Finding Gravitational Waves from Space in Space
LISA Pathfinder paves the way for detecting gravitational waves from space.
It’s been only a few months since astronomers triumphantly announced the first detection of gravitational waves – ripples in space and time caused by the movement of massive objects – but already they’re testing out a new, space-based way of ‘hearing’ gravitational waves.
Europe’s LISA Pathfinder is a proof-of-concept satellite that has just exceeded all expectations and proved that detecting gravitational waves from space is possible.
Launched in December 2015, LISA Pathfinder’s challenge was to hold perfectly still so that any ripple from a passing gravitational wave could be detected. It has just passed this challenge with flying colours, paving the way for a full-scale array of gravitational detectors flying in precise formation, listening for gravitational waves from distant collisions.
What are gravitational waves and why do they matter?
Imagine a distant corner of the universe where two black holes are locked in a tight dance, caught by each other’s mutual gravity. Over time this pair of black holes spirals inward faster and faster and closer and closer until eventually the two merge into one and form a bigger black hole.
Collisions on this scale don’t go unnoticed. At least not any more. When massive objects like black holes and neutron stars collide, they create characteristic ripples in the fabric of the universe that travel out in all directions.
These ripples are called gravitational waves, and were first described by Albert Einstein back in 1916 as part of his theory of general relativity.
The idea is this: distances in space and intervals of time are not fixed quantities. Instead they can be warped and stretched, like a fabric, depending on the size of nearby objects. The bigger the object, the more it warps space and time. This is like the fabric of a trampoline, where a bowling ball creates a bigger dip in the trampoline than a marble. And in fact, the marble wants to roll towards the bowling ball because of the curvature of the trampoline – this explains the pull of gravity.
But Einstein’s theory doesn’t stop there. Einstein realised that if space and time are like a fabric, then matter moving around on the fabric will create ripples, just like someone bouncing up and down on a trampoline. He called these ripples gravitational waves.
So gravitational waves are created by any objects moving around – accelerating – in space. Waving your hand produces (very small) gravitational waves. Black holes spiralling into each other produce much larger waves.
Until last year (2015) we didn’t have the technology capable of detecting such ripples, even from black holes. Now, thanks to a ground-based detector called LIGO in the United States, we do. In February 2016, LIGO announced the first detection of gravitational waves.
It’s like gravitational waves have given us a new sense. For all of history we’ve been using just our eyes to understand the universe, but now it’s like we can hear the universe as well, with all its collisions and explosions and activity. It’s like we’ve been watching an orchestra perform without sound and now we can hear the music as well.
Finding gravitational waves from space
But just like for traditional telescopes that use light, it often helps to get away from Earth. Looking for gravitational waves from the ground is like trying to hear a pin drop in a crowded environment – any minor earth tremor or even a passing lorry truck adds to the background noise – but in space, things are much quieter.
This is the next step for gravitational wave astronomy and will allow us to hear much fainter collisions from different types of objects, including supermassive black holes lurking in the centres of galaxies.
The challenge comes in keeping all parts of the space detectors flying exactly in unison so that any extra movement is due to a passing gravitational wave. LISA Pathfinder is a single satellite that contains two cubes of gold and platinum, floating freely in space and unperturbed by any external forces.
LISA Pathfinder exceeds expectations
On 7 June 2016, the first results from LISA Pathfinder showed that it is performing five times better than originally required. The two cubes are so still that femtometre-sized movements can be detected, which is about the same size as the nucleus of an atom.
This level of precision is more than enough to detect a whole new range of gravitational waves, and paves the way for an array of satellites like LISA Pathfinder flying in unison. This array, called LISA, is a proposed European Space Agency mission that will truly begin the hunt for gravitational waves from space.