Image Credit: NASA/CXC/M. Weiss
One question about black holes that we astronomers are most often asked is, "If a black hole traps everything, even light, how do we know it is there?"
This is not a stupid question. In fact, as I'll talk about below, it's the whole issue to proving that something is a black hole! But let's start with how we detect likely black holes in the first place.
The picture above is an artist's conception of what a typical black hole we can detect from Earth might look like up close. These black holes have companion stars that are close enough to the black hole that material from the companion star falls toward the black hole.
But, before getting to the black hole, the gas tends to form a disk (like a frisbee) around the black hole. In this disk, gas jostles around and heats up to temperatures of hundreds of thousands of degrees. At this temperature, the gas glows in X-rays, which we detect using X-ray telescopes like the Chandra X-ray Observatory. The gas in the disk slowly loses energy and spirals into the black hole.
So, the light that we see does not come from inside the black hole, as that is impossible. The light comes from outside the black hole. And that is how we can "see" the black hole.
But the picture above would be the same if, instead of a black hole, we had a neutron star or even a white dwarf. So, how can we be certain that the thing at the middle of the disk is a black hole?
To date, the evidence is typically indirect. We can measure how fast the companion star is orbiting the hidden object, and if that hidden object has more than three times the mass of the sun, we think it is a black hole. But that is only because theory has trouble making neutron stars that massive. However, our theories could be wrong.
Last week and this week, we've had a visitor in our department named Ramesh Narayan, an astronomy theoretician from Harvard. Dr. Narayan spoke of a clever scheme to tell if a black hole is truly present or not. This idea relies on the fact that light cannot escape a black hole.
When something falls onto something else (like a window falling out of a building, or a meteor hitting a planet), there is a tremendous crash. That crash is a release of energy -- the falling object has a lot of energy until it hits the ground, when it has to release all that energy. The energy may go into sound (as in the window shattering), it may go into moving dirt around (like a meteor making a crater), or it can go into heat. Actually, quite a lot of energy goes into heat, but for most things we deal with, the crash is not energetic enough to really heat things up a lot.
Meteorite impacts are big enough to start releasing lots of heat. Many meteor craters have little bits of glass sprinkled around them, because the heat of the impact melted the dirt, which cools as glass. The asteroid which killed off the dinosaurs released enough heat to cause tremendous forest fires around the entire planet!
Now, if the Earth's gravity is enough to accelerate an asteroid to those very high speeds to release a lot of heat, imagine what the gravity of a neutron star (all the mass of the sun squeezed into a ball about 30 miles across) or a black hole (the mass of the sun squeezed into something smaller than three miles across) could do! The gas that we see emitting X-rays heats up even more as it begins its final plunge onto the central object.
In the gas falls onto a white dwarf, a neutron star, or anything else with a surface, the impact will release a lot of heat which we would be able to see in X-rays. In fact, in many X-ray bright objetcs, we see both the heat from gas in the disk and the heat of the gas impact on the star. But in some (those that we think might be black holes), Narayan and his collaborators see the heat of the gas disk, but they don't see heat from an impact of the gas onto a surface. It's like the gas stars to fall and then just vanishes.
And that, what we should see but don't see, is some of the best proof of the existence of black holes. So, going back to the original question -- we know black holes exist because we see places where matter is being swallowed up by something, and we can see the gas until very close to the bitter end, but then it just vanishes without a trace. In this case, not seeing is believing.