The Twilight Zone

by Nicholas Mee on January 18, 2022

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Often black holes are portrayed as the most mysterious objects in the universe. Black holes are certainly bizarre and astonishing. Even Einstein thought that such outlandish objects could not really exist. But I am going to argue that rather than being totally mysterious, black holes are actually the most well-understood objects in the universe. First, let’s look a little closer to home.


The planets of the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune are all very different – each has its own distinct characteristics. The same is true of the many satellites of these worlds. Our Moon is quite different to Jupiter’s sulphurous volcanic moon Io, for instance, and both are completely different to Saturn’s gloomy Titan with its thick atmosphere and its lakes of methane.

Huygens at Titan. Copyright: Emile-Raphael Franco.

Each of these bodies could be mapped – and many have been. But to know such a body perfectly would be impossible. We have excellent maps of the Earth, but even our oceans deeps have barely been explored. Perfect knowledge is on another scale altogether. It would require knowledge of the position and identity of every one of the countless molecules of which the Earth is composed. Even then our knowledge would be fleeting as the Earth is a dynamic ever-changing system. The same is, of course, true of all the other worlds in the solar system – and no doubt those in every other star system.

I might be stating the obvious. But I am emphasizing this point to highlight the contrast with what we know about black holes. In 1963, Roy Kerr found a remarkable solution to Einstein’s equation that describes the shape of spacetime outside a spinning spherical body. Its most important application is to describe the spacetime in and around a black hole.

Nobel Prize winning astrophysicist Subrahmanyan Chandrasekhar.

What is truly astonishing is that, according to general relativity, the Kerr solution describes a black hole exactly. The great Indian astrophysicist Chandrasekhar found this revelation overwhelming. He recorded the impact it had on him in the following words:

In my entire scientific life, extending over forty-five years, the most shattering experience has been the realization that an exact solution of Einstein’s equations of general relativity, discovered by the New Zealand mathematician Roy Kerr, provides the absolute exact representation of untold numbers of massive black holes that populate the universe.


Artist’s impression of the Cygnus X-1 black hole.

NASA’s orbiting X-ray observatory is called Chandra and it is named after Chandrasekhar. This is the Chandra Deep Field South image – produced by pointing the X-ray telescope at a patch of sky the size of the Full Moon continuously for over six weeks. In this one small patch, Chandra gathered X-ray data from the blazing-hot accretion discs of five thousand supermassive black holes in ultra-distant galaxies.

Ultra Deep X-ray image produced by the Chandra space X-ray telescope.

Here the Chandra data is combined with data from the Hubble Space Telescope to produce the deepest ever combined X-ray, optical and infrared view of the sky. The X-ray sources detected by Chandra are shown in blue.

All these black holes are described exactly by the Kerr solution. Remarkably, the Kerr solution depends on just two parameters – the black hole’s mass and the rate at which its spins – or its angular momentum. So given these two quantities we know everything there is to know about a black hole. They can have no other features whatsoever – well, in principle they could also carry an electric charge, but there is no known way of giving a black hole a significant charge. The physicist John Wheeler summed this up with a pithy statement:

Black Holes Have No Hair!

The Kerr solution might seem rather complicated and intimidating – and it certainly isn’t straightforward to understand exactly what it means. But the magnitude of this result is totally mind-boggling. It is a short mathematical statement that encodes the exact structure of all the black holes in the universe – this is what Chandrasekhar found so staggering. The insight that this short statement gives us into black holes must be contrasted with the impossibly vast number of quantities that would be necessary to completely encapsulate the structure of the Earth or any other macroscopic body. Black holes are the only objects for which exact knowledge is possible.

Furthermore, the structure of a black hole does not evolve. It only changes due to outside influences, such as when material falls into the black hole increasing its mass and altering the rate at which it spins. So the Kerr solution describes a black hole for all time. This means that a black hole shows no scars of its earlier history. We cannot tell by looking at a black hole whether it formed from a collapsing star, from the collision of two neutron stars or some other esoteric process – the implosion of a giant bag of marshmallows, perhaps. Its entire prior history has been erased.

You might be sceptical, objecting that the Kerr solution is a mathematical result that does not necessarily describe the real world, and until recently you would have had a point. Plenty of simple mathematical results are used to model physical phenomena without representing them exactly. So how do we know that black holes really are described by the Kerr solution? In 2015, the LIGO gravitational wave observatories in the United States detected the first gravitational wave signals. Hundreds of such signals have now been received – most of which were produced by black hole mergers.

LIGO Gravitational Wave Observatory, Livingston, Louisiana.

These signals offer us the first opportunity to test general relativity in ultra-strong gravitational fields. Indeed, computer models based on general relativity are required to interpret the data received by LIGO. Physicists have generated a library of computer simulations of the gravitational waves produced in a range of astrophysical scenarios. When LIGO receives a signal it is matched to a simulation in the library and this reveals vital information about the objects – usually black holes – whose collision produced the gravitational wave signal.

Schematic illustration of gravitational waves from binary black hole system.

The good new is that general relativity has passed every test and it has enabled physicists to extract a great deal of information from these signals – such as the masses of the colliding black holes and the distances to the merger events. So it seems that general relativity is accurate even in the most extreme gravitational scenarios, which means that the Kerr solution really does give a precise picture of a black hole.

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