LIGO: The set of arms that feels space-time waves

( A follow-up to my last story about what makes things fall, composed using the Up-Goer Five text editor)

What sort of thing can we build to see the waves? It has to be able to feel those tiny pulls and pushes. So we make a thing with two long arms. Imagine putting one arm up over your head, and one arm straight out to your side, and then lie down on the ground. That's how the arms need to be. When a wave coming from the sky pulls you taller, the arm above your head would become longer than the one out to your side. Then, as the wave makes you wider, the arm out to your side gets longer than the one over your head. We need to know about how the arms get longer or shorter than each other.

The longer the arms are, the bigger the change the waves make by pushing or pulling on them. So we make very long arms, arms that would go across half a hundred city blocks. And still the arm gets longer or shorter by a very tiny bit: a bit that is as much smaller than a human hair is wide as the human hair is smaller than  sun is wide. That is crazy small!

This is why we don't feel any of these waves going through us.

So how do we know if the arms got longer or shorter?

We use the fact that light is also made of waves, and light waves will go up and down many many times as they go along the arms. We start the waves going down the arms with their ups and downs matched. Then they hit a mirror at the end of the arm and come back. We line the mirrors up so, when there are no space-time waves, the ups and downs in the light from one arm come back in such a way that they end up going down and up instead when they reach the middle. In the middle we add the light from the two arms together. We add down with up and up with down, and end up with nothing: no light gets out.

But if a wave goes through, one of the waves has farther to go because one arm is longer. That means it gets through a little bit more of its up and down on the way. So now not-quite-down-anymore adds with up and not-quite-up-anymore adds with down and a little bit of light can escape the middle. This little light tells us a wave is going past.

There are a lot more parts to our set of arms to make it feel space-time waves better.  Like, the light gets turned around to go through the arms lots of times before it comes back to the middle, and we carefully make sure all the parts are very still, and we're also starting to use some other weird things that light can do. But adding the light from two arms is the biggest idea.

It's still very hard to do this, so we make more than one of these sets of arms. Then we can check the light from the two sets against each other. In case something that's not a space-time wave lets light through one of them, the other one will say "No! That's not a real wave. If it was, I would have felt it too." We also have friends around the world with their own sets of arms and we share what we know about the waves going past.

We also think a lot about what kind of waves will come from the sky. If we know what kind of wave we expect to see, we can look more carefully for exactly that type of wave and ignore some of the other things going on.

We haven't seen any space-time waves yet, but people are hard at work making our sets of arms even better. If we're right about what's going on in space, we should be able to notice some of the space-time waves before five more years have passed!

Gravity and gravitational waves in the most common words

This is a story about what makes things fall down.

(Written using the Up-Goer Five text editor)

1. The world and the sun

On our world, when you drop something, it falls down to ground. This is because the whole world - and by that I mean everything under the ground - is pulling on it. It turns out that all the things we know about pull on each other in this way.

All the things around us wants to fall toward other things. The closer to each other and the heavier they are, the more they want to fall together. In our day-to-day lives we only notice stuff falling towards the ground, because the world is by far the heaviest thing that's close to us.

But this same falling also explains how our world goes around the sun every year. Our world is much bigger than we are, so it doesn't really notice the pull from things we do. Instead, our world falls toward the biggest, heaviest thing it sees: the sun.

(We're actually falling toward the sun too, but since the world and everything on it is falling together, we don't really notice.)

Our world is moving through space, and with nothing in space to slow it down, it will keep moving for all time. Alone in space it would keep moving in a straight line, but there are other things around it. Since the sun is the heaviest thing around, the world starts to fall toward the sun.

If our world started out still, then it would fall toward the sun the same way a rock falls toward the ground. Instead, since it's moving pretty fast, it doesn't fall enough to hit the sun. It just makes a bit of a turn toward the sun. This keeps happening as it goes along and the turns add up along the way. The world ends up going around and around the sun, always falling and always missing, always as fast and always as far away.

(Well, actually, it gets a little bit closer and faster and then a little bit farther and slower as each year passes, but that's just a small change from how far away it usually is.)

2. Falling and falling

Things only stop falling if something gets in the way. Like when you drop a rock, it stops falling when it hits the ground. So why isn't the ground falling? Under the ground are more things, like rocks, and the ground can't fall past those rocks. When stuff falling onto them presses the rocks together, they push back. The stuff of the world pushes back against the falling, all the way down to the middle, deep under our feet.

Some other worlds are made of other things, like air, but even air pushes back if you try and make it small, like if you force a lot of it into a small bag. For big stars, like the sun, the middle part gets really hot and sends out a lot of light, which pushes on the outside bits and stops them from falling in further.


Most of the things we see in the sky, the sun and stars and other worlds going around the sun, are as big as they are because that's where the stuff getting pressed together pushes back and stops things from falling in any further.

3. Pulling from far away

So this explains most of the falling and not-falling we see. But there's still one weird thing: The sun doesn't touch our world, so how does it pull on it?

It turns out that heavy things actually change the space and time around them.  The changes are stronger near the heavy thing and then settle down far away. When the world tries to move straight ahead, it's doesn't exactly feel the pull of a far-away sun, it just goes through the space that the sun has changed a bit. The change in the space makes the world move a little bit toward the sun when it's trying to go straight ahead.

It's kind of like if you drove over a big dog and your car got pushed a little to one side. Imagine that space near the sun is full of dogs that push you toward the sun as you drive, bigger dogs close to the sun and smaller dogs far away. And if you try and stay still the dogs push you toward the sun anyway. It's only really far away from the sun you can drive straight ahead again.

(There aren't actually any dogs, I'm just trying to explain how the space makes you move and I don't have a lot of words for things that push you as you go).

Everything that moves through the space around the sun, even light, gets pushed a little as it passes. Light moves really fast, so it makes a very small turn, but it makes stars look like they're in different places than where they are when when the sun doesn't get in the way. People were first able to see this about a hundred years ago, during one of the times the sun got covered and went dark during the day.

For any round thing, like the sun or the world, the changes in space and time look a lot alike when you're outside it. They're the same kind of changes, they just get bigger and reach further out as the thing gets heavier. Even things much bigger than the sun have the same sort of change in space and time outside of them. We can sometimes see bright far-away things looking really weird in space because their light has come past some really heavy thing, like a huge group of stars. It can look like there are four of the same bright spot if the bright spot's light went around the heavy thing  to our eyes in four different ways. Or sometimes the light gets pushed around so that the bright spot looks long, kind of like an eyebrow, instead of small and round.

4. Falling toward nothing

If you have a round thing that is heavy and really small, the changes in space and time get really strong close to it. Even light, which moves as fast as anything can move, can get pushed around a lot as it passes. If the heavy thing is small enough, any light that passes too close will fall in toward the middle.  A heavy thing can even get so small that there is a place, quite close to the middle of the heavy thing, where if light tried to go away from it, moving straight out and as fast as anything can go, it would still end up falling back toward the middle, never to be seen again.

Now, as soon as something gets small enough that we could find that place, it doesn't look like any other kind of world or sun or star. If light can't stop from falling in, we know that nothing else can stop from falling either: not hard rocks, not air pushing back, no matter how hot it gets or how fast it moves. Nothing can escape. All the stuff that made the heavy thing will keep falling toward the middle, getting pushed closer and closer together with no way to stop. And we can't see what happens then, because none of the light can get out to tell us. We don't see any stuff, we just see a dark place in space that things fall toward, like they are falling toward nothing.

We know these dark places are out in space, because we can see other stars move around them, getting pulled enough that we know they are going around something really heavy. And sometimes things like air and tiny rocks falling into a dark place rub against each other, and they get really hot and bright and we see light from that. So we're pretty sure the dark heavy places are really out there. Let's call them dark stars.

5. Waves in space and time

Remember how the sun didn't just pull on the world, but changed the space and time outside so the world moved in a different way? This means that if the sun suddenly moved, the world doesn't know right away. It takes some time for the new changes to go from near the sun to where the world is.

If heavy things move the right way, the changes in space and time become like waves. Waves go out from the moving things, kind of like if you throw a rock into the water and waves go out from it in all directions. The waves can go along for a long time, but they get smaller as they go.

If you were in the way of a strong space-time wave and you were looking at the moving thing that it's coming from, you might feel something pull on your head and feet and push your sides in, then push you shorter and pull you wider. You'd feel a pull and push, pull and push, as the wave is going past you.


Everything we know about that makes these waves is far enough away that we only see very small waves on our world, so small that we'll never notice them. We have to carefully build a thing with lots of parts that can see the waves.

(Read more about how we see the waves here: LIGO: A set of arms that can feel space-time waves)

Science in the most common words

My friend Dave linked me to this text editor which restricts you to the 1000 most commonly used words in the English language, as inspired by Randall Munroe's Up Goer Five. I wrote up a summary of my research:

There are some tiny but heavy stars that are left over after normal stars die. Imagine our whole sun---hundreds of hundreds of our worlds---forced down to fit inside a city. We don't know exactly what's inside the tiny stars because the stuff there is pretty weird, but we have some ideas. 

Sometimes two of these tiny stars go around each other, like our world goes around the sun. When they go around really fast and close together they make waves in space and time that we try to see from our world. As the waves go out the stars move closer and closer together until they hit each other. 

The stuff inside the stars gets moved around when they're close together, so different kinds of stuff make different waves. Also, when they hit each other, sometimes they make a new star and sometimes they fall in to nothing (falling into nothing is another cool story). If we see these waves, the way they look at the end will tell us something about the weird stuff inside these tiny stars.