“Why it is that of all the billions and billions of strange objects in the Cosmos — novas, quasars, pulsars, black holes — you are beyond doubt the strangest?” –Walker Percy
Black holes. You’ve all heard them before, and you can visualize them pretty easily. How so? Start by thinking about the Earth.
Held together by the immense force of gravity, the Earth is a difficult world to leave.
What exactly do I mean by that?
It takes a tremendous amount of energy to get off of the planet Earth. If you were at the surface of the Earth, you’d have to be moving at around 40,000 km/hr (or 25,000 mi/hr) to escape from the Earth’s gravity!
Not even a patriot missile, above, has the juice to escape from the Earth.
A Saturn V rocket could do it, though. In fact, we’ve sent quite a few objects out of Earth’s gravity, and even a couple out of the Solar System itself!
But what if you made something so massive and so dense that, forgetting about rockets for a minute, not even light could escape from it! What would that do?
Well, in theory, that would be a black hole. But how would we know, for certain, that an object was a black hole?
Well, we’d have to get an object that emitted no light, that was super-massive, and that we could, with great certainty, exclude all other possibilities.
There’s one surefire place to look.
Go to the center of our galaxy! If there’s one place in space where we expect there to be a bigger concentration of mass close by than any other, it’s the center of our spiral galaxy. After all, looking at other spiral galaxies, it’s pretty obvious that’s where the greatest, largest, densest area lives.
Well, we look at the center of our galaxy, and, unsurprisingly, there are a bunch of stars there. But, as astronomers, we know the virtue of being patient.
Imagine taking the very center of the galaxy, and watching the innermost stars over the timespan of years.
What do you find? A massive star making a beautiful elliptical orbit — just like Kepler predicts — with a very heavy mass at one point. What is that mass? Between 2.7 and 4.0 million Suns. But where’s the light?
There is none!
But you know scientists. We’re never convinced by one example. So we turn to the UCLA galactic center group, where they’ve been tracking many stars, over more than a decade, in their orbits.
(It may take a bit to load; movie download here.)
When you can see all of these different stars, and they’re all orbiting the same point, and that “point” has millions of Solar Masses there, what else could it be?
Not only does it have to be a black hole, but there are no other ideas even out there. So if you didn’t believe in black holes, I hope this convinces you, and if you aren’t convinced, then please, tell me what that thing is!