In February last year, scientists announced the first detection of gravitational waves, 100 years after Albert Einstein proposed their existence in his landmark general theory of relativity. The detection of the waves, which happened in September 2015 but was announced in February 2016, resulted in this year’s Nobel Prize in Physics for three scientists who had made it possible.
But even before the Nobel Committee had disclosed its decision earlier this month, scientists were getting ready to make another big bang announcement. On Monday, they said they had, on August 17 this year, made another detection of gravitational waves, this time arising from the collision of two neutron stars. The gravitational waves detected in September 2015, and on three subsequent occasions, were all produced by the merger of two black holes.
The scientific community is convinced that the latest detection is as significant an achievement as the first one. There are several reasons for the immense excitement.
It is not just that the source of gravitational waves detected this time is different, although that in itself is no less important. All heavy moving bodies in the universe produce gravitational waves — ripples in the fabric of spacetime that can be imagined as something similar to waves produced in water by a moving boat. Gravitational waves carry signature signals of the event they originate from. They can, therefore, be a very powerful medium to understand the event, and the source.
So, gravitational waves that originate from the merger of black holes can reveal a lot of information about the merger, as well as about the black holes. Similarly, the ones that originate from the collision of neutron stars can throw more light on the properties of neutron stars. Gravitational waves can, therefore, act in ways that are very similar to electromagnetic waves — such as light, or x-rays, or gamma rays — that, too, carry the signatures of the source from which they have been emitted. The detection of gravitational waves from newer sources would help scientists understand these sources better.
However, there was much more to the latest detection than just this. This was the first time that scientists were able to record two different kinds of signals from the same celestial event — gravitational signals, as well as the more traditional electromagnetic signals. This is what happened on August 17.
A gravitational wave signal was picked up by the US-based LIGO (Laser Interferometer Gravitational-Wave Observatory) facility — the same one that had detected the first gravitational wave in September 2015 and all subsequent events after that — and another similar detector in Italy, called VIRGO. The signal was very different from the ones received earlier, and lasted much longer. The signals from the mergers of black holes had continued for a few fractions of a second. In contrast, this time, the gravitational wave that had arrived lasted for about 100 seconds.
It was enough indication that the source of these waves was something comparatively less massive than black holes. Scientists were very quickly able to identify it as a pair of colliding neutron stars, both between 1.1 and 1.6 times as massive as the Sun.
But, almost simultaneously with the arrival of gravitational waves, observatories around the world — the Fermi gamma ray telescope was the first one — also picked up a short gamma ray burst, similar to a flash of light but not in the visible range. The difference in the arrival of gravitational waves at LIGO detectors and the recording of electromagnetic gamma ray bursts was just 1.7 seconds. And they were both coming from the same location in the universe, about 130 million light years away from Earth. Scientists were able to discern that both the signals were emanating from the same cosmic event, something that had never been observed before. LIGO scientists later compared the situation to watching a movie that had both video and audio signals.
More predictions come true
Scientists have for years associated these kinds of high-energy gamma ray bursts with colliding neutron stars. Neutron stars are some of the smallest and densest stars in the universe, and consist only of neutrons, electrically-neutral atomic particles. In the August 17 event, signals from the gravitational waves revealed that the masses of the source objects were much less than black holes, and consistent with masses of neutron stars.
The electromagnetic signals from gamma rays assured scientists again that they were looking at a neutron star collision, since black hole mergers do not produce any light. The two different signals, therefore, provided evidence supporting each other. This is what scientists are now calling the new world of “multi-messenger astronomy”, in which gravitational waves join other traditionally-detectable information-carriers like light and other electromagnetic waves, thermal radiations, and special particles like neutrinos, in revealing the nature of the universe.
The near simultaneous arrival of gravitational waves and gamma rays from the August 17 event, which happened 130 million years ago, also gave the first direct confirmation that gravitational waves indeed travel at the speed of light, something that Einstein had proposed 100 years ago. The 1.7 second difference in the arrival of the two signals was because gravitational waves are probably produced slightly earlier than the gamma ray burst.
Almost 70 observatories, both ground-based as well as space-based, recorded the August 17 event, making it one of the most widely observed celestial events ever. As they are analysed, the signals are likely to continue to reveal new information for months, probably years, to come.
As is common in any new scientific finding, while providing answers to some known puzzles, the August 17 event has thrown up some unexplained facts as well. The gamma ray burst was the closest to Earth ever recorded — just 130 million light years away — and yet the signals were very weak, weaker than what the scientists would have expected. Scientists have already begun probing why this could be so.
Unlike in the earlier cases of inward-spiralling black hole pairs, which merged to form bigger black holes, it is not yet clear what happened to the two neutron stars after their collision. They could have merged to form a bigger neutron star, or could have formed a black hole. Clues could emerge from further data analysis and lead to a much better understanding of the life of a neutron star.