Want to know everything about time? Read Jim Al-Khalili’s new book

Theoretical physicist Jim Al-Khalili probes the physics of time in his new book

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I wonder what you believe about time? Do you, for example, think the past exists, but the future doesn’t? Perhaps, if pressed, you might say the present is real, but the past is gone and the future is yet to come. Or maybe, like Albert Einstein, you think all moments are equally real and we simply move through them.

If pondering such deep questions is your kind of thing, then physicist and broadcaster Jim Al-Khalili’s latest book might be for you. A word of warning, though. On Time: The physics that makes the universe tick (UK | US) is a book you should read if you want to understand more or less everything there is to know about time. But if you just want to dip your toe into the subject, then there are easier places to start.

Al-Khalili begins with a rundown of the many interwoven strands that thinking about time involves, separating them into two categories. First, there are the problems of manifest time, or time as we experience it. These include questions like: why do we feel time flowing, why does that flow feel continuous like a river, and what exactly is the difference between the past and the future?

Second, we have the problems of physical time, which are to do with the different ways various fields of physics interpret time and the efforts to resolve the tensions these produce. For instance, some equations in physics treat time as a label, writes Al-Khalili, while Einstein’s general theory of relativity tells us it is a dimension, and thermodynamics paints it as an arrow. So, what is time, really? How do we reconcile all these contradictory views? And can the problems of physical time help us with the problems of manifest time?

General relativity tells us time is a dimension, and thermodynamics paints it as an arrow. So, what is it really?

I found the early part of the book that disentangles these questions supremely helpful. As a New Scientist editor, I have read plenty about the mysteries of time in my, er, time, but Al-Khalili sets it all out with great skill and clarity.

From there, the book reveals what we know about time and tackles the various problems, step by step. If this sounds like a lot of ground to cover, it is. And this is where I would sound a note of caution. Some popular books about physics seek to beguile with the poetic mysteries and grandeur of reality, sweeping you along in the wonder of it all. There is certainly some of that here – these are lofty questions, to be sure. But Al-Khalili is known as a brilliant explainer of science and his focus here is explanation, not entertainment and narrative drive. He is at pains to be clear, fair and methodical. Some will relish this (as, indeed, I mostly did), but I still felt some of the book’s 350 or so pages could have been trimmed to help the story move with a touch more pace.

That aside, there are plenty of passages in which Al-Khalili’s clarity of mind impresses. Take the simple, compelling argument he makes in discussing whether time or “change” is more fundamental. “Sure, if we don’t experience change then we don’t have any perception of time passing,” he writes. “But… for us to perceive that something isn’t changing we need to perceive time to be passing in the background in order to be able to say that something remains the same at different times.”

There are some new ideas in the book, too, and we will get to those anon, but much of the value comes from Al-Khalili’s elucidation of what the past 100 years of physics has taught us about time.

One implication of general relativity, for instance, is that there is no such thing as a universal “now”, because different observers can disagree on the order of events if they are moving extremely fast. This is heady stuff, but Al-Khalili’s characterisation is one of the clearest I have read.

He is admirably forthright in giving us, yes, all the facts and competing arguments, but also his opinions. For example, he believes our perception of time’s flow is an illusion. In addition, he isn’t prepared to agree with the increasingly popular stance among physicists that time itself is an emergent property of nature (like, say, temperature), rather than a fundamental ingredient of reality (like, say, a quantum field).

Delightfully, Jim Al-Khalili includes a ‘bonus chapter’ that asks whether time travel is possible

It is also common to read in the physics literature that time’s direction emerges from the second law of thermodynamics, which says that things tend to get more disordered over time. But Al-Khalili shows us this is perhaps a touch too neat, arguing that the arrow of time may instead spring from what he says is the only truly irreversible process, the “decoherence” of quantum systems when they lose their quantumness due to disturbances from their environment.

He also has a fresh and interesting way of thinking about something called the past hypothesis, though I fear his argument is too complicated to summarise here.

Physicists describe the daily experience of time as “manifest time”

Roger Viollet via Getty Images

Delightfully, Al-Khalili also includes what he describes as a “bonus chapter” at the end of the book that asks whether time travel is possible. Intended as “a tasty but self-indulgent dessert after a big meal”, he executes this skilfully, treading the line between having fun with this wild topic and staying true to himself as a serious physicist.

Among other things, he covers the well-known grandfather paradox, where a time traveller goes back in time and kills their grandfather, raising questions about how they could have been born in the first place. But he does explore several variants of the paradox, and at length. Again, he goes deeper than most others who write about time travel and helps us think about it in a nuanced way.

There’s such a lot packed into this book that it is hard to know quite how to sum it up – and I suspect each reader will take away something different. One thing is for sure: if you are looking for easy answers about the nature of time, you won’t find them here – because there are none.

And here’s a thought. What if that’s actually OK? Earlier in my career as a journalist, covering physics, I tended to think the discipline’s job was to explain reality. That’s not wrong, but surely there are limits. Near the start of On Time, Al-Khalili levels with his readers: “Maybe there are some aspects of the universe we will never be able to explain, or that turn out not to require explanation because they just are.”

I certainly know an awful lot more about how to think about time after reading this book. But will we ever fully understand it? I am more doubtful of that than before I opened the first page.

Hear Jim Al-Khalili talk about the physics of time on 11 October at New Scientist Live
newscientist.com/nslmag

Three more recommendations about time

In Search of Now (UK | US)
by Jo Marchant

Former New Scientist staffer Jo Marchant explores the contradictions of the present moment. It is all that ever exists, yet what is “now” really? In this book, she draws on a wide range of fields in search of answers.

Interstellar
by Christopher Nolan

If you really want your mind blown, try this epic sci-fi movie. A father travels to a supermassive black hole, where time dilation means he ages more slowly than his earthbound daughter. It gets even stranger from there…

The Order of Time
by Carlo Rovelli

This is a more lyrical (and shorter) book than Jim Al-Khalili’s. In it, Carlo Rovelli argues that time isn’t a fundamental property of reality, but something that emerges as we look at it from a particular perspective.

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