How did scientists know about the existance of black holes, how they behave etc… long before getting the very first image of one
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It started off as a mathematical hypothesis using Einstein’s Theory of General Relativity. Basically, Einstein showed that mass warps space time. They plugged some ridiculously large masses into the equations and came out with an escape velocity that was greater than the speed of light.
After that, they started looking for things where we could see the gravitational effects of things but couldn’t see what that massive thing was.
For instance, we can see these stars orbiting around the supermassive black hole at the center of our galaxy, Sagittarius A*… but we can’t see SagA* directly: https://youtu.be/VP3uMtP4kIw
By plotting the orbits of those stars, we can get a good idea of the location and mass of whatever is holding them in orbit.
First, you design a theory that explains what you see and the evidences you have. This mathematical theory, if it explain everything perfectly, can then be extended to make predictions. That’s why, today, we constantly find stuff that was predicted years ago by the math. At least that gives us good indications where and how to look.
You asked how they know before seeing it. And that’s the answer. It’s been predicted by mathematical models and we know they had to exist. And there won’t ever be an image of one since they absorb all light. But we can see their effects around them clearly.
Before you took your first step, you had already learned allot about gravity. When you pushed your sippy cup off the table, you got mom’s attention. Cause and effect.
The same way you know a person was there when you see footsteps in snow. They have an impact on their environment that can be measured.
There are two things going on here.
First, before the first image of a black hole (produced in 2019), we had other experimental evidence for them. Thanks to studying the centre of our galaxy over long time periods, we can see that stars are orbiting something very massive but non-luminous: [https://www.space.com/41291-relativity-revealed-milky-way-core.html](https://www.space.com/41291-relativity-revealed-milky-way-core.html). Also, experiments to detect gravitational waves were successful, and detected signatures matching predictions of black holes spiralling into and colliding with one another: [https://www.scientificamerican.com/article/gravitational-waves-discovered-from-colliding-black-holes1/](https://www.scientificamerican.com/article/gravitational-waves-discovered-from-colliding-black-holes1/).
Second, before either of these things, nobody “knew”. They did, however, have interesting predictions made by playing around with the equations of General Relativity. GR had been experimentally verified to high precision – for example, GPS works (when launched, the satellites had the option to enable GR corrections or not, and they had to enable the corrections else the positions drifted rapidly thanks to the effects of the satellites moving fast and being in weaker gravity than on the earth’s surface). But if you try and work out what happens with a very dense blob of matter, you realise that if you get close enough to it that not even light can escape its gravity. Such an object is a totally black hole in space… hence the name. Scientists then did more calculations and predictions, ran models of galaxy formation, etc etc, and guided the above searches to look for them.
The first X-ray telescopes in the 1960s and 1970s showed a few hundred stars glowed very brightly in X-rays. Some of the spots where X-rays were found were where there were binary star systems. We couldn’t tell the two stars apart by sight but by how their motion shifted their colors. In some cases only one star could be detected moving about an unseen companion. Some of those star systems contained neutron stars. Neutron stars have a maximum mass of something like 2 or 3 times the Sun’s mass, according to our theories.
But some star systems like Cygnus X-1 showed bright X-rays and that the visible star was being pulled on by something MUCH more massive than is possible for a neutron star (the mass of the unseen object can be known from how fast it makes the visible star orbit around.) Stephen Hawking actually bet AGAINST black holes being real because he figured he’d win either way: either he’d win the bet, or black holes would be real and his life work would be meaningful! He eventually had to admit that the evidence that star systems like Cygnus X-1 were black holes was so convincing that there was no doubt he lost the bet. This was long before we were able to make images showing the event horizon.
Also there are some galaxies that are REALLY bright in their centers. These are known as quasars. How could so much energy come from such a small space? The only way anyone could figure out to make so much energy in such a small space was if gas was falling onto a really massive black hole. Starting with the Hubble Space Telescope we were also able to measure how fast gas was spinning around galaxy centers, and from that figure how massive those galaxy centers were. In some cases, there were billions of times the Sun’s mass coming from areas comparable to our solar system. The only way that could happen would be if there were a black hole there.
And in the center of the Milky Way, Nobel Prize Winner Andrea Ghez and her team were able to make movies of stars orbiting an unseen object over decades, and from the speeds of those orbits it was clear there was an unseen object millions of times more massive than our Sun in the center of the Milky Way.
You start out with a fact of reality, mass attracts eachother. No matter if it’s an apple, or the sun, everything that has mass attracts other things that do have mass. But naturally, the larger the mass, the larger the attraction.
And then you observe and do math.
To make the math very simple (but still pretty difficult for a 5 year old), it’s akin to a problem like “2 + X = 5”.
You observe the movement of planets you know of, which is the result “5” in the equation. But when you do the math for the planets you know of, the result is “2”. And 2 is not 5, so there must be an unknown there somewhere. So you reverse your problem, “5 – 2 = X”. You find X, which in this case is 3, and start looking if you find 3 in reality.
If you do, Great! You found out something new about reality. If not, you go a step back again – what else than 3 could X be? It could be (1 + 2), or (2 + 1), or maybe even (1 + 1 + 1). So again, you go back to looking if you can find any of these.
Which is how we first [discovered the planet Neptune](https://en.wikipedia.org/wiki/Discovery_of_Neptune), and it’s the same to find black holes without needing to be able to see them, but even more complicated.
It’s the cool thing about science. You don’t know, but you come up with what you think is a pretty reasonable guess. Then you test for it and see if it fits. Then, you refine and try to reproduce your results. Prediction can be quite good, once you’ve really tested for it, and actually seeing it is irrelevant.
Goes for species, physics, economics, climate, sociology, etc.
The Einstein field equations predicted their existence and were a consequence of his theory of general relativity. When you solve the equations, which aren’t trivial mind you and if I recall correctly there’s 10 solutions in total of which not all are related to black holes, you end up with four possible solutions for blackholes: a charged rotating black hole, an uncharged rotating black hole, a charged non-rotating black hole and an uncharged non-rotating black hole.
In the theory, the two non-rotating black holes are a prediction but in reality we haven’t found any black holes that fulfill the criteria. The black holes we do know of are of the uncharged rotating type otherwise known as Kerr black holes. We have yet to discover any charged rotating black holes, known as Kerr-Newman black holes however they are useful for studies into stellar mass blackholes.
We don’t expect to ever find non-rotating black holes as from our current understanding all black holes have formed from previously rotating masses like stars. Or in short, we expect all blackholes to have angular momentum.
Another consequence of general relativity is the phenomena called “white holes”. They too are a prediction from general relativity but there is a consensus that they don’t really exist.
**Tl;Dr the math told us they should exist, scientists went looking for them and found them thus confirming their existence.**
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