Notes from Seven Brief Lessons on Physics

A list of notes from Seven Brief Lessons on Physics by Carlo Rovelli.

Ever since we discovered that the Earth is round and turns like a mad spinning-top we have understood that reality is not as it appears to us.

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.
It’s a moment of enlightenment. A momentous simplification of the world: space is no longer something distinct from matter, it is one of the ‘material’ components of the world.
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’s equation shows that space cannot stand still; it must be expanding.
The same equation predicts that the expansion ought to have been triggered by the explosion of a young, extremely small and extremely hot universe: by what we now know as the ‘Big Bang’.
In short, the theory describes a colourful and amazing world where universes explode, space collapses into bottomless holes, time sags and slows near a planet, and the unbounded extensions of interstellar space ripple and sway like the surface of the sea …

The German physicist Max Planck calculated the electric field in equilibrium in a hot box. To do this he used a trick: he imagined that the energy of the field is distributed in ‘quanta’, that is, in packets or lumps of energy.
Einstein showed that light is made of packets: particles of light. Today we call these ‘photons’.
Why are precisely these elements listed there, and why does the periodic table have this particular structure, with these periods, and with the elements having these specific properties? The answer is that each element corresponds to one solution of the main equation of quantum mechanics. The whole of chemistry emerges from a single equation.
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.
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.
In quantum mechanics no object has a definite position, except when colliding headlong with something else.
The equations of quantum mechanics and their consequences are used daily in widely varying fields: by physicists, engineers, chemists and biologists. Yet they remain mysterious. For they do not describe what happens to a physical system, but only how a physical system affects another physical system.

Scientific thought is fed by the capacity to ‘see’ things differently than they have previously been seen.