Einstein's Enigma: The Black Hole Story

Authors: Shreya Pithva & Sibsankar Palit

SpaceCrew Working Group, InnovaSpace

Albert Einstein: (14/03/1879 - 18/04/1955) © Nobel Foundation archive.

​Space is vast and unexplored. And in its vastness, there lie mysterious corners. Black holes are one among those less-known parts of our universe that go beyond our comprehension. In simple words, black holes are areas where spacetime is so strongly drawn due to gravitational force that nothing, literally nothing, not even light, can escape it. The very idea of a black hole was first proposed by Albert Einstein based on his General Theory of Relativity in 1915. His equations indicated that if a mass were compact enough, it would warp spacetime so much that a black hole would be created. It was even astonishing to Einstein himself, who was not so convinced initially, to accept that mass could collapse into a singularity or a point in spacetime with infinite density. He even expressed his doubts to French physicists during the 1920s, suggesting that singularities could be a defect in his own proposed theory.

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​Radio astronomy, started in the 1930s when Karl Jansky discovered radio waves from the Milky Way. This discovery was pivotal in the development of our knowledge about black holes. Technology made it possible for astronomers by the 1950s to map out the sky more accurately. The Cambridge Radio Telescope and Jodrell Bank Observatory detected unusual radio sources, which were point-like objects showing unusual brightness in the radio spectrum. A breakthrough finding came in the late 1950s when radio sources such as 3C 273, a quasar in the Virgo constellation, were discovered not to have any corresponding visible objects. A quasar is an extremely luminous active galactic nucleus (AGN) powered by a supermassive black hole at the centre of a distant galaxy. Optical observation found faint, stellar counterparts with mysterious emission lines. These objects produced large amounts of radiation at varied frequencies, but no source was seen visually, except for a very faint, point-like object looking like a star at a distant place. The spectral lines, which normally signified the existence of chemical elements, were mysterious. In addition, these objects displayed quick luminosity changes in both optical and X-ray regimes.

Complex enough? Ok! To understand this, let’s use the Einstein technique. Let’s perform a thought experiment! Imagine seeing an object in the sky that suddenly changes its brightness. From the perspective of an observer on Earth, luminosity increases to its ultimate value gradually because photons from the front of the object reach earlier than those from the back. By timing how long the luminosity would take to settle, astronomers would be able to estimate the object's size—the principle of "light travel time and variability." From these observations, it was seen that while these objects were no bigger than our solar system, they contained the light of a whole galaxy, signifying an extremely high power density.

Astronomer Maarten Schmidt proved that the lines were very probably the usual spectral lines of hydrogen, which had been 15.8% redshifted i.e. stretched to longer wavelengths towards reddish regions. This was, at the time, an incredibly high redshift, and only a handful of much less luminous galaxies were known to have higher redshifts. If this redshift was caused by the physical motion of the "star," then 3C 273 was traveling at a mind-boggling speed of approximately 47,000 km/s, many times faster than any star and frankly quite impossible to explain. In addition, that extreme speed would not be responsible for the extraordinary radio emissions from the 3C 273 quasar.  But, assuming the redshift was indeed cosmological—an assumption now considered to be valid—the huge distance meant that 3C 273 was so much brighter than any galaxy but a great deal smaller. The general understanding today is that this process results from material in an accretion disk i.e. a rotating disk of gas and dust, collapsing into a supermassive black hole at its centre.

Fig 1: Image of quasar 3C 273 taken by Hubble Space Telescope (HST)

Quasars, which are also known as "quasi-stellar radio sources," are some of the brightest objects in the universe and reside in the centres of some galaxies. Quasars are fueled by supermassive black holes that are actively accumulating and coalescing matter. When material is pulled into a black hole, it settles into an accretion disk, releasing angular momentum and spiraling toward the compact object. The accretion disk gets heated to millions of Kelvins from the dissipation or fading away of gravitational energy and radiates radiation from radio waves to X-rays. By the 1970s, theories describing the energy generation in accretion disks were formulated, and indirect evidence for black holes was coming forward. Observations also reported that most galaxies, including our Milky Way galaxy, contain supermassive black holes at their center. Quasars, which are more common in the early universe, were found to be AGN fuelled by matter accreting into supermassive black holes.

Fig 2: Image (above) and simulation (below video) made by 2020 Physics Nobel Laureate Andrea Ghez and her group using Keck Telescope on the black hole (Sagittarius A*) at the centre of our Milky Way galaxy.

Fig 3: The supermassive black hole imaged by the Event Horizon Telescope (EHT) is located in the centre of the elliptical galaxy M87.

To calculate the mass of these black holes, astronomers observed the motion of their nearby stars. Applying Kepler's laws of motion, they calculated the orbital periods and distances of stars that revolve around the black hole. This information is enough for them to calculate the gravitational force and, in turn, the mass of the black hole. For example, the Milky Way's central black hole, Sagittarius A*, has a mass of approximately 4.1 million times that of our Sun. Unbelievable, isn’t it? That’s all with the tale of black holes, from Einstein doubting prophecies to the finding of quasars and the historical picture of the black hole in galaxy M87, goes on unfolding, showing the incredible power and enigma of the universe we live in.
​No doubt why Einstein quoted for a reason:

“The most beautiful experience we can have is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. Whoever does not know it and can no longer wonder, no longer marvel, is as good as dead and his eyes are dimmed."

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