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First observation of gravitational waves

The first direct observation of gravitational waves was on September 14, 2015.[2] The LIGO and Virgo groups announced it on February 11, 2016.[2][5] Before this, scientists had only seen the effects of gravitational waves indirectly.[2] They saw these effects in how some stars, called pulsars, moved.[2]

GW150914
Distancec. 1.4 billion ly[1]
Progenitor2 black holes[2]
Total energy output3.0+0.5
−0.5 M☉ × c2[3]
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The wave that was found came from two black holes.[2] The black holes were spiraling inward and joined together.[2] The signal was seen by both of the LIGO detectors.[6] It looked just like what general relativity said it would look like.[7][8][9] The signal was named GW150914.[10] The name comes from "Gravitational Wave" and the date it was found, 2015-09-14.[2]

This was also the first time a binary black hole (two black holes orbiting each other) was seen merging.[2] It proved that systems with two black holes of that size exist.[2] It also proved that they can merge in the lifetime of the universe.[2]

Finding the waves was a very big success.[2] Scientists had tried to find them directly for over fifty years.[11] The waves are so small that Albert Einstein did not think they would ever be found.[11] The wave from GW150914 was a ripple in spacetime.[10] It changed the length of a part of the LIGO detector by a thousandth of the width of a proton.[10][12] The energy the wave released was huge.[2] For a few moments, the power of the wave was greater than all the light from all the stars in the known universe put together.[2][13]

This discovery was the last time a prediction of general relativity was proven right after not being directly seen.[2] It began a new type of astronomy called gravitational-wave astronomy.[14] This new type of astronomy lets scientists see events in space that were impossible to see before.[15] It might let them see the very beginning of the universe.[16]

Gravitational waves

Main page: Gravitational wave

Albert Einstein first predicted gravitational waves in 1916.[11] He came up with the idea from his theory of general relativity.[11] General relativity says that gravity is caused by mass bending spacetime.[11] When masses move or speed up, they can make ripples in spacetime.[11] These ripples travel out from the source at the speed of light.[11]

Einstein thought this was interesting, but he knew the ripples would be much too small to find with the technology of his time.[11] When two objects in orbit, like stars or black holes, give off gravitational waves, they lose energy.[17] This makes them slowly spiral closer to each other.[17] This effect is also usually very small.[17]

The waves are strongest when two very dense objects, like neutron stars or black holes, merge.[18] In the last moments before they join, a large part of their mass can be changed into gravitational energy.[18] This makes the waves easier to find.[18]

How they are seen

Main page: Gravitational-wave astronomy

Indirectly

The first proof of gravitational waves was found in 1974.[19] It came from studying a pair of stars called PSR B1913+16.[19] In this pair, one star is a pulsar.[19] A pulsar sends out radio waves at very regular times.[19] Scientists Russell Hulse and Joseph Taylor saw that the time between pulses got shorter over the years.[19] This meant the stars were spiraling toward each other.[19] The amount of energy they were losing matched what Einstein's theory said would be lost to gravitational waves.[19] Hulse and Taylor won the 1993 Nobel Prize in Physics for this work.[20]

Directly

Main page: LIGO
The north arm of the LIGO detector in Hanford, Washington

Finding gravitational waves directly was hard because the effect is so small.[2] In the 1960s, a method called interferometry was suggested.[21] The technology got better, and it became possible.[21]

LIGO uses interferometers. Here is how they work: 

 * A laser beam is split in two.[22]
 * The two beams travel down long, separate paths. Then they are brought back together.[22]
 * If a gravitational wave passes by, it changes the length of the paths.[22]
 * This change causes the two light beams to no longer line up perfectly when they come back together. This creates a pattern that can be measured.[22]

This method is very sensitive.[22] An interferometer with arms that are 4 km long can find a change in spacetime that is a tiny piece of the size of a proton.[2] To be sure it is a real wave, there need to be at least two detectors far apart.[22] A real gravitational wave will be seen by both, but other shaking and noise will not.[22]

The Laser Interferometer Gravitational-Wave Observatory (LIGO) project was started in 1992.[23] LIGO has two observatories that work together.[24] One is in Livingston, Louisiana, and the other is at the Hanford Site in Washington.[24] They are 3,002 km (1,865 mi) apart.[24] The first LIGO search from 2002 to 2010 did not find any gravitational waves.[25] After that, the detectors were shut down and made much better.[26] The new, better version was called "Advanced LIGO."[26]

On September 14, 2015, the new detectors were being tested.[27] At that time, the instruments saw a possible gravitational wave.[27] This event was named GW150914.[28]

First Observation Of Gravitational Waves Media

  • Slow motion computer simulation of the black hole binary system GW150914 as seen by a nearby observer, during 0.33 s of its final inspiral, merge, and ringdown. The star field behind the black holes is being heavily distorted and appears to rotate and move, due to extreme gravitational lensing, as space-time itself is distorted and dragged around by the rotating black holes.

  • Simulation of merging black holes radiating gravitational waves

  • GW150914 announcement paper –
    click to access

References

  1. ↑ The LIGO Scientific Collaboration and The Virgo Collaboration. An improved analysis of GW150914 using a fully spin-precessing waveform model. Physical Review X 6 (4) (2016). p. 041014. doi:10.1103/PhysRevX.6.041014.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 Abbott, Benjamin P.. Observation of Gravitational Waves from a Binary Black Hole Merger. Phys. Rev. Lett. 116 (6) (2016). p. 061102. doi:10.1103/PhysRevLett.116.061102.
  3. ↑ 3.0 3.1 Abbott, Benjamin P.. Properties of the binary black hole merger GW150914. Physical Review Letters 116 (24) (2016). p. 241102. doi:10.1103/PhysRevLett.116.241102.
  4. ↑ GW150914: LIGO Detects Gravitational Waves. Black-holes.org. Retrieved 16 February 2016.[dead link]
  5. ↑ Castelvecchi, Davide. Einstein's gravitational waves found at last. Nature News (11 February 2016). doi:10.1038/nature.2016.19361. Retrieved 11 February 2016.
  6. ↑ "Einstein's gravitational waves 'seen' from black holes". BBC News. 11 February 2016. https://www.bbc.co.uk/news/science-environment-35524440(https://www.bbc.co.uk/news/science-environment-35524440). 
  7. ↑ Pretorius, Frans. Evolution of Binary Black-Hole Spacetimes. Physical Review Letters 95 (12) (2005). p. 121101. doi:10.1103/PhysRevLett.95.121101.
  8. ↑ Campanelli, M.. Accurate Evolutions of Orbiting Black-Hole Binaries without Excision. Physical Review Letters 96 (11) (2006). p. 111101. doi:10.1103/PhysRevLett.96.111101.
  9. ↑ Baker, John G.. Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes. Physical Review Letters 96 (11) (2006). p. 111102. doi:10.1103/PhysRevLett.96.111102.
  10. ↑ 10.0 10.1 10.2 Naeye, Robert (11 February 2016). "Gravitational Wave Detection Heralds New Era of Science". Sky and Telescope. http://www.skyandtelescope.com/astronomy-news/gravitational-wave-detection-heralds-new-era-of-science-0211201644/(http://www.skyandtelescope.com/astronomy-news/gravitational-wave-detection-heralds-new-era-of-science-0211201644/). Retrieved 11 February 2016. 
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 11.7 Blum, Alexander. The long road towards evidence. Max Planck Society (12 February 2016). Retrieved 15 February 2016.[dead link]
  12. ↑ Radford, Tim (11 February 2016). Gravitational waves: breakthrough discovery after a century of expectation. https://www.theguardian.com/science/2016/feb/11/gravitational-waves-discovery-hailed-as-breakthrough-of-the-century(https://www.theguardian.com/science/2016/feb/11/gravitational-waves-discovery-hailed-as-breakthrough-of-the-century). Retrieved 19 February 2016. 
  13. ↑ Harwood, W.. Einstein was right: Scientists detect gravitational waves in breakthrough. CBS News (11 February 2016). Retrieved 12 February 2016.
  14. ↑ [1](https://edition.cnn.com/2016/02/12/opinions/gravity-wave-team-conversation/) CNN quoting Prof. Martin Hendry (University of Glasgow, LIGO) – "Detecting gravitational waves will help us to probe the most extreme corners of the cosmos – the event horizon of a black hole, the innermost heart of a supernova, the internal structure of a neutron star: regions that are completely inaccessible to electromagnetic telescopes."
  15. ↑ Abbott, Benjamin P.. Astrophysical implications of the binary black-hole merger GW150914. The Astrophysical Journal 818 (2) (20 February 2016). p. L22. doi:10.3847/2041-8205/818/2/L22.
  16. ↑ Ghosh, Pallab (11 February 2016). "Einstein's gravitational waves 'seen' from black holes". BBC News. https://www.bbc.com/news/science-environment-35524440(https://www.bbc.com/news/science-environment-35524440). Retrieved 19 February 2016. "With gravitational waves, we do expect eventually to see the Big Bang itself.". [dead link]
  17. ↑ 17.0 17.1 17.2 Schutz, Bernard. A First Course in General Relativity (31 May 2009)Cambridge University Press. p. [2](https://archive.org/details/firstcourseingen00bern_0/page/234) 234, 241. ISBN 978-0-521-88705-2.
  18. ↑ 18.0 18.1 18.2 Commissariat, Tushna; Harris, Margaret (11 February 2016). "LIGO detects first ever gravitational waves – from two merging black holes". Physics World. http://physicsworld.com/cws/article/news/2016/feb/11/ligo-detects-first-ever-gravitational-waves-from-two-merging-black-holes(http://physicsworld.com/cws/article/news/2016/feb/11/ligo-detects-first-ever-gravitational-waves-from-two-merging-black-holes). Retrieved 19 February 2016. 
  19. ↑ 19.0 19.1 19.2 19.3 19.4 19.5 19.6 Weisberg, J. M.. Gravitational waves from an orbiting pulsar. Scientific American 245 (4) (October 1981). p. 74–82. doi:10.1038/scientificamerican1081-74.
  20. ↑ Press Release: The Nobel Prize in Physics 1993 (13 October 1993)Nobel Prize. Retrieved 6 May 2014.
  21. ↑ 21.0 21.1 Baker, John G.. Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes. Physical Review Letters 96 (11) (2006). p. 111102. doi:10.1103/PhysRevLett.96.111102.
  22. ↑ 22.0 22.1 22.2 22.3 22.4 22.5 22.6 Staats, Kai. Detecting Ripples in Space-Time, with a Little Help from Einstein. Space.com (8 August 2015). Retrieved 16 February 2016.[dead link]
  23. ↑ LIGO Scientific Collaboration – FAQ. Retrieved 16 February 2016.
  24. ↑ 24.0 24.1 24.2 LIGO Hanford's H1 Achieves Two-Hour Full Lock (February 2015). Retrieved 11 February 2016.
  25. ↑ Gravitational wave detection a step closer with Advanced LIGOSPIE Newsroom. Retrieved 4 January 2016.
  26. ↑ 26.0 26.1 Gravitational wave detection a step closer with Advanced LIGOSPIE Newsroom. Retrieved 4 January 2016.
  27. ↑ 27.0 27.1 Castelvecchi, Davide. Gravitational waves: How LIGO forged the path to victory. Nature 530 (7590) (16 February 2016). p. 261–262. doi:10.1038/530261a.
  28. ↑ Castelvecchi, Davide (12 January 2016). "Gravitational-wave rumours in overdrive". Nature News. doi:10.1038/nature.2016.19161 . http://www.nature.com/news/gravitational-wave-rumours-in-overdrive-1.19161(http://www.nature.com/news/gravitational-wave-rumours-in-overdrive-1.19161). Retrieved 11 February 2016. 
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