At the dawn of gravitational-wave astronomy: the second observation of the merger of black holes
Today, the scientific collaboration LIGO-Virgo announced the discovery of gravitational waves from a second source and published the results of the first scientific cycle of observations (only three events in four months of observations). Article published in Physical Review Letters .
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In February, scientists announced the first direct observation of gravitational waves, and now the results of the second event, which happened on December 26, 2015 and is tentatively called GW151226, are published. This time the signal was not so clear (signal-to-noise ratio 13), but the confidence is still more than 5σ. The signal appeared when two black holes with masses of 14.2 and 7.5 solar are merged into one - with a mass of 20 solar. The difference in mass (2 solar) was converted to the energy of gravitational waves.
A big difference compared to the previous discovery is the need for an additional matched filter to extract the signal from the noise. As seen in the picture, in the noise it is difficult to see the signal directly, so scientists use knowledge of the noise of detectors and models for the process of merging black holes.
The second difference is the signal itself - the mass of the black hole system is much smaller, and the merging process takes longer than about 1 second and 45 black hole hits around each other (for comparison, in the previous event the merger lasted only 0.2 seconds).
As before, a signal came to both detectors (Livingstone and Hanford), which made it possible to eliminate local errors and also to make an estimate for the distance to the object — about 440 MPc (by triangulation).
In January 2016, the first scientific cycle of the detectors was completed and they are now undergoing a renewal procedure - the laser power will be increased and other changes made, which will significantly increase the sensitivity. In just four months of operation of the detector, three events were recorded corresponding to the merging of two black holes: two with a certainty greater than 5σ, and one with low confidence (87%). Both major events are in excellent agreement with the predictions of the General Theory of Relativity.
These discoveries make it possible to test many of the predictions that GR gives, as well as to give estimates for the parameters of the systems we are observing, and thus to test certain extensions of GR (and other gravity theories).
How do we know that we really discovered gravitational waves
In fact, the signal must meet many criteria so that we can claim that this is indeed a gravitational wave. First, the amplitude of the signal must be significantly more noise in the system. The detector itself is extremely sensitive, and many different sources of noise interfere with measurements: these are seismic noise, electronic, laser, thermal, and many others. The detectors are carefully characterized for susceptibility to these noises, and the noises themselves are measured continuously. This data is then used to filter the signal.
Secondly, you need to make sure that nothing else could cause such a signal. There are many non-stationary phenomena (glitches) that occur occasionally and for a short time and may have a shape very similar to a grav. wave Scientists are looking for possible sources of such phenomena, study them, reproduce, check the response of the system to them and classify them. This makes it possible to compare the registered signal with known sources and to conclude that it is similar to glitch. By the way, you can contribute to the search for glitches!
Finally, measurements are made on two independent detectors separated by several thousand kilometers. Any local phenomenon would manifest only one of them, and the gravitational wave acts on both at once.
As a result, the key to success is to know very well both the detector itself and all possible sources of noise around it, and to make independent measurements through several channels.
Opening Grav. waves. was undoubtedly one of the most important events in modern physics. During these few months, the founders of LIGO have already received 4 awards, including the Milner Prize for a breakthrough in science , and the Nobel Prize is just around the corner. But the second observation is in a sense even more important - this means we can really observe dozens of events a year. This is not just luck, but scientific progress. The detectors will be updated, a new one built in India , launched after the upgrade of Virgo in Italy, and an underground cryogenic KAGRA in Japan - and we will be able to observe not only the fusion of black holes, but also pair neutron stars, and supernova explosions ... Recent successes in the space probe interferometer - LISA Pathfinder - give us hope for the construction of giant space detectors to monitor low-frequency signals - supermassive black holes in the center of galaxies.
Now it remains to wait a bit - and hope that the results of these observations will not coincide with any of our theories, and will force us to move forward again and look for a deeper understanding of the laws of nature.
UPD And here is a good video arrived:
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GW151226 in detail
In February, scientists announced the first direct observation of gravitational waves, and now the results of the second event, which happened on December 26, 2015 and is tentatively called GW151226, are published. This time the signal was not so clear (signal-to-noise ratio 13), but the confidence is still more than 5σ. The signal appeared when two black holes with masses of 14.2 and 7.5 solar are merged into one - with a mass of 20 solar. The difference in mass (2 solar) was converted to the energy of gravitational waves.
A big difference compared to the previous discovery is the need for an additional matched filter to extract the signal from the noise. As seen in the picture, in the noise it is difficult to see the signal directly, so scientists use knowledge of the noise of detectors and models for the process of merging black holes.
The second difference is the signal itself - the mass of the black hole system is much smaller, and the merging process takes longer than about 1 second and 45 black hole hits around each other (for comparison, in the previous event the merger lasted only 0.2 seconds).
As before, a signal came to both detectors (Livingstone and Hanford), which made it possible to eliminate local errors and also to make an estimate for the distance to the object — about 440 MPc (by triangulation).
The first scientific cycle
In January 2016, the first scientific cycle of the detectors was completed and they are now undergoing a renewal procedure - the laser power will be increased and other changes made, which will significantly increase the sensitivity. In just four months of operation of the detector, three events were recorded corresponding to the merging of two black holes: two with a certainty greater than 5σ, and one with low confidence (87%). Both major events are in excellent agreement with the predictions of the General Theory of Relativity.
These discoveries make it possible to test many of the predictions that GR gives, as well as to give estimates for the parameters of the systems we are observing, and thus to test certain extensions of GR (and other gravity theories).
How do we know that we really discovered gravitational waves
In fact, the signal must meet many criteria so that we can claim that this is indeed a gravitational wave. First, the amplitude of the signal must be significantly more noise in the system. The detector itself is extremely sensitive, and many different sources of noise interfere with measurements: these are seismic noise, electronic, laser, thermal, and many others. The detectors are carefully characterized for susceptibility to these noises, and the noises themselves are measured continuously. This data is then used to filter the signal.
Secondly, you need to make sure that nothing else could cause such a signal. There are many non-stationary phenomena (glitches) that occur occasionally and for a short time and may have a shape very similar to a grav. wave Scientists are looking for possible sources of such phenomena, study them, reproduce, check the response of the system to them and classify them. This makes it possible to compare the registered signal with known sources and to conclude that it is similar to glitch. By the way, you can contribute to the search for glitches!
Finally, measurements are made on two independent detectors separated by several thousand kilometers. Any local phenomenon would manifest only one of them, and the gravitational wave acts on both at once.
As a result, the key to success is to know very well both the detector itself and all possible sources of noise around it, and to make independent measurements through several channels.
Now what?
Opening Grav. waves. was undoubtedly one of the most important events in modern physics. During these few months, the founders of LIGO have already received 4 awards, including the Milner Prize for a breakthrough in science , and the Nobel Prize is just around the corner. But the second observation is in a sense even more important - this means we can really observe dozens of events a year. This is not just luck, but scientific progress. The detectors will be updated, a new one built in India , launched after the upgrade of Virgo in Italy, and an underground cryogenic KAGRA in Japan - and we will be able to observe not only the fusion of black holes, but also pair neutron stars, and supernova explosions ... Recent successes in the space probe interferometer - LISA Pathfinder - give us hope for the construction of giant space detectors to monitor low-frequency signals - supermassive black holes in the center of galaxies.
Now it remains to wait a bit - and hope that the results of these observations will not coincide with any of our theories, and will force us to move forward again and look for a deeper understanding of the laws of nature.
More about gravity waves
- First detection article and excellent review with details.
- LIGO official website about the first event, and details of detection
- Recent interesting interview with Kip Thorne
UPD And here is a good video arrived:
Source: https://habr.com/ru/post/372373/
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