Time — A Very Short Introduction is my favourite in the VSI series but it was not at all what I was expecting. I was expecting clocks and train timetables and calendars. This book is about Newton and Einstein and thermodynamics and the human spirit.

It’s a beautiful book about how time shapes our universe and our lives. Go and read it if you are interested in physics. Here’s a quick summary if you are not.

Chapter 1
In the beginning, every civilisation had its own way of keeping track of time but they reset their calendar to zero every time the king died. One day, the Seleucids decided not to reset their calendar and used the same year counter forevermore. A few centuries later, we reset our calendar one last time and decided that 1 AD should be four years after Jesus was born. There is no Year Zero.

That’s all the book has to say about timekeeping. The rest is about physics and philosophy.

Aristotle divided the earth from the heavens each was made from different stuff. The universe is finite with us at the centre.

Newton said the universe is infinite and has no centre. Everything everywhere is made of the same stuff and follows the same laws. Time and space exist independently of the objects that are in them and motion is just the change of place over time.

Leibniz disagreed. Space and time are relations between objects.

Galileo showed that if you do experiments with balls, you will get the same result whether you are moving at constant velocity or at rest. But not if you are accelerating.

Chapter 2
The speed of light turns out to be the same for everyone however fast they are travelling. Einstein’s theory of relativity explains this by treating time as just another dimension in spacetime. Distances and time intervals are different for different observers. It makes no sense to say that one event in spacetime happened before or after another event or that one person is moving while another is at rest. It’s all relative to the observer.

Einstein’s general theory of relativity describes how spacetime is warped by massive objects and Minkowski gave us the maths to describe it. Warped spacetime gives us black holes where time appears to stop. But it doesn’t.

Chapter 3
Our commonsense understanding of time conflicts with the consequences of relativity. That’s why we need philosophy!

Chapter 4
The equations that describe the physical world are reversible. If you throw a ball, it will travel in a parabola. If you reverse time, the ball will follow the same trajectory in the other direction. Physical equations have no opinion on which direction the clocks run. All physical equations except one: the second law of thermodynamics says you can’t unstir your coffee.

The author describes the second law in terms of microstates and macrostates. Microstates — like the molecules of milk and coffee — don’t care about the arrow of time. Macrostates — cups of coffee — do. There are more ways to arrange the microstates of the milk and coffee if they are mixed than if they are kept separate and you can’t unmix them. Stirred coffee stays stirred. The same rule applies to omelettes, steam engines and star systems. Any random change to the microstates results in greater entropy and time’s river flows to the sea.

Given enough time and a lot of mixing, complex structures formed. Some structures found a way to copy themselves and the miracle of life began. Life can unmix the coffee for a brief period and uses energy from the external environment to change its internal environment. Eventually the copies mutate and the best mutations survive, giving evolution a way to swim upstream in time’s river and we end up with bumble bees, cabbages and gazelles.

Chapter 5
Einstein’s world suggests that time is an illusion. All of spacetime was laid out in advance and we seem to be travelling across it in a predefined route. But we humans don’t experience time this way.

Henri Bergson argued that Einstein’s world leaves out the features of time that are essential to human understanding. Where is our sense of self in Einstein’s world? What happened to that little boy who gazed up at the stars? For Bergson, time is more like a river that flows through a past, a present and a future.

Neuroscience sheds light on these mysteries. The unconscious brain builds a model of the world informed by our memories and experiences and presents it to our consciousness. Our experience of the flow of time informs our model and guides our actions. It tells us where to run to return a tennis serve and when to hide our nuts in storage for the coming winter.

We see our own lives from multiple perspectives. We live in anticipation, in experience and in our collection of the memories, perceptions, hopes and fears that shape who we become. We look forward to branching possibilities and we look back on the thin, hard line of fact.

The author gets lyrical in chapter 5 and muses on penguins recognising their sole chick in a sea of thousands and little children struggling with third-grade maths. He tells of the doctor who does not see, of the old woman on his operating table and of the young mother who fastened our buttons against the cold on the first day of school.

I thought I was reading a book about time but it turned out to be about all of creation and all of humanity. I wish all science books and philosophy books could be written this way. I will read it again and again.