So, here it is, finally, the LIGO blog. If you haven’t read my other blogs or ever read the news, then just for you LIGO stands for Laser Interferometry Gravity-wave Observatory, which I think is a little cheeky. I think that because when you think of an observatory, you think of a big telescope with a little eyepiece allowing you to see with your own eyes far, far into outer space. But that’s not exactly what this does. It does allow us to “observe” some of the furthest off reaches of the universe, but much in the same way that looking at leaves allows us to observe the wind. We’re not really looking at it, we’re watching its effects on other things. In this case, giant lasers!
If you want all the gory details then you should stroll through all the links on this website. That’s not what you come here for though, is it? I promised this would be easy to swallow, so here it is, (fairly) fully explained, in a way that I hope every visitor of this website can understand.
Everything with mass produces gravity. The Earth does, you do, your cup of tea does (all to different degrees, of course). The heavier, or more massive, something is, the more gravity it produces. Most scientists will tell you that you can think of space as being like a massive rubber sheet. I think I’ve said this myself in a previous blog. But I’m going back on this now. Instead, you can think of space as being like a big marshmallow. Confused? Me too, but bear with me, I think this will all work out.
So, imagine you have a box filled up with marshmallow stuff.Now, if you shove a ball, like a marble or something into the box and close the lid, what happens?
Well, you’ve got the same amount of marshmallow, but it needs to occupy less volume (the volume of the box, minus the volume of the marble). So, how does it do that? It compresses, which means in gets more dense, which means the same amount of stuff in a smaller volume. This is like when you squeeze a sponge, all the holes close up so you’ve got the same amount of sponge but squeezed into a smaller space.
Ok, so we’ve pushed our marble into the fluff and the fluff has compressed. Where? Well, mostly it will compress around the marble, and it will be less compressed the further from the marble you get. So the greyer area of the animation above is the denser marshmallow. If the box was infinitely big, you would eventually get to a point where the density of the fluff is unchanged by the presence of the marble.
It’s the same with gravity. Massive objects (by which I mean objects with mass, not necessarily very big objects) need to squeeze themselves into space, and by doing this, they warp it around themselves in all dimensions and this produces gravity (just like it produced density in the marshmallow box). The closer you are to the Earth, the more space is warped (like the squeezed fluff) and the stronger the gravity. So, we now have a correlation, more dense fluff = more gravity.
Now let’s imagine we put in another marble, near the first one, and they start spinning around each other.
As one of the marbles gets closer to one side of the box, it will compress the mallow, then as it moves away, the mallow will relax again, and then the next one moves towards the edge and the mallow recompresses. If you put some sort of device for measuring density at the edge of the box, it would look like waves of compression passing over it. And remember, more compression means more gravity. So these fluctuations up and down in density are really gravity waves.
As the marbles orbit each other (like black holes orbiting each other in space) they get closer and closer and spin faster and faster, until eventually they collide and… kind of explode… let’s say they’re exploding marbles… can you get those? I dunno. You can now. So, they explode a bit, but because the marbles were spinning around each other, the explosion kind of spins as it happens and so you get even bigger waves, moving even faster. Then the explosion dies away and you don’t see the waves anymore.
Ok then, so the marbles are black holes, and the marshmallow is space and the compression waves are gravity waves. This leaves one big question. How do lasers help us see them? The answer to this is simple, even light gets pulled in by gravity. It seems unbelievable. Light just doesn’t seem solid enough to us to feel gravity, but in fact it does. This has been observed, by actual observatories, with pretty pictures and everything, the way it’s supposed to be! We have seen what is called gravitational lensing, which is where the light from objects very far away gets pulled in by objects merely far away and we see a warped and distorted version of that object, often, in fact, multiple warped and distorted versions all at once. That’s because light going in two different directions get pulled in by these large objects and it ends up hitting the same place in our observatory.
What we can do then, is we shine a laser and we see how it changes as the gravitational waves pass over it. Easy! Well, actually, no, not so easy. These massive events which cause the gravitational waves (like when the marbles explode) happen a very long way away, and as we saw with our marshmallow fluff, if you are very, very far away from the source of the waves, they become smaller and smaller until they are barely there at all. So to see these waves, you need a very, very sensitive way of detecting them.
The way we do this is by taking advantage of the fact that light acts like a wave. Imagine you have an auditorium full of people. At one end of the hall someone starts a Mexican wave going, (and we’ll assume he keeps getting up and down to keep the wave going constantly), but at the other end, someone else starts a wave going in the other direction. You are sat directly in the middle. By the time the wave gets to you, it happens that the person on one side has stood up, so it’s you turn next, but the person on the other side, is sat down, because it’s their turn next. So you and your partner get up at the same time and there is no one after you to carry on the wave. The two waves have cancelled each other out.
Now, if the second wave had started just half a second earlier, then the person to your left and to your right would stand and the same time, and you’d have to do two waves at once, so you jump twice as high out of your seat, and then the wave carries on on both sides of you.
This is exactly what happens with light.The laser beam gets split into two beams. They travel for a really long way then get reflected and come back together at the point they were split and are recombined and sent into a detector.
If the waves arrive at the same time, you get a bigger wave, and see brighter light, if they arrive out of synch, they cancel out and you see no light. The waves interfere with each, that’s why it’s called an interferometer.
Light waves are really small (the ones used in LIGO are 0.000001 meters long) so we can use this technique to detect really small changes in the path the light travels. As gravity waves pass over LIGO one path is stretched compared to the other and the light goes from cancelling itself out, to enhancing itself, just like the Mexican wave, and we can measure that change in signal.
But it’s still not that easy. Earthquakes, heavy traffic, or just temperature fluctuations in the air can make bigger changes to the signal than gravity waves will ever make, they are so weak by the time they get to us. So the real genius of LIGO is the technology used to keep it as close to completely stable as possible. The laser travels through a vacuum so that air cannot affect the signal, and special dampening devices are used to prevent any shaking of the laser whatsoever. That way, the scientists know that if they see a wobble in the signal, it must have come from gravity waves. But to double check, there are actually two LIGO facilities at opposite ends of the USA. If they see a signal at both detectors at almost (but not quite) the same time, then they can say they’ve seen gravity waves. And that’s exactly what has happened.
Millions of light years away, two massive black holes, each one thirty times more massive than our own sun, have collided with each other, and that has caused ginormous ripples in the very fabric of space itself to occur. They’ve travelled all the way to the Earth, getting weaker and weaker as they go until finally, we see them as a teeny tiny variation in laser light. One of the most sensitive pieces of equipment ever made was needed to see one of the biggest, most energetic events that ever take place in our universe, once again showing how the most fundamental, microscopic science impacts of the cosmically gargantuan. I still think that is pretty cool!