lpetrich
Contributor
LIGO-P1800307-v6: GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs (
List of gravitational wave observations) The LIGO and Virgo teams have decided that eleven of their observed events are real gravitational-wave events.
Of them, ten are observations of black-hole mergers and one an observation of a neutron-star merger. None of them were mergers of a neutron star and a black hole.
The black holes had masses from 8 to 50 solar masses, and the more massive one of each pair was 1.2 to 1.8 times more massive than the less massive one. Their distances were from 320 to 2750 megaparsecs, or from 1 to 9 billion light years.
The first one observed was GW150914, and its two black holes had masses 36 and 31 times the Sun's mass. It radiated about 3 solar masses of gravitational-wave energy, with a peak luminosity roughly comparable to that of all the stars in the observable Universe. Its distance was 430 megaparsecs or 1.4 billion light years, it was not accompanied by any gamma-ray burst, and its galaxy is unknown.
The only neutron-star merger of these events is GW170817, and its two neutron stars had masses 1.3 and 1.5 solar masses. It radiated some 0.04 solar masses of gravitational-wave energy, and it is not very certain whether the two neutron stars produced a neutron star or a black hole. About 1.7 seconds later, a gamma-ray burst was observed in essentially the same spot, GRB 150101B. The material spewed off by the merger was then observed in radio, infrared, visible light, ultraviolet, and X-rays, and its expansion speed is around 0.1 c, as one might expect from this kind of event. No neutrinos were observed, however. The stars' home galaxy was found, NGC 4993, in a search where the remnant's glow was an astronomical transient originally called SSS17a and then AT 2017gfo. That galaxy's distance is about 44 megaparsecs or 144 million light years, consistent with the G-wave estimated distance of 40 megaparsecs.
As more events are observed, then I think that one can do tests of general relativity itself. The G-wave events look like an increasing and speeding-up oscillation from the final bit of inspiral, followed by an oscillation from the final object settling down. Both neutron stars and and black holes will do that. Both phases offer potential tests of GR. Does the inspiral speed up faster or slower during its last moments? Can one observe overtones in the final object's ringdown?
Polarization of gravitational-waves are another potential test. Electromagnetic waves are polarized in the perpendicular direction, meaning that their polarization has a combination of 2 out of 3 directions. It is usually given as the electric field's direction, with the magnetic field's direction readily inferred. G-waves are more complicated. They involve changing the separation of two separated objects, where the direction of change need not be along the direction of separation. The G-wave field is thus a "tensor", a multivector sort of mathematical object. Vectors are 1-tensors, and this is a 2-tensor.
The full distortion tensor has 3*3 = 9 components. Three combinations of give rotations:
Separation = +x, distortion = +y
Separation = +y, distortion = -x
Leaving out these antisymmetric combinations leaves 6 symmetric combinations of the distortion tensor components. These are what are predicted for GR and alternatives to it. They are one combination that's all parallel to the G-wave direction of travel, two combinations that are part-parallel part-perpendicular, and three combinations that are all perpendicular. Of these three, one combination of them is expansion and contraction, and the other two are shear combinations.
GR predicts only the two perpendicular shear combinations ("transverse traceless"), while some alternatives predict more combinations.
Currently, there are only two operating G-wave observatories: LIGO and Virgo. The LiGO one has two detectors in the United States, one at Livingston LA, and one at Hanford WA. Virgo has one near Pisa, Italy. This means three detectors. There are some ones under development in Japan, India, and China, but these ones will not be in operation any time soon.
The detectors work by comparing the travel times of light in different directions. The light is sent down long evacuated tubes and bounced back with mirrors. G-waves pull on the mirrors, making the travel times different.
With one detector, one gets no directional information and only one direction combination of polarization. With two detectors, travel-time difference translates into a ring of directions on the sky, and in most cases, enough information to find both combinations of GR-prediction polarization. With three detectors, travel-time difference yields two directions in the sky, and the possibility of testing for non-GR G-wave polarization. With four or more detectors, one can test the speed of propagation of G-waves across the Earth, and improve one's polarization tests.
So there are some interesting times ahead.
List of gravitational wave observations) The LIGO and Virgo teams have decided that eleven of their observed events are real gravitational-wave events.Of them, ten are observations of black-hole mergers and one an observation of a neutron-star merger. None of them were mergers of a neutron star and a black hole.
The black holes had masses from 8 to 50 solar masses, and the more massive one of each pair was 1.2 to 1.8 times more massive than the less massive one. Their distances were from 320 to 2750 megaparsecs, or from 1 to 9 billion light years.
The first one observed was GW150914, and its two black holes had masses 36 and 31 times the Sun's mass. It radiated about 3 solar masses of gravitational-wave energy, with a peak luminosity roughly comparable to that of all the stars in the observable Universe. Its distance was 430 megaparsecs or 1.4 billion light years, it was not accompanied by any gamma-ray burst, and its galaxy is unknown.
The only neutron-star merger of these events is GW170817, and its two neutron stars had masses 1.3 and 1.5 solar masses. It radiated some 0.04 solar masses of gravitational-wave energy, and it is not very certain whether the two neutron stars produced a neutron star or a black hole. About 1.7 seconds later, a gamma-ray burst was observed in essentially the same spot, GRB 150101B. The material spewed off by the merger was then observed in radio, infrared, visible light, ultraviolet, and X-rays, and its expansion speed is around 0.1 c, as one might expect from this kind of event. No neutrinos were observed, however. The stars' home galaxy was found, NGC 4993, in a search where the remnant's glow was an astronomical transient originally called SSS17a and then AT 2017gfo. That galaxy's distance is about 44 megaparsecs or 144 million light years, consistent with the G-wave estimated distance of 40 megaparsecs.
As more events are observed, then I think that one can do tests of general relativity itself. The G-wave events look like an increasing and speeding-up oscillation from the final bit of inspiral, followed by an oscillation from the final object settling down. Both neutron stars and and black holes will do that. Both phases offer potential tests of GR. Does the inspiral speed up faster or slower during its last moments? Can one observe overtones in the final object's ringdown?
Polarization of gravitational-waves are another potential test. Electromagnetic waves are polarized in the perpendicular direction, meaning that their polarization has a combination of 2 out of 3 directions. It is usually given as the electric field's direction, with the magnetic field's direction readily inferred. G-waves are more complicated. They involve changing the separation of two separated objects, where the direction of change need not be along the direction of separation. The G-wave field is thus a "tensor", a multivector sort of mathematical object. Vectors are 1-tensors, and this is a 2-tensor.
The full distortion tensor has 3*3 = 9 components. Three combinations of give rotations:
Separation = +x, distortion = +y
Separation = +y, distortion = -x
Leaving out these antisymmetric combinations leaves 6 symmetric combinations of the distortion tensor components. These are what are predicted for GR and alternatives to it. They are one combination that's all parallel to the G-wave direction of travel, two combinations that are part-parallel part-perpendicular, and three combinations that are all perpendicular. Of these three, one combination of them is expansion and contraction, and the other two are shear combinations.
GR predicts only the two perpendicular shear combinations ("transverse traceless"), while some alternatives predict more combinations.
Currently, there are only two operating G-wave observatories: LIGO and Virgo. The LiGO one has two detectors in the United States, one at Livingston LA, and one at Hanford WA. Virgo has one near Pisa, Italy. This means three detectors. There are some ones under development in Japan, India, and China, but these ones will not be in operation any time soon.
The detectors work by comparing the travel times of light in different directions. The light is sent down long evacuated tubes and bounced back with mirrors. G-waves pull on the mirrors, making the travel times different.
With one detector, one gets no directional information and only one direction combination of polarization. With two detectors, travel-time difference translates into a ring of directions on the sky, and in most cases, enough information to find both combinations of GR-prediction polarization. With three detectors, travel-time difference yields two directions in the sky, and the possibility of testing for non-GR G-wave polarization. With four or more detectors, one can test the speed of propagation of G-waves across the Earth, and improve one's polarization tests.
So there are some interesting times ahead.