Date: November 1, 2009
Title: Listening to Gravitational Waves: A New Window on the Universe
Podcaster: Gareth Jones
Organization: Gravitational Physics Group, Cardiff School of Physics and Astronomy
http://www.astro.cf.ac.uk/research/gravity/
Description: In this podcast we provide a quick introduction to gravitational waves. A prediction of Einstein’s theory of General Relativity, gravitational waves are generated by the acceleration of massive objects that cause ripples in the warping of spacetime that travel at the speed of light. Gravitational waves will provide us another way to view the universe, allowing us to directly observe black holes and look deep into the heart of exciting phenomena such as supernovae and gamma-ray bursts, beyond what is possible with traditional telescopes. We will describe the world-wide efforts to detect these elusive signals, discuss astronomical sources of gravitational waves including the violent collision of binary black hole systems and give listeners a chance to “hear” the gravitational wave signals for themselves!
Bio: Gareth Jones is a research assistant within the Gravitational Physics Group and a member of the LIGO (Laser Interferometric Gravitational-wave Observatory) Scientific Collaboration. He has published papers on the search for spinning black hole binaries and data analysis techniques for the proposed space-based gravitational wave detector LISA (Laser Interferometer Space Antenna). He is currently looking for gravitational wave signals associated with gamma-ray bursts.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the American Association of Variable Star Observers, the world’s leader in variable star data and information, bringing professional and amateur astronomers together to observe and analyze variable stars, and promoting research and education using variable star data. Visit the AAVSO on the web at www.aavso.org.
Transcript:
===== Gravitational Waves – a new window on the Universe ========
(Script by Gareth Jones, read by James Clark and Ian Harry, all of Cardiff University)
A: Hello, my name is James Clark, I’m a researcher at Cardiff University and in this podcast I am going to talk about the search for gravitational waves.
You are probably familiar with the idea of gravity as the force which keeps us on the Earth, the planets orbiting the sun and the sun orbiting about the galaxy and so on but you may not be as familiar with the idea of gravitational waves.
B: So what are they?
A: I find the easiest way to picture gravitational waves is to imagine that space – and time – is a large sheet of rubber. On that sheet imagine different sized marbles, these are planets and stars, asteroids, people, anything you want. Each marble causes the a dip in the rubber sheet. The heavier the marble, the larger the dip in the rubber sheet. We can imagine one large marble, put some smaller marbles nearby and they will roll down the dip caused by the first marble. This is an analogy of how gravity keeps us on the Earth.
B: The Earth is the large marble, we are the small marbles
A: Exactly, and the dip in the rubber mat corresponds to curvature of space and time which we call gravity. We can imagine an even larger marble to represent the sun. If the earth-marble has enough speed, rather than fall into the sun-marble it will orbit around and around it.
B: So that is a nice description of gravity but where are the waves?
A: Okay, now imagine we toss a marble onto the rubber sheet or roll it really fast along the rubber sheet. It will create ripples on the rubber sheet which will move outwards from the marble – like ripples on a lake. These ripples represent gravitational waves are caused when ever anything with mass – people, stars, planets accelerate.
B: Okay, gravity keeps us on the Earth and the Earth orbiting the sun but what do gravitational waves do?
A: When a gravitational wave passes through something it causes its shape to change. If a gravitational wave passed through a book say, it would get taller and narrower at first, then it would get shorter and wider and would keep doing this until the wave passed.
B: Right, it causes things to stretch in one direction and get compressed in another direction and then the other way round. I can imagine that, but I don’t see it happening much – why not?
A: The reason we do not see these effects in everyday life is because space and time, that’s the rubber sheet in our analogy, is very stiff. So that even some of the most cataclysmic events in the universe, such as super-massive black holes colliding only cause very tiny ripples in the space-time. Even very massive objects – like stars and black holes – need to be moving incredibly fast to create gravitational waves that can be detected using modern equipment.
B: Alright, so you are saying that even though I can create a gravitational wave by waving my arms around that it’s too small for us to even notice?
A: Yup, to detect gravitational waves caused by something huge, like a star exploding in our galaxy we still need to construct amazingly sensitive instruments. I work with detectors called laser interferometers.
B: So, to detect gravitational waves you cannot use normal telescopes, you need your own detectors?
A: That’s right, there are a handful currently working, GEO600, a 600m long detector situated near Hannover in Germany, Virgo, a 3km long detector near Pisa, in Italy and LIGO which consists of three detectors across two sites in the United States of America: One site is in the Washington desert where there are 2 detectors, one of which is 4 kilometers long and another which is 2 kilometers long. The other site is in the Louisiana swamps where there is a single 4km long detector.
Essentially, these act as really sensitive rulers allowing us to measure the stretching and compressing caused by gravitational waves, we can measure changes in length less than a billionth of a diameter of an atom.
B: Wow, that’s tiny but it sounds pretty difficult. Why is it so important?
A: There are many reasons why we should try and detect gravitational waves. Firstly, they are predicted by Einstein’s theory of General Relativity, detecting gravitational waves would provide more evidence that his theory is correct.
All our information from about the distant universe comes from observation of electromagnetic radiation, that’s light, radio waves, x-rays and from detecting cosmic particles, like neutrinos. That’s like watching the television with the sound down. It’s still interesting, you can learn a lot but you don’t get the whole story. Observing gravitational waves be like suddenly turning up the sound, you get the whole story, a fuller picture of the universe.
B: Give me an example
A: Okay, right now we believe that there are super massive black holes in the centre of most galaxies including our own galaxy, the milky way. We know this by tracking the positions of stars by observing the light they emit and from this we can infer there is a something massive they are orbiting around.
B: Right
A: Using gravitational waves we could directly measure the presence of the black hole and estimate its mass and how fast it is spinning.
B: What else?
A: Well, we can help clear up some mysteries that have been troubling astronomers for a while now. We can help figure out what is behind gamma-ray bursts, the most violent explosions in the universe since the big bang itself. In addition, gravitational wave observations may prove crucial in pinning down the nature of dark energy.
B: Well, that sounds cool. So how is all this going?
A: Currently, gravitational wave astronomy is in its early days. The detectors, that is the laser interferometers I mentioned before have been built and have recently reached their design sensitivity. We haven’t made any detections yet but with sensitivity improving every year and with plans to launch a detector called LISA into space the future looks bright!
B: Okay, I think I know what gravitational waves are and I’m beginning to see why you want to detect them, how exactly do you help in all this?
A: Ha, well the output from the detectors is noisy, things like earthquakes, variations in the laser power and even passing cars can help hide a gravitational wave signal. If we turn our data into a sound wave it sounds a bit like this:
[Sound of detector noise]
B: Okay sounds, like radio static
A: If we take a gravitational wave signal, say from two black holes orbiting each other they sound like this:
[Sound of inspiral chirp waveform]
B: Sounds funny!
A: We call these chirps as they sound a bit like birds. As the black holes orbit they emit gravitational waves they lose energy and fall in closer and closer together. As they get closer they orbit faster and the gravitational wave get more frequent and louder, that’s why the sound I played gets higher pitched and louder.
B: Okay, that makes sense. What exactly do you do again?
A: Well, our data is noisy and we are trying to pick our a quiet signal from that noise – a bit like trying to hear someone whispering in a noisy room. I write computer code that looks at the data and tries to pick the quiet gravitational wave signal out of the noise.
B: Okay, we’re nearly out of time. This sounds pretty interesting, can I do anything to help out?
A: Well, there sure is. You can get involved by visiting www.Einsteinathome.org, where you can download a screensaver so your computer can help out searching to gravitational waves when you’re not using it.
As well as helping our own efforts, you can play our online game, download gravitational wave ringtones and learn more about gravitational wave astronomy at www.Blackholehunter.org
A: One more thing before we go, remember the sound of the gravitational waves from orbiting black holes we played you earlier?
[Sound of inspiral chirp]
We’ve stitched some of these together to make ringtones for you mobile phone all of which are all available at www.blackholehunter.org – here’s what they sound like!
B: Thanks for listening!
[Ringtones play]
End of podcast:
365 Days of Astronomy
=====================
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