Genre: Information/Explanation

Nearly everything we know about the universe comes from electromagnetic radiation – that is , light . Astronomy began with visible light and then expanded to the rest of the electromagnetic spectrum . By using the spectrum , from the short wavelengths of gamma rays to the long wavelengths of radio waves , astronomers have discovered strange and wondrous things . Now a new form of astronomy , gravitational-wave astronomy , has come . It watches not light but movements in space-time . Along with the new astronomy has come a new kind of observatory , the Laser Interferometer Gravitational-Wave Observatory ( LIGO ) . 6 Gravitational Waves Are Ripples in Space-Time According to Albert Einstein's theory of general relativity , gravity is not a force reaching out through the universe . It 's a bending of space-time . When an object accelerates , it distorts the space-time around it , and that distortion travels away from the source at the speed of light . 5 Gravitational Waves Come from Really Heavy Objects So how massive an object are we talking about ? The first proof that gravitational waves actually exist came from a binary pulsar – two neutron stars , each about the mass of the Sun , that orbit each other . The pulsars' orbit is gradually shrinking , so the pulsars are losing energy . That energy is exactly the amount that general relativity predicts that the pulsars would give off in gravitational waves . 4 The Effect of Gravitational Waves Is Very , Very Small Since gravitational waves are a ripple in space-time , they cause the distance between two points to change ever so slightly . How slightly ? LIGO must be able to measure distances as small as 10 − 19 meter . The proton has a radius of about 0.85 × 10 − 15 meter , or 10,000 times larger . 3 Measuring Gravitational Waves Is Tricky To detect a change in distance much smaller than the proton requires great precision . Each LIGO installation is a laser interferometer made up of two underground pipes , each 1.3 meters ( 4 . 3 feet ) wide and 4 km ( 2 . 5 miles ) long , set in an L-shape . The inside of the pipes is a vacuum . When a gravitational wave passes through LIGO , one arm of the instrument gets longer and the other gets shorter . A laser beam is split in half , sent down the two pipes , reflected back , and then recombined so the two beams cancel each other out in destructive interference if there is no gravitational wave . If there is a gravitational wave , the beams wo n't cancel each other out . A 4-km long beam is still not enough to detect a gravitational wave , so the beams are bounced back and forth about 400 times so the light travels a distance of 1,600 km ( 1 , 000 miles ) . 2 LIGO Is Very Sensitive LIGO detects such a small change in distance that it can detect a lot of other vibrations too . For example , the speed limit at LIGO is 16 km ( 10 miles ) per hour to minimize vibrations from nearby cars . One source of noise is gravity gradient noise , which is the minute change in Earth's gravitational field when a vibration passes through the ground near the mirrors . The mirrors that reflect the light weigh 40 kg ( 88 pounds ) and hang by silica fibers in a complex suspension system . To make sure that LIGO does detect gravitational waves and not just passing cars , there are two LIGO installations – one in Livingston , Louisiana , and the other in Hanford , Washington . A gravitational wave would show up at both installations . 1 Gravitational-Wave Astronomy Can See an Entirely New Side of the Universe If supermassive black holes ( black holes one million times more massive than the Sun ) merged in a distant galaxy , LIGO could observe it . Scientists also expect that if a neutron star is slightly nonspherical , the gravitational waves could be observed and thus reveal much about the star's structure . Every time astronomers have been able to look at the universe in a new way , they 've always observed something unexpected , and gravitational-wave astronomy will likely show something not yet thought of .