Stellar Trashpiles?

[mtvessel]

Apr 2023
A Brief History of Black Holes - Dr Becky Smethurst – Macmillan, 2022
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The author is an Oxford University Junior Research Fellow in astrophysics who, in the way of modern academics, is also a YouTuber with over 700,000 subscribers (including me). Her channel covers all of space news, but her research focuses on the super-massive black holes that are now thought to exist at the centres of most galaxies. This is a subject I know little about - supermassive black holes not being a thing during my formative years - so I was interested to see if her breezy and enthusiastic presenting style, which is excellent for explaining the minutiae of scientific papers, would translate well to the page. On the whole, it does, though there were a few annoyances.

The book starts with the question of what mechanism powers the sun and the historical process by which this was worked out. After some amusingly wrong answers (Smethurst cites a Scientific American article from 1863 entitled "Experts doubt the Sun is actually burning coal"), we get to the process of fusion under gravity that is now generally accepted to be case. The following chapters show how scaling up stars and considering what happens at the end of their lives leads to the idea of a "gravitationally completely collapsed object", unfortunately dubbed a black hole by the American physicist Robert H. Dicke after the British imperialist tale of the black hole of Calcutta (Smethurst prefers the term "dark star", or even "dark mountain").

But how do you show that an object that, by definition, gives off no light (but see below), actually exists? This involves a detour into the fortuitous discovery of x-rays and the subsequent detection of x-ray sources that were too powerful to be caused by anything other than a black hole sloughing matter off neighbouring stars and spinning it into an accretion disk, which ironically makes black holes some of the brightest objects in the universe. Then there is the recent detection of the gravity waves emitted by two black holes spiralling into each other by the amazing detectors LIGO and VIRGO, which can detect changes in spatial dimensions that are less than the diameter of a proton. Both of these means of detection rely on the fact that black holes have more than one form of energy - they may not emit light, but they have kinetic energy and angular momentum, just like any other object with mass.

After a diversion into whether the proposed "Planet Nine" suggested to explain the unusual orbits of extreme trans-Neptunian objects such as the dwarf planet Sedna could in fact be a tennis-ball sized black hole operating in the Oort cloud (probably not, she concludes reluctantly), Smethurst focuses in on the opposite end of the size spectrum - the supermassive black holes now thought to exist at the centres of most galaxies. We know that there must be one at the heart of the Milky Way because it is the only plausible explanation for the extremely rapid orbits (over 11 million km/h) of nearby stars. The fascinating question - and the subject of Smethurst's research - is how supermassive black holes co-evolve with galaxies. Are they, in effect, trash piles of dead stars, or are they the kernels around which galaxies accrete?

Unsurprisingly, the answer, so far at least, is "it's complicated". There is a well-established correlation (the Magorrian Relation) between the mass of a supermassive black hole and the mass of the stars in the galactic bulge that surrounds it. This is not caused by the black hole's gravitational influence - unlike the solar system, where the sun forms 99.8% of the total mass, a supermassive black hole is only 0.006% of the mass of a galaxy - so the cause of this correlation is unclear. One possibility is that the mechanics of colliding galaxies creates the relationship; the more stars each galaxy has, the more their orbits are disrupted by the collision and the more material the black hole at the centre of the merged galaxy has to feed on. Another is the accretion disk itself - radiation pressure from it acts as a limiter on the rate at which the black hole can grow by pushing material away (the Eddington limit), and it is possible that the accretion disk outflows also serve to "poison" the creation of new stars, creating a negative feedback effect on galaxy development. Time will tell which if any of these theories are correct.

The final question that Smethurst addresses is whether or not black holes are eternal. After all, if nothing can escape a black hole's event horizon, surely there is nothing to stop it from existing as a permanent knot in spacetime? Ah, but that would break the second law of thermodynamics. Black holes act as entropy hoovers, locking disordered matter away from the rest of the universe and thereby increasing order in their vicinity, and the second law states that the entropy of the universe as a whole must increase. So where does the entropy of the mass absorbed into a black hole go? The solution to the paradox was arrived at by Princeton PhD student Jacob Bekenstein, who realised that as a black hole absorbs more mass, the surface area of its event horizon increases. The entropy of a black hole must be linked in some way to its event horizon. But increasing entropy is also associated with heat transfer from hot to cold, which means that it cannot be true that black holes are completely black - their event horizons must be emitting energy somehow.

Enter Professor Stephen Hawking, who worked out that a black hole would disrupt quantum oscillation modes of the vacuum state of the space in which it sits, causing energy to be emitted as light with wavelengths similar to the diameter of the black hole's event horizon and a spectrum equivalent to a black body emitting heat into its surroundings. This Hawking radiation is emitted at a very low rate, but the energy for it comes from the black hole's mass. So eventually a black hole will evaporate. It's worth noting that this is entirely theoretical - there is not yet a shred of actual evidence for Hawking radiation, since you are only likely to detect it in the last 0.1 seconds of a black hole's existence - but astrophysicists live in hope that it will be seen one day.

Smethurst explains all this with her customary clarity, though she falls into the all-too-common trap of adding jokey pop-culture references that will date horribly (dear authors, when doing a final edit of your MS, please consider whether it will be comprehensible to readers in 20 or 50 years' time). I would also have liked a little more detail on some of the processes involved and the observations that led to them being worked out. Her enthusiasm for this fascinating subject, however, cannot be denied, and I enjoyed reading it.