Gravitational waves are a logical inference from Albert Einstein’s theory of general relativity. Explaining this without getting wrapped around the axle of very complex mathematics is difficult; WC apologizes to any Real Physicists out there who are offended by gross oversimplification. Or analogies.
Gravitational waves are ripples on the curvature of spacetime, somewhat like ripples on a pond. Under current theories, they should be generated by binary star systems like pairs of white dwarfs, neutron stars and black holes.
Scientists have set out to detect gravitational waves. It may be that the universe is sloshing with random gravitational waves, making a wave motion difficult to detect for the same reasons you can’t find waves in a randomly sloshing body of water. But the scientists are trying. So far, only indirect observations have succeeded.
One approach – the technical term is ground-based interferometry – is to have extremely precisely measured objects here on earth, and measure the very tiny changes in the distance between them predicted to be caused by gravitational waves.The difficulty there is separating the signal from the noise; the measurements have to be controlled for even very weak effects like the influence of Jupiter’s gravity on Earth, as well as stronger forces like the tide from the Moon and the Sun. So far, the results of those ground-based measurements are inconclusive.
Another, and perhaps more promising approach, involved pulsars. Pulsars are very rapidly rotating stars. As they rotate, they beam radiation like a lighthouse across the universe. We see it here in radio telescopes as a blinking light. Pulsars are extremely regular. Some kinds of pulsars are precise to intervals of 20 nanoseconds – that’s 20 billionths of a second. Theory predicts that gravitational waves should disrupt that regularity.
Unfortunately, there are other known things that can disrupt that regularity, things like dust clouds and passing planets and stars. But you can control for that by observing a number of pulsars that are near each other. Gravitational waves are predicted to travel at the speed of light. If a pulsar show variation in its rotation rate, and then five years later another pulsar five light years away shows a similar disruption, you’d have pretty clear evidence for gravitational waves. This would be useful for measuring the strongest (and therefore most easily detected) gravitational waves, those generated by binary supermassive black holes.
And thats what the Parkes Pulsar Timing Array did. Parkes watched 24 different pulsars for a period of 11 years, with special focus on four extremely accurate pulsars.
And they found no evidence for gravitational waves. And they found no evidence at a 91-99.7% confidence level.
So either the theoretical structure underlying gravitational waves is wrong or the theories for supermassive black holes is wrong. Or, perhaps, both.
The thing to remember about physics is that negative proof is just as exciting as confirmation. Disproving a theory is as cool as proving it. But something about our current theories of gravity is wrong, and it will be very interesting to find out what it is, and what replaces it.