Universe is a Jungle and Gravitational Waves are sounds of the animals in it
Close your eyes and imagine you are in a helicopter flying over a very big beautiful jungle in the heart of Africa. Open your eyes now! What you see is like the following pictures: a lot of trees and plants which you see them as a big, green picture and call it “jungle”!
What you see from your helicopter above a huge jungle in Africa
The ground is so covered by trees and plants such that there is no way to see the animals who are living in this jungle from your helicopter flying ~500m above the ground. You also can’t hear their voices. Even though you do believe that lions, tigers, elephants, monkeys, and etc are living down there and make some sounds some times naturally, based of what Mr. Einstein have told you. You have never seen these animals before but according to Mr. Einstein, they should sound like followings: tigers [listen], lions [listen], elephants [listen], monkeys [listen].
There are some animals under the jungle trees that there is no way to see them from your helicopter, you just can hear their voices
However you can not see the lions in the jungle because they have hidden by many trees, but you are able to see if some birds are flying around your helicopter or even very far away, but above the trees.
You are able to see if some birds are flying around your helicopter or even far away, but above the jungle trees.
Unwanted sounds from your helicopter: noises!
Unfortunately, you can not hear any animal voice neither birds nor lions. Noise of your helicopter engine [listen] is the only sound that you can hear (however you probably can hear your friend beside you when he is shouting in your ear). With the assumption of a completely silent helicopter, not all the animals have strong voices which can reach you at the helicopter. Even if the animals make strong enough voices, in reality, what you hear is helicopter noise plus an extremely weak voice of an animal. Therefore it would be so difficult to recognize the animal voice in presence of such a high level of noise. In the best circumstances, you will need a super-duper high-tech artificial ears to filter out animal voice from helicopter noise. To do this, indeed, it will be required to know how does the animal voice that you are looking for look like. You can not use voice of monkey and look for voice of lion in the data! Fortunately, by experience, we do know what does the voice of a typical lion look like, however the African lion voice might be a little different. But it doesn’t matter that much, it perfectly works for our purpose.
In fact, voice, or basically sound wave, is not more than some simple mechanical waves in the air. When you speak, your vocal cords shake the air around and make some mechanical waves in air which can be heard by your friend’s ears. Waves, in all forms, need an environment to travel through. If you speak in vacuum [suppose your body doesn’t implode in vacuum!] nobody can hear you. Gravitational waves, in the other hand, are some kind of waves which are produced by moving super-massive sky-objects like black holes, and neutron stars. Instead of air in the case of sound waves, gravitational waves travel trough the fabric of spacetime and affect masses in the field. Gravitational waves are prediction of Einstein’s general theory of relativity. If strong gravitational waves go through your body, you will experience a situation like the following picture. Although, effects are super exaggerated in this picture.
When strong gravitational waves go through your body. (different polarization)
When you speak your vocal cords shake the air around and make sound waves
Getting back to the analogy of Gravitational Waves
Above described circumstances in jungle is very much similar to what we study in the field of gravitational waves. Universe is the jungle, and Earth is your helicopter in this case. You can look at the sky and are able to see some celestial objects like planets, stars, comets and etc with naked eye or even with modern telescopes. They are the birds above the trees that we can see them but can’t hear them. But this is not everything which exists in the jungle. There are some animals hidden by trees that there is no way to see them even with modern telescopes [Gravitational Wave Sources]. The only way to detect them is listening to their voices [Gravitational Wave Signals]; however they are extremely weak compared to helicopter noise [Detector Noise]. You need to use your super-duper high-tech artificial ears [Gravitational Wave’s Detectors] to get some data, which is basically background noise plus some signal. People use Matched Filtering methods to filter out the signal from data. Post-Newtonian theory, for example, is your knowledge about the voice of a typical lion and if you want to know exactly the model of African lion voice you should use Numerical Relativity.
Universe as a jungle
Celestial objects as jungle animals including birds: stars, supernovas, galaxies, and lions [can’t see but can hear]: black holes and neutron star binaries
We actually use Post-Newtonian formalism to study the sound of a particular source: two angry lions fighting! [compact binary systems, e.g. two extremely massive black holes orbiting each other]. This is the most promising source of gravitational waves that ground current detectors on the earth can detect. One of our motivations to study this system is to find out if Mr. Einstein was right [2]. Gravitational wave astronomy will open a new window to the universe. In addition to the animals that we expect to detect their voices, some strange creatures, might be heard by this new generation of astronomy that we never expect. Somebody may has been “LOST” in spacetime from many years ago. Who knows?
We might hear voice of strange creatures (like super-massive BHs) in the jungle (universe) that we have never been aware of them before Gravitational Waves Astronomy
A good source: two angry lions fighting! [compact binaries]
Imagine in 3014, when kids will be learning Quantum Mechanics at kindergartens, they will learn the sounds of cosmic creatures at elementary schools. Latest modelings show that the voice of some sky animals are expected to be as followings.
Tigers: Equal mass binary gravitational waves: Two black holes, each of 50 solar masses [Listen]
Lions: Extreme mass ratio binary gravitational waves: Initially circular orbit, into rapidly spinning BHs [Listen]
Elephants: Extreme mass ratio binary gravitational waves: Initially circular orbit, into slowly spinning BHs [Listen]
Monkeys: Extreme mass ratio binary : Initially highly eccentric orbit, into rapidly spinning BH [Listen]
Next generation of astronomers will “listen” to the sky instead of looking. Photo: My imaginary son which is an old fashion astronomer at right, and his son which is a modern astronomer at left, looking and listening to the sky from MIR-III space station. ©Photo by: my lovely great-granddaughter, who is not an astronomer but a poet.
References
[1] The title is borrowed from a Bernard Shutz's talk, "Gravitational Waves: Listening to the Music of the Spheres", public talk at Washington University in St. Louis, 2010.
[2] Title of a book by C.M. Will, “Was Einstein Right?”, 2nd Edition, Basic Books, New York, 1993.
