LIGO ( Laser Interferometer Gravitational-Wave Observatory / Laser-Interferometer Gravitational Wave Observatory ) is an observatory that was used to detect gravitational waves for the first time . Originally founded in 1992 by Kip Thorne , Ronald Drever ( Caltech ) and Rainer Weiss ( MIT ), the project now employs hundreds of scientists in over 40 institutes worldwide. Researchers in this group received the Nobel Prize in Physics in 2017 .
The data obtained with LIGO are evaluated by various working groups according to types of possible sources of gravitational waves. These are:
- stochastic sources (e.g. from the big bang ),
- Pulsars (sources with continuous emission),
- Merging of two compact objects such as neutron stars and black holes ,
- collapsing stars ( supernovae ),
- exotic sources.
It all started at Caltech in the late 1970s, initiated by Kip Thorne and Rochus (Robbie) Vogt and with Ronnie Drever and Stanley E. Whitcomb . The collaboration with MIT developed in the early 1990s. Important leaders of the project were Whitcomb and Barry Barish , who were awarded the Henry Draper Medal for this in 2017 . The spokesman for the LIGO Scientific Collaboration (LSC) is currently Patrick Brady (election in 2019 for 2 years), David Reitze has been the managing director of the LIGO Laboratory since 2011 . As elected speakers for LIGO, Gabriela González , David Reitzer and Peter Saulson (the first elected speaker) received the NAS Award for Scientific Discovery in 2016 . One of the leading scientists is Peter Fritschel from MIT. The LIGO research association, including the Max Planck Institute for Gravitational Physics and the Laser Center Hannover , was also awarded the Breakthrough Prize in Fundamental Physics .
The main task of the LIGO is the direct measurement of gravitational waves of cosmic origin. These waves are predicted by Albert Einstein's general theory of relativity . These gravitational waves could be confirmed for the first time by researchers of the LIGO collaboration through the first successful direct measurement of gravitational waves in September 2015 due to a collision of two black holes , as announced in February 2016.
An indirect indication of the existence of these waves is given by the double pulsar PSR 1913 + 16 discovered by Russell Hulse in 1974 . The variations in the orbit of this double system agree exactly with the predictions of the general theory of relativity for the emission of gravitational waves. For this discovery, Russell Hulse received the Nobel Prize in Physics in 1993 .
The direct detection of gravitational waves is a very active research area in modern physics, as it would enable a whole new kind of astronomy , in addition to astronomy in the electromagnetic field and neutrino astronomy. For this reason, attempts were made as early as the 1960s to measure gravitational waves using resonance cylinders, above all by Joseph Weber . In the 1970s, Rainer Weiss realized the possibility of using interferometers for this search.
LIGO was founded in 1992 and construction work on both detectors was completed in 1999. After the first tests and fine adjustments of the systems, the first scientific measurement period took place in August 2002. The fifth measurement period ended at the end of 2007 after two years of data with a previously unique sensitivity had been obtained. On February 11, 2016, the LIGO and VIRGO collaborations announced in a press release that they had directly detected gravitational waves for the first time on September 14, 2015. A second event was observed on December 26, 2015, as announced on June 15, 2016. A total of four more events were observed by the end of the second observation period in August 2017. The next run started in April 2019.
LIGO operates two observatories in Hanford ( Washington ) and Livingston ( Louisiana are located) and about 3000 km apart have been removed. Light takes 10 ms to travel between the two stations. Since gravitational waves propagate at the speed of light , the difference in time between at least three signals measured in these observatories can be used to determine the position of the actual source in the sky. In addition, numerous earthly disturbances that spread more slowly (such as vibrations, distant earthquakes, etc.) can be excluded.
Each observatory has an L-shaped ultra-high vacuum system with a leg length of four kilometers each, in which a laser interferometer is housed. The Hanford observatory has a second interferometer with a leg length of two kilometers, housed in the same vacuum system.
At the main station of the observatory (the corner of the L where the two arms cross) a stabilized laser beam of 200 W is first sent through a mirror, which allows the laser light into the system but not in the opposite direction ( power -recycling mirror ). This increases the power of the laser light in the interferometer to 700 kW, which increases the sensitivity.
Then the beam hits a beam splitter , where the beam is split and half is sent to the two 4 km long arms (or the 2 km long arms in the second interferometer in Hanford). A Fabry-Pérot resonator is housed in each arm , consisting of two mirrors (one of which is partially transparent), so that the light travels about 280 times this distance before it passes through the partially transparent mirror and hits the beam splitter again. This technique of multiple reflections increases the effective running length of the light to 1120 km, which in turn increases the sensitivity of the instrument.
At the beam splitter in the corner station, both partial beams are directed onto a photodiode , which measures the intensity of the light arriving there. The interferometer, in particular the adjustable mirrors at the ends of the two arms, is now set so that the two partial beams just extinguish each other (see interference ) and thus ideally no light arrives at the photodiode. Due to numerous external, but also internal influences, this is not permanently possible, so that the entire system has to be constantly adjusted in order to achieve the extinction of the two partial beams.
When a gravitational wave passes through the observatory, the relative lengths of the arms of the interferometer change: One or both arms can lengthen or shorten (by different amounts). This causes a phase shift of the two partial waves of the laser light and their interference changes the intensity of the measured light.
Due to the combination of mirrors, the laser intensity and the Fabry-Pérot cavity within the system, the observatories are able to measure a relative difference between the two arm lengths of 10 −22 . This corresponds to about a thousandth of a proton radius over arm's length .
The entire measurement technology therefore reacts very sensitively to external influences such as movements in the ground (earthquakes, waves on distant beaches), weather-related effects (wind), road traffic and internal influences such as thermal movements of the atoms in the mirrors, light scattered in the tunnels etc. One of the tasks of data analysts is to filter out a gravitational signal from these interference effects.
Sources of gravitational waves
|modeled||Pulsars||Fusion of compact objects|
|unmodeled||Gravitational wave background radiation (
|Bursts (other types of transient bursts)|
There are a variety of signals to look for. These can be grouped into continuous signals (search for pulsars and cosmic gravitational background radiation) and into transient signals (merging of compact objects and unclassifiable outbursts). However, these four signals can also be classified by modeling the signal (see table).
Pulsars are neutron stars that have a strong magnetic field and rotate around their own axis at up to 716 revolutions per second. If these pulsars show asymmetries in their mass distribution (e.g. due to a small elevation on their surface), according to the theory they emit gravitational waves, which reduces their rotational frequency. An example is the Crab Nebula pulsar , which rotates about 30 times per second.
Gravitational wave background radiation
Many models of the universe predict strong gravitational waves that arose shortly after the Big Bang . These gravitational waves have a broad spectrum and make it possible, if these waves are detected, to look much further into the history of the universe than is possible with the cosmic microwave background radiation .
Fusion of compact objects
If two compact objects such as two neutron stars or two black holes (or combinations thereof) orbit each other, they also emit gravitational waves according to the theory. This causes the system to lose energy, so that both bodies slowly approach each other. As a result, stronger gravitational waves are emitted, so that this process accelerates until both bodies collide and merge to form a black hole.
This was indirectly proven in the above-mentioned double pulsar PSR 1913 + 16 , and the measurements exactly match the predictions of the general theory of relativity. Although the two bodies in this system approach each other by 3.5 m annually, the two neutron stars will only merge in about 300 million years.
The expected signals for such a scenario can be calculated very precisely so that a targeted search for such gravitational waves in the data can be carried out.
On September 14, 2015, a signal from the merging of two black holes in the two LIGO detectors was detected for the first time.
On June 15, 2016, the LIGO collaboration announced the observation of a second such event on December 26, 2015. The event is called GW151226, after the English name for December 26th, it is also called the Boxing Day Event by scientists .
On October 16, 2017, LIGO announced the observation of the collision of two neutron stars ( GW170817 ), which was also subsequently observed by other telescopes in the optical and other wavelength ranges such as the Fermi gamma-ray telescope (a short gamma-ray flash).
Burst signals are short, unmodeled signals such as those used in B. in a supernova, the collapse of a very heavy star, could arise. Such signals can also arise from the merging of two very heavy black holes.
The first measurements were carried out between 2002 and 2007. Thereafter, the sensitivity and thus the range was doubled and further data was collected between 2009 and 2011. The Franco-Italian Virgo detector was also included in this measurement period .
Since the approximately two-year measurement period with enhanced LIGO, the instruments have again been extensively improved, so that the sensitivity is to be improved by a factor of 10 compared to the original device. In other words, a thousand times the volume can be examined with the same sensitivity. This renovation was completed on May 19, 2015.
Doubts have been expressed about some Ligo results since 2017. In particular, a Danish group of scientists criticized an insufficiently documented and potentially error-prone separation of the actual signal and random interference. Members of the LIGO consortium have admitted that illustrations in the publication for the first detection of gravitational waves (Abbott et al., 2015) have been adapted by hand and by eye for educational reasons (“hand-tuned for pedagogical purposes”) were without disclosing this.
- GEO600 , a German gravitational wave detector near Hanover
- Virgo , a European gravitational wave detector near Pisa / Italy
- Einstein telescope , a concept for a much more accurate detector
- LISA (Laser Interferometer Space Antenna) , a gravitational wave detector in space
- LIGO website (English)
- Advanced LIGO website (English)
- Einstein's Messengers , film about gravitational waves and LIGO (English)
- Livingston Observatory with Google Maps
- Hanford Observatory with Google Maps
- Einstein @ Home: Search for gravitational waves in LIGO data
- LIGO Detection full movie , David Reitze, Reiner Weiss explaining the measurement. 2017-02-08.
- A Brief History of LIGO. (pdf) (No longer available online.) ligo.org, 2016, archived from the original on July 3, 2017 ; accessed on April 25, 2019 .
- 2017 Draper Medal: Barry C. Barish and Stanley E. Whitcomb. National Academy of Sciences , accessed April 25, 2019 .
- UW-Milwaukee astrophysicist elected spokesperson of the LIGO Scientific Collaboration. newswise.com, April 3, 2019, accessed April 24, 2019 .
- New LIGO Executive Director Named. caltech.edu, August 24, 2011, archived from the original on December 6, 2017 ; accessed on October 20, 2018 .
- Special price for gravitational wave detection
- The proof is there: Einstein's gravitational waves are proven . Zeit Online , February 11, 2016.
- Kathy Svitil et al. : Gravitational Waves Detected 100 Years After Einstein's Prediction. February 11, 2016, accessed June 18, 2016 .
- B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration): Observation of Gravitational Waves from a Binary Black Hole Merger . In: Phys. Rev. Lett. February 11, 2016, p. 061102 , doi : 10.1103 / PhysRevLett.116.061102 , arxiv : 1602.03837 ( ligo.org ).
- Davide Castelvecchi, Alexandra Witze: Einstein's gravitational waves found at last . In: Nature . February 11, 2016, doi : 10.1038 / nature.2016.19361 .
- B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration): GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence . In: Phys. Rev. Lett. tape 116 , June 15, 2016, p. 241103 , doi : 10.1103 / PhysRevLett.116.241103 , arxiv : 1606.04855 ( ligo.org ).
- Davide Castelvecchi: LIGO detects whispers of another black-hole merger . In: Nature . June 15, 2016, doi : 10.1038 / nature.2016.20093 .
- LIGO Does It Again: A Second Robust Binary Black Hole Coalescence Observed. June 15, 2016, accessed June 18, 2016 .
- Detection papers. ligo.org, accessed October 20, 2018 .
- LSC news. (PDF; 7.8 MB) March 2019, accessed on August 16, 2019 .
- LIGO's interferometer. Retrieved July 24, 2019 .
- BP Abbott et al. a .: GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral, Phys. Rev. Lett., Volume 119, 2017, p. 161101, abstract
- Alexander Pawlak: Progressive Search for Gravitational Waves. May 20, 2015, accessed May 21, 2015 .
- S. Hossenfeder (2017) Was It All Just Noise? Independent Analysis Casts Doubt On LIGO's Detections, Forbes, June 16, 2017; 
- J. Creswell, p. V. Hausegger, AD Jackson, H. Liu, P. Naselsky (2017) On the time lags of the LIGO signals; August 2017
- M. Brooks (2018) Exclusive: Grave doubts over LIGO's discovery of gravitational waves. Scientist , October 31, 2018