is 'eat' a reasonable metaphor?

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    Quote Originally Posted by Robert Tulip View Post
    A further science fiction question: if an atom-sized black hole were created on the earth, and it escaped from the lab, would it eat the earth, then send the moon crashing out of orbit, then eventually eat the whole solar system? And is 'eat' a reasonable metaphor?
    I did see a fairly detailed answer to this (but it was a long time ago, I doubt I could find it again - it may have been related to the LHC starting up and some rather fanciful claims it could create a microscopic black hole).

    You are right in principle. It would fall through the Earth absorbing matter as it went. It would fall past the centre towards the surface on the other side and then fall back again. However, the amount of matter it could absorb is limited by its surface area (and, in large black holes, by the energy released by the infalling matter pushing stuff away - I don't know how relevant this is on this scale). As it absorbed matter, its surface area would increase, allowing it to "feed" more quickly.

    But from what I remember, the timescales involved in it eating the Earth mean we wouldn't actually need to worry too much.

    And the Moon would not change it's orbit because if it absorbed the whole Earth (and was about 2cm in diameter) it would still have the same mass. So the Moon would continue to orbit as if nothing had changed. (I suppose there might be some small effect because of the change in tidal losses in the Earth?)

    And "eat" is a reasonable metaphor. It seems fairly standard for "active" black holes to be described as "feeding".

    ETA. Googling this produces all sorts of interesting facts. For example:
    Assuming it eats every proton, neutron, or electron that it comes in contact with — and also taking into account its gravity, to see what it attracts — it will eat about 66,000 protons and neutrons per second. Of course, 66,000 protons-and-neutrons is a tiny amount in terms of mass: 1.1 x 10^-25 grams. That rate-of-growth will be constant until the black hole becomes quite large; only at about one billion metric tonnes will the black hole will start to grow faster than this rate, as it takes that long for its cross-section to increase. Capturing 66,000 nucleons per second, how long will it take to get the black hole up to even one kilogram? Three trillion years, which is much longer than the lifetime of the Sun or even the age of the Universe.
    https://medium.com/starts-with-a-ban...e-886d9e600c28

    Another important point to note is that if your initial black hole has the mass of, say, an oxygen atom then it will evaporate in about 10-93 seconds (Hawking radiation).
    Last edited by Strange; 2019-Dec-10 at 01:36 PM.

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