“The difference between past and future is not a fundamental property of the laws of nature. It arises only when we look at the world from a privileged perspective, that is, from our ignorance of the microscopic state of things.”Carlo Rovelli · The Order of Time · 2017 · The Order of Time, Part I: The Crumbling of Time, Chapter 2: The Direction of Time
The arrow of time is a consequence of our microscopic ignorance, not a fundamental law of nature.
Rovelli shows that fundamental laws of physics, both classical and quantum, are time-symmetric. The difference between past and future emerges only when we look at macroscopic systems, where entropy increases. For instance, a cup of coffee cools down but never heats up spontaneously. This happens because there are so many microscopic states that the probability of finding the system in an ordered state decreases. Rovelli calls this privileged perspective our ignorance, since we cannot track every molecule. Thus, thermal time, defined by entropy increase, replaces fundamental time.
Why it matters This idea explains why sudden market fluctuations seem unpredictable: they are effects of our ignorance of the microscopic details of transactions.