Observing Gravitational Waves
Ben - In 1916, Albert Einstein predicted that Gravitational Waves, these are ripples in the very fabric of space and time, must exist. We now know that they do, but we still can't observe them directly. Professor B. S. Sathyaprakash (who prefers to be called Sathya), from Cardiff University, explained more about these mysterious ripples.
Sathya - Gravitational waves are really a consequence of combining Newton's gravity with Einstein's special theory of relativity. In Einstein's special relativity, he's sure that nothing can move at speeds greater than the speed of light. But according to Newton's gravity, gravity will have to travel instantaneously and that's impossible according to Einstein's special relativity. So in a way, when you marry Newton's gravity with Einstein's relativity, you have to have gravitational waves. So in a few more words, gravitational waves are simply ripples in the very fabric of space and time which travel from their sources at the speed of light.
Ben - So, light does it come in photons or is it a little bit more complicated?
Sathya - It's actually not so much more complicated at all because light, we can think of them as particles only when the wavelength is tiny, which means energy is extremely high. When we are talking about gravitational waves, the wave length is extremely large. If you have very high energetic gravitational waves, we would probably think of them as gravitons which are the names given to particles of gravitational waves. But the correct way of thinking about them is really that they are ripples. You know, you throw a stone in a pond that produces ripples outwards and similarly, there is an exploding star that sends out ripples of gravity and those are gravitational waves.
Ben - Are they entirely theoretical or have we experimental observations of them?
Sathya - Until about 30 years ago, they were completely theoretical and in fact, even Einstein is supposed to have said, "Oh, they are an artefact of my theory. There is really no way to generate them or detect them." But lo and behold! Astronomical observations have actually given us a system. Namely, it consists of a pair of stars. They are very, very dense stars and they're going around each other. What would happen in Newton's law of gravity is that they will go on like that forever, not changing their period, not doing anything much. Whereas in Einstein's theory of gravity, they're churning the space time in which they are moving and therefore, they're generating gravitational waves. And gravitational waves carry energy and they carry rotational energy from the system and that energy has to actually come from the rotational energy which means that the period of the binary is going to decrease. This has been observed. The agreement between observation and theory is so wonderful, there is no other explanation for the decrease in period other than gravity waves. So we know for sure gravity waves exist.
Ben - So is there or will there be a direct way of observing them rather than just observing the impact that they have?
Sathya - I think you put it very, very rightly. There is no way of concluding from this binary that we have directly observed gravitational waves. We have to find a way of detecting them indirectly and currently, world over, there are many gravitational wave detectors that are currently being built, and they are actually taking data and this data is being analysed. So we may or may not detect them now, but they're also being upgraded. Upgraded to the level where we are guaranteed of detecting gravitational waves within the next 5 to 10 years or so.
Ben - What can we do with them once we measure them? What can they tell us about the universe?
Sathya - We might be able to learn a lot more about the way the universe works especially about what we call the energetic universe, the universe in which there is so much energy that you can't see the object in photons. You can't see the object in radio waves or light and that's because it shrouded with a lot of matter, very dense matter, and the only way we might be able to learn about them is actually by observing gravitational waves. One example that comes to mind immediately is a pair of black holes. Now black holes are really dark only in light and radio waves, etcetera, but they're not dark in gravitational waves. If you compress a black hole, energy that you put in will be emitted in gravitational waves and that black hole will regain its original shape. Now the way you can compress is to take another black hole and hit it! Hit this first one with the second black hole and we don't need to really do that ourselves. Nature provides for us what are called binary black holes. These are two black holes that are going around. Originally of course, both of these black holes were ordinary stars. They evolved and they were massive, and when they went through their evolutionary stage, they became black holes and they are going around each other and they reduce in size as in the case of binary neutron stars. Eventually, the two black holes will collide and in the collision process, you can actually see these black holes in the gravitational window.
Ben - So gravitational wave astronomy could let us observe things that we've never been able to see before. That was B. S. Sathyaprakash, from Cardiff University.