Wednesday, June 20, 2007

*sigh* relativity is complicated

The Bad Astronomer discusses a paper which basically adds a new......let's say perspective, to black holes. I'm far too non-technically-learned to read the paper, but by what the Bad Astronomer says, it's one of those conflicting reference point things that makes relativity a real pain in the butt. It basically says two people could see something different, based on their motion or acceleration, and they're both right. Here are some of the juicy parts of the BA's post:

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.

So, confusing thing number one is that to someone outside the event horizon, a black hole never forms. As it compresses, time gets slower and slower, so it will never QUITE reach the event horizon and become a black hole. However, from the black hole's perspective, it shrinks down through the event horizon and becomes a singularity. Next quote:

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.
This isn't a really confusing thing, but a necessary part of this post. I can't tell you the details about Hawking Radiation either, because I frankly have a few questions about it. But it's basically: starve a black hole long enough, it'll evaporate until it doesn't exist anymore. The crazy part comes next:

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.

Hawking Radiation doesn't care whether this is a black hole or just a really really dense thing. The thing is, since a black hole never actually GETS to being a black hole, it should evaporate away before becoming a de facto black hole. Also, since time gets slower as you approach a "black hole", any debris that falls into the black hole doesn't actually REACH it, because it slows down and slows down, always a little bit behind the actual black hole. So, according to this, black holes CAN'T eat, they just waste away due to Hawking Radiation. So, we SHOULDN'T have black holes.

Unfortunately, according to many of our observations, we find objects that behave exactly as black holes behave. But we consult relativity on black holes, it says, "It depends on how you look at it." I'm really interested in how the physics community would deal with that, so BA, keep us informed, please.

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