The difference between past and future... cause and effect... memory and hope... regret and intention... in… — Carlo Rovelli

In the elementary equations of the world, the arrow of time appears only where there is heat. The link between time and heat is... fundamental: every time a difference is manifested between the past and the future, heat is involved.

When two friends meet... after one has lived in the mountains and the other at sea level, the watches... will show different times. ...Neither is truer than the other. ...Times are legion: a different one for every point in space. ...there is a vast multitude of them. ...Every clock has its proper time. ...Einstein has shown how to calculate the difference... The world is not like a platoon advancing at the pace of a single commander. It is a network of events affecting 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.

Time has lost... its unity.

Children grow up and discover that the world is not as it seemed within the four walls of their homes. Humankind as a whole does the same.