February 12, 2016, by Lindsay Brooke
Ripples in time and space – the dance of death!
It was quite a day for the LIGO (Laser Interferometer Gravitational-Wave Observatory) team yesterday. They announced the first direct detection of gravity waves today. This is Nobel Prize winning stuff! Gravity waves have been searched for now for several years (LIGO started in 1992), and there has even been a previous Nobel Prize for indirect detection of gravity waves. So it’s quite a hot topic!
What did they find?
In this case the two LIGO detectors both found a signature of two black holes orbiting each other faster and faster until they combined into one. So, this is two objects, both with a mass of around 30 times our Sun, yes 30 Suns, each compressed into something about the size of Nottinghamshire. These two objects were whizzing around each other at a very substantial fraction of the speed of light. So, basically you have two inconceivably massive objects, moving at inconceivable speeds, packed into a really tiny space, doing a dance of death that lasts a ⅕th of a second, at a distance of over a billion light years away throwing off gravity waves that reached Earth to move a mirror a fraction of a protons width. (Did you know that astronomers LOVE big numbers?).
So – what are gravity waves?
Gravity waves are a prediction from Einstein, and his theory of relativity. It’s a fact from fundamental physics that if you shake things, you get waves. You shake a rope back and forth, you get waves travelling down the rope. This also applies at a more fundamental level, with all the basic bits of physics. So if you shake an electron back and forth, or “accelerate it” in the jargon, it gives off waves of light. This is how a radio transmitter works for instance, giving off radio waves – which are just lightwaves of very low energy – by shaking electrons about in an aerial. A similar thing happens with the Higgs field and the Higgs boson, although in this case it was the opposite way around, getting the Higgs field to emit a Higgs boson.
Now Einstein said similar things should happen if you accelerate masses, they will give off gravity waves. So just jumping up and down should give off a source of gravity waves, as would bouncing a ball and so on.
These waves are ripples in space, a bit like ripples on a pond when you drop a pebble in. They ripple out in all directions, gently stretching and contracting space by tiny tiny amounts as they ripple across the universe.
Why can’t we detect them?
The trouble with gravity is that it is a very very very weak force. It is staggeringly weak, and no one is quite sure why, but it’s lucky for us it is. The electromagnetic force is what locks together atoms, and is responsible largely for chemistry. However a few atoms locked together in the floorboards are plenty good enough to stop the whole of the Earth’s gravity trying to pull me through the floor. In fact gravity is about 1/1000000000000000000000000000000000000th the strength of the electric force. So – while it’s quite easy to get waves from electrons and detect them (in our eyes, in radio sets etc), detecting gravity waves is vastly more challenging. The gravity waves you will give off by jumping up and down are so insignificant, we’ll probably struggle to ever detect them.
So what can we detect?
Just like if we want to take a photograph, it’s easier to take them if you have lots of light, it’s the same with gravity waves. Trying to detect people jumping up and down isn’t going to cut it, so we need something on a much grander scale. The biggest scales we can think of. We first need something that makes a huge dent in space, like a star. Then we need to violently accelerate it to get it to give off waves. This is not something we can do with our own star, the Sun, and probably just as well. The risk assessment alone would run to volumes.
However there are natural systems we know of that have huge masses accelerating. A favourite is two neutron stars orbiting closely around each other. A neutron star weighs somewhat more than our Sun, but is squashed down into a size about that of the width of Nottingham. So if you get two of these orbiting around each other, they are undergoing continual acceleration whilst making large grooves in the fabric of space. These are ideal candidates for generating gravitational waves. In fact they give off so much energy in gravitational waves, that they spiral in on each other until they collide. Indeed this is what the first gravity wave Nobel prize was for – noticing that two neutrons stars, which also happen to act like very precise clocks (just sometimes the universe gives you a break), were speeding up and getting closer at just the rate you would predict if they were emitting gravitational waves.
You can go even further with more exotic objects, such as black holes – which can be smaller and heavier still. So two black holes rotating around each other would also be a good source of them, and indeed that is what LIGO detected. In this case the frantic dance of death was so intense that the two black holes weighing 36 and 29 solar masses each, ended up combining to form a single black hole of 62 solar masses. Wait a moment, 36+29 is not 62, its 65! Yes, during this final dance of death, they managed to radiate away a colossal 3 suns worth of mass as energy in gravitational waves! So that’s why LIGO had a chance of detecting it, 3 Suns evaporated in a ⅕ of a second! Three suns worth of mass is enough energy to power the death star!
How do you detect them?
This is where LIGO comes in. What they’ve done is to make a very precise measuring device, with some fantastically clever engineering. They effectively measure very precisely a distance across the Earth of 4km in two tunnels at 90 degrees to each other. As a gravity wave passes through the detector, one arm of the detector will slightly lengthen, and the other arm slightly contract. The effect is a tiny tiny amount, much less than the width of an atom, so very precise measurements are needed using lasers. In fact even the 4km distance is too small to let this setup detect them, but by bouncing the laser beams back and forth down the tubes, up to 400 times, and then using very precise measurements comparing one beam with another, they can detect this tiny tiny ripple.
There are three such gravity wave “telescopes”, two in the US in Louisiana and Washington, and one in Germany which isn’t yet quite running – and some others in the planning stage. There are also proposed space based versions, of which eLISA was recently launched to test out the technology, although it won’t be able to detect waves until the full version is launched many years from now.
It’s not without issues though, as you can imagine, any vibration in the mirrors will swamp the signal, and anything from a truck, to a person, to geological movements, and even waves breaking on distant beaches are all enough to cause issues. So this is where the clever engineering has to come in.
What LIGO looks like in concept.
However, just one detector is not that useful. Firstly when you are looking for a tiny signal in a lot of noise, it’s good to have confirmation from another experiment that this isn’t just a random bit of noise. Second, all these detectors can tell is a ripple passed by. They can tell a lot from the ripple – such as how big the thing was and so on, but not where it is. With two detectors they can get a rough idea of where on the sky it happened. With three or more detectors you can really start to pin down just where the event happened, perhaps with enough resolution to point a telescope.
Where does it get us?
Up to now we’ve had to use mostly light in all it’s forms to study the universe. We’ve used radio, microwaves, infra-red, visible, ultra violet, x-rays and gamma rays to view it with. These are all forms of the same thing, just light with different energies. Gravitational waves are completely different, they are not light of any form. They literally open a new window onto the universe, allowing us to see gravitational events that we can’t see with light. It is hoped that in at least some cases we can bring both to bear, watch for light waves from two merging neutron stars whilst observing their gravity waves. It will be like having sound and vision at the same time, we will get a more enhanced idea of what is going on from either one alone. It’s like we grew a sixth sense.
Image courtesy of NASA
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