Physicist Nergis Mavalvala on the general theory of relativity, Hulse-Taylor system, and the way gravitational wave can be measured using a mirror, a laser, and a clock
How can gravitational waves be measured? What is the Hulse-Taylor binary? Massachusetts Institute of Technology Professor of Astrophysics Nergis Mavalvala speaks on the work of the Laser Interferometer Gravitational Wave Observatory.
Gravitational waves are a prediction of Einstein’s general theory of relativity. When Einstein first proposed general relativity in 1916, an essential piece of that theory was the existence of gravitational waves. What are these gravitational waves? They are essentially ripples of space-time itself. If you recall, Einstein’s method of looking at gravity was not to think of it as a force, but to think of it as a curvature of space-time. The simplest analogy of course one can think of, which is not completely accurate, but it works pretty well, is to think of a membrane or a cushion and in the center of that you put a bowling ball, and what happens is that the cushion curves inwards and if you put a little playing marble at the edge, that playing marble falls in towards the bowling ball. That was Einstein’s way of thinking of gravity: gravity as geometry.
In 1973, I think it was, Taylor and Hulse discovered a pulsar — pulsars are beautiful objects in the sky, they are neutron stars. How they are different from ordinary stars? Well, neutron stars are half the same mass as our sun almost but they are only ten kilometers in radius. They are very, very small, which means that they are extremely dense. One of the things that happens with these very dense neutron stars is that they have very strong magnetic fields. Those magnetic fields make the light that’s being emitted by the star get very heavily beamed. It’s like a little lighthouse. As the neutron star spins on its axis, the lighthouse goes through our line of sight. Hulse and Taylor were looking at these light pulsars using a radio telescope, the Arecibo telescope, and they noticed something very interesting. Usually these pulsars are very good time keepers; the light beam crosses your line of sight at a very regular interval. But this pulsar that they noticed it had a periodicity where those pulses were not exactly regular — they would speed up and then about 8 hours later they would slow down, they would speed up again. And when they looked at this system more carefully they realized that this pulsar was part of a binary system: it had a companion star.
The very simplest principle of the measurement is: imagine for a moment that I have a laser and I have a mirror and, in the case of LIGO, that mirror is sitting four kilometers away from the laser and I have really, really good clock, so all I do is I shine the laser light on the mirror, the light reflects off the mirror and comes back to me, and I have a really good clock that can tell me how long it took the light to travel that distance. If a gravitational wave comes by, that distance changes and my clock will measure a different time, and that’s the principle of our measurements. If you conceptually boil it down to how to make such a measurement, we have only two really difficult things to do. One is we have to make our mirror so, so still, more still than 10-18 meters, so that its motion is dominated by the passing gravitational wave and not by everything else that tries to shake it.