When flying, it's sometimes possible to look down the domes of ordinary observatories such as in Hawaii or Chile, or onto radio telescopes in Puerto Rico or Jodrell Bank, but now, looking down on Italy or Germany or Louisiana or Washington state, it is possible, if not flying too high, to see L-shaped labs which are observatories for gravity waves. The arms of those 'L's are typically ~2 miles long each, and they house the laser beams that travel inside them.
The setup in each lab is a powerful laser beam that immediately gets split into two beams, one traveling along each mutually perpendicular arm. At the far ends each beam gets reflected off a mirror and comes back to interfere with the other returning beam. For a simple illustration, it's like 2 runners, twins who run at identical speeds, who leave their common starting point and run to a post at the end of each arm, one north and the other west, say, and then turn round and meet again at the starting point.
This kind of setup was first used with light on a much smaller scale in the 1880s, and is called a Michelson interferometer. It was used for the famous 'ether wind' experiment, many people expecting the runners not to arrive back at exactly the same time because of the 'wind'. However, it always yielded a null (or negative) result, which confirmed the suspicions of the young Einstein, in 1905, that there really hadn't ever been such a thing as an 'ether'.
Now fast forward 10 years. It's 1915, Europe is deep into it's killing mode in WW1, and Einstein is a 36 year old professor working quietly in Berlin. He has further developed his special relativity into his general theory, and has predicted that gravitational waves must exist -- waves that must travel at the same speed that light does, but which would be far too small to detect.
What are these waves? Einstein famously said that we live in a 4-dimensional space-time continuum.' This continuum is often represented by a sheet, which can get slightly distorted by heavy masses. If something violent is going on with these masses, the ripple in the sheet propagate outwards.
Another century; it is 2015. Experiments to find these waves, by seeing if anything changes when nearby masses are moved -- for example Joseph Weber's experiments in the 1960s -- have shown nothing. Now, since the early 2000s, the L-shaped labs have been built -- Italy, Germany, America -- but again nothing has been detected. There has then been a hiatus, while NSF has funded expensive improvements, and now, in September 2015, the improved detectors have yielded small but undeniable signals. A gravity ripple has ever so slightly moved the goal-post, so to speak, and the interferometer has detected this. A wave has passed the L shaped Louisiana lab, and, yes, 7 msec later the same signals have passed the L-shaped observatory in Washington state, just like a tsunami traveling across an ocean.
The pretty graphs of the signals have been in all the newspapers, but how do we know that what was measured came from a black hole pair, rather than something else, like a much nearer binary pulsar? How do we know that this event recorded in September 2015 happened 1.3 billion years ago, before even life on earth?
We don't. But LIGO physicists (LIGO stands for Laser Interferometer Gravity-wave Observatory) have made templates of expected wave patterns for different scenarios, such as exploding stars and other violent events, and last September's pattern seems to resemble the template they have for an inspiralling black hole pair. As for the "1.3 billion years", as opposed to just, say, fifty ,or five hundred years back, all we can say with confidence is 'maybe'. There is always that phrase "evidence points closely to".
Finally, the Italian lab, VIRGO, near Pisa, is a collaboration between Italy and France, and there will soon be labs working in Australia and India. All these labs have agreed to share results, so that the direction and power of these sources may be obtained more accurately.
