The Order of Time

by Carlo Rovelli

a mass slows down time around itself. The Earth is a large mass and slows down time in its vicinity. It does so more in the plains and less in the mountains, because the plains are closer to it.

If things fall, it is due to this slowing down of time. Where time passes uniformly, in interplanetary space, things do not fall. They float, without falling. Here on the surface of our planet, on the other hand, the movement of things inclines naturally towards where time passes more slowly, as when we run down the beach into the sea and the resistance of the water on our legs makes us fall headfirst into the waves. Things fall downwards because, down there, time is slowed by the Earth.

Only where there is heat is there a distinction between past and future.

This reads: ‘Delta S is always greater than or equal to zero’, and we call this ‘the second principle of thermodynamics’ (the first being the conservation of energy). Its nub is the fact that heat passes only from hot bodies to cold, never the other way round.

Hot tea is tea in which the molecules are very agitated. Cold tea is tea in which the molecules are only a little agitated.

Ten years before understanding that time is slowed down by mass,1 Einstein had realized that it was slowed down by speed.

Time passes more slowly for the one who keeps moving.

‘Proper time’ depends not only on where you are and your degree of proximity to masses; it depends also on the speed at which you move.

Remember the clocks in Chapter 1 that slow down in the vicinity of a mass? They slow down because there is, in a precise sense, ‘less’ gravitational field there. There is less time there.

This image illustrates what physicists call ‘curved’ spacetime. ‘Curved’ because it is distorted: distances are stretched and contracted, just like the elastic sheet when it is pulled.