“The arrow of time is a consequence of the extremely special initial conditions of the universe, particularly the low entropy of the Big Bang. Without this condition, time would be symmetric, and past and future would be indistinguishable.”Roger Penrose · The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics · 1989 · The Emperor's New Mind, Chapter 7: Cosmology and the Arrow of Time
The arrow of time stems not from physical laws, but from the universe's extremely special initial condition: the low entropy of the Big Bang.
Penrose argues that the second law of thermodynamics — entropy increases over time — does not explain time's direction. The key is that the universe started in a state of incredibly low entropy, far lower than a black hole of the same mass. For instance, at the Big Bang, the universe's entropy was about 10^10 times smaller than the maximum possible. This extremely special initial condition creates the arrow of time: things evolve from order to disorder. Without it, processes would be reversible, and memory and causality would vanish. Penrose calls this the initial entropy problem, which he considers unsolved by current physics.
Why it matters The Rovelli–Penrose debate over the universe's initial entropy is central to our shelf, and this book explains Penrose's view that the initial conditions are a fundamental mystery, not a byproduct of quantum gravity.