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Existential Physics - Sabine Hossenfelder *****

If I had six stars to give this book, I'd do it. Sabine Hossenfelder's first book for the general public, Lost in Math, showed just how much some aspects of theoretical physics were based on maths-driven speculation. That was arguably one for the science buffs only - but in Existential Physics she takes on questions that really matter to all of us.

Many of these questions hover on the boundary between science and philosophy - but this is no repeat of a book like Hawking and Mlodinow's unimpressive The Grand Design, which attempted to show that we no longer needed philosophy or religion because science can do it all. Rather, Hossenfelder manages to show where science can tell us things we didn't expect... and where it does not give any helpful contribution to answering a question.

Delightfully, these answers are not at all what you might expect. For example, Hossenfelder makes it clear that the various 'how did we get from the Big Bang to here' theories, such as inflation, are really not usefully scientific - because with different parameters the models could predict pretty much anything - while some apparently unscientific ideas aren't actually excluded by science, even if any sensible person would be likely to think them wrong.

The most startling example of this was in an interview Hossenfelder gives with climate scientist Tim Palmer. Each is either atheist or agnostic, yet there is an impressive argument given that it is possible for the idea of creation happening a few thousand years ago to be compatible with all existing science. I stress that neither of them thinks this is true - there is absolutely no evidence to support this idea, but equally science can't say it's untrue. What the interview demonstrates is that those who dismiss other people's beliefs with a sweeping 'science proves it can't be true' don't understand what science can truly prove or disprove. (Read the book if you want to know the ingenious argument that makes a 6,000-year-ago creation scientifically possible.)

Along the way, Hossenfelder explores a whole gamut of big questions - the sort of things that children like to ask us, we ponder on in the long dark teatime of the soul, and philosophers spend their time exploring. But all this is done from a solid, physics-based viewpoint. You'll find coverage of the distinction between past, present and future, the beginning and end of the universe, consciousness, Boltzmann brains, free will and much more. All the way through, Hossenfelder's light, slightly cynical voice makes it feel like a discussion in the pub rather than a lecture. 

One comment early on made me laugh out loud - in the preface we read 'physicists are really good at answering questions' - in my long experience of talking to them, most physicists are absolutely terrible at answering questions. This is partly because they often don't understand what the questioner is asking, and partly because they don't know how to frame the answer in a way the questioner can understand. Hossenfelder admits that, like all modern physicists, her worldview is primarily mathematical and as such she finds it difficult to communicate in an accessible way because the maths is often all there is. It's a fair point, though thankfully in this book she mostly succeeds in overcoming that barrier with no mathematics explicitly involved.

The answers Hossenfelder gives to the many questions she covers in the book are put with such conviction and so convincingly, it's easy to get the feeling these are all the 'right' answers. It's important to remember that science doesn't really work like that - apart from anything else, new evidence can always require a change of theory. But bearing in mind she doesn't always agree with many big name physicists, it's a reminder that sometimes even experts in the field can be wrong. As it happens, I mostly agree with Hossenfelder over many of the famous names she mentions - but one of the nice things about a book like this is that you can actually consider your own beliefs and either change them or be prepared to argue back.

One example where I think this applies - I would suggest that Hossenfelder is a touch over-enthusiastic about the application of Occam's razor (without actually using the term). In several cases she suggests that something is not needed because everything observed can be explained without needing the extra something. This is absolutely true, but that doesn't prevent it existing. The simplest explanation is not always the correct one. (To be fair, this is made clear in several cases.)

The big thing that Hossenfelder grasps but many public-facing scientists don't is that there aren't two categories of theory - scientific and unscientific. Instead there are three: scientific, unscientific and ascientific. (Arguably there is a fourth - pre-scientific, where someone holds a theory for which there is not yet evidence, but for which evidence will eventually be found.) While some ideas are downright unscientific because there is good evidence that they are not true, many others are ascientific because there is no evidence for or against them. Where that's the case, Hossenfelder tells us, we are welcome to hold these beliefs, and it's not good for scientists to argue against them. This is especially the case because there are plenty of beliefs held by some scientists (the many worlds hypothesis, for example, or cosmic inflation) which are ascientific.

All in all, both a thought-provoking exploration of questions that are important to most people from a physics viewpoint and a useful counter to scientists who spend too much time on speculative theories with no hope of ever having evidence to back them up. Highly recommended.

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Review by Brian Clegg - See all of Brian's online articles or subscribe to a weekly digest for free here
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