This stems from the fact that some equations of quantum gravity such as the Wheeler—DeWitt equation, which assigns quantum states to the Universe can be written without any reference to time at all.
The book is split into three parts. The malleability of space and time mean that two events occurring far apart might even happen in one order when viewed by one observer, and in the opposite order when viewed by another. Rovelli gives good descriptions of the classical physics of Newton and Ludwig Boltzmann, and of modern physics through the lenses of Einstein and quantum mechanics.
There are parallels with thermodynamics and Bayesian probability theory, which both rely on the concept of entropy, and might therefore be used to argue that the flow of time is a subjective feature of the Universe, not an objective part of the physical description. For example, it is far from certain that space-time is quantized, in the sense of space and time being packaged in minimal lengths or periods the Planck length or time.
Rather, our understanding peters out at those very small intervals for which we need both quantum mechanics and relativity to explain things. Quantum uncertainty means we cannot know the positions and speeds of all the particles in the Universe. If we could, there would be no entropy, and no unravelling of time. The Order of Time is a compact and elegant book. And the writing, translated from Italian by Erica Segre and Simon Carnell, is more stylish than that in most physics books.
Occasionally, the writing strays into floweriness. And this book alone would not give a lay reader enough information to render judgement. Together they have developed a framework to show how the thing we experience as time might emerge from a more fundamental, timeless reality. Realizing that his explanation may only be deepening the mystery of time, Rovelli says that much of the knowledge that we now take for granted was once considered equally perplexing.
But this is what fundamental physics is about: finding new ways of thinking about the world and proposing them and seeing if they work. I think that when Galileo said that the Earth was spinning crazily around, it was utterly incomprehensible in the same manner. Space for Copernicus was not the same as space for Newton, and space for Newton was not the same as space for Einstein.
We always learn a little bit more. Einstein, for one, found solace in his revolutionary sense of time. That means nothing. People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion. Rovelli senses another temporal breakthrough just around the corner. When the dust settles, time — whatever it may be — could turn out to be even stranger and more illusory than even Einstein could imagine.
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