Sunday, February 6, 2011

A Brief Note on the Myth of the Astrophysical Vacuum Cleaner

I've done some reporting on physics but have yet to write about my favorite science topic, gravity, here on the 'blog. Today, I want to tell you a little bit about something I know a fair bit about, black holes.

The popular perception of black holes seems to be that they are cosmic vacuum cleaners, inexorably sucking things in. The idea that nothing can escape from inside a black hole—only sort of true—seems to have morphed into the idea that black holes have sharp pointy teeth and some sort of magic power that pulls things from far away deep into its sharp pointy black hole stomach.

Well, that's partly true. Black holes do have a sort of magic power, but you've known it all your life—it's gravity, and every massive object from a paperclip to Sgr A*, the supermassive black hole at the center of the milky way, influences every other through gravity.

To understand what I mean, it helps to understand how gravity works on Earth first. Let's say you find yourself at, say, 100 meters above the surface of the earth, except that there's nothing underneath you. With nothing to stop your fall, you fall 100 meters, whereupon you smash into the aforementioned surface of the earth and come to a rather unpleasant stop. Did gravity stop? Of course not. The planet stopped your fall. We don't usually think of it that way, but that's what happened: the planet stopped your fall.

Now, let's suppose we collapse the entire planet down to one millimeter, and now you fall from 6899.999 meters above the (new) surface of the earth. There's nothing there at 6800 meters, where the surface of the Earth had been, to stop your fall, so you'll just keep falling. Remember, while the New Earth is really small, it still has all the mass of the Old Earth, and gravity works just the same—for a while. Around about 8.7mm, strange things will happen. Notably, people watching your fall from afar will see you stop and stay there forever.

That's because you just entered a black hole. You crossed a threshold, called the event horizon, from which you can't return, or at least not in your present form. (The reason for this is often misunderstood. The popular perception is that gravity is just really, really strong, and you can't fight it. The real reason is vastly more beautiful. You know how you can only go forward in time? Inside a black hole, time and space reverse their roles, and you have no choice but to go forward in space, toward the center of the black hole.)

So weird stuff does happen in black holes, but let's be clear about something: outside, everything's totally normal. Gravity works as it always does. You can even orbit a black hole. It's easy. We've got satellites hovering above the earth now at, let's say, about 100km. They can hover above the collapsed earth at about 6900km—in other words, right where they were—just fine. It's not a black hole because gravity is bizarrely strong. The weird stuff about black holes that we all hear about happens there's a whole lot of matter in a very small space, which just means that you can, in principle, get really close to a whole of matter.

And that's all there is to it, really. Once you realize this, you realize that black holes aren't scary, and that they aren't going to necessarily eat up the whole universe. They might eventually, but it's a long ways off, and it's not because they're superstrong or something. And the LHC holes aren't going to eat you up, either. They evaporate too quickly...but I digress.

Stay tuned for a deeper explanation of what gravity is all about. Preview: things always go in straight lines, but the straight lines might be curved.