What we can learn from gravitational waves

Why is gravity so weak - and do black holes emit gamma rays? Gravitational waves are set to shake up our understanding of the universe.

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Don’t be fooled by the comparisons between the discovery of the Higgs boson and that of gravitational waves. Yes, both were the result of searches that began in the 1960s – proof of a tenacity shown in few fields outside science. Yes, both have taught us something fundamental about the nature of the universe.

Yet the Higgs boson discovery closed a chapter. It was the final piece in the puzzle of particle physics and marked the completion of a project to understand the stuff out of which the universe is made. The announcement that we now have devices that can see gravitational waves, on the other hand, marks a new era.

Gravitational waves are ripples in the fabric of the universe. We play out our lives on a stage made up of three dimensions of space and one of time. Our presence – and the presence of any mass or energy, for that matter – distorts that fabric. Any repetitive movement changes the distortions in a way that makes waves emanate out from the origin. If you jump up and down, you create tiny gravitational waves; shake your fist and you shake the universe.

Human beings are too insignificant to make detectable waves but there are plenty of things in the cosmos that can. This is why physicists are so excited. The recent observation of a gravitational wave signal from two merging black holes was just the beginning.

We can now investigate the structure of neutron stars, for instance. The dance of these ultra-dense objects (a teaspoon of their material would weigh about ten million tonnes on Earth) produces gravitational waves that should make our detectors chime. We will be able to interpret those signals to infer the heavy stars’ properties. The same is true of black holes. After all, we have assumed that they exist, but the latest gravitational wave discovery was by far the best evidence we have ever gathered. Now we can analyse those waves to find out more about these mysterious objects.

Interpretations of gravitational waves will teach us about gravity. It is our least understood force and the waves will tell us whether its influence travels at the speed of light, say, and perhaps even something about the “graviton”: the hypothetical particle that transmits the force. Gravity is strangely weak. It is many billions of times weaker than the electromagnetic force that holds atoms together. Understanding gravitational waves might eventually give us clues as to why this is.

Gravitational waves also open a window on the “dark energy” that constitutes nearly 70 per cent of the universe. We know nothing about this stuff but we have a new reason for optimism. Dark energy is speeding up the expansion of space. As gravitational waves travel through space, they may well pick up characteristics that provide clues about the nature and origins of dark energy.

It is likely that our new technology for investigating the universe will shower us with insights that are as puzzling as they are welcome. We already have one: just after the information about the gravitational waves came in from the colliding black holes, we saw a burst of gamma rays – high-energy particles – that seemed to be associated with the merger of the holes. However, black holes do not, as far as we know, emit gamma rays.

George Bernard Shaw once observed that science rarely answers a question without posing ten more. Watch this space: gravitational waves will shake our universe in more ways than one.

Michael Brooks holds a PhD in quantum physics. His most recent book is At the Edge of Uncertainty: 11 Discoveries Taking Science by Surprise.

This article appears in the 18 February 2016 issue of the New Statesman, A storm is coming

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