What are gravitational waves?
Gravitational waves are ripples that carry energy across the universe. They were predicted to exist by Albert Einstein in 1916 as a consequence of his General Theory of Relativity. Although there is strong circumstantial evidence for their existence, gravitational waves have not been directly detected before. This is because they are minuscule – a million times smaller than an atom. They are like tiny waves on a lake – from far away, the lake’s surface looks glassy smooth; only up very close can the details of the surface be seen.
Particularly exciting are “primordial” gravitational waves, which were generated in the first moments of the universe’s birth. These carry vital information about how the universe began.
What is general relativity?
In 1916, Albert Einstein discovered a mathematical way to explain gravity. He called it his general theory of relativity. It relied on a set of coordinates that described space and time together, known as the space-time continuum.
Matter and energy warp the space-time continuum like heavy weight on a mattress. The warping creates the force of gravity. Gravitational waves are ripples in the space-time continuum (instead of an ordinary mattress, think of a waterbed).
It isn’t all esoteric mathematics. General relativity tells us how gravity affects time, which must be taken into account by your satnav to tell you accurately where you are.
What is the significance of this discovery?
If scientists at Harvard University have detected gravitational waves, it is significant for two reasons. First, this opens up a whole new way of studying the Universe, allowing scientists to infer the processes at work that produced the waves. Second, it proves a hypothesis called inflation. This can be used to give us information about the origin of the universe, known as the big bang.
How can gravitational waves be detected?
A telescope at the south pole, called Bicep (Background Imaging of cosmic Extragalactic Polarisation), has been searching for evidence of gravitational waves by detecting a subtle property of the cosmic microwave background radiation. This radiation was produced in the big bang. It was originally discovered by American scientists in 1964 using a radio telescope and has been called the “echo” of the big bang. Bicep has measured the large-scale polarisation of this microwave radiation. Only primordial gravitational waves can imprint such a pattern, and only then if they have been amplified by inflation.
What is inflation?
The big bang was originally hypothesised by Belgian priest and physicist Georges Lemaître. He called it “the day without yesterday” because it was the moment when time and space began.
But the big bang does not fit all astronomers’ observations. The distribution of matter across space is too uniform to have come from the big bang as originally conceived. So in the 1970s, cosmologists postulated a sudden enlargement of the universe, called inflation, that occurred in the first minuscule fraction of a second after the big bang. But confirming the idea has proved difficult. Only inflation can amplify the primordial gravitational wave signal enough to make it detectable. If primordial gravitational waves have been seen, it means that inflation must have taken place.
What next? Do cosmologists just pack up and go home?
No way. Now the work really begins. Einstein knew that general relativity did not mesh with another theory of physics called quantum mechanics. Whereas general relativity talks about gravity and the universe as a whole, quantum mechanics talks about the small scale of particles and the other forces of nature, the strong and weak nuclear forces, and electromagnetism. Despite almost a century of effort, the world’s physicists have not been able to show how these theories work together. The primordial gravitational waves were generated when gravity and the universe were working on the same scale as particles and the other forces of nature. This detection and the subsequent analysis will hopefully tell us how. If it does, this could lead to what physics wistfully call “the theory of everything”.