What do you think?
Rate this book
On Time: The Physics That Makes the Universe Tick
From quantum physicist, New York Times bestselling author, and BBC host Jim Al-Khalili, essential insights into the meaning and fundamental nature of time
While we have come a long way toward deciphering the laws of the universe, many physicists still regard time to be one of the greatest unsolved mysteries. On Time provides answers to the questions about time that philosophers and scientists have been asking for millennia, such as whether it has a beginning and an end, why it only points toward the future and not the past, whether it can be stretched and squeezed or even reversed, and whether it is even real.
In this mind-altering book, Jim Al-Khalili tackles the enduring problems of time head-on, carefully examining them across the three central pillars of physics—quantum mechanics, general relativity, and thermodynamics. He shows how, in each domain, time can be understood as a coordinate or label, as an inseparable dimension of space-time, or as a direction or “arrow.” He explains why it can be difficult to reconcile these different perspectives and describes how theorists at the frontiers of physics are attempting to do just that. Al-Khalili makes the persuasive case that time is real, not illusory, and that a master arrow of time is not merely a trick of our senses but is rather a fundamental feature of reality that was baked into the structure of our universe from the very beginning.
Sharing fresh insights into some of the most complex and profound questions in science, On Time will forever change the way you think about the passage of time and your place within it.
While we have come a long way toward deciphering the laws of the universe, many physicists still regard time to be one of the greatest unsolved mysteries. On Time provides answers to the questions about time that philosophers and scientists have been asking for millennia, such as whether it has a beginning and an end, why it only points toward the future and not the past, whether it can be stretched and squeezed or even reversed, and whether it is even real.
In this mind-altering book, Jim Al-Khalili tackles the enduring problems of time head-on, carefully examining them across the three central pillars of physics—quantum mechanics, general relativity, and thermodynamics. He shows how, in each domain, time can be understood as a coordinate or label, as an inseparable dimension of space-time, or as a direction or “arrow.” He explains why it can be difficult to reconcile these different perspectives and describes how theorists at the frontiers of physics are attempting to do just that. Al-Khalili makes the persuasive case that time is real, not illusory, and that a master arrow of time is not merely a trick of our senses but is rather a fundamental feature of reality that was baked into the structure of our universe from the very beginning.
Sharing fresh insights into some of the most complex and profound questions in science, On Time will forever change the way you think about the passage of time and your place within it.
- GenresSciencePhysicsNonfiction
350 pages, Kindle Edition
Published September 1, 2026
About the author
Jim Al-Khalili
50 books788 followersDr. Jameel Sadik "Jim" Al-Khalili is an Iraqi-British theoretical physicist, author and broadcaster. He is professor of theoretical physics and chair in the public engagement in science at the University of Surrey. He is a regular broadcaster and presenter of science programmes on BBC radio and television, and a frequent commentator about science in other British media.
In 2014, Al-Khalili was named as a RISE (Recognising Inspirational Scientists and Engineers) leader by the UK's Engineering and Physical Sciences Research Council (EPSRC). He was President of Humanists UK between January 2013 and January 2016.
In 2014, Al-Khalili was named as a RISE (Recognising Inspirational Scientists and Engineers) leader by the UK's Engineering and Physical Sciences Research Council (EPSRC). He was President of Humanists UK between January 2013 and January 2016.
Ratings & Reviews
Friends & Following
Create a free account to discover what your friends think of this book!
Community Reviews
Displaying 1 - 8 of 8 reviews
September 2, 2026
By a small margin Jim Al-Khalili's The World According to Physics is my favourite of his books, both because the 'three pillars' concept at its heart works so well - but also because it's short. On Time is somewhat heavier, but thankfully not a wrist-buster. It might seem at first sight odd that a book on a subset of physics is considerably longer than one covering the whole thing, but time is one of those subjects that inevitably involves considerable complexities and needs depth.
As Al-Khalili establishes, it's hard to think of any other scientific concept where our perception of it - what he refers to as manifest time or psychological time - is so different from the view from physics. Some physicists go so far as to suggest that time doesn't exist at all. In the book, Al-Khalili sets out (to my mind quite successfully) both to explain why they think this is the case, but also why it's not the best interpretation of time. I'd certainly note that I've never yet come across a time-denying physicist who doesn't recognise the existence of dinner time.
We begin with what's meant by time (not as easy to establish as you might think), which leads through to predictions about the future (in essence what most science involves). There's plenty here on the distinction between past, present and future and what each of these really is. We then get relativistic, treating time as a dimension, introducing spacetime and the implications of first the special and then the general theory of relativity. (John Gribbin would be irritated that Al-Khalili refers to these as special and general relativity rather than special and general theories.) We then get onto entropy and its definition of time's arrow - with implications for a beginning and end of time - before taking in time as an emergent property and concluding that time is real.
If I have one criticism it is that throughout Al-Khalili keeps things at a descriptive level rather than allowing any mathematics to enter the fray: I'm not expecting equations, but there can be an in-between stance where we understand that the verbal description doesn't quite hack it (and, for instance, I've always thought it's a good idea to have at least an appendix with a mathematical example of time dilation, as the maths is so simple and makes the unlikely outcome more clear).
This extends to a degree to some of the examples given where, without a bit more depth what's said sounds a touch unlikely. So, for instance, we are given examples of changes that don't involve time: 'one can easily think of examples involving things that change, but which are not necessarily changing over time. Atmospheric pressure changes when you climb to higher altitudes, the speed of a car changes when you press your foot on the accelerator pedal...' Yet these examples happen over time: neither can be instantaneous.
There's also the usual description of the way that systems evolving with time don't have a direction, because 'such equations run equally well forwards and backwards in time'. But (at least at this point in the book) there is no consideration given to this being 'spherical cow' rather than real world physics. A classic example is a pair of billiard balls colliding. You can run the 'movie' of what you see backwards and it's indistinguishable. But this is only true on a non-existent frictionless table - and if you cherry pick an isolated system in time, excluding the bits where the motion starts and ends - though if, like me, you find this cheat very irritating, it is resolved in the final chapter.
These are small moans, though. Al-Khalili takes us through mind-bending subject with panache, making what could be a dense and difficult topic approachable and engaging.
As Al-Khalili establishes, it's hard to think of any other scientific concept where our perception of it - what he refers to as manifest time or psychological time - is so different from the view from physics. Some physicists go so far as to suggest that time doesn't exist at all. In the book, Al-Khalili sets out (to my mind quite successfully) both to explain why they think this is the case, but also why it's not the best interpretation of time. I'd certainly note that I've never yet come across a time-denying physicist who doesn't recognise the existence of dinner time.
We begin with what's meant by time (not as easy to establish as you might think), which leads through to predictions about the future (in essence what most science involves). There's plenty here on the distinction between past, present and future and what each of these really is. We then get relativistic, treating time as a dimension, introducing spacetime and the implications of first the special and then the general theory of relativity. (John Gribbin would be irritated that Al-Khalili refers to these as special and general relativity rather than special and general theories.) We then get onto entropy and its definition of time's arrow - with implications for a beginning and end of time - before taking in time as an emergent property and concluding that time is real.
If I have one criticism it is that throughout Al-Khalili keeps things at a descriptive level rather than allowing any mathematics to enter the fray: I'm not expecting equations, but there can be an in-between stance where we understand that the verbal description doesn't quite hack it (and, for instance, I've always thought it's a good idea to have at least an appendix with a mathematical example of time dilation, as the maths is so simple and makes the unlikely outcome more clear).
This extends to a degree to some of the examples given where, without a bit more depth what's said sounds a touch unlikely. So, for instance, we are given examples of changes that don't involve time: 'one can easily think of examples involving things that change, but which are not necessarily changing over time. Atmospheric pressure changes when you climb to higher altitudes, the speed of a car changes when you press your foot on the accelerator pedal...' Yet these examples happen over time: neither can be instantaneous.
There's also the usual description of the way that systems evolving with time don't have a direction, because 'such equations run equally well forwards and backwards in time'. But (at least at this point in the book) there is no consideration given to this being 'spherical cow' rather than real world physics. A classic example is a pair of billiard balls colliding. You can run the 'movie' of what you see backwards and it's indistinguishable. But this is only true on a non-existent frictionless table - and if you cherry pick an isolated system in time, excluding the bits where the motion starts and ends - though if, like me, you find this cheat very irritating, it is resolved in the final chapter.
These are small moans, though. Al-Khalili takes us through mind-bending subject with panache, making what could be a dense and difficult topic approachable and engaging.
October 25, 2026
I found the book surprisingly dull. It was filled with dry explanations of obvious facts and inconclusive hypotheses, and very little of the content was based on new or original research.
October 24, 2026
Most popular books on time cover familiar ground. Moving clocks run slow. Entropy rises. The universe began in a special low-entropy state that gave time its direction. Jim Al-Khalili covers this material briskly. He then devotes most of the book to the questions those standard accounts leave unresolved. That choice makes On Time one of the most rewarding books on the subject for readers who have already read the usual titles.
The first strength is structural. The author examines time separately in each of the three pillars of physics. In quantum mechanics it is a coordinate, a label in the equations. In relativity it is a dimension woven into spacetime. In thermodynamics it is a direction. Keeping the three apart shows exactly why they resist reconciliation, a difficulty most popular accounts blur.
The second is his separation of the arrow of time from the flow of time. Popular books usually treat “why does time pass” and “why does entropy increase” as one question. The book shows they are distinct. The arrow is a physical asymmetry with a physical explanation. The flow appears nowhere in the equations of physics. With that distinction in place, the debates over the block universe, eternalism versus presentism, and whether any “now” is special become far sharper.
The third is his treatment of entropy. Where most authors rely on a single loose notion of disorder, he distinguishes thermodynamic, statistical, informational, and entanglement entropy, each measuring something different. He then questions whether the low-entropy Big Bang is the deepest explanation of the arrow at all. He makes the case that it may trace back to very low initial entanglement, with decoherence making every measurement irreversible. Few popular books reach this territory. Fewer still are written by someone who has contributed to the research. The book is also exceptional in describing the factors that lead to lowering entropy events we see all around.
The depth holds elsewhere too. Time dilation is grounded in experimental evidence such as muon decay. The problem of time in quantum gravity, including the possibility that time emerges from entanglement, gets a serious hearing. Even the closing chapter on time travel stays disciplined and skeptical.
The book also takes a position. It argues that time is real and that its arrow is a fundamental feature of the universe. The book even posits the possibility of starting the flow as an axiomatic reality. Readers may disagree, but the book carries a genuine argument from start to finish where most titles in the genre offer a survey. Some chapters, particularly on quantum gravity, demand a second reading.
The first strength is structural. The author examines time separately in each of the three pillars of physics. In quantum mechanics it is a coordinate, a label in the equations. In relativity it is a dimension woven into spacetime. In thermodynamics it is a direction. Keeping the three apart shows exactly why they resist reconciliation, a difficulty most popular accounts blur.
The second is his separation of the arrow of time from the flow of time. Popular books usually treat “why does time pass” and “why does entropy increase” as one question. The book shows they are distinct. The arrow is a physical asymmetry with a physical explanation. The flow appears nowhere in the equations of physics. With that distinction in place, the debates over the block universe, eternalism versus presentism, and whether any “now” is special become far sharper.
The third is his treatment of entropy. Where most authors rely on a single loose notion of disorder, he distinguishes thermodynamic, statistical, informational, and entanglement entropy, each measuring something different. He then questions whether the low-entropy Big Bang is the deepest explanation of the arrow at all. He makes the case that it may trace back to very low initial entanglement, with decoherence making every measurement irreversible. Few popular books reach this territory. Fewer still are written by someone who has contributed to the research. The book is also exceptional in describing the factors that lead to lowering entropy events we see all around.
The depth holds elsewhere too. Time dilation is grounded in experimental evidence such as muon decay. The problem of time in quantum gravity, including the possibility that time emerges from entanglement, gets a serious hearing. Even the closing chapter on time travel stays disciplined and skeptical.
The book also takes a position. It argues that time is real and that its arrow is a fundamental feature of the universe. The book even posits the possibility of starting the flow as an axiomatic reality. Readers may disagree, but the book carries a genuine argument from start to finish where most titles in the genre offer a survey. Some chapters, particularly on quantum gravity, demand a second reading.
September 15, 2026
I generally only provide a review to a book that makes me think about something differently with a concise explanation. I general read most books 2x but im on the 3rd read of this. I enjoyed reading it.
October 22, 2026
My passion of late is to dig into Quantum Theory to understand it better, and more specifically to form a life philosophy from what it may have to say.
Reading this book was a bit of a heavy lift for a few reasons. Time is a heavy lift. Any delve into physics is generally a heavy lift. Following the writing of most physicists is a heavy lift. Many of them are not the best of writers. Never the less, I always learn something.
I pretty much follow Relational Quantum Field Theory in the way I see things. Everything is relational to its environment, everything is relative to everything else. It is an endless relational net that continues out forever, one affecting the next, which is being affected by the first, and all the rest. Everything is a process, being affected by other processes, being influenced by yet others. It is complexity. Built on relational complexity. We are not separate. We are part and parcel of it. As are all things. Simply put, nothing is strictly individual.
A simple way to understand a probability wave is thus: in the macro world I hear a bird, up in a tree somewhere. I look around searching for where… I settle on a likely tree as the source of the sound… I spot the bird. The probability wave collapses. The the quantum world of the smallest of smalls difference is that the bird is smeared out in all possible positions at once until a measurement (spotting the bird) resolves a location. The macro world is simply like a gross averaging of what occurs in the quantum world.
As far as time goes this is how I see Now: Now is simply the relative and continuous collapsing of the wave function of probability into reality for each individual, as we walk down the street. Those probabilities and that reality is influenced by all that is around us, in our reference frame, and perhaps outside that frame as well, for all is relational and relative. It is also a matter of perspective.
My recommendations for reading are Carlo Rovelli’s “Helgoland” and for what the heck the quantum world is made out of I highly suggest Matt Strassler’s book “Wave in an Impossible Sea”!
Reading this book was a bit of a heavy lift for a few reasons. Time is a heavy lift. Any delve into physics is generally a heavy lift. Following the writing of most physicists is a heavy lift. Many of them are not the best of writers. Never the less, I always learn something.
I pretty much follow Relational Quantum Field Theory in the way I see things. Everything is relational to its environment, everything is relative to everything else. It is an endless relational net that continues out forever, one affecting the next, which is being affected by the first, and all the rest. Everything is a process, being affected by other processes, being influenced by yet others. It is complexity. Built on relational complexity. We are not separate. We are part and parcel of it. As are all things. Simply put, nothing is strictly individual.
A simple way to understand a probability wave is thus: in the macro world I hear a bird, up in a tree somewhere. I look around searching for where… I settle on a likely tree as the source of the sound… I spot the bird. The probability wave collapses. The the quantum world of the smallest of smalls difference is that the bird is smeared out in all possible positions at once until a measurement (spotting the bird) resolves a location. The macro world is simply like a gross averaging of what occurs in the quantum world.
As far as time goes this is how I see Now: Now is simply the relative and continuous collapsing of the wave function of probability into reality for each individual, as we walk down the street. Those probabilities and that reality is influenced by all that is around us, in our reference frame, and perhaps outside that frame as well, for all is relational and relative. It is also a matter of perspective.
My recommendations for reading are Carlo Rovelli’s “Helgoland” and for what the heck the quantum world is made out of I highly suggest Matt Strassler’s book “Wave in an Impossible Sea”!
September 17, 2026
This book is an excellent overview of the state of physics. While it purports to be about time, and it is, Khalili is clear that time cannot be explained without reference to three different kinds of time - quantum time (which means he is clear on quantum physics), general relativity time (clear on astrophysics) and 'manifest' time (time as we see it - clear on entropy). These clarities and their reconcilability of otherwise make up the bulk of the book, and are explained so well that inevitably the book becomes a physics primer for the perplexed. A wonderful, enlightening and educational read.
October 2, 2026
I have read two previous books by Jim Al-Khalili, watched him in tv documentaries, and attended two of his public lectures. As a non-specialist, I am grateful to have learned so much about modern physics. I am thoroughly enjoying this, his latest, book. It's helping me consolidate my previous learning and acquire and assimilate new knowledge at just the right pace. I think I would be struggling without my previous exposure to the subject matter, which can be a bit challenging, not to say at times baffling, to understand. So, completely untutored readers may need firstly to familiarise themselves with the subjects of relativity and quantum physics by reading a suitable layperson's primer.
September 23, 2026
A good overview of a ton of relevant concepts, with many simple examples and plenty of repetition in case you zone out or skip around — which you might be tempted to do because the way the material is presented is not especially engaging. It attempts to briefly touch on so many diverse theories, ideas, and related fields of study that it ends up dismissing some without fair consideration and skimming over others without due elaboration. Still, as a rapid-pace hop over the contemporary landscape of the physics of time, it's probably an decent travel guide into the subject matter.
Displaying 1 - 8 of 8 reviews