Seven Brief Lessons on Physics

by Carlo Rovelli

the gravitational field is not diffused through space; the gravitational field is that space itself. This is the idea of the theory of general relativity. Newton’s ‘space’, through which things move, and the ‘gravitational field’ are one and the same thing.

The sun bends space around itself and the Earth does not turn around it because of a mysterious force but because it is racing directly in a space which inclines, like a marble that rolls in a funnel. There are no mysterious forces generated at the centre of the funnel; it is the curved nature of the walls which causes the marble to roll. Planets circle around the sun, and things fall, because space curves.

To begin with, the equation describes how space bends around a star. Due to this curvature, not only do planets orbit round the star, but light stops moving in a straight line and deviates. Einstein predicted that the sun causes light to deviate. In 1919 this deviation was measured, and the prediction verified. But it isn’t only space that curves; time does too.

Rab − ½ R gab = Tab

In accordance with the assumption to be considered here, the energy of a light ray spreading out from a point source is not continuously distributed over an increasing space but consists of a finite number of ‘energy quanta’ which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units.

Heisenberg imagined that electrons do not always exist. They only exist when someone or something watches them, or better, when they are interacting with something else. They materialize in a place, with a calculable probability, when colliding with something else. The ‘quantum leaps’ from one orbit to another are the only means they have of being ‘real’: an electron is a set of jumps from one interaction to another. When nothing disturbs it, it is not in any precise place. It is not in a ‘place’ at all.

For now, this is what we know of matter: A handful of types of elementary particles, which vibrate and fluctuate constantly between existence and non-existence and swarm in space even when it seems that there is nothing there, combine together to infinity like the letters of a cosmic alphabet to tell the immense history of galaxies, of the innumerable stars, of sunlight, of mountains, woods and fields of grain, of the smiling faces of the young at parties, and of the night sky studded with stars.

Our universe may have been born from a bounce in a prior phase, passing through an intermediate phase in which there was neither space nor time.

The difference between past and future only exists when there is heat. The fundamental phenomenon that distinguishes the future from the past is the fact that heat passes from things that are hotter to things that are colder.

The branch of science which clarifies these things is called statistical physics, and one of its triumphs, beginning with Boltzmann, has been to understand the probabilistic nature of heat and temperature, that is to say, thermodynamics.

To trust immediate intuitions rather than collective examination that is rational, careful and intelligent is not wisdom: it is the presumption of an old man who refuses to believe that the great world outside his village is any different from the one which he has always known.

‘We’, human beings, are first and foremost the subjects who do the observing of this world; the collective makers of the photograph of reality

During the great period of German idealism, Schelling could think that humanity represented the summit of nature, the highest point, where reality becomes conscious of itself.

All things are continually interacting with each other, and in doing so each bears the traces of that with which it has interacted:

To be free doesn’t mean that our behaviour is not determined by the laws of nature. It means that it is determined by the laws of nature acting in our brains.

There is not an ‘I’ and ‘the neurons in my brain’. They are the same thing. An individual is a process: complex, tightly integrated.

There are frontiers where we are learning, and our desire for knowledge burns. They are in the most minute reaches of the fabric of space, at the origins of the cosmos, in the nature of time, in the phenomenon of black holes, and in the workings of our own thought processes. Here, on the edge of what we know, in contact with the ocean of the unknown, shines the mystery and the beauty of the world. And it’s breathtaking.