# Gravitational Waves

The big breaking physics news is the detection of gravitational waves. These waves are distortions in space-time, caused by a large mass doing something spectacular (two colliding black holes in this case)  that propagate across the universe and create tiny changes in space when they reach us. The commentary here describes what goes on. Essentially, things change their length/width. When a gravitational wave passes through my office (say ceiling to floor) one can imagine the length of the office increasing slightly, coupled with a decrease in the width of the office, followed by the reverse – a decrease in the length and an increase in the width.  But its not just that the bricks that make up the room vibrate (e.g. as in a seismic wave) – its the whole of space that does it.

These waves were predicted by Einstein in 1916, just after the publication of his theory of General Relativity. Their discovery is further evidence for the theory. But it's not just about Einstein. Gravitational waves provide another way of observing the universe – 'seeing' what's going on. Up to now, we've been stuck with light-based observations (be it visible light, infra-red, microwave – they all are electromagnetic waves). There are neutrino observations too, but these aren't exactly easy. But gravitational waves are something else – it's like seeing AND hearing something, rather than just seeing it.

So how are they detected? The concept is rather simple, as explained in the commentary. Build a (large, meaning 4 km in the case of LIGO) interferometer with two arms. Pass light up and down each arm. The light from the two different paths will interfere – such interference could be constructive (if a peak from one arm comes at the same time as a peak from the other) or destructive (if a peak comes with a trough). If everything is stable, the interference is stable. But when a gravitational wave passes, the arms change their lengths. Not by much. The light takes longer to pass up and down one arm, and shorter to pass up and down the other. Now the timings of the peaks and the troughs change, and the interference signal changes. We detect a gravitational wave.

The difficulty to now has been detecting the tiny signals amongst larger 'noise' signals, but a recent upgrade to the LIGO detector has done its job. Well done LIGO team!