Gravitational Radiation

Reading: Chapter 22 (Neutron Stars and Black Holes)

In the 1600's, Issac Newton developed his Universal Theory of Gravitation and his three laws of motion. This way of looking at the Universe works quite nicely for the motions of the planets and most of our everyday experiences. However, under certain circumstances, this picture is inadequate. To fix-up some problems, Albert Einstein developed his General Theory of Relativity which brought a new persepctive to our thinking about space, time, and gravitation.

To get a feel for some of Einstein's ideas, we must start thinking about the space-time of the Universe. Suppose that I tell you that Astronomy 122 meets in 110 Willamette Hall, Is this enough to get you to class? Well, no, because I didn't tell you when the class meets. In order for you to show up for class, you must know not only where it meets but also when it meets. This is true for all events in the Universe; you must know not only where the event occurs (its spatial position) but also when the event occurs (its temporal position). The space and time positions are equally important and we should think about events in the Universe in terms of their space-time positions. An interesting property of space-time in the Universe is that it has structure and it is not rigid.

Question: What are some consequences of viewing the Universe in this manner?

The path an object rolling on a table top follows is determined by the shape of the surface (it rolls on the table top). The table top defines the space-time for the rolling object. For objects rolling in the Universe, a similar idea holds in that the paths of objects follow the shape of the shape-time.

Locally, the space-time in this room is fairly flat and so, unless you push on an object, its free motion (unforced motion) is in a straight line. If I were to place a large chunk of mass into the room, the mass would distort the shape of the space-time. In two-dimensions, this is easy to visualize. Imagine a rubber sheet onto which you place a bb. The bb causes a depression to form in the rubber sheet.


This is analogous to what mass does to the structure of space-time. It causes a depression to form so that if an object rolls toward it, it falls into the pit and is captured. (This, by the way, is how Einstein envisioned how gravity works. Mass distorts the space-time causing particles to roll toward the mass. Note that the objects follow the shape of the space-time and in this sense are following an unforced motion! That is, there is no gravitational force, objects are simply following their natural motions.)

Return to the rubber sheet analogy. If I drop a bb on the sheet and it bounces, ripples in the sheet are produced which propagate away from the disturbance. These ripples in the space-time are referred to as gravitational waves.

If I disturb mass in the Universe, I can cause ripples to form in the space-time which propagate away from the disturbance. These ripples are referred to as gravity waves.

They almost certainly exist, but there have been no direct measurements of their existence (there is only indirect evidence that they exist).

Possible sources for gravity waves are supernovae, merging neutron stars or black holes ( low amplitude waves, high amplitude waves), unstable rotating neutron stars, ... .