China has the world’s largest telescope, but why wasn’t it used to help capture the world’s first black hole pictures?

China has the world’s largest telescope, but why wasn’t it used to help capture the world’s first black hole pictures?

The 500-metre aperture spherical telescope in Guizhou province is the largest single radio dish on the planet, but was not designed to look at the signals that black holes emit


The world's largest telescope is located in Pingtan, Guizhou province in China.
Photo: Xinhua

To create the first photo of a black hole, scientists analysed signals from some 55 million light-years away that crossed intergalactic space to reach an array of telescopes spanning the globe, from Hawaii to the South Pole.

On April 10, the world saw the first photo of a black hole, an image made possible because telescopes around the planet were joined in a two-year project to create a virtual radio antenna almost the size of the Earth itself, enabling astronomers to pick up faint signals from the far reaches of the universe.

None of the telescopes, however, is in China. This is despite the country having a 500-metre aperture spherical telescope (Fast) in Pingtan, Guizhou province, the largest single radio dish on the planet; and the Guoshoujing telescope in Meiyun, Beijing, which can scan the night sky quicker than any competitor.

The reason China did not contribute hardware to the black hole imaging project, known as the Event Horizon Telescope (EHT), is that none of the country’s telescopes was built for the task, the Chinese astronomers involved in the project said.

Black holes cannot be seen because their gravitational pull is so strong that light cannot escape. To draw a clear boundary of a black hole, astronomers turned their attention to light-emitting materials that were sucked into it. To detect these materials, especially those close to the black hole’s event horizon, where gravity is at its strongest, a telescope must be able to pick up very high frequency radio waves to produce a sharp image.

Professor Wu Xuebing, director of the department of astronomy in the school of physics at Peking University, said most Chinese telescopes were designed to detect centimetre waves, but the international project is working on energy where frequency is measured in millimetres.

“They are looking at different things,” he said. The Fast telescope, for instance, could pick up signals from a pulsar, the kind of star that emits a fixed pattern of electromagnetic waves, but Fast is almost deaf to radio signals from a black hole. “The amount of data processed is enormous. It is an achievement not possible for any single country, but took global collaboration,” Wu said.

A photo of a black hole was only possible with the co-operation of many different space agencies.
Photo: AFP

Bu Defu, an astronomer who studied black hole physics at the Chinese Academy of Sciences’ Shanghai Astronomical Observatory but who was not involved in EHT, said the photo could be important because it might contain valuable insights into the physics governing the evolution of our universe. “It will put theory of general relativity to the test,” he added.

Albert Einstein’s theories posited the existence of black holes. Although scientists have obtained much indirect evidence that suggested the existence of such celestial bodies, no one has seen one.

But Professor Yang Wei, an astrophysicist with Chinese University of Science and Technology in Hefei, Anhui province, said that the image, even at the highest possible resolution, would still leave some fundamental questions unanswered.

For example, the very centre of the black hole is believed to be smaller than an atom. The subatomic world, however, is governed by quantum physics, a set of rules completely different from the equations laid out for the universe by Einstein.

The black hole photo could tell us little, if anything, inside the shadow of the black hole, Yang said. “The ultimate truth will still remain in the dark,” he said.

Edited by Doris Wai

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