#028: The Absurdity of Gravitational Waves
Two neutron stars colliding made headlines again, in no small part because of the role gravitational waves played. This is the fifth observation of a stellar event via gravitational waves. But it did little to highlight the absurdity of detecting gravitational waves.
In principle, it doesn’t sound complicated. When you throw a stone into a calm pond, you get a nice, circular wave from where it enters the water. So, if all you can look at is the edge of the pond, you can tell if someone threw a stone in the water. And, if you look at how big the waves are, you can even calculate how big the stone was.
Even if you factor in wind also creating ripples on the surface of the water, it doesn’t sound hard to do.
Except detecting gravitational waves is so very much harder. To begin with, all they change is the very fabric of space-time itself. There’s no equivalence to the “edge of the pond” we could look at to see the waves. Usual methods of measurement also fail, because gravitational waves stretch and compress them when passing through them.
So what scientists came up with was something called a laser interferometer. It works by sending a laser beam through a beam splitter, and then each laser half through two perpendicular arms. Each arm has a mirror at the end, reflecting the laser. The laser beams travel back to the beam splitter, joining the two laser beams, and arrives at a detector.
The idea goes that if a gravitational wave passes through the arms, it changes the length of the arms, and the laser light will interfere with itself, which the detector can tell by looking at how light or how dark the laser is when it arrives vs. what it should be.
And even that doesn’t quite convey the ridiculousness of the actual setup. Because gravitational waves change space-time so little, the whole detector has to be extremely sensitive. It has to detect changes in length of less than 1/10.000th the width of a proton.
The mirrors are the smoothest ever created, suspended by tiny threads to reduce outside influences. Each arm of the interferometer is 4 km long, and has to be as perfect a vacuum as we can make it. And the laser light has been tuned so it only has one and only one wave length.
And even then, two detectors where build far apart. Just to make sure environmental factors could be factored out. Even then, the software has to account for all the noise that still makes it through. It’s why the first detection of gravitational waves was announced 6 months after the observation, because scientists wanted to be absolutely sure.
So, the next time you read about gravitational waves, remember how absurd it is that we can even detect them in the first place.