Black Holes

by Brian Cox and Jeff Forshaw

A Review

‘Black holes are perfect for learning about physics because understanding them requires pretty much all of it.’

‘What happens at the singularity of a black hole is beyond current understanding.’

Sometimes you think you know quite a lot about a subject, only to find, after reading another book, that you know a lot less than you thought.

Cox and Foreshaw’s latest offering, Black Holes, is such a book. Marketed in some ads as A Brief History of Time for the 21st Century, probably with justification, it offers a complete history of the subject from the late 18th century to the cutting edge mathematical physics of today. Perhaps it’s only me, but I found the original Hawking A Brief History of Time a whole lot easier.

I should be careful here, because the term black hole did not exist when I studied physics back in 1960; it wasn’t even on the curriculum. Scientists had known about the theoretical stuff for some time, that there could be astronomical objects capable of trapping light. They knew about white dwarves, and – from the work of Chandrasekhar in the 1930s – that if a star was sufficiently massive, the electron pressure inside could no longer resist gravity and it would collapse to a point. The idea was treated with scorn by some physicists and remained largely theory until Jocelyn Bell discovered pulsars in 1967. Now we know there are billions of black holes throughout space; it seems the universe is teeming with these things, and there are even ‘super massive black holes’ at the centre of galaxies.

The quotations at the head of this article sum up the book very well. Here, Cox and Forshaw take us on a roller coaster ride, to quote Star Trek, where no one has gone before. Using Penrose diagrams, graphical representations of how spacetime behaves, but more particularly in the presence of strong gravitation, they follow a road from Einstein’s theories of relativity through quantum entanglement to modern information theory. They use analogy to describe what intrepid astronauts (the immortals) would experience on and around the event horizon, and even beyond it en route to the singularity itself.

‘All the immortals in flat spacetime live forever, no matter how they move. In Schwarzschild spacetime, every worldline that enters the upper triangle must end on the singularity.’

Beginning with a general review of black hole history, the book moves on to present the idea of spacetime represented on a two dimensional surface. It’s rather like the one used in a Mercator atlas, in which lines of longitude are flattened out and distort the sizes of the continents. After a discussion of light cones and Minkowski (flat) spacetime, the next chapters introduce the warping of spacetime by gravity, Schwarzschild black holes and the paradoxes that arise in distorted space. There is a chapter on white holes and wormholes. After that, we go on to the more difficult stuff – how “real” black holes form, thermodynamics, so-called Kerr (spinning) black holes and Hawking radiation. The paradoxes mount up.

‘From the perspective of someone falling through the horizon, they feel nothing and are oblivious to …. their fate [to be] both spaghettified in the singularity (from their own perspective) and burnt up on the horizon (from an outsider’s perspective). This is the essence of black hole complimentarity resolution to the black hole information paradox. It is nonsense.’

How, I used to wonder, if black holes swallow up everything that approaches too closely, does the radiation escape? The professors explain this very well, so now I know, but it takes careful reading.

The final chapters of the book are really tricky and deal with entanglement, holograms and the encoding of information. From studying quantum theory, I sort of get entanglement, but quantum computing, q-bits and a holographic universe are pretty much new territory.

Black Holes is a good read, but you have to be really interested in the physics of relativity, quantum theory and (I think) mathematics first. Cox and Forshaw seem to go out of their way to explain all aspects of their subject. Unlike Stephen Hawking, they don’t mind using equations to amplify the text. There is humour too occasionally, some of it political, which lightens the serious mood; one can almost hear Brian Cox’s voice!

However, I found parts of the book difficult, especially the last couple of chapters. There is no skipping lines anywhere; if you want to understand, you must pay close attention. It was a pity some of the black and white diagrams were so poor – the colour plates were better – and too small for proper study. But this is probably the fault of the publishers, as were a couple of mathematical glitches in the footnotes. Surely, for example, log 2 to base 2 is 1???

‘According to general relativity, the singularity marks the end of time for anything unfortunate enough to meet it.’


5 thoughts on “Black Holes

  1. Neville Lockhart

    A great review. Made me order two copies of the book, one for self and one for daughter (along with your summary) to help her keep up with her son doing maths and physics towards cosmology at uni. Many scientist colleagues from the 60s and 70s are fascinated by black holes, dark matter, dark energy etc and are interested in books between the popular level (no maths) and the specialist level with difficult maths.

    Liked by 1 person

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