It seems like yesterday, but it’s already been a year since LIGO announced they found the elusive gravitational waves, that stretch and squeeze space (a tiny bit) in their wake.
But what does it mean “they stretch and squeeze space”? and how do you actually measure something like this? Let’s answer these important, very deep questions!
What does it even mean to “stretch and squeeze space”?
That is kind of a hard question, and I also struggle a bit with it. I’ll try my best, and please do correct me if I get it wrong.
Let’s consider an adorable universe where all of space consists of the picture of this puppy:
So cute. If we squeeze space in one direction, we make all distances smaller, we literally squeeze points together. Like this:Notice that we are scaling down distances, not chopping space off. The puppy is still all there, its ears got closer together. All the universe we started with is still there. Gravitational waves squeeze bits of space, while stretching others. Kind of like this¹:
Basically, stretching a portion of space means pulling everything in that portion of space away from everything else, by the same proportion.
Needless to say, spacetime is more complicated than puppies, but this should give an idea.
How do you measure a change of space itself?
If every length changes the same way, you can’t use rulers to measure changes in space, because they change too. To explain how it’s done, I might need some math. I promise it won’t hurt and I’ll use puppies to make it better.
Let’s say we have two perfectly identical puppies², Brian and Stella.
They run to fetch a ball 50m away from us. They always run at 10 meters per second (freakin fast puppies), so it takes them 10 seconds to fetch the ball: 5 seconds to cross the 50m to the ball, and another 5 on the way back.
These puppies also breathe very regularly, taking a breath every two seconds (one second breathe in, one breathe out). So, if they both start running with a breath in, by the time they come back with the ball, each of them will have taken exactly 5 breaths (one every two seconds), and they both arrive breathing out.
Good girl! Good boy!
Ok, now we throw the ball again, but an amazingly strong gravitational wave passes through, stretching Brian’s way path to 100m, and squeezing Stella’s to 25m. If they leave at the exact same time, breathing in, and running at the same speed, they will not come back together anymore: Stella arrives first, after 5 seconds (half as much as before, because she has half the distance), breathing in, whereas Brian arrives after 20 seconds breathing out.
And that is how we measure if space itself changed. We can tell the two paths are not the same length anymore, because identical puppies come back out of sync.
Real dogs, of course, don’t breathe that regularly, or change speed instantly, regardless of how much you train them. You know what behaves like that, though? Light. Light travels at the same speed no matter what, and each “color” has its constant frequency (it breathes regularly in time).
LIGO sends trains of perfectly identical “laser puppies” on perfectly identical paths and measures any teeny tiny difference in their “breathing” (their wave phase) when they come back.
Bonus (without puppies): doesn’t the laser get blue- or red-shifted?
It should indeed. Like the stretching of space between us and distant galaxies shifts their light towards longer wavelengths, so the passing of gravitational waves shifts the light of the laser inside LIGO.
However, despite the scientists’ best efforts, lasers at LIGO can only produce a range of wavelength. They’re extremely close to each other, like between 1063.99999999999999999nm and 1064.00000000000000001nm. Yet, gravitational waves change them by ten times less than that, so it’s impossible to distinguish the effect from the instrument uncertainty.
Also, the effect the time it takes light to cross space is waaaaaay larger, so it’s “easier” to measure.
A huge thank you to my friend Leila, who patiently and graciously explained all this stuff to me
If you want more
… then you must really be into gravitational waves! I got a few posts for you, and this great video on how freakin hard it is to make the measurements.
¹Gravitational waves don’t quite work like that. It’s just an example, please don’t take it literally.
²These are not real puppies, these are example puppies. Please, for goodness’ sake, PLEASE don’t take this literally.