Home Electromagnetic Up Close and Personal with a Gravitational Beast – The Wire Science

Up Close and Personal with a Gravitational Beast – The Wire Science

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Inset: the image of Sagittarius A* from the EHT. Bottom: Four of the first ALMA telescopes on the Chajnantor plateau in Chile. Image: EHT/ALMA


  • Astrophysicists revealed the first images of the hidden black hole at the heart of our galaxy, the Milky Way, last week during simultaneous press conferences in seven countries.
  • Scientists had to literally chase shadows into deep space to get a close look at the black hole – a collapsed star from which not even light can escape due to its incredible gravity.
  • The electromagnetic snapshots of Sagittarius A* will help astrophysicists unravel some of cosmology’s greatest mysteries.
  • The project had a negligible contribution from India despite operating at the gigahertz frequencies that JC Bose first developed in Calcutta over 125 years ago.

Astrophysicists revealed the first images of the gargantuan black hole that lurks at the heart of our galaxy, the Milky Way, last Thursday during simultaneous press conferences in the United States, Germany, China, Mexico, Chile, the Japan and Taiwan.

Scientists had to literally chase shadows into deep space to get a close look at the black hole – a collapsed star from which not even light can escape due to its incredible gravity. He is named Sagittarius A*, or SgrA*.

The photos were obtained by an international team using the Event Horizon Telescope (EHT) – a planetary array of radio telescopes tuned deep in space to detect radio sources near black holes.

The radio telescopes in Hawaii, Mexico, Spain, Chile and Antarctica synchronized their observations with the rotation of the Earth, so that they became a gigantic virtual radio telescope whose baseline (the distance maximum between the telescopes) was equivalent to the diameter of the planet. This increased the resolution of the EHT so much that it could identify objects the size of ping-pong balls on the Moon!

Of course, radio telescopes don’t ‘see’ things. Instead, their satellite dishes “listen” for radio frequency emissions from outer space. They extract these signals from the hiss of static electricity and convert them into data and images on computer screens.

The EHT team sifted through the vast amounts of data thus produced, on their supercomputers, for months to finally create the shadow of SgrA*, located some 27,000 light-years from Earth. The Milky Way is a spiral galaxy with over 100 billion stars and seen from the front looks like a pinwheel firecracker. The Sun and its procession of planets are located on one of its spiral arms while SgrA* is in the center.

This isn’t the first time EHT has clicked images 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 center of another galaxy called Messier 87; it was therefore named M87*. The image showed a glowing halo of red, yellow and white around a dark center – much like the SgrA* snapshot the world saw the other day.

While black holes are virtually invisible entities, it is possible for light to pass near the boundary of a black hole, or event horizon, to evade capture and instead simply be bent. Matter falling into the black hole is also heated and emits radiation. These emissions cause the surroundings of the black hole to glow, forming what looks like a halo around a dark central region. EHT researchers searched for these shadows to find the gravitational beast’s lair at the heart of our galaxy.

SgrA*’s electromagnetic snapshots will help astrophysicists unravel some of cosmology’s greatest mysteries.

“These unprecedented observations have significantly improved our understanding of what is happening at the very center of our galaxy and offer new insights into how these giant black holes interact with their environment,” said EHT project scientist Geoffrey Bower. from the Taipei Institute of Astronomy and Astrophysics. Taiwan, said in a statement.

For example, the EHT team confirmed that the size of the ring that revealed the location of SgrA* is consistent with one of the cornerstones of modern physics: Albert Einstein’s theory of general relativity. This theory insists that the speed of light is equal to that of gravity and that mass distorts space, causing light rays to bend in the presence of a gravitational field.

This phenomenon of each object distorting light by its gravity – known as gravitational lensing – is very small for our Sun, but has been measured. The gravitational lens is much stronger with more massive and distant objects in space. Efforts to observe the gravitational lens near a black hole or to directly photograph the dark disk around it were unsuccessful, however – until the EHT team imaged M87* in 2019.

More than a century after Einstein developed the concept, the theory of general relativity remains a cornucopia of elusive possibilities, all defying attempts to confirm or rule them out. So the images of black hole 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 huge gravitational field of the black hole deflects light to the same extent predicted by theory. As Bower said, “We were amazed at how well the ring size matched the GRT’s predictions.”

SgrA*’s worldwide celebration of halo and shadow photography 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 on the very gigahertz frequencies that Jagadish Chandra Bose first developed in Calcutta more than 125 years ago.

Black hole research draws on accretionary astrophysics, an area in which a number of Indian scientists have made their mark. However, India is not part of the EHT project.

“Astrophysicists in India need to join the EHT project where they have an important role,” says Tapas Kumar Das, associate professor of astrophysics at the Harish Chandra Research Institute in Kolkata. “A detailed knowledge of accretionary astrophysics is important for the future goals of the EHT, and the Indian astrophysical community has enormous potential to contribute to this in the near future.”

Prakash Chandra is a science writer.