Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Pseudoscience-is-malarkey on 10/06/2016 07:08:29
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Are black holes just gigantic stars that gravity will not allow us to see?
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Are black holes just gigantic stars that gravity will not allow us to see?
Some are and some aren't. A black hole need not have a mass as great as a star although theory suggests that some black holes are burned out stars. However micro black holes have a mass much smaller than a star. See:
https://en.wikipedia.org/wiki/Micro_black_hole#Minimum_mass_of_a_black_hole
Notice what it says
Minimum mass of a black hole
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In principle, a black hole can have any mass equal to or above the Planck mass (about 22 micrograms).
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Interesting idea. I'd not looked at it like that before.
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Note that micro black holes (https://en.wikipedia.org/wiki/Micro_black_hole#Minimum_mass_of_a_black_hole) are "hypothetical tiny black holes". There is no evidence that they exist. However there is good evidence that stellar (https://en.wikipedia.org/wiki/Stellar_black_hole#Candidates) and supermassive (http://hubblesite.org/reference_desk/faq/answer.php.id=64&cat=exotic) black holes exist.
Are black holes just gigantic stars that gravity will not allow us to see?
No. A black hole is very different to a star. Its exact nature is the subject of some debate, but what isn't, is the infinite gravitational time dilation. Any process occurring within or at the event horizon is subject to infinite time dilation and therefore does not occur. This includes the upward motion of a vertical light beam. Consider this scenario:
You're standing on a gedanken planet holding a laser pointer straight up. The light doesn't curve round, or slow down as it ascends, or fall down. It goes straight up. Now I wave my magic wand and make the planet denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. I make the planet even denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. I make the planet even denser and more massive, and take it to the limit such that it's a black hole. At no point did the light ever curve round, or slow down as it ascends, or fall down. So why doesn't the light get out?
The answer is that the "coordinate" speed of light at the event horizon is zero. However IMHO it's simpler to take a tip from Einstein (see the second paragraph here (http://einsteinpapers.press.princeton.edu/vol7-trans/156?highlightText=%22spatially%20variable%22)) and say the speed of light is zero. A place where light doesn't move is a rather strange place. There's no motion so there's no heat, and there's no way to measure distance or time. And because light doesn't move and nothing goes faster than light, nothing moves, so I can't see how there can even be any gravity. All this reminds me of the "void in the fabric of space and time" you can see mentioned in the Wikipedia gravastar (https://en.wikipedia.org/wiki/Gravastar) article. It also makes me wonder if the early universe was once like this. Not a point singularity, but a "hole in space". Imagine a blue-grey balloon, then imagine a hole in it, then check out this CCASA depiction of a black hole by cosmologist Alain Riazuelo (https://commons.wikimedia.org/wiki/File:BH_LMC.png):
(https://upload.wikimedia.org/wikipedia/commons/thumb/5/5e/BH_LMC.png/220px-BH_LMC.png)
It's on the Wikipedia black hole page (https://en.wikipedia.org/wiki/Black_hole). It looks like a hole in space, but this hole in nothing isn't nothing, it's something. One day we might learn that this is raw energy in its most fundamental form.
Or something else!