In this week's "Ask a Physicist," we tackle a general relativistic paradox: If time slows down near the event horizon of a black hole, how does anything ever fall in?
I've been enjoying reading all of your questions to "Ask a Physicist." As an added twist in the coming weeks, I'd be interested in hearing any questions you have about physics and cosmology in the news, especially those along the lines of, "Is this real, or just bullshit?" As always, please send your queries to email@example.com
Today's question comes to us from David Sirola who asks:
If a black hole warps space-time to such a degree to slow and stop time, how can anything ever disappear past the event horizon (or whatever point t=0)? It would seem to me in my superficial understanding, that ultimately, after all the fun at the outer edges of the hole, that nothing ever really happens, since happens implies a time/cause/effect relationship.
What am I missing here?
Let's get one thing out of the way from the outset: Black holes are awesome. They are the only major disturbance of space-time which have the advantage of actually being known to exist. Almost every large galaxy, including our own, seems to have a supermassive black hole at the center.
And black holes are ridiculously simple objects — or at least the non-rotating ones are, which are the only ones I'm going to talk about here. They basically consist of an infinitely compact "singularity" at the center and an outer boundary known as an "event horizon" from which nothing can escape (and here's where I'm supposed to use an ominously spooky voice) not even light. These guys are tiny, astronomically speaking. Were our sun to become a black hole, it would be smaller in radius than the city of Philadelphia. Even the 3 million solar mass black hole at the center of the Milky Way could comfortably fit inside the orbit of Mercury.
Okay, you probably knew all of that. I still need to dispel a few myths before we get into the hardcore space-warping.
- Black holes don't suck.
- Suppose the sun were to suddenly turn into a black hole. Would you notice? Sure you would. The sun would blink out of existence and you'd quickly freeze to death. But in your dying moments, you'd no doubt be struck by the fact that J.J. Abrams lied to you. Rather than get pulled into the black-hole sun, the earth would just keep orbiting that seemingly empty point in the sky, exactly as it always had. Only icier.
- You can't actually see them.
- Black holes are called that because they don't give off any light. I don't want to get into a nerd-fight here, partly because my mom says I'm not allowed, but mostly because things will go more smoothly if I anticipate a few objections. Somebody is likely to point out that we do, indeed, "see" black holes in the form of quasars in other galaxies. But this isn't quite right. What you're really seeing is hot, glowing gas falling onto the black hole or even larger glowing gas clouds surrounding the whole shebang. And by the way, with the exception of giant radio jets, we can't even generally resolve these clouds. When you see detailed accretion disks in news stories about black holes, that's somebody using MS Paint or whatever they use these days to make artist's conceptions.
- Let me further anticipate a black-belt level nerd who might introduce an even better possibility: Hawking Radiation. This is one of the coolest ideas in astrophysics, and one that most physicists believe, even though we've never observed it. Near the event horizon of black holes, particles and antiparticles are constantly being created in pairs. Every now and again, one of the particles escapes and creates some radiation (and takes with it some of the mass-energy of the black hole). But here's the deal: Hawking radiation is far too dim, and far too long of wavelength to ever be seen directly.