Gravitational waves were first predicted by Einstein in 1916 as part of his general theory of relativity, but their detection continues to elude scientists. This is not for lack of effort; many experiments have attempted to detect this enigmatic form of radiation. So, what are these mysterious waves, how do we search for them, and why do we believe they're even real? Read on to find out!
What are gravitational waves, anyway?
When electrically-charged particles (like electrons or protons) accelerate, decelerate, or change directions, they emit rays of light. Similarly, when objects that have mass accelerate in certain ways, gravitational waves are emitted. Imagine space as a big sheet of fabric. All objects - planets, stars, galaxies - sit within it. Each object makes a dimple in the fabric, and the more mass an object has, the deeper the dimple. If you think of a bowling ball and a golf ball sitting on your bed, the bowling ball will make a bigger indent in the sheets. When objects move through space, they can cause ripples in this fabric that travel outward. These ripples are gravitational waves.
How do we detect gravitational waves?
Gravitational wave detectors attempt to measure the little distortions in the curvature of space caused by a passing gravitational wave. The Laser Interferometer Gravitational-Wave Observatory (LIGO), attempted to detect them by using laser beams. Each LIGO site has two 4-km tubes that sit at a right angle to each other and have mirrors at both ends. A laser beam is generated, then split into two beams. One beam is sent into each tube, where it bounces back and forth between the mirrors. If a gravitational wave passes through a beam, the distance the beam travels between the mirrors will be altered slightly. When the beams are recombined, scientists can determine if one or both have experienced a change in their path length. There are three of these sites, in order to allow scientists to triangulate any signal they detect and find out what direction it came from. Unfortunately, LIGO (which operated from 2002 to 2010), did not detect any gravitational waves. This does not necessarily mean that they're not out there, just that their signal is too faint for our technology to pick up on.
So we haven't found them yet. Do we have any evidence for their existence?
Though we have not directly detected gravitational waves, we do have some evidence for their existence from the Hulse-Taylor binary system. A binary system is any system containing two objects that orbit around a shared point in between the two. The Hulse-Taylor system consists of a neutron star (an extremely dense core left behind after the deaths of some stars), and a pulsar (a rapidly rotating neutron star whose emitted light we observe in pulses). Scientists predict that, in binary systems like these, as gravitational waves are emitted, the objects should start to shrink their orbits so they get closer to this central point, eventually merging in the center. Astronomers have been able to observe this so-called "orbital decay" in the Hulse-Taylor binary, giving us indirect evidence for the existence of gravitational waves.
Is there hope for detecting them in the future?
The future is bright for gravitational waves! LIGO is getting an upgrade, which will improve the sensitivity of LIGO's instruments by a factor of ten. Advanced LIGO should begin operations during 2014. Additionally, another LIGO site is planned to be built in India, which will improve localization of detected signals.
Farther down the road, the European Space Agency is planning to launch the Evolved Laser Interferometer Space Antenna (eLISA). You can think of eLISA as a space-based version of LIGO. It will consist of three spacecraft orbiting the sun in a triangle formation. Lasers will connect the three spacecraft, and if a gravitational wave passes through, scientists will detect a slight change in the distance between the spacecraft. The advantage to doing this in space is that you can put a much bigger distance between the spacecraft, which makes the instruments sensitive to a much wider range of gravitational waves. eLISA's spacecraft will be separated by 1 million kilometers (much bigger than the 4 kilometer separation of LIGO's mirrors). eLISA is tentatively scheduled for launch in 2034, but the LISA Pathfinder mission, a small mission that will test the technology necessary for LISA, is scheduled to launch in 2015.
Have more questions about gravitational waves? Ask in the comments!