One of my undergraduate students has been researching gravitational waves this year. Last Friday, he gave a nice presentation on the subject.
Gravitational waves are one of the many examples of waves in physics. We are perhaps more used to waves on the surface of water, or waves along a guitar string, or electromagnetic waves (such as radio waves and light), and, in many ways, gravitational waves aren’t much different.
But they are a little strange. Whereas a radio wave travels through space and time, a gravitational wave (caused for example by a supernova) travels ‘on’ space and time, rather like a water wave (caused by throwing a stone into a pond,) travels on the surface of water. This means that space-time distorts as the wave goes past. When a gravitational wave hits us front on, we will alternately squash in height and expand widthways, before squashing widthways and growing in height (the preferred option for most of us).
These changes in lengths are not some mathematical construction, they are real. At least, they are predicted to be real, but, to date, no-one has actually detected a gravitational wave. The problem is, that unless you are standing next to a supernova, the changes in length due to gravitational waves are very small indeed. Imagine a rod the same length as the distance from the earth and the sun. Now imagine it growing in length by about the width of an atom. That is the sort of distortion we are talking about. Not surprising that no-one has built a detector sensitive enough yet.
But that doesn’t stop people trying. Detectors on earth are limited by, for example, seismic vibrations, and the constraints on how large an object you can build. But space doesn’t have those problems. And so there is the LISA concept; three satellites in a large triangle 5 million kilometres apart, following the earth in its orbit around the sun, linked with laser beams. And you thought the Large Hadron Collider was ambitious.