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If black holes are so dense that they do not allow light to pass through them then how can we see them ? We can see anything if it reflects light but it is not in the case with black holes.

Qmechanic
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MathsWiz
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    What makes you think that we do really see them? – JMac Sep 12 '18 at 13:21
  • You are right that we cannot actually see directly them. Can you thing of indirect ways to detect them? – Ozz Sep 12 '18 at 13:24
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    Possible duplicate of Direct observations of a black hole? – ProfRob Sep 12 '18 at 14:21
  • Which black holes have been seen? We've seen lots of evidence of things that are almost certainly black holes, eg gravitational lensing, radiation from accretion disks, and stars orbiting the galactic centre, but AFAIK, nobody has claimed to have produced an image of a black hole itself. – PM 2Ring Sep 12 '18 at 14:42
  • @PM2Ring The Event Horizon Telescope team has produced preliminary images showing a concentration of matter at 3 gravitational radii for Sagittarius A* closely matching the 2.5 radii prediction of general relativity. – safesphere Sep 12 '18 at 14:55
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    @safesphere Which is just more evidence of a black hole, not an image of a black hole itself. – JMac Sep 12 '18 at 20:10
  • @JMac Yeah, it's more like a shadow, but it shows the size, which is a critical evidence. – safesphere Sep 12 '18 at 20:24

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For the last few hundred years it has been impossible to directly view/observe atoms, but we had insurmountable evidence from indirect methods that atoms do exist (only recently have we been able to resolve individual atoms). So there is nothing wrong with using indirect methods, in principle.

A black hole is a perfect black body absorber, that is, it absorbs all radiation perfectly. So we won't see any light reflecting off of the black hole's event horizon.

The currently best indirect method that we have to observe black holes, and to constrain their properties, is by observing the gravitational radiation emitted in the ringdown and merger of black hole binaries.

Daddy Kropotkin
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  • Merging black hole binaries must be a very small fraction of the total number of black holes, though. Have we observed more than one? – D. Halsey Sep 12 '18 at 21:24
  • Sooooo, two main formation channels: 1) start with two isolated stars that independently collapse into BH's then they come together to form binary, 2) start with stellar binary that evolves into binary BH (BBH). Our sample size of detections is too small right now (LIGO has released 6 total BBH detections thus far) to say which of the two formation channels dominates... But, it is observationally true that most stars exist in binaries, so it's reasonable to expect that BBH's compose a non-negligible portion of all black holes in the universe. The link above discusses all six detections. – Daddy Kropotkin Sep 13 '18 at 00:24
  • And it's kind of tricky. Are you counting supermassive black holes in your "total number of black holes" ? Because our current detectors don't detect gravitational waves from them. – Daddy Kropotkin Sep 13 '18 at 00:28