Astronomers unravel a blast that sent ripples in space-time

While dinosaurs were reigning the earth, two neutron stars collided in a galaxy 130 million light years away, causing a blast so fierce, gravitational waves were formed that shook up space-time. A plethora of electromagnetic radiation was generated too, which are now being studied by radio astronomers from around the world, amongst whom Leonid Gurvits of the Delft Astrodynamics and Space missions group.

For the first time, scientists can now confirm what has been predicted by theorists all along. “When Einstein developed his theory of relativity in 1916, he predicted that the collision of two massive bodies would distort space-time,” says Gurvits, one of the authors of an article that appeared in Science earlier this month describing the radio emission generated in the aftermath of the collision.

What are neutron stars?

Neutron stars are ultra-dense objects, roughly the same mass as the Sun, but similar in size (diameter) to a city like Amsterdam. Ripples of the gravitational waves, caused by the collision of two such stars, were first detected by scientists at the Laser Interferometer Gravitational-Wave Observatory in the US and Virgo in Italy on 17 August 2017. In the following days and months, radio astronomers were able to pick up radio signals and create a clearer picture of the event that created the gravitational waves.

Gravitational waves are an exotic phenomenon, to say the least. Everybody knows the three dimensions, you see them every time you look at the corner of a room – the three axes that together form the Euclidian space. But there is a fourth dimension, and that is time. The term for all four dimensions is space-time. Gravitational waves cause distortions of the metric of space-time. Imagine gravitational waves somewhere in your room. These would result in fluctuations in the length of two fixed points. And time too would fluctuate. For Albert Einstein this all made a lot of sense.

Why is the discovery of these gravitational waves and the ripples in space-time important?

“Usually, gravitational waves have small amplitudes and are extremely hard to detect. But the phenomenon is of paramount importance for the foundation of physics. And for many decades physicists tried to detect these waves to verify the theory of general relativity.”

But didn’t scientists already detect gravitational waves in 2015?

“Yes they did. In 2015 gravitational waves were detected that resulted from a collision of supermassive black holes. That discovery  was a huge leap, and its three main actors – Rainer Weiss, Barry Barish and Kip Thorne – justly received the 2017 Nobel prize in physics. Both discoveries, the first ever direct detection of gravitational waves in 2015, and the one of August 2017 were made with the Laser Interferometer Gravitational-Wave Observatory (LIGO) built in the US. The 2015 discovery was an event of another physical nature, much more powerful than the 2017 one, as it included the collision of supermassive black holes, each of the order of 100 millions of solar masses. That collision, however, was much less informative. The major observational difference between the two types of collisions lies in the products of the collisions. In the case of black holes, the resulting product is yet another black hole, and black holes don’t emit anything immediately detectable on their own. But the neutron star merger produces materials emitting electromagnetic waves (e.g. radio, optics, X-rays and gamma-rays). The gravitational wave phenomenon of 17 August 2017 allows us to study the electromagnetic aftermath much more thoroughly.

In fact, the collision produced a plethora of electromagnetic ‘smoke’. A very prominent component of the ‘smoke’ is a so-called jet. This is a flow of plasma, a stream of charged hot gas. This hot gas can be detected in radio and optical domains of the electromagnetic spectrum. By studying this jet we can learn a lot about gravitational waves.”

So if the collision is a gun, you and your radio astronomer colleagues study the smoke after a shot, not the shot itself. That radio emission (the smoke) is unimaginably weak. How did you manage to detect it?

“The combination of dozens of radio telescopes from around the world allow us to obtain an extremely high angular resolution. This is achieved by a technique called Very Long Baseline Interferometry (VLBI). The vision of VLBI is so sharp that if people were playing table tennis on the Moon we would see the moves of the ping pong ball. Data recorded by 33 radio telescopes were sent to our facility in Dwingeloo which is designed to process such data and produce a very sharp image of the radio source.”

What does the image look like?

“In short, we created an image of the jet, which for non-experts looks like a strange pattern with some spots. The shape, its evolution over time and the spectral properties of the jet tell us a lot about the physics of neutron star collisions and generation of gravitational waves. During such events, massive amounts of heavy metals are formed. This collision, for instance, produced a lump of gold with the mass of our planet in just a split second. And it produced many other chemical elements. Eventually, they will end up somewhere in the stars or in planets. The gold and many other heavy elements that we have here on our planet are produced in this kind of event or by the explosion of supernovae.

We hope to find out how common neutron star collisions are. That way we will better understand the evolution of galaxies and stars, the formation and evolution of planetary systems and ultimately also the evolution of conditions suitable for life. We are at the beginning of an exciting scientific journey.”

The VLBI technique is useful for many other purposes. Can you explain?

“It allows us to do many things, for example, measure tectonic plate movements as we can measure distances between radio telescopes. We found that the distance between radio telescopes located on different continents change by millimetres in a year. This has lots of implications for Earth’s tectonics, including events such as earthquakes. 

And we can use VLBI to observe and locate artificial radio sources such as interplanetary spacecraft. This is of great interest for space mission navigators and planetary scientists. ESA’s Jupiter Icy Satellites Explorer mission (Juice) will be launched in 2022. On the way to Jupiter and inside the Jupiter system, we will be able to pinpoint the Juice spacecraft position with 30-70 metre precision. Translated to earthly conditions, that would be comparable to a navigation system in your car pinpointing your location on the road with micron level precision. This part of the Juice mission is called Planetary Radio Interferometry and Doppler Experiment (Pride), and is led by Jive and TU Delft.”

• More information on the VLBI detection of the neutron star collision can be found in: G. Ghirlanda el al., Compact radio emission indicates a structured jet was produced by a binary neutron star merger, Science (2019).