From Galileo's first telescope to today's most sensitive neutrino telescopes, astronomers have been developing new eyes with which to see the night sky, allowing them to discover new worlds while better understanding our own. Now, for the first time, astronomers are creating new ears with which to hear the Universe around us.
The sounds we hear in our ears are carried through the air around us. Anything giving off sound gives the air more pressure, then less pressure. These changes in pressure travel as waves, until they reach our ears and push on our eardrums. The waves don't move our heads very much, but they move our eardrums, which allows the delicate mechanisms in our ears to pick up these movements relative to our heads.
Since sound needs air (or some other matter) to compress, sound can't travel through empty space. Gravitational waves, on the other hand, don't need air to travel; they just need spacetime. They travel across the Universe from its deepest reaches, never stopping or slowing down—regardless of the presence or absence of air. Nonetheless, they have a similar effect on our ears. As a gravitational wave passes through your head, the positions of your eardrums change relative to the position of your head. Again, the delicate mechanisms in your ears would pick up these movements, and your brain would turn them into sounds. But why aren't we kept up at night with the noise from black holes everywhere falling into each other?
It turns out that, by the time gravitational waves from these distant sources reach us, they are incredibly quiet. The smallest sound that a human with good ears can hear is roughly the sound of a mosquito buzzing 10 feet away. Gravitational waves reaching the Earth are typically another thousand billion billion times quieter than this. So how can physicists hope to hear such amazingly small sounds? There are three main tricks: Use a really sensitive microphone, make that microphone enormous, and keep everything really quiet.
| Decibel level |
Movement of eardrum |
Source |
| 180 | 10-3 feet | Rocket from 100 feet |
| 150 | | Jet engine from 100 feet |
| 130 | | Threshold of pain |
| 120 | 10-6 feet | Rock concert |
| 110 | | Chainsaw |
| 100 | 10-7 feet | Jackhammer from 6 feet |
| 80 | 10-8 feet | Vacuum cleaner from 3 feet |
| 60 | 10-9 feet | Office or restaurant inside |
| 40 | 10-10 feet | Residential area at night |
| 30 | | Theater, no talking |
| 20 | 10-11 feet | Rustling of leaves |
| 10 | | Human breathing from 10 feet |
| 0 | 10-12 feet | Threshold of hearing (human with good ears) |
| -220 | 10-23 feet | The typical sound of the Universe in
gravitational waves on Earth |
Basic Interferometers
A wave is a disturbance which brings some "medium" higher or lower than that medium would be without any waves. For example, a wave in water has the water as its medium, and it carries the water higher or lower than the undisturbed water. Now, it might happen that two different waves are traveling along the medium and meet. They might try to move the medium in different ways. When one wave tries to bring the medium higher, and the other tries to bring the medium lower, they cancel each other out, and there is simply no change to the medium. This is called "destructive interference." On the other hand, the waves may try to change the medium in the same way. In this case, they will add to each other, and disturb the medium more than either wave could manage alone. This is called "constructive interference."
Whether the waves are interfering constructively or destructively depends on whether their peaks match up at the same place at the same time, or not. If we imagine keeping one wave in place, and shifting the other by just a little bit—half a Wavelength, for instance—the waves will switch between interfering constructively and destructively.
[Some picture of waves matching up, and interfering constructively; another picture of waves clashing, and interfering destructively.]
This allows us to use a very clever trick to measure distances very precisely. It turns out that half a wavelength of light is roughly one one-hundred-thousandth (1/100,000) of an inch. By "moving" a light wave by just this tiny distance, we could see its interference change from completely constructive to completely destructive. A light wave can be moved by bouncing it off of a mirror, and moving the mirror. Thus, we could set it up so that light bounces off a mirror and interferes with another light wave. Moving the mirror by these tiny distances we would see the total light wave would go from light to dark. One clever instrument to accomplish this is an Interferometer.
The Laser Interferometer Gravitational Wave Observatory
[enormous microphone, fairly quiet]
The Laser Interferometer Space Antenna
[more enormous microphone, really quiet]
