By Kumar David –

Prof. Kumar David

“Black holes are regions in space where gravity is so strong that nothing that enters can escape, including light” ~ Stephen Hawking as a rising physicist.

“Black holes aren’t as black as painted. They are not the eternal prisons once thought. Things can get out on the outside and possibly to another universe inside” ~ Stephen Hawing, Commun. Math. Phys. 43, April, 1975

That Black Holes (BH) exist was foreshadowed in Einstein’s General Relativity paper of 25 Nov. 2015, though it was incorrectly attributed to Stephen Hawing. What Hawing proposed was a theory that they emit radiation which can be detected and this makes an observational quest for a BH Holy Grail, meaningful. A BH is a weird thing; it starts when a massive star, usually more massive than our sun explodes in a supernova and then collapses sucking in nearby stars and portions of its galaxy. It is the most powerful event known in the universe apart from the Big Bang 14 billion years ago. The edge of a BH is the Event Horizon and nothing that crosses it ever comes out; it emits no light it reflects no light; trillions of tons of matter, millions or billions of stars disappear into who knows where, it is a one way street. An Information Paradox emerges – information ceases to exist and without data, measurement and chronology the laws of physics go belly-up. Where the hell does everything go? Maybe to hell indeed establishing the realty of hell to the delight of the faithful! Some like Hawking speculate that maybe the anus of a BH could be another parallel universe but to the common sense (quotidian) rational mind such strangeness even cast in the symbols of mathematics is no different from the rantings of religious quacks. But theoretical physics is not my speciality so I will draw up my paper and say no more on these strange philosophical disputations.

Supermassive black-hole at centre of the M87, 53 million light-years away.
(Image Credit: Event Horizon Research Group, April 2019)

Stephen Hawking is renowned for his thesis that “black holes aren’t as black as painted; they are not the eternal prisons once thought. Things can get out.” Well not quite, but at the event horizon matter may fizz; what is now called Hawking Radiation is released because of quantum effects at the event horizon; matter degenerates into pairs, photons and neutrinos, particle and anti-particle etc. It’s a bold attempt to marry general relativity (colossal gravity) with quantum physics (particle physics). Let’s stop the weird stuff here and get to a real story. Irrespective of parallel universe speculations and whether information can really leak out of the deepest entrails of a BH, there is one interesting point. Searching for Hawking Radiation is a way of searching for a BH event horizon  – you see though the existence of BHs have been theorised from the time of General Relativity none have actually been seen (cannot be seen) by a telescopic or any device. Eureka let’s search for Hawking Radiation, and if we finds it we can surmise that a BH lies hidden behind its veil.

The problem is that to look sufficiently far into the cosmos – and correspondingly back in time – and see something even as massive as a BH requires a gigantic telescope, one as big as the earth itself. Hooray, the Event Horizon Telescope (EHT) to the rescue! The EHT consists of eight massive telescopes distributed in different parts of the world – two in continental USA and one each in Hawaii, Mexico and the South Pole and two in Chile. All carefully coordinated and synchronised to peer exactly at the same point in space all at once. It’s like smashing a mirror into pieces, distributing the pieces in different locations, collecting the individual images and using clever algorithms and super computers to fill the gaps and build a composite image.

Visualization of a BH. Matter falling in first collects in a thin hot accretion disk.
(Source: Harvard-Smithsonian Centre for Astrophysics)

The BH of choice was the one at the centre of M87 a supergiant elliptical galaxy with several trillion stars, about a thousand times larger than our Milky Way. It is 53 million light-years away but one of the brightest radio sources in the sky. True the BH at the centre of the Milky Way is only 26,000 light years away so why was it not selected? Well although M87 is 2000 times farther away the BH at its centre is humongous, about 1500 times larger and its mass is equal to 6 billion suns! Anyway ours is not to question why the boffins in charge of the project – I believe in consultation with Hawking – decided on the M87 BH. Imaging and data collection using the globally distributed EHT array started in 2017 but it took a long time to get all the observations together and the final processed image was released on 10 April 2019. For the first time an image of a BH Event Horizon was seen. Interestingly this is almost 100 years to within weeks of when Arthur Eddington looked at the stars beyond a solar eclipse from Principe Island, West Africa on 29 May 1919. Edington was so overjoyed by observations that confirmed Einstein’s theory that he parodied Omar Khayyam in his notebook:

Oh leave the Wise our measures to collate

One thing at least is certain, light has weight

One thing is certain and the rest debate

Light rays, when near the Sun, do not go straight.

April 2019 is not as breath-taking for science as was May 1919 but it does make a point and opens a can of worms. A Princeton PhD student Jacob Bekenstein was dissatisfied with Hawking’s original formulations and reasoned that if a BH emits radiation it does not violate the sacrosanct Second Law of Thermodynamics. Meaning in laymen’s words that it cools with time and eventually disappears. That opens a Pandora’s Box; what about the alternative universe at the anus? If BHs radiate and eventually evaporate and disappear does all the information that fell in simply vanish? If ‘yes’ it violates quantum mechanics (entanglement); if ‘no’ it violates general relativity (gravity).

The EHT experiments and observations of 2016-19 did not resolve these conundrums but they did establish that Hawing radiation exists, like tell-tale loose hairs sticking out of a bald pate and can be used to target a BH. Perhaps Hawking’s two greatest contributions were the prediction that black holes emit radiation and the attempt to fuse quantum physics and relativity, if not as a general theory at least in the examination of a particular problem. He will be remembered as the Twentieth Century’s foremost physicist next to Einstein. The great paradoxes however remains unsolved – but the EHT experiments were not designed to deal with them.

When pairs of particles and antiparticles spawn in the event horizon, quantum physics says that the pair share a connection, an entanglement; this is information. But one is released as Hawking radiation while the other falls into the BH and everything about it is forever lost. The Information Paradox is reborn. Some physicists lamely hold that information is preserved because new correlations are created among the particles in the Hawking radiation. But from the perspective of the particle falling freely into the BH this is b-s. Gravity disappears for a free-falling observer hence it sees no event horizon and therefore remains entangled with its soul-mate. Quantising gravity, as Hawking did, has created a new set of unresolved contradictions. Whenever science solves a riddle it opens the door to ten others; therefore scientists are never unemployed.

[Note: I consider it the responsibility of ageing academics from time to time to present new trends in technology and science in a form intelligible to a wide readership. I have undertaken, I think successful interventions in electrical energy issues. I am not so sure about this piece which attempts to update the general reader about the Event Horizon Telescope and the involvement of 250 scientists from across the world from 2016 to 2019. Will it hold a lay reader’s attention to the end?  If those with an interest in modern science and school kids with an exposure to A-Level physics keep reading right to the end, I will not ask for more].