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He looked for a plausible explanation and found one by imagining that the sun and the Earth do not attract each other directly but that each of the two gradually acts on that which is between them. And since what lies between them is only space and time, he imagined that the sun and the Earth each modified the space and time that surrounded them, just as a body immersed in water displaces the water around it. This modification of the structure of time influences in turn the movement of bodies, causing them to “fall” toward each other.4 What does it mean, this “modification of the structure of
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We do not describe how the world evolves in time: we describe how things evolve in local time, and how local times evolve relative to each other.
Physics does not describe how things evolve “in time” but how things evolve in their own times, and how “times” evolve relative to each other.*
If nothing else around it changes, heat cannot pass from a cold body to a hot one. The crucial point here is the difference from what happens with falling bodies: a ball may fall, but it can also come back up, by rebounding, for instance. Heat cannot. This is the only basic law of physics that distinguishes the past from the future.
In the elementary equations of the world,13 the arrow of time appears only where there is heat.* The link between time and heat is therefore fundamental: every time a difference is manifested between the past and the future, heat is involved. In every sequence of events that becomes absurd if projected backward, there is something that is heating up.
The notion of “particularity” is born only at the moment we begin to see the universe in a blurred and approximate way.
The notion of “the present” refers to things that are close to us, not to anything that is far away. Our “present” does not extend throughout the universe. It is like a bubble around us.
