"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 general theory of relativity. Newton’s “space,” through which things move, and the “gravitational field” are one and the same thing.
• A momentous simplification of the world: space is no longer something distinct from matter—it is one of the “material” components of the world.
• Planets circle around the sun, and things fall, because space curves.
• Space curves where there is matter.
• The whole of space can expand and contract.
• Einstein’s equation shows that space cannot stand still; it must be expanding.
• space and gravitational field are the same thing.
• Einstein showed that light is made of packets: particles of light. Today we call these “photons.”
• In quantum mechanics no object has a definite position, except when colliding headlong with something else.
• Science begins with a vision.
• Scientific thought is fed by the capacity to “see” things differently than they have previously been seen.
• The things we see are made of atoms. Every atom consists of a nucleus surrounded by electrons. Every nucleus consists of tightly packed protons and neutrons.
• Electrons, quarks, photons, and gluons are the components of everything that sways in the space around us. They are the “elementary particles” studied in particle physics.
• There is no such thing as a real void, one that is completely empty.
• Quantum mechanics and experiments with particles have taught us that the world is a continuous, restless swarming of things, a continuous coming to light and disappearance of ephemeral entities.
• Our world may have actually been born from a preceding universe that contracted under its own weight until it was squeezed into a tiny space before “bouncing” out and beginning to re-expand, thus becoming the expanding universe that we observe around us.
• A hot substance is a substance in which atoms move more quickly.
• Cold air is air in which atoms, or rather molecules, move more slowly. Hot air is air in which molecules move more rapidly.
• Heat, as we know, always moves from hot things to cold.
• Friction produces heat.
• The difference between past and future exists only 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.
• Heat does not move from hot things to cold things due to an absolute law: it does so only with a large degree of probability. The reason for this is that it is statistically more probable that a quickly moving atom of the hot substance collides with a cold one and leaves it a little of its energy, rather than vice versa.
• It is not impossible for a hot body to become hotter through contact with a colder one: it is just extremely improbable.
• 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 that he has always known.
• There is a detectable difference between the past and the future only when there is the flow of heat.
• To be free doesn’t mean that our behavior is not determined by the laws of nature. It means that it is determined by the laws of nature acting in our brains.
• Our free decisions are freely determined by the results of the rich and fleeting interactions among the billion neurons in our brain: they are free to the extent that the interaction of these neurons allows and determines.
• There is not an “I” and “the neurons in my brain.” They are the same thing. An individual is a process: complex, tightly integrated.
• It is not against nature to be curious: it is in our nature to be so.
• Life is precious to us because it is ephemeral.