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How a neutron star collision proves Einstein's 100-year-old General Relativity prediction

University of Johannesburg


IMAGE: Professor Soebur Razzaque's Astroparticle Physics Group in the UJ Department of Physics conducts Gamma Ray Burst (GRB) and Gravitational Wave (GW) research. Prof Razzaque is a coordinator of the... view more 

Credit: Image provided by the University of Johannesburg

A global scientific project has finally observed the collision of neutron stars, directly, for the first time, with great significance for cracking enduring challenges in astrophysics around gravitational waves and gamma ray bursts. Prof Soebur Razzaque from the University of Johannesburg's (UJ) contributed theoretical modelling of the expected behavior of gamma rays when neutron stars collide to the discovery. Prof Razzaque, from the UJ Department of Physics, based his work on data provided by the global Fermi-LAT research collaboration, which UJ is a member of.

More than 3000 researchers across the world contributed to the groundbreaking event, which started with a prediction from a genius in the previous century, and took until now to prove.

"In 1916, Albert Einstein predicted gravitational waves or ripples in space-time, squeezing and squashing of dimensions, due to violent movement of massive objects in the universe. Einstein predicted gravitational waves as part of his General Relativity Theory, in which he sought to predict how the force of gravity works in space and time," says Prof Razzaque.

"However, gravitational waves are very faint and their detection is extremely challenging. It was only on September 14, 2015, that the first Gravitational Wave event, known among researchers as GW150914, was finally detected. Two instruments in the USA, called ALIGO, picked up the signals created by the collision of two huge black holes," he adds.

"The one black hole was 36 times the mass of the sun, and the other 29 times the mass of the sun. After that, ALIGO detected several more black hole mergers. But a key puzzle piece to understand gravitational waves remained missing: the ability to detect the collision, or merger, of two neutron stars," adds Razzaque.

Prof Razzaque, along with the thousands of other scientists working on the challenge, expected that a neutron star merger would produce gravitational waves and electromagnetic radiation, in the form of a burst of gamma rays emitted during the collision. However, none of the scientific teams detected neutron star collisions, so the puzzle piece remained missing.

Everything changed on Thursday 17 August 2017, when the gravitational-wave event GW170817 was observed by the ALIGO detector in the USA and the Virgo detector in Italy, he says.

"Just 1.7 seconds after GW170817, the Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope detected a burst of gamma rays (named event GRB170817A) from the same direction. This confirmed a long-standing theoretical view that mergers of neutron stars produce short-duration Gamma-Ray Bursts. These bursts have been routinely detected by space-borne telescopes since 1960s, but no-one could prove what created them," says Prof Razzaque.

Since 17 August, teams working with detection instruments at more than 70 observatories across the world and in space, decided to try and detect more such events, in all electromagnetic wavelengths.

When the data from all the detected events was combined, it became clear that the 17 August events (GW170817 and GRB170817A) took place in the galaxy NGC4993, which is about 130 million light years away from us.

(That means the galaxy is so far away, light will take 130 million years to travel from that galaxy to Earth.)

"Finally, the puzzle piece Einstein has been looking for came to light as it were. The combined data also showed that the 17 August gamma ray burst, which only lasted a few seconds, was created by the merging of two neutron stars, which then produced an explosion, called a kilonova," says Prof Razzaque.

"Next, the kilonova emitted visible light from the burning of radio-active materials of the stars for several days. In that burning, which was a nuclear reaction taking place in a short period of time, gold and platinum were produced. The process is called rapid nuclear synthesis, the main mechanism to produce Gold and Platinum in the universe," concludes Prof Razzaque.


Prof Razzaque's Astroparticle Physics Group in the UJ Department of Physics conducts Gamma Ray Burst (GRB) and Gravitational Wave (GW) research. Prof Razzaque is a coordinator of the GRB and GW science group of the Fermi-Large Area Telescope (LAT) Collaboration. The University of Johannesburg is the only Full Member Institute of the Fermi-LAT Collaboration on the African continent.

Prof Razzaque's group is also involved in theoretical modelling of GRBs and binary mergers. Prof Razzaque is a co-author of the GW170817/GRB170817A discovery paper as well as the Fermi-LAT paper on observations of this event.

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