In direct analogy to the unlikely case of 15 billiard balls, randomly launched, finding their way into a triangle configuration, the trillion-trillion molecules following random paths between two conjoined rooms are unimaginably unlikely to find their way all into a single room. The statistics of this are overwhelming: the odds of finding all the molecules in one room are worse than one in a very large number—a one followed by a billion-trillion zeros. This latter number is larger than anything we encounter in economics, astronomy, etc: it is the kind of number you get when you ask how many different ways you can arrange 10²⁴ molecules in a room. If we take a tally of the positions of molecules every nanosecond (the amount of time it takes a molecule to travel about a micron), then the amount of time one would have to wait to catch all the molecules in one room is a similarly very large number times the age of the universe. That is to say, Scenario 2 is truly irreversible: the molecules will never find their way back into a single room.

The irreversibility of phenomena like Scenario 2 is summed up in the Second Law of Thermo­dynamics. The Second Law is unusual among physical laws because, unlike the microscopic laws of physics (say in mechanics or electromagnetism), the Second Law is essentially a statistical state­ment: given the unimaginably large number of particles associated with the average macroscopic physical system in the universe, these systems, once disorganized, never reorganize if left on their own. The fact that the Second Law is a purely statistical statement also makes it unlikely to be revised. While the microscopic laws of physics may be rehauled or even repealed based on new data, the simple (and overwhelming) statistical arguments about macroscopic systems that lead to the Second Law are so straight forward and manifest in nature as to make the conclusions immutable. Disorganization in nature is irreversible. Things fall apart. The center cannot hold, at least not in the long run. In this way, the Second Law establishes the arrow of time in nature, and the arrow points in the direction of disintegration.

One can do work to reorganize a subset of the universe. For instance, in Scenario 2 one could use a pump that moved all the molecules back into only one room. However, the pump would release enough heat into the rest of the universe such that the organization reintroduced between the two rooms would be offset by the increased randomness elsewhere. Similarly one can release the energy in well-organized chemical bonds by burning carbon-based fuel to power a turbine that runs a refrigerator that freezes disordered water molecules into a well-ordered lattice of ice. Though a subset of the universe (the water) has been ordered into an ice lattice, the overall disorder in the universe is increased through the burning (disintegration) of the carbon-based fuel. The full process, from fuel burning to water freezing is irreversible. To put the Second Law another way, energy is destined to become less and less useful.

The ultimate source of useful energy for Earth is the sun. Over the course of billions of years the sun has provided the energy to form the carbon-based molecules we burn for energy. Solar power also drives the orderly flow of wind and ocean currents that can provide useful energy. In this way the sun is responsible for the ultimate organizer: evolution. By creating ever more impressive and differentiated forms of life, evolution stands in apparent rebellion against the Second Law of Thermodynamics. To observe evolution on earth in the absence of the sun would be like observing molecules in a lukewarm glass of water assemble themselves into an icy lattice in the absence of refrigeration. The sun is analogous to the diesel-burning turbine that taps useful energy for the refrigerator. But just as the burning of carbon fuel to organize water molecules into an icy lattice is irreversible, so is the burning of the sun to organize organic molecules into living creatures. And eventually in both cases the fuel will run out, the ice will melt, life will reduce to its random molecular origins. It is our good fortune that the sun (though likely not carbon fuel) will be burning for billions of years yet. This observation brings us naturally to the second perspective on time.