Gravitational waves are ripples in the curvature of spacetime which propagate as waves outward from the source. Their existence was first proposed by Henri Poincaré in 1905 and subsequently predicted in 1916 by Albert Einstein on the basis of his general theory of relativity. Gravitational waves cannot exist in Newtonian gravity, since in that theory gravitational interactions propagate instantaneously.
On 11 February 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration announced the first direct detection of gravitational waves, emitted from a pair of merging black holes over one billion years ago. This discovery was made possible by advances in laser interferometry technology pioneered over many decades by scientists and engineers worldwide.
The LIGO detectors are located at Hanford, Washington, USA and Livingston, Louisiana, USA; each consists of two 4 km long arms forming an “L” shape. Laser light is bounced back and forth down these arms; when a gravitational wave passes by, it stretches one arm while compressing the other causing a very slight change in the length of each arm which is detected as a change in the interference pattern between beams reflected back up the arms to be recombined at their point of origin. By precisely measuring this change using extremely sensitive laser interferometers LIGO can infer the presence of a passing gravitational wave. The signal seen by LIGO on 14 September 2015 lasted for about 0.2 seconds and corresponded to two black holes coalescing into one; during this time 3 solar masses were converted into energy equivalent to that radiated by about 2 suns worth of x-rays over its entire lifetime!
The Advanced LIGO detectors are now regularly making measurements with unprecedented sensitivity; they are currently observing at frequencies around 100 Hz with plans to operate at even higher frequencies in future years. In addition to searching for signals from binary black hole mergers like that seen on 14 September 2015, Advanced LIGO will also be sensitive to signals from binary neutron star mergers which are thought to be much more common than black hole binaries. These events release large amounts of electromagnetic radiation across the spectrum as well as gravitational radiation; because both types of signals arrive at Earth simultaneously they can be used to study each other and improve our understanding of astrophysical phenomena such as gamma ray bursts.
Gravitational waves were first proposed theoretically by Henri Poincaré in 1905 and subsequently predicted in 1916 by Albert Einstein on the basis of his newly developed general theory of relativity. In 1978 Ruslan Adrenov suggested that if there existed any sources for detectable amounts of GW emission within our galaxy then Doppler tracking might enable their detection via very long baseline radio interferometry. However it was Joseph Weber’s claimed detections beginning in 1969, quickly followed publications purporting null results, which finally led most scientists to conclude that GWs could not be reliably detected with ground-based instrumentation – until recently!.
Observations made using Laser Interferometer Gravitational-Wave Observatory (LIGO), have shown that GWs do indeed exist! On September 14th 2015 at 09:50:45 UTCthe twin detectors observed a transient GW signal producedby two colliding black holes 1.3 billion light years away. One detector locatedin Hanford WA (USA) recorded a strain amplitudeof 1×10−21while its sister detector situated 1800 miles awayin Livingston LA (USA) measured 10% lessat 9×10−22.:7 This kind offaithful replication allowed scientists toreconstruct not only propertiesaboutGWsand their sources but also test variousaspects aboutEinstein’s theory itself! :2–4
Figure 1: The strain amplitude vs time recording(left panel) from Livingston LA alongwiththe corresponding Fast Fourier Transform(right panel).:7 From this we seea clear chirp signal consistent with twocoalescingblack holes.#x25cf#x25cf