Four new detections of gravity waves have been announced at the Gravitational Waves Physics and Astronomy Workshop at the University…
Four new detections of gravity waves have been announced at the Gravitational Waves Physics and Astronomy Workshop at the University of Maryland in the United States.
This gives the total number of discoveries to 11 since the first back in 2015.
Ten are from binary black hole fusions and one from the merger of two neutron stars, which are the dense residues of star explosions. A black hole fusion was extraordinarily distant and the most powerful explosion ever observed in astronomy.
The latest news comes only one month after doubts about the first discovery.
At the end of October an article in New Scientist entitled Exclusive: Grave of LIGO’s Detection of Gravity Waves raised the thought that it could have become an illusion.
So how sure are we discovering gravity waves and not seeing an illusion
All good scientists understand that review and skepticism is the power of science.
All theories and all knowledge are preliminary, because science slowly incorporates our best understanding of truth. There is no security, only probability and statistical significance.
Years ago, the Gravitational Wave Observatory (LIGO) team determined the levels of statistical significance required to make an alleged statement. [1
9659002] For each signal, we determine the false alarm rate. This tells you how many years you need to wait before you have a smooth chance that a random signal mimics your real signal.
The weakest signal so far has a fake alarm rate every five years, so there is still a chance that it could have been inadvertently.
Other signals are much stronger. For the three strongest signals so far, you have to wait from 1000 times to 10 billion times times the age of the universe for the signals to happen by chance.
Detection of gravity waves is a bit like acoustic ornithology.
Imagine studying birds and want to determine the population of birds in a forest. You know the conversations of the bird birds.
When a bird call matches your predetermined call, you jump with tension. Its highness tells you how far it is. If it was very weak against the background noise, you may be uncertain.
But you have to consider the lion birds that mimic other species. How do you know that the sound of a kookaburra is not actually made of a shelter?
You must be very careful before claiming that there is a kookaburra in the woods. Even then you can only be sure if you make further detections.
In gravity waves we use memorized sounds called templates. There is a unique sound for the merging of every possible combination of black holes and spins. Each template has been developed using Einstein’s theory of gravity wave emissions.
In the pursuit of gravity waves, we search for these rare sounds with two LIGO detectors in the USA and a third detector, Virgo, in Italy.
To avoid failed signals or claim false positive effects, the utmost accuracy is required to analyze data. Large layer looks over data, searches for shortcomings, criticizes each other, reviews computer codes, and finally reviews proposed publications for accuracy.
Separate teams use different analytical methods and finally compare results.
Next, reproducibility, the same results will be played over and over again. Reproducibility is a critical part of science.
Before LIGO made its first public message on gravity waves, two signals were detected, each of which was divided into two detectors.
This increased our confidence and told us that there is a population that collides black holes out there, not just a single event that can be false.
The first detected gravity wave was surprisingly high and matched a predetermined template. It was so good that LIGO spent many weeks trying to train if it was possible that there had been a prank, intentionally injected by a hacker.
While LIGO researchers finally convinced that the event was correct, further discoveries greatly increased our confidence.
In August 2017, a signal of the two LIGO detectors and the Virgo detector was discovered in Italy.
On August 17 last year, a completely different but long predicted type of signal from a coalescing pair of neutron stars was observed, accompanied by the predicted burst of gamma rays and light.
Now, the LIGO-Virgo collaboration has conducted the analysis of all data since September 2015.
For each signal, we determine the mass of the two colliding black holes , the mass of the new black hole they create, and pretty much the distance and direction.
Each signal has been seen in two or three ectors almost simultaneously (they separated by milliseconds).
Eight of the 20 first black holes have masses between 30 and 40 soles, six are in the 20s, three are in teens and only two are as low as 7 to 8 Suns.
Only one is close to 50, the largest black colletet found so far.
These are the numbers that help us work out where all these black holes were made, how they were done and how many are out there. To answer these big questions, we need many more signals.
19659002] The weakest of the new signals GW170729 was discovered July 29, 2017. It was the collision of a black hole 50 times the mass of the sun, with another 34 times the mass of the sun.
This was by far the most remote event that took place, most likely 5 billion years ago – before the birth of the earth and the sun 4.6 billion years ago. Despite the weak signal, it was the most powerful gravity explosion detected so far.
However, because the signal was weak, it is detection with false alarm speed every five years.
LIGO and Virgo improve their sensitivity year after year and will find many more events.
With planned new detectors, we predict ten times more sensitivity. Then we expect to discover new signals every five minutes.
David Blair, Emeritus Professor, ARC Center of Excellence for Gravitational Wave Discovery, OzGrav, University of Western Australia.
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