Time is more than just an emotionless variable. It gives rise to the fear of death, memory and the flow of the universe. Carlo Rovelli depicts time from this human perspective in his 2017 book.
We commonly view time as something absolute and uniform across the universe. The author shows that time is much more flexible and dependent on other variables in reality. Time passes differently depending on the speed of the observer or the strength of the gravitational field. This is Einstein’s brilliant idea of relativity. Time passes faster the further away you are from the source of a gravitational field. Thus, clocks move a tiny bit faster in the mountains relative to sea level. This was experimentally verified by flying an atomic clock in an airplane. In short, time is just perspective.
But it gets even stranger. The idea of past and future is determined only by entropy. Time flows from low to high entropy, as things tend to get more disordered. However, Rovelli states that entropy is not an objective quantity. Imagine a standard deck of cards, with half of the cards marked red and half marked black. Let’s arrange them so that the first half of the deck is all red cards, and the second half of the deck is black cards. If we shuffle them, you might say that they are less ordered. However, that is only because we assumed that this half red half black configuration is what we consider order. Perhaps, the shuffled cards are now in a particular state that another person considers ordered. Therefore, entropy also depends solely on perspective.
This doesn’t mean that I can see a different entropy than you. Simply, the state of the universe that we observe manifests this particular entropy that we can use to differentiate past from the future. But this is not absolute, and the author proposes that we feel the flow of time only because of our unique vantage point. Throughout the book, the familiar concept of time completely crumbles.
The Order of Time is a great read for anyone interested in physics. The author sprinkles in bits from Greek poetry and Proust, in order to contrast our intuition with our equations. He shows that time is beautiful and fundamental to the human experience, since we are formed from memories. I really recommend this book although, you might get disappointed if you are looking for a lot of equations and diagrams.