A Detail of LIGO
What is Gravitational Wave？
Before discussing gravitational waves, we should recall the background raised by Einstein: general relativity. John Wheeler said in his book, Geons, Black Holes, and Quantum Foam: "matter tells Spacetime how to curve, and Spacetime tells matter how to move.” This is a very ingenious description on general relativity. Scientific magazines and websites describe gravitational waves as “ripples in space-time”. The bigger the mass is, according to general relativity, larger the twist it would bring to space-time. While such big mass as a binary neutron star system, the ripples they bring to space-time appears to be relatively big and easier to detect for us, we distant and curious human beings.
Why is it so hard to detect?
It took us such a long time to finally detect gravitational waves because it is so trivial. The intensity of gravitational waves is h0 =40×G×I×f^2×e/（c^4*D), which leaves us “1e-23” scale due to G (gravitation constant~6.67e-11) and c (speed of light~3e8) in the equation. Even if the source is binary neutron star system with enormous mass and large density, it is still too small for scientists to detect. Such small signal could be easily interfered by other sources. For instance, even a truck passing by an ordinary detector could lead to errors.
How did we solve this?
To eliminate errors brought by motorcycles, sea waves, rock music players…etc., Scientists built two giant distant detectors, LIGO (Laser Interferometer Gravitational-Wave Observatory). One in Hanford Washington and the other in Livingston, Louisiana — operated in unison as a single observatory. Putting multiple observatories apart ensure one thing: if they receive a unique signal at the same time, that signal must came from distant universe. The basic idea of LIGO can be simply understood in this way: four test masses are suspended from the ceiling, a monochrome laser with fixed frequency emitted and divided into two groups with equal intensity on the spectrometer, a beam reflected through the spectrometer into the Y arm of the interferometer, and another beam pass through the spectrometer into another X arm perpendicular to it. After the same time, two beams of light reflected back and meet again on the spectrometer to produce interference. By adjusting the length of the X and Y arms, we are able to control the two beams to ensure there is no light signal on the photodiode. If a gravitational wave enters from the direction perpendicular to the ceiling, one of the arms is elongated and the other is compressed, so that two beams of light changes, the original coherent phase elimination condition is destroyed, and a certain amount of light will enter the detector to create gravitational wave signal.
What applications?
The reason why people are so excited about the detection is that, this accomplishment opened another pair of “eyes” for human. The discovery of ripples in space-time has vindicated Einstein’s prediction — but it can also do so much more. Before the release of black hole’s image in April 10th, detection of gravitational waves confirms that black hole does exist-- at least as the perfectly round objects made of pure, empty, warped space-time that are predicted by general relativity. Also, it answers the question whether neutron stars are rugged. As we all know, neutron stars collapsed from bigger stars, having surprisingly big density. A rapidly rotating giant mass with giant density is supposed to be a very smooth, nearly perfect ball. But some scientists theorized there are tiny mountains on it. Those asymmetric distribution of mass would deform space-time and produce a continuous gravitational-wave signal in the shape of a sine wave, which would radiate energy and slow down the star’s spin. We now could figure out whether neutron stars are perfectly spherical through detection of gravitational waves. Detection of gravitational waves also answers the question: how fast does our universe expand? If several gravitational-wave detectors across the world (like VIRGO in Europe and KAGRA in Japan) detect signals from the same neutron-star merger, together they will be able to provide an estimate of the absolute loudness of the signal, which will reveal how far away the merger occurred. They will also be able to estimate the direction it came from. Comparing that galaxy’s redshift, those data would offer a more accurate comparison. Besides those applications, the detection of gravitational waves give supports, or answers more science questions like whether gravitational waves travels at the speed of light (slower if gravitons have mass); what make stars explode; relation between cosmic strings and space-time…