Frontiers and Controversies in Astrophysics: Lecture 15 Transcript
March 8, 2007
Professor Charles Bailyn:
Okay, this is the last class of the section on black holes and relativity. I should
say, this particular section of the course has gone in a direction I didn't quite expect. We've done a little
more--gone a little more deeply into the theory. We haven't done quite as much on actual observed objects.
That's not such a terrible thing. I'm quite happy with that. I just want to point out a couple of things that were
on the syllabus that we didn't get to, because they're very interesting things. You can look them up on the
little black hole website and find out all sorts of things about them.
And one of them is, there exists a category of what's called supermassive black holes. I mentioned the black
holes that come from stars, and I'll talk about that much more today. But I don't think I'm going to have a
chance to talk about the supermassive black holes. These are black holes that live in the centers of galaxies
and can have masses of hundreds of thousands, millions, sometimes even billions of times the mass of the
Sun. So, very massive black holes in the centers of galaxies--including our own galaxy, by the way.
And it is gas falling into these black holes that powers the quasars. We saw a gravitationally lensed quasar the
other day. These are very powerful sources of emission that are located in the centers of galaxies, sometimes
called active galactic nuclei, and they're caused by these very massive black holes. So, that's one thing we're
probably not going to get to.
Another thing is, I had originally intended to talk a little bit about gravitational waves. I mentioned that in the
context of the fact that the emission of these waves causes the orbital periods of things like the binary pulsar,
and indeed all orbits, in principle, to gradually becomes shorter. The orbital period, the semi-major axis,
gradually becomes shorter. But there's also the hope--it hasn't yet been done--that you could detect these
waves directly, that you could make a kind of telescope that would actually observe gravitational waves. This
is now in progress. It hasn't yet succeeded. So, these can, in principle, be directly detected. That hasn't been
done yet, but it will be soon, I think.
There's something called the Laser Interferometer Gravity Observatory, abbreviated LIGO, which is basically
a kilometer-long hunk of metal, the length of which can be measured to some fraction of the size of an atom.
And when a gravitational wave rolls over this, what you expect to happen is you should see the thing getting
slightly longer and slightly shorter as the wave goes over it. The problem with this is that the effect of passing
trucks on highways ten miles away is many, many times greater than the effect of passing gravitational
waves. And so, what they've done is they've built two of these things, one in Washington State and one in
Louisiana. And the plan is to operate them simultaneously, so that you can see, things that happen in both
places might be attributable to some cosmic source.