Inset: The EHT’s image of Sagittarius A*. Bottom: Four of the first ALMA telescopes on the Chajnantor plateau in Chile. Image: EHT/ALMA
- Astrophysicists revealed the first images of the black hole lurking in the heart of our Milky Way galaxy last week at simultaneous press conferences in seven countries.
- Scientists had to literally chase shadows in deep space to get a close look at the black hole – a collapsed star from which not even light can escape because of its incredible gravity.
- The electromagnetic snapshots of Sagittarius A* will help astrophysicists unlock some of the greatest mysteries in cosmology.
- The project had negligible contribution from India despite it operating at the gigahertz frequencies that J.C. Bose first worked out in Calcutta more than 125 years ago.
Astrophysicists revealed the first images of the gargantuan black hole lurking in the heart of our Milky Way galaxy last Thursday at simultaneous press conferences in the US, Germany, China, Mexico, Chile, Japan and Taiwan.
Scientists had to literally chase shadows in deep space to get a close look at the black hole – a collapsed star from which not even light can escape because of its incredible gravity. It is named Sagittarius A*, or SgrA*.
The photos were obtained by an international team using the Event Horizon Telescope (EHT) – a planet-wide network of radio telescopes tuned into the depths of space to detect radio sources near black holes.
Radio telescopes in Hawaii, Mexico, Spain, Chile and Antarctica synchronised their observations with Earth’s rotation so that they became a virtual, gigantic radio telescope whose baseline (the maximum distance between telescopes) equalled the planet’s diameter. This increased the EHT’s resolution so much that it could identify objects the size of a ping pong ball on the Moon!
Radio telescopes of course do not ‘see’ things. Instead, their dish antennae ‘listen’ to the radio-frequency emissions from outer space. They extract these signals from the hiss of static and convert them to data and images on computer screens.
The EHT team sifted through vast amounts of data produced this way, on their supercomputers, for months on end to finally create the shadow of SgrA*, situated some 27,000 lightyears from Earth. The Milky Way is a spiral galaxy of more than 100 billion stars and seen face on looks like a pinwheel firecracker. The Sun and its retinue of planets are located on one of its spiral arms while SgrA* sits at the centre.
This is not the first time that EHT has clicked pictures of a black hole. In 2019, researchers used the EHT to take the first-ever photo of a black hole: a colossal monster at the centre of another galaxy called Messier 87; it was accordingly named M87*. The image showed a bright halo of red, yellow and white around a dark centre – not unlike the snapshot of SgrA* the world saw the other day.
While black holes are virtually invisible entities, it is possible for light passing near a black hole’s boundary, or event horizon, to escape capture and instead just be bent. Matter falling into the black hole is also heated up and emits radiation. These emissions together make the black hole’s surroundings glow, forming what looks like a halo around a dark central region. The EHT researchers hunted for these shadows to find the lair of the gravitational beast at the core of our galaxy.
The electromagnetic snapshots of SgrA* will help astrophysicists unlock some of the greatest mysteries in cosmology.
“These unprecedented observations have greatly improved our understanding of what happens at the very centre of our galaxy and offer new insights on how these giant black holes interact with their surroundings,” EHT project scientist Geoffrey Bower of the Institute of Astronomy and Astrophysics in Taipei, Taiwan, said in a statement.
For example, the EHT team confirmed that the size of the ring that gave away SgrA*’s whereabouts is consistent with one of the cornerstones of modern physics: Albert Einstein’s general theory of relativity. This theory insists that the speed of light equals that of gravity and also that mass distorts space, causing rays of light to bend in the presence of a gravitational field.
This phenomenon of every object bending light through its gravity – known as gravitational lensing – is very weak for our Sun, but has been measured. Gravitational lensing is much stronger with more massive and distant objects in space. Efforts made to observe gravitational lensing near a black hole or to directly photograph the dark disk surrounding it, however, were in vain – until the EHT team imaged M87* in 2019.
More than a century after Einstein developed the concept, the general theory of relativity remains a cornucopia of elusive possibilities, all of them defying attempts to confirm or rule them out. So the pictures of black holes’ shadows – one of nature’s most inscrutable creations – taken by the EHT could be ideal observational test-beds for the theory.
Thus, the image of SgrA* captured by EHT proves that the immense gravitational field of the black hole does bend light to the same extent anticipated by the theory. As Bower said, “We were stunned by how well the size of the ring agreed with predictions from the GRT.”
The global celebration of the photograph of SgrA*’s halo and shadow would perhaps be incomplete without a postscript from India. The EHT collaboration has had negligible contribution from the subcontinent, despite the fact that the project operates at the very gigahertz frequencies that Jagadish Chandra Bose first worked out in Calcutta more than 125 years ago.
Black hole research is built on accretion astrophysics, which is a field where a number of scientists in India have made their mark. Yet India is not part of the EHT project.
“Astrophysicists in India must join the EHT project where they have an important role,” says Tapas Kumar Das, an associate professor of astrophysics at the Harish Chandra Research Institute in Kolkata. “Detailed knowledge on accretion astrophysics is important for the future goals of the EHT, and the Indian astrophysics community has enormous potential to contribute to it in the near future.”
Prakash Chandra is a science writer.