Time: is the future already decided?

The central drive in physics recently is to fuse the two main theories, quantum theory and relativity. Both are well tested, but among the things they disagree about is time. That means two different notions of time must now somehow be reconciled - it has brought to the surface long-standing contradictions about time, and some now see time as the main obstacle in attempts to reach quantum gravity. So a puzzle that for decades was set to one side has shifted from the edge to the centre, and become a key issue.

One interesting part of the puzzle is the contradiction between the two main theories about whether the future exists yet. Quantum theory tells us the future is unformed and undecided, because of a deep-seated randomness about events at the particle scale. But special relativity has led to the idea that the entire future already exists, and is unalterable. Both theories are well tested - the mathematics of both is certainly right. But it seems our understanding of at least one of them must be wrong somewhere.

Some of the clues to the time mystery are so weird that people hardly talked about them until recently, and now new clues are being found. The standard view of time, in as far as there is one (there's little consensus on any of the issues), has been failing in laboratories since 2015.

Some things we know for sure don't fit with any view of time. We know matter gets out of sync with other matter - constantly, everywhere. Across the wide universe, dust grains that move differently for a few microseconds age at slightly different rates, and are left with permanent age differences. This is from basic special relativity, and has been measured, but it doesn't fit with the standard view of time, and nor do some of the other clues.

The aim of this book is to set out more of the puzzle than has been set out before. The book is for both popular science readers and physicists. It explains the main clues - including rarely mentioned ones - with an engaging frankness, also the avenues of thought that lead off from where we are now, and what different kinds of solution might work. Surprisingly few books have come near to setting out the whole mystery: some areas of it are so baffling that physicists had a reluctance to talk about it even amongst themselves, let alone to the public. But in the 21^st century there's a new openness, and we're questioning everything about time.

Jonathan Kerr is the British physicist whose explanation for the weird behaviour of light and matter in quantum mechanics led to a documentary, The Interactions Avenue, in which he discusses it with Carlo Rovelli and Neil Turok, two of the world's best known physicists. They both became interested in his theory - there was a Sunday Telegraph article about his work, and his first book, The Unsolved Puzzle. The same theory also has a lateral new approach to time.

The first part of this book simply sets out the time mystery, with new aspects of the puzzle. But in the later part of it he explains his own solution. Other books usually offer nothing genuinely new: some suggest minor tweaks that smooth the problems with time back into the present picture, in order to rescue it - skating over the best clues for the same reason. But this one brings a truly new solution, and breaks several taboos about the puzzle. And in the present situation that's important, as we may now need to solve a very old mystery in order to move forward.