listen to part of a lecture in an astronomy class.

listen to part of a lecture in an astronomy class. P: Last time we began talking about the theory of relativity, which Albert Einstein proposed over a century ago. We said that the formulas and mathematics in Einstein\'s theory led to predictions that light, rays of light, could be bent by gravity. Question, Laurie? L: Based on what you said last time, I can\'t help thinking about something. When you say rays of light can bend, so once, when I saw a rock lying in a shallow pond, it looked like it was in one place, but when I reached into the water to touch it, it wasn\'t where I expected it to be. Is this a good analogy for what you\'re talking about? P: In a way, yes, because water, like gravity, also alters the path of light. But here\'s an example where gravity\'s involved. During solar eclipses, when the moon briefly blocks the sun, observers have described seeing a star that\'s directly behind the sun and the moon. Nonetheless, the star was visible alongside them. Such observations support Einstein\'s theory that gravity, in this case, the gravity of the sun, affects the path of the light from that star, bends it or curves it. And sort of like that rock you saw in the water, the star appeared to be in a different location from where it actually was. Okay, so wherever there\'s an enormous concentration of matter in space, you get a significant gravitational effect. And as you might imagine, lots of concentrated areas out there, places where galaxies are clustered together, for example. Think about that, why might that be a problem for us as we try to get a clear picture of the universe? Mike? M: Uh, because each one would be distorting our view of the stars and galaxies behind it. P: Right, and we might not even realize how much. Okay, think of a pair of eyeglasses. Hopefully, if the eye doctor gave you the right prescription, the lenses in your glasses refocus the image you see, correcting distortions to improve your vision. Well, in order to create a prescription for seeing the universe more clearly, we need to map out where light from objects in the sky is being distorted. M: So we need to note where there are large concentrations of objects exerting a lot of gravity. P: Exactly. Where massive amounts of matter produce enough gravity to bend light, we want to measure this distortion. To do that, we need a common standard. And it turns out there\'s a particular kind of supernova that can help. It\'s called a type one a supernova. A supernova, you may recall, is an exploding star. Type one a supernovas always shine with a predictable brightness. Our telescopes have found a number of these. By examining the light from each one, astronomers can estimate how far the light from the exploding star has traveled to reach us, and the extent to which the light\'s been distorted by gravity on its path to Earth, gravity from clusters of matter. So now, when we look into a particular region of the sky, we can calculate how the light from other stars and galaxies in that region has been affected. We don\'t yet have enough information to correct our image of the entire sky, but this is a start. Now, these high mass, high gravity regions of space, I don\'t want to suggest that they\'re only a problem to be corrected, because as their gravity bends the light from distant objects, they actually make those objects appear much closer, and that gives us useful information. For example, a cluster of galaxies created one of these distortions that allowed astronomers to see a single star halfway across the universe. And elsewhere another galaxy cluster made it possible to observe an extremely distant, very young galaxy, one so far away that the light from it must have left the galaxy billions of years ago, not long after the very beginning of the universe. So what we\'re seeing is how that young galaxy appeared way back then, and this gives us the opportunity to collect data about what the young universe looked like.