Making Waves

Gravitational waves are already the science story of the week but if the rumours hold up they will one of the science stories of the century. We’ll know soon enough, as there will be a press conference in Washington DC at 10:30am (local time) on Thursday. And this revolution will be broadcast; you can catch a livestream on Youtube.

The rumour doing the rounds is that the LIGO team will announce the detection of gravitational waves emitted during the merger of two black holes. Here’s a quick explainer as we head into the (we hope!) big day…

What Are Gravitational Waves? Gravitational waves are waves that travel through the fabric of space, just as ripples move across the surface of a pond.

Waves In Space?  Yep. By detecting gravitational waves we are watching space bend and stretch.

Gravitational fields are encoded in the curvature of space CC BY-SA 3.0

Gravitational fields are encoded in the curvature of space CC BY-SA 3.0

Really? Waves in Space? Yep, Really. Once upon a time, physicists thought space was rigid and unchanging. However, in 1915 Einstein’s General Theory of Relativity told us that gravitational forces are communicated via the curvature of space. This is often described via the “rubber sheet model”; curved space is analogous to a rubber sheet warped by massive objects that sit upon it. But what really matters for gravitational waves is not just that space (or, more properly, spacetime) can curve, but that its curvature can change. As stars and planets move the curvature of space must adapt itself to their new positions. If the curvature didn’t change the universe would be a very strange place as a moving object would leave its gravitational field behind, a little like Peter Pan losing his shadow. Mathematically, the ability of space to bend and stretch means that waves can move through it, and this led Einstein to predict that gravitational waves could exist.

Why Is This So Exciting?  Science waited 100 years for this; who wouldn’t be excited? For physicists, LIGO is testing a key prediction of General Relativity, which is one of the most fundamental theories we have. On top of that, if LIGO sees gravitational waves emitted by a pair of black holes as they collide and merge we will have ringside seats to some of the most remarkable events in the universe. And a detection by LIGO will mark the culmination of decades of work by a cast of thousands who have built what is probably the world’s most sensitive scientific instrument.

How Does LIGO Work? LIGO has two giant L-shaped detectors; one in Washington State and the other in Louisiana, on the other side of the United States. Each detector is 4 kilometres on a side. Gravitational waves always stretch space in one direction while squeezing it in another, so a passing gravitational wave expands one side of the “L” while shrinking the other. Powerful lasers then pick up the resulting change in the lengths of the arms. The stretching and squeezing is tiny – each arm may grow and shrink by only a quadrillionth of a millimetre, far less than the diameter of a single atom. By having two detectors LIGO rules out spurious signals from local vibrations, traffic or tiny earthquakes; LIGO also pools its data with two smaller European experiments, GEO and VIRGO.

Spacetime near two orbiting black holesImage: Swinburne University

Spacetime near two orbiting black holesImage: Swinburne University

How Are Gravitational Waves Made? Two black holes (or any pair of orbiting objects) stir up space as they circle one another, creating gravitational waves. If the black holes are far apart the gravitational waves are unimaginably small. But gravitational waves carry away energy, and that energy has to come from somewhere – so the orbit slowly shrinks. But a smaller orbit is a faster orbit, increasing the output of gravitational radiation and the orbit shrinks faster and faster. This is the inspiral, and can take hundreds of millions or even billions of years. But eventually the two black holes are orbiting one other at a decent fraction of the speed of light, churning space like an out-of-control cosmic egg-beater. This phase lasts seconds but produces a huge burst of gravitational waves: this is the signal that LIGO detects. The black holes then plunge towards a merger, followed by the ringdown as the new black hole settles into a stable shape.

Didn’t Everyone Get All Excited About Gravitational Waves A Couple of Years Ago? We did, and it was a false alarm. Several things are different this time, though. That claim was made by BICEP2, a telescope that looks at the microwave background, fossil light from the Big Bang. BICEP2 did not observe gravitational waves directly, as LIGO does. The signal BICEP2 saw turned out to be associated with dust in our own galaxy; this was quickly realized as astrophysicists checked and re-checked the results. (I blogged about the latest news from BICEP2; it is producing lovely data and starting to test a number of different theories about the Big Bang.) Moreover, the LIGO team has built a reputation for caution – going so far as to do “signal injections”, where the analysis teams are unknowingly fed synthetic data to test their ability to extract real gravitational waves from the experimental noise. Finally, the rumour is that their results have been through peer review, and will have stood up to scrutiny from independent scientists.

What Next? Physicists will use LIGO to make stringent tests General Relativity: do its predictions match the behavior of spacetime seen during black holes mergers? And for astronomers it will like growing a new set of eyes: LIGO is an entirely new kind of telescope that lets us explore the universe with gravitational waves. Watch this space.