Viewing black holes?

Question

How could you possibly see a black hole when there are stars etc in front of it and then the matter being drawn into it , all you would view surely would be the objects in front of the black hole ?

Answer 1

With the emergence of gravitational wave astronomy, it has become possible to obtain a direct view of black holes via their gravitational-wave emissions.

Anything undergoing acceleration emits gravitational waves. In ordinary experience this emission is very small, but when closely packed massive objects interact via their gravity the emissions become large enough to detect even at a distance of a billion light years.

The first gravitational-wave observation in 2015 came from a merger of two black holes in a galaxy $1.4\pm0.6$ billion light years away. The observed gravitational wave signature matched closely with general relativistic calculations for such a merger. The waveforms are shown below (Wikimedia link):

The involved black holes came in with masses of about 36 and 29 solar masses and the merged black hole came away with about 62 solar masses, the remaining three solar masses being the energy released in the gravitational waves.

Note carefully that although we see the black holes directly via this method, we see them only around the time of their actual interaction. We still need black holes to interact in order to see them, even where gravitational-wave astronomy enables viewing without the use of auxiliary matter or electromagnetic radiation.

Answer 2

Yesish. Of course you see these foreground objects. Of the black hole itself you can see the photon sphere and its shadow. Imaging this was awarded with a Nobel prize in 2020. A more elaborate explanation is on the telescopes website: Astronomers Reveal First Image of the Black Hole at the Heart of Our Galaxy

Answer 3

Black holes themselves are... black. So you don't see them directly, but indirectly from the effect they have on light rays that pass close to them.

These effects include the bending and distortion of light coming from objects behind them and even light rays, emitted from material near the black hole, orbiting the black hole once or more times before escaping towards the observer.

Light from foreground sources will be unaffected by a background black hole. However, if you consider a cone extending into space, filled with a uniform density of stars. It is quite easy to see that if you put the black hole half way along the cone, there will be far more stars behind the black hole than there are in front. The trick to observing a black hole is therefore just to make sure it is adequately illuminated, either from light sources behind it, or from sources of light in its immediate vicinity.

A simulated example of a black hole in front of a dense star field is shown below. In this case, we are so close to the black hole that there are no intervening sources of light. Almost all the light is coming from sources behind the black hole and the light rays from these are distorted in a characteristic pattern that leaves a "shadow" on the foreground field. You can imagine what this would look like if there were a few stars in front of the black hole - it wouldn't really make much difference.

Another example is the Event Horizon Telescope's image (in microwaves) of the supermassive black hole in M87. Here, the illumination is provided by the plasma immediately around the black hole. Some of this is indeed in front of the black hole, but that is of low surface brightness compared to the bright ring caused by light forced into tight orbit(s) around the black hole and what is chiefly visible in this image.