Frontiers and Controversies in Astrophysics: Lecture 14 Transcript
March 6, 2007
Professor Charles Bailyn:
I've been talking about general relativity, and in particular, the post-Newtonian,
relativistic effects. These are the first things that happen as the gravitational field becomes strong enough that
Newton's laws don't perfectly apply. And let me just summarize those, because we're going to see them all in
action in just a minute.
So, here are the post-Newtonian effects of general relativity. And the first of them is the precession of the
perihelion. I'm just going to list these. And this was seen in the nineteenth century already, and correctly
interpreted by Einstein. It had to do with Mercury's orbit.
I should say, this word, perihelion, that means closest approach to the Sun. And so, that only applies for
planets if you have the same--you can have the same kind of effect in binary stars or in other things, in which
case, you might--they call it a periastron. That's the closest approach to a star that isn't the Sun. Or, here's a
favorite word of mine: perigalacticon, I like that one. That's the closest approach to a galaxy. You can
have--the other side of the orbit that is 180 degrees away from the perigalacticon, that's called the
anti-perigalacticon. And you are challenged to use that in a sentence at any time over break. Anyone who
successfully does that, send me an email recounting the circumstances, and I'll give you an extra point or
All right, so, precession of the perihelion, that's one post-Newtonian effect. Another post-Newtonian effect is
the deflection of light by mass, just because mass curves space-time, and so, light will follow a curved path
also. This was first demonstrated in 1919 by the famous eclipse observations of Eddington. And it also has
been manifested recently in the discovery of so-called gravitational lenses, of various kinds, in which the
appearance of an object is distorted by the fact that the light from that object has to pass by some other mass
on its way to your eye.
And so, I have a couple examples, here, which I'd like to show you of gravitational lenses. This is the famous
Einstein cross, so-called. And what this is, this is just a picture, I think, from the Hubble Space Telescope.
And these four objects, here, are the same object. That's a quasar. It's just, for this purpose, a point of light in
the sky. But it's located--its true location is behind this thing. This is much closer to you than any of these
guys. This is a galaxy about partway between us and the quasar. And what has happened is, if there were no
deflection of light, you'd see this quasar, kind of, right behind the galaxy. And in fact, it would be hard to
distinguish between the two. But what has happened is, because there's this galaxy in the way, the light from
the quasar starts coming this way and then bends around the galaxy and comes back toward us. And the same
is true up here and down here, and down here.