How we found them: Inside a giant gravitational wave detector

It took decades of work to prove that gravitational waves are real.  New Scientist goes inside the story – and the awesome tool that did it, LIGO

Enrico Sacchetti

By Daniel Cossins

In 1969, Rainer Weiss was a young MIT professor.  At the time gravitational waves were a theoretical curiosity: Einstein himself took years to be convinced by his own prediction that moving cosmic bodies would send out ripples through space-time.  Then physicist Joseph Weber claimed to have recorded one on a xylophone-like instrument he called a resonant bar detector.  Weiss takes up the story. 

The students on my course were fascinated by the idea that gravitational waves might exist.  I didn’t know much about them at all, and for the life of me I could not understand how a bar interacts with a gravitational wave. 

I kept thinking, well, there’s one way I can explain how gravitational waves interact with matter.  I said, suppose you take a light – I was thinking of just light bulbs because, in those days, lasers were not yet really there – and sent a light pulse between two masses.  Then you do the same when there’s a gravitational wave. 

Lo and behold, you see that the time it takes light to go from one mass to the other changes because of the wave.  If the wave is getting bigger, it causes the time to grow a little bit.  If the wave is trying to contract, it reduces it a little bit.  So, you can see this oscillation in time on the clock. 

I was hiding in this little office in Building 20 at MIT and for about three months I thought about how you might do this.  First I thought you couldn’t get clocks good enough.  But we did some experiments and I learned you ...

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