A Totally New Kind of Observatory
Measuring the Tiniest of Fluctuations in Spacetime (Spacetime: A concept in physics which merges our usual notion of space with our usual notion of time.)
From Galileo's first telescope to today's most sensitive neutrino (Neutrino: A type of particle which has no charge and an extremely small mass. It is a Fermion, and is extremely difficult to stop or to detect. Nonetheless, they are produced in large numbers. The Sun, for example, sends 30 million neutrinos (Neutrino: A type of particle which has no charge and an extremely small mass. It is a Fermion, and is extremely difficult to stop or to detect. Nonetheless, they are produced in large numbers. The Sun, for example, sends 30 million neutrinos through every square inch of the Earth every single second. They are so hard to stop, however, that if a neutrino were sent through a solid light year of lead, it would still have a 50:50 chance of flying right through without stopping. ) through every square inch of the Earth every single second. They are so hard to stop, however, that if a neutrino were sent through a solid light year of lead, it would still have a 50:50 chance of flying right through without stopping. ) 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 (Gravitational Wave: A gravitational disturbance that travels through space like a wave. This type of wave is analogous to an Electromagnetic Wave. Gravitational waves are given off by most movements of anything with mass. Usually, however, they are quite difficult to detect. Physicists are currently working hard to directly detect gravitational waves. Experiments like LIGO and LISA are designed for this purpose. ), 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 (Gravitational Wave: A gravitational disturbance that travels through space like a wave. This type of wave is analogous to an Electromagnetic Wave. Gravitational waves are given off by most movements of anything with mass. Usually, however, they are quite difficult to detect. Physicists are currently working hard to directly detect gravitational waves. Experiments like LIGO and LISA are designed for this purpose. ) 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 (Black Hole: A region of spacetime where the warpage of both space and time (gravity) is so intense that nothing — even light — can ever escape. Objects may fall in to the Black Hole, but once they pass the Event Horizon (Event Horizon: A surface — like the one surrounding a Black Hole — enclosing a region of space from which nothing (even light) can ever escape.), they can never escape again. Most Black Holes believed to exist are thought to be formed in the collapse of very large stars, or the collision of stars or other Black Holes. )})|blackholes}) 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 three trillion times quieter than this. To put it another way, consider the sound of an atomic blast 20 feet away. That sound (though you wouldn't be around to hear it if you were there) is as much louder than the mosquito as gravitational waves are quieter.
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 | Eardrum Movement | Source |
|---|---|---|
| 250 | 100 inches | Atomic bomb blast 20 feet away |
| 150 | 10-3 inches | Loudest rock concert |
| 130 | 10-4 inches | Threshold of pain |
| 110 | 10-5 inches | Symphony orchestra |
| 90 | 10-6 inches | Jackhammer from 6 feet |
| 70 | 10-7 inches | Vacuum cleaner from 3 feet |
| 50 | 10-8 inches | Office or restaurant inside |
| 30 | 10-9 inches | Residential area at night |
| 10 | 10-10 inches | Rustling of leaves in soft breeze |
| 0 | 3×10-11 inches | Threshold of human hearing |
| -250 | 10-23 inches | The typical sound of the Universe in gravitational waves, as measured on Earth |
Basic Interferometers
Scientists are recycling an old idea — one which was crucial in the earliest stages of Relativity theory — to try to detect gravitational waves. The Michelson interferometer (Interferometer: A scientific device which makes use of the Interference (Interference: A phenomenon which can occur any time there is any type of wave, which amounts to two waves canceling each other out. If two different waves meet at the same place, and one would hit its peak while the other would hit its trough, the waves will cancel, and there will be no disturbance. Alternatively, if both waves would hit their peaks at the same time, the waves will boost each other, so that there is a greater disturbance.) of waves — typically, light waves. This type of device can measure changes in length with extraordinary precision, and forms the basis of modern gravitational wave detectors.) is a very sensitive instrument which originally showed that the speed (Speed: For a wave, the speed of a particular point (such as its crest).) of light (Speed of Light: A constant of Nature. This speed is precisely 299,792,458 meters per second, or roughly 670,616,629 miles per hour. One of the most unusual discoveries of science has been the fact that all Observers measure light as moving at exactly this speed, even if those observers are moving relative to each other. This fact is one of the basic ingredients in Einstein's Special Theory of Relativity (Special Theory of Relativity: Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity (Special Theory of Relativity: Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity becomes very nearly the same as Newton's theory, which is much easier to use. ) becomes very nearly the same as Newton's theory, which is much easier to use. ).) is the same, no matter which way it goes. Now, that same sensitivity is being used to detect the tiny fluctuations due to gravitational waves. The detector relies on interference between light waves. 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 (Wavelength: The distance between two neighboring peaks or troughs of a wave. )}), for instance — the waves will switch between interfering constructively and destructively. That is, they will switch between producing very large waves and producing no waves at all.
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 interferometer works by splitting a single laser beam into two parts, sending them along different paths, and then recombining the two parts, allowing them to interfere with each other. In the picture above, the two separate paths of the laser are shown colored blue and red, just for illustration. If the distance along the two paths is exactly the same, the two halves of the laser beam will interfere constructively, giving the original beam back; if the distance along the paths is slightly different, the two halves can interfere destructively. Clearly, if we change the position of one of the mirrors by just half a wavelength, the two paths will have different lengths, and the final beam at the detector will change between constructive and destructive interference. With sensitive electronics, it is possible to detect very slight dimming of the final laser beam, so a shift of only a small fraction of a wavelength is noticeable. This is a very sensitive instrument.
Recall the effect of a gravitational wave. If the wave were passing directly through the interferometer from above, we can see that the path of one beam would get shorter as the other got longer, and vice versa. This is exactly the type of motion that would change the final beam between destructive and constructive interference. This trick is fundamental to the design of gravitational-wave detectors now being designed and built.
The Laser Interferometer Gravitational-Wave Observatory
An American collaboration of physicists have built and begun operating the largest and most sensitive gravitational-wave detector that currently exists: the Laser Interferometer Gravitational-wave Observatory, or LIGO for short. There are actually two totally separate LIGOs, one in Louisiana and one in Washington state. The two separate instruments are necessary because a possible signal found by one might just be noise that happens to look like a gravitational wave, but if that signal is also found by the other instrument, it's almost certainly real.
Each arm of the interferometers in LIGO is four kilometers long (almost two and a half miles long). Why so big? Well, remember that the amplitude (Amplitude: The height of the peak of a wave, measured relative to its center. Equivalently, the depth of the trough of a wave.), of a gravitational wave is the percentage of stretch or squeeze by which it distorts objects. Now, we can't change the amplitude of the waves we detect, because they're just given to us by Nature. If each interferometer arm were just one meter long, a gravitational wave of amplitude 10-23 would change the distance each half of the laser beam goes by just 10-23 meters. On the other hand, if each arm were one kilometer long, the same gravitational wave would change the distance each half of the beam goes by 10-20 meters. This is still tiny, but is much better. So, we see that the longer the arms of the interferometer are, the easier it will be to detect gravitational waves.
In fact, LIGO uses another trick to basically make its arms even longer than four kilometers: it bounces the laser back and forth hundreds of times. This is like folding up hundreds of interferometer arms into the concrete tubes of the instrument. But this trick can only get us so far. It's just not practical to make the arms too long, or to bounce the laser beams back and forth too many times; there's simply too much noise — earthquakes on the other side of the planet, passing trains, falling trees, even waves crashing on the shore hundreds of miles away. Instead, scientists are proposing to make another interferometer, which will be immune to some of the biggest problems LIGO faces.
The Laser Interferometer Space Antenna
Removed from all the noise of earth and given all the room it needs, an interferometer in space will have be able to detect gravitational waves far more sensitively than one on earth. Scientists at NASA and ESA (the European Space Agency) are designing a system they call the Laser Interferometer Space Antenna — LISA. It will consist of three separate spacecraft, separated by 5 million kilometers! The three spacecraft, basically, form the three important ends of an interferometer.
Because of the enormous distances, the laser beam won't be able to bounce back and forth between the spacecraft — it would get too dim. Instead, the beam sent from one will be detected by a second spacecraft, and delicate electronics will make a laser on that second craft perfectly match the incoming beam, and shoot it back to the first spacecraft. Still, the basic principle is the same; LISA is just an enormous interferometric gravitational-wave detector. And in fact, because it forms a complete triangle, there will be three interferometers — each with its center in a different craft — just to make sure that a failure of one won't doom the entire project.