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Carlo Rovelli is a physicist who tries to explain the current state of physics to the layman. In the first part of the book, Rovelli explains the history of physics starting with the Greek Democritus who concludes that the world

“Is made up of a finite number of discrete pieces that are indivisible, each one having finite size: the atoms.” [p23]

Unfortunately, as Rovelli sees it, Aristotle and Plato, gum things up with their theories of ideals and monotheism threw scientific study off the rails. We then go through Ptolemy, Copernicus, Kepler, Galileo up to Newton and Einstein. Each of these scientists improve our understanding of the universe.  He concludes with the current theory of quantum mechanics.

Newton saw the universe made up of three things: space, time, and particle. Through the years this list was subdivided then regrouped. In 1905 Einstein described the universe as Spacetime, Fields, and Particles; grouping Space and Time into 1 thing. 

I took away three theories of the universe:

1. Time is meaningless at the smallest reaches of the universe; and, 
2. Electrons vanish and reappear (I have a hard time with this)
3. There is a limit to how small the building blocks of the universe are – the Planck scale – a really, really, REALLY small number.

Through Rovelli’s explanations I understand how there is not a universal “now”. Time is relative and changed by gravity. Experiments have been done with extremely accurate clocks. One taken to sea level, and one to a mountain top. When brought back together the clocks are no longer in sync because time passes more slowly at sea level because it was more affected by the earth’s gravity. 

“[T]ime is not universal and fixed; it is something that expands and shrinks, according to the vicinity of masses: Earth, like all masses distorts spacetime, slowing down time in its vicinity.” [p 86]

In fact, physicists can explain the universe without the variable of time. We only perceive time because we are observing the universe at a much more macro, synthesized level.

Heisenberg – of the Heisenberg Uncertainty Principle – determined that we can’t know the movement of particles, we can only know their position when they interact with something else. 

[Particles] “are not described by their position at every moment, but only by their position at a particular instant – the instants in which they interact with something else. This is a second cornerstone of quantum mechanics, its hardest key: the relational aspect of things. Electrons don’t always exist. They exist when they interact.” [p 119]

I have a hard time with this. While the motion of an electron around its nucleus may seem random to us, just because we only know it is at a particular location when we look for it, it could very well exist at other times. Rovelli seems to admit this later when he says:

“Perhaps because we must not confuse what we know about a system with the absolute state of the same system.” [p p253]

As for a lower limit to the universe, Rovelli argues “loop theory specifies that volume … cannot be arbitrarily small. A minimum volume exists.” [p 169] 

“The world is not made up of tiny pebbles [i.e. atoms]. It is a world of vibrations, a continuous fluctuation, a microscopic swarming of fleeting microevents.” [p 132]

As I understand it, these microevents are the overlapping intersections of the loops. I may have misunderstood that.

Through it all, quantum mechanics has pulled everything – Newton’s space, time, and particles –  together now into 1 entity: quantum fields. 

“The world, particles, light, energy, space, and time – all of this is nothing but the manifestation of a single type of entity: covariant quantum fields.”[p 193]

This gives us a unified theory pulling gravity into relationship with actions and reactions. If I remember correctly, Einstein was frustrated that his special relativity and general relativity theories could not fully account for gravity.

On a frivolous side note, Rovelli likes his universe “loopy” unlike Sheldon on the Big Bang Theory who likes his reality “stringy”. In fact Sheldon and Leslie Van Winkle have this very argument in an early episode of the TV series. In his book, Rovelli takes a little bit of delight that the collisions in the large Hadron collider have not revealed super symmetry that string theory predicts. In seasons 7 and 8 of the TV show, Sheldon realizes that string theory might be a dead end. How about that – we actually get insight into the scientific world from a SitCom.  

I found this book by Rovell, as well as the other two I’ve read, interesting even though I’m sure I don’t follow it all. It is a short book and easy enough for me – an English major – to get through. I especially liked how Rovelli traces through the history of physics from the Greeks to modern times; it was nice to see that the earliest Greeks looked at the structure of the world and the explosive growth in our knowledge in the 20th and 21st centuries.