How to detect black holes

A black hole is a huge ‘gravity well’ where matter distorts the structure of space and time so severely that even light rays can no longer travel in a straight line, but rather follow a trajectory curving back towards the hole. Because light cannot escape from black holes, we cannot see them. If we cannot see them, then how do we know they are out there somewhere in the Universe?
We can detect black holes indirectly by measuring their effects on objects around them. Among the many techniques used by astronomers, the most commonly used ones are: analysis of radiation emitted from the neighborhood of black holes, mass estimates of objects orbiting a black hole, gravitational lens effects caused by black holes and detection of gravity waves from merging black holes.
It is often found that a black hole existing near other stars betrays its presence by attracting a great deal of material towards itself from the neighbouring star. This gas gets accelerated to a tremendous speed by the huge gravity of the black hole. As a result, friction within the gas streams causes it to heat up to a temperature of roughly 10 million degrees Celsius, high enough to emit blazing X-rays. These X-rays, emitted not from inside the black hole but from just near its edge, loudly announce its existence.
The space-based Chandra X-ray Observatory, named after the Nobel astrophysicist Subramanyan Chandrashekhar and launched by NASA in 1999, has been specially designed to detect the X-rays emitted from the vicinity of black holes. The most distant black hole discovered by Chandra is some 12.4 billion light-years from Earth, giving us a peek at the life of black holes formed not long after the Big Bang.
Another way to detect supermassive black holes is by observing the motions of stars near the galactic centre.  Because black holes are massive, nearby stars will feel a strong gravity.  As a result, these stars will move quite fast. The motions of these stars are detectable by the ground-based telescope Gemini in Hawaii and the space-based Hubble telescope. These telescopes uncovered a supermassive black hole, 17 billion times the solar mass, in the center of a galaxy in a sparsely populated area of the Universe 200 million light years away. The current record holder tips the scale at 21 billion solar mass and resides 380 million light years away from Earth in the crowded Coma galaxy cluster.
At the centre of our own Milky Way galaxy, astronomers have charted the motions of individual stars revolving around the galaxy center for over two decades. Their motions provided unambiguous evidence for a black hole four million times the solar mass.
It is also possible to detect black holes by the way they bend light – a phenomenon known as ‘gravitational lensing’. The approach draws on Einstein’s prediction that space around massive objects is warped. Surely, it has been observed that the intense gravity from extra-bright galaxies with huge black holes at their core warps space so much that they bend light appreciably to create cosmic magnifying glasses.
Gravitational lensing is particularly useful in detecting supermassive black holes because the amount of bending is directly related to the foreground galaxies’ mass. The greater is the mass, the stronger is the gravity and thus, greater is the lensing effect. Using the awesome power of the Hubble Space Telescope, astronomers have documented many cases of light-bending black holes, particularly in Quasars – brightest objects in the Universe. Until now, astronomers have discovered about 40 Quasars – each with a black hole ranging from 1 to 12 billion times the solar mass.
When a black hole swallows a neutron star or when two black holes merge or just orbit each other very closely, they emit ripples in spacetime known as gravity waves. Hence, probes sitting stationary in space or on Earth will jiggle slightly when a gravity wave rolls by. Using this technique, astronomers have detected gravity waves in two instances. A ground-based observatory with two installations situated in Hanford, Washington and Livingston, Louisiana and called the Laser Interferometer Gravitational-wave Observatory did detect in 2014 and 2016 gravitational waves produced by two massive black holes when they merged 1.3 and 1.4 billion years ago, respectively.
The evidences for the existence of black holes are more convincing now than ever before. We can no longer say that black holes are the concoction of theorists whose mathematics got away from them. They are real and astronomers are not only unveiling new black holes all over the cosmos, they are also challenging our prevailing ideas about how they are formed, thereby providing us with a fertile ground for studying the structure of the Universe.

The writer is Professor of Physics at Fordham University, New York.

How to detect black holes

A black hole is a huge gravity well where matter distorts the structure of space and time so severely that even light rays can no longer travel in a straight line, but rather follow a trajectory curving back towards the hole. Because light cannot escape from black holes, we cannot see them. If we cannot see them, then how do we know they are out there somewhere in the Universe? We can detect black holes indirectly by measuring their effects on objects around them. Among the many techniques used by astronomers, the most commonly used ones are: analysis of radiation emitted from the neighborhood of black holes, mass estimates of objects orbiting a black hole, gravitational lens effects caused by black holes and detection of gravity waves from merging black holes. It is often found that a black hole existing near other stars betrays its presence by attracting a great deal of material towards itself from the neighbouring star. This gas gets accelerated to a tremendous speed by the huge gravity of the black hole. As a result, friction within the gas streams causes it to heat up to a temperature of roughly 10 million degrees Celsius, high enough to emit blazing X-rays. These X-rays, emitted not from inside the black hole but from just near its edge, loudly announce its existence. The space-based Chandra X-ray Observatory, named after the Nobel astrophysicist Subramanyan Chandrashekhar and launched by NASA in 1999, has been specially designed to detect the X-rays emitted from the vicinity of black holes. The most distant black hole discovered by Chandra is some 12.4 billion light-years from Earth, giving us a peek at the life of black holes formed not long after the Big Bang. Another way to detect supermassive black holes is by observing the motions of stars near the galactic centre. Because black holes are massive, nearby stars will feel a strong gravity. As a result, these stars will move quite fast. The motions of these stars are detectable by the ground-based telescope Gemini in Hawaii and the space-based Hubble telescope. These telescopes uncovered a supermassive black hole, 17 billion times the solar mass, in the center of a galaxy in a sparsely populated area of the Universe 200 million light years away. The current record holder tips the scale at 21 billion solar mass and resides 380 million light years away from Earth in the crowded Coma galaxy cluster. At the centre of our own Milky Way galaxy, astronomers have charted the motions of individual stars revolving around the galaxy center for over two decades. Their motions provided unambiguous evidence for a black hole four million times the solar mass. It is also possible to detect black holes by the way they bend light a phenomenon known as gravitational lensing. The approach draws on Einsteins prediction that space around massive objects is warped. Surely, it has been observed that the intense gravity from extra-bright galaxies with huge black holes at their core warps space so much that they bend light appreciably to create cosmic magnifying glasses. Gravitational lensing is particularly useful in detecting supermassive black holes because the amount of bending is directly related to the foreground galaxies mass. The greater is the mass, the stronger is the gravity and thus, greater is the lensing effect. Using the awesome power of the Hubble Space Telescope, astronomers have documented many cases of light-bending black holes, particularly in Quasars brightest objects in the Universe. Until now, astronomers have discovered about 40 Quasars each with a black hole ranging from 1 to 12 billion times the solar mass. When a black hole swallows a neutron star or when two black holes merge or just orbit each other very closely, they emit ripples in spacetime known as gravity waves. Hence, probes sitting stationary in space or on Earth will jiggle slightly when a gravity wave rolls by. Using this technique, astronomers have detected gravity waves in two instances. A ground-based observatory with two installations situated in Hanford, Washington and Livingston, Louisiana and called the Laser Interferometer Gravitational-wave Observatory did detect in 2014 and 2016 gravitational waves produced by two massive black holes when they merged 1.3 and 1.4 billion years ago, respectively. The evidences for the existence of black holes are more convincing now than ever before. We can no longer say that black holes are the concoction of theorists whose mathematics got away from them. They are real and astronomers are not only unveiling new black holes all over the cosmos, they are also challenging our prevailing ideas about how they are formed, thereby providing us with a fertile ground for studying the structure of the Universe. The writer is Professor of Physics at Fordham University, New York.