Was it really a black hole that the EHT imaged in 2019

Context: "Was it really a black hole that the EHT imaged in 2019?" is an important topic for UPSE GS Paper 3 and Prelims.

The debate among physicists takes off over alternative possibilities such as naked singularities into the realms of time travel and loops of time

• In 2019, astronomers of the Event Horizon Telescope captured the first-ever image of a supermassive black hole (M87*) which was located at the centre of a galaxy Messier 87 (M87).
• This black hole is calculated to be 6.5 billion times the Sun’s mass and is 55 million light-years away from the Earth. The discovery set the world of astronomy on fire and also found a mention in the “popular information” section of the announcement of the Nobel Prize in physics for 2020.
• Now, a paper published in The European Physical Journal C brings in an alternative explanation for the compact object that was imaged by the Event Horizon Telescope. The authors say it (M87*) is not necessarily a black hole but could even be a “naked singularity with a gravitomagnetic monopole.”
• When stars much more massive than the Sun reach the end of their lives, they collapse under their gravity, and the product of this collapse, most astronomers believe is a black hole.
• A black hole has two parts: At its core is a singularity – an infinitely dense point, as all the remnant mass of the star, is compressed into this point.
• Then there is the event horizon – an imaginary surface surrounding the singularity, and the gravity of the object is such that once anything enters this surface, it is trapped forever. Not even light can escape the pull of the singularity once it crosses the event horizon.
• That is why we cannot see the singularity at the heart of a black hole but only see points outside the event horizon.

Troublesome sibling

• In many scenarios of stellar collapse, the event horizon does not form, and the singularity is exposed to the outside, without any event horizon shielding it.
• Pankaj S. Joshi, a specialist in the general theory of relativity and cosmology, calls this “naked singularity” a “troublesome sibling” of a black hole in an article by him in Scientific American.
• If this weird object should exist, observers can, in principle, see the bare, or “naked” singularity as it is called.
• While many physicists object because of some seriously problematic issues associated with the solutions, these nevertheless exist, puzzling researchers, perhaps embarrassing them.
• Prof. Joshi is the Founding Director of International Center for Cosmology and Advisor, Charusat University, Anand, in Gujarat, and is not involved in the work under discussion.
• In the above paper, Chandrachur Chakraborty, from the Department of Physics, Indian Institute of Science, Bengaluru, and Kavli Institute of Astronomy and Astrophysics, Peking University, Beijing, China, and his collaborators show that M87* could be either a black hole or a naked singularity and each of these possibilities could be plain or coupled with what is called a gravitomagnetic monopole. In all, that leaves four possibilities in principle.

Monopoles and gravity

• In the nineteenth century, James Clerk Maxwell unified electricity and magnetism as one combined phenomenon, showing that light is an electromagnetic wave.
• But there is an asymmetry between electricity and magnetism. While positive and negative electric charges can be found to exist independently, the poles of a magnet are always found in pairs, north and southbound together. Dr Chakraborty draws upon an analogy between gravitational force and electromagnetism to explain that in 1963, Newman, Tamburino and Unit (NUT) proposed a theoretical concept called a “gravitomagnetic charge” also called a gravitomagnetic monopole.
• “We have shown that M87* could be a black hole (with or without gravitomagnetic monopole) or a naked singularity (with or without gravitomagnetic monopole) … The Event Horizon Telescope collaboration mapped it to a black hole only and did also not consider that it might contain a gravitomagnetic monopole,” says Dr Chakraborty.
• But this is where a difficult situation arises. When you allow the NUT-like solutions, there is a possibility of “closed timelike loops.”
• These are regions in parameter space of spacetime in which the past merges with the future allowing many bizarre situations. Hence, this solution comes in for strong criticism from physicists.
• “The Kerr and the Schwarzschild solutions of Einstein's equations [namely black holes with spin and having an event horizon] are considered appropriate for describing astrophysical black holes, like the one that has been recently imaged in M87.
• However, the Kerr-NUT solutions describing gravitomagnetic monopoles are not regarded in the same way,” says Joseph Samuel, a physicist from Bengaluru-based International Centre of Theoretical Sciences, who has worked in General Theory of Relativity and Cosmology. “These solutions suffer from a pathology known as closed timelike curves.
• In such spacetimes, time is a periodic variable [that is, everything repeats after some time]. One can travel into the past and create logical contradictions by killing off one's ancestors,” Prof. Samuel adds.
• Prof. Joshi has this to say. “Some important solutions to Einstein equations do admit closed timelike curves. Despite this, the solutions have been studied because they have some other useful or important or interesting features. So, you have to take them in that spirit. This is why the so-called wormhole solutions that also admit closed timelike curves are very much under study today.”

Another approach

• There is another approach. Why discard the entire solution? Only those regions of parameter space that give rise to the closed timelike loops could be thrown away. This is suggested by Prof. Joshi. “Not all solutions of Einstein equations have closed timelike curves, but some have… If you do not like closed timelike curves then you have to discard away these solutions. It is as simple as that. That is the status of the theory right now,” he says, clarifying that not all naked singularity solutions have closed timelike curves, many of them do respect causality.
• Thus, an old debate resurfaces with this paper: to throw out these solutions or keep them and handle them carefully.