Surfing Gravity’s Waves

On 11th February 2016 the world rejoiced in the astounding news that the Laser Interferometer Gravitational-Wave Observatory (LIGO) had confirmed the first ever direct observation of gravitational waves, predicted a hundred years ago by Albert Einstein in his theory of general relativity. This is hailed as the breakthrough of the century, like the Holy Grail of scientific discoveries.

But what is this gravitational wave I speak of? First we will discuss gravitational fields.

Everything that has mass also has a gravitational field, an area around it where it can attract other objects with mass. The greater the mass, the stronger the field; the closer you are to the mass, the more strongly that you’ll be attracted to it. A typical person has a mass of 50 to 100 kg, so the gravitational attraction between two people is extremely weak. However, the Earth has a mass of around 6×10²⁴ kg, which is many, many times greater. Because of this, the Earth has a very strong gravitational field, and therefore pulls us down to the ground (thankfully).

Now we can delve into the concept known as spacetime.

Spacetime is the (metaphorical) fabric on which everything exists; it combines space and time (unsurprisingly) into one plane. A common analogy is to represent spacetime as a stretched-out sheet, like that on a trampoline. This sheet is a two-dimensional representation of spacetime.

When you place, say, a ball on the sheet, it will cause a slight dip. This is representative of the ball’s gravitational field. An object with greater mass will cause a larger dip, which is representative of a stronger gravitational field. If you place a smaller object near it, for example a marble, it will roll towards the ball; this corresponds to the gravitational attraction between the two objects.

Image: ESA – C. Carreau.

This ‘dip’ in the sheet is not only representative of the attractive force of gravity, but also shows how a gravitational field causes a distortion in the fabric of spacetime. Distances can become shorter or longer, and time can slow down or speed up. With small masses like the Earth, this distortion is not greatly significant, but still measurable. For example, if you were to measure a second at sea level, and then to measure a second at the top of Mount Everest, you would find that a second is slightly longer at sea level. This is called gravitational time dilation, and is explained by Einstein’s theory of general relativity (which may be a post for another time).

Events such as physical collisions can cause the fabric of spacetime to ripple; these ripples are what we call gravitational waves. These gravitational waves propagate at the speed of light. Only the most violent events that involve huge masses and immense energy can release strong gravitational waves, for example the collision between black holes, or the collapse of stellar cores into a supernova.

The gravitational waves that LIGO detected on 14th September 2015 were caused by the collision of two black holes, both around thirty times the mass of our Sun, at a distance of 1.3 billion light-years away, i.e. it has taken 1.3 billion years for the waves to propagate from the collision to us. The detected oscillations were extremely tiny, thousands of times less than the width of the nucleus of an atom, yet still detectable. Over the course of the following months, LIGO confirmed that the pattern in the waves is exactly as predicted by general relativity.

The discovery of gravitational waves is so significant that it deserves to be spread over two posts – watch this space for a hopefully thrilling second part, where I will explore how the apparatus that detected the waves actually works, and why this discovery deserves the commotion it has caused.

Yanhao