News: Do black holes really exist?

The entire universe in blog form
June 19 2007 9:28 AM

News: Do black holes really exist?

Black holes are weird. Really weird.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


But you know this. Infinitely small, with huge gravity, warpers of time and space; they're simply cool to astronomer and the public alike.

Everything about them is interesting. They form when massive stars explode in titanic supernovae, they sit in the centers of big galaxies (like ours!) with masses millions or billions of times that of the Sun, and if they're feeding on material around them they can form disks of swirling material which can easily outshine the rest of the galaxy combined.

But do they really exist?

No astronomer doubts that these gravitational objects actually exist. But astronomers Tanmay Vachaspati, Dejan Stojkovic, and Lawrence Krauss at Case Western Reserve University have written a new paper which has thrown a monkey in the wrench about the exact, well, "surface" of a black hole. Here's how this works.

When a massive star ends its life, the core collapses. As the core shrinks, its surface gravity increases (that is, the gravity you would feel if you were standing on its surface). This means the escape velocity increases as well -- this is how fast an object would have to move to be able to break free and escape to infinity. For the Earth it's about 11 km/sec (7 miles/sec), and for the Sun it's about 600 km/sec (400 miles/sec). The escape velocity of a body depends on how massive it is and how big it is. For a given mass, a smaller body has a higher escape velocity.

So as the core of the star shrinks, the escape velocity increases. At some point, if the core has enough mass, the escape velocity reaches the speed of light. This means that if you are standing there on the core's surface, you would need to move at the speed of light to escape (actually, the situation is more complicated than this, but I'm simplifying). It's like an infinitely deep hole; any matter in it cannot get out.

If the core shrinks just a wee bit more, not even light can escape. To an outside observer, the core becomes black. So let's see, it's a hole, and it's black. What should we call such a thing?

Anyway, the theory is that the mass inside the black hole shrinks all the way to a point, an object of infinitely small size, called a singularity. The region around it where the escape velocity equals the speed of light is called the event horizon. And this is where things get sticky.

Einstein showed that as gravity increases, your clock runs slower. Literally, if you have two people, one guy up high above a black hole, and another guy close in, the guy outside sees the close-in guy's clock running slower. Literally, time flows more slowly near an object with gravity, and the stronger the gravity the slower time flows relative to an outside observer. For a black hole, time literally stretches to infinity at the event horizon. Clocks stop. Update: Well, I was being glib. Actually they continue to slow, ever approaching stopping but never actually reaching it. I was trying to simplify, but oversimplified -- I make similar comments below in this entry, so where you read that things stop, think of it as "slowing almost to but never quite reaching zero". Read the comments thread below for details.

This brings up a very interesting situation. If time takes forever to flow, then how does a black hole ever form? Imagine the core collapsing, and you're looking at it from far away. You see it getting smaller, but the collapse also appears to be going more slowly because of the time dilation. Like Zeno's paradox, you see the escape velocity approach the speed of light, but you'll never see it actually get to the speed of light! Time would stretch out infinitely, and the collapse of the core would appear to you to stop.

No black hole.

But it gets worse. Years ago, Stephen Hawking discovered that black holes can in fact "leak" out mass. It's very complicated, and has to do with entropy and quantum mechanics, so forgive me if I leave out details. Let's just say that black holes can evaporate, and go from there.

From the black hole's viewpoint, time flows just fine. It starts to form, and it starts to very slowly lose mass through Hawking radiation. Over time, billions of years or more, it eventually evaporates away.

But from your point of view, high above the black hole, the event horizon never quite actually forms. It gets closer and closer, remember, but slower and slower. Yet the Hawking radiation isn't really affected by this. So the two effects compete: the event horizon never totally forms because it would take an infinite amount of time, but during that time the hole is losing mass. So the black hole will actually evaporate before it ever really becomes a black hole.

If you throw something, let's say a wad of paper, into the black hole, you would actually see the black hole evaporate (if you could wait long enough) before you'd see the paper wad get to the event horizon. So the black holes loses mass faster than it can gain mass and the event horizon can never actually form.

This idea makes scientists nuts. And this is what the new paper is about. Some people have thought that if you take quantum mechanics into account, this paradox may be resolved. What the authors appear to have shown is that QM doesn't help. The black hole itself, the event horizon, never really forms.

However, have a care here: there is still a massive, dense, highly gravitational object there! So we still have what are essentially black holes in the cores of galaxies and forming when stars explode and all that, it's just that, technically, well, they aren't actually black holes.

Get it?

I will note that this is how I understand the situation, and I may have it wrong. This is very complicated stuff! This paper is by no means the last word on the subject -- even the experts argue incessantly about it, and I'm no expert. This is a very interesting situation, and I'm quite sure that it is nowhere near being resolved. I have many friends who study black holes and I'm sure they'll have quite the reaction to this story. If I hear more I'll post again. I guarantee that this idea won't, ah, evaporate on its own anytime soon.


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