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Gravitational Waves WARNING!!!!

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Gravitational Waves WARNING!!!! Gravitational Waves WARNING!!!! ⚫ Terminology is treacherous! ⚫ There are gravitational waves (our topic) and there are gravity waves (a topic for a surfing class). Mix them to your peril. Gravitational waves ⚫ Movie The History ⚫ The history of gravitational waves is rocky. Einstein argued in 1916 that gravitational waves must exist – if the space-time is dynamic, there must be ripples on it. ⚫ Einstein derived the approximate formula for the gravitational waves from two orbiting bodies. ⚫ In 1922 Eddington (that one) argued that gravitational waves were not real and just a mathematical artefact. The History ⚫ In 1936 Einstein wrote a paper with Nathan Rosen reversing his original view; they concluded that all gravitational waves should collapse into black holes. ⚫ The paper was submitted to Physics Review but was returned after the referee pointed out a mistake in it. ⚫ Einstein got berserk in response, writing an angry letter to the editor and promising never to publish in Physics Review again. The History ⚫ Einstein was later persuaded by Infeld of his mistake and corrected the paper. Rosen never conceded. ⚫ In 1957 Richard Feymann (a very famous particle physicist) presented a “sticky bead” argument that convinced most people that gravitational waves are real. The Challenge ⚫ The primary challenge with gravitational waves is that in GR there is no (unambiguous) mathematical expression for its energy. ⚫ Since locally space-time is Minkowski, in the local inertial reference frame the energy of gravitational waves is zero. I.e., that energy is non-local, one cannot claim that the energy of gravitational waves is … at his location, only that the total energy is such and such. Detecting Gravitational Waves 1919-2000 ⚫ The experimental pioneer was Joseph Weber (also a discoverer of the physical principle for a laser). ⚫ He spent most of his life building more and more sophisticated devices for detecting gravitational waves, but never succeeded (although he claimed that he did and was publicly called “insane” for that). ⚫ One of his apparatus even went to the moon on Apollo 17 mission. First Indirect Detection ⚫ In 1993, Russell Hulse and Joseph Taylor received the Nobel Prize in Physics for the discovery and observation of the Hulse-Taylor binary pulsar, which offered the first indirect evidence of the existence of gravitational waves. The End of the Road ⚫ A 50-year long searched ended on Feb 11, 2016! Rainer Weiss Barry Barish Kip Thorne LIGO ⚫ Laser Interferometer Gravitational Observatory LIGO: How It Works ⚫ The main principle is the same as in the Michelson-Morley interferometer. ⚫ Arm length is 4km. ⚫ Initial laser power is 20W, eventually amplified to 100 kW. ⚫ Number of times light bounces back and forth: 280. LIGO ⚫ How it works: LIGO ⚫ Started in 2002. Nothing was detected by 2010. ⚫ Had a major refurbishing into Advanced LIGO until 2015. ⚫ Advanced LIGO had the first detection announced in 2016. ⚫ By 2020 some ~30 detections have been made. ⚫ https://www.gw-openscience.org/eventapi/html/ LIGO: How It Measures GW ⚫ The change of the length of a 4 km arm is 10-18 m. ⚫ This precision is just barely enough to find the signal. ⚫ https://blackholehunter.org/game.html LIGO: Results ⚫ T LIGO: Results ⚫ Why does LIGO find only big BHs and X-ray observers find only small ones? Astronomer’s Bane: Selection Effects ⚫ With one method we always see just one side of the whole story. LIGO: Kilonova ⚫ The GW170817 event had an optical counterpart – they called it a “kilonova”. Future ⚫ LIGO-India (INDIGO), ~2024 ⚫ LIGO A+, ~2026 ⚫ Second refurbishing of LIGO: “LIGO Voyager”, 2028 ⚫ “Einstein Telescope” (Europe), 10 km arms ⚫ LISA: Light Interferometer Space Antenna, ~2034 LISA ⚫ Three spacecraft “flying-in-formation”, arm length is 2.5 million km. ⚫ The arm length is not fixed, variations in length need to be accounted for all the time. ⚫ Light travel time between satellites is significant. ⚫ Science goals: detect mergers of supermassive black holes, compact object binaries before merger, cosmic background. LISA Pathfinder ⚫ A small satellite launched in 2015 to test some of LISA technologies, including measuring positions of free-floating test masses to 0.01 nanometers. ⚫ Exceeded expectations by almost a factor of 10..
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