This article is a supplement to the feature "Cosmic Time Arrow: Does Time Run Backward in Other Universes?" from the June 2008 issue of Scientific American

If entropy always increases, then how do low-entropy objects such as eggs form in the first place?
The law of entropy applies to closed systems. It does not forbid decreases in entropy in open systems, including chickens. A hen takes in energy and goes through a great deal of effort to produce an egg.

Don’t some particle processes have a built-in arrow of time?
The decays of some elementary particles, such as neutral kaons, happen more frequently in one direction of time than the other. (Physicists do not need to travel backward in time to observe this asymmetry; they infer it from experiments on related particle properties.) But these processes are reversible, unlike the growth of entropy, so they do not explain the arrow of time. The Standard Model of particle physics does not seem to be of any help in explaining the low entropy of the early universe.

Doesn’t quantum mechanics have an arrow of time?
According to the standard interpretation of quantum mechanics, the measurement of a system causes its wave function to “collapse,” a process that is asymmetric in time. But the reason wave functions collapse yet never uncollapse is the same reason that eggs break yet never unbreak—namely, because collapse increases the entropy of the universe. Quantum mechanics does not help explain why the entropy was low in the first place.

Why do we remember the past but not the future?
To form a reliable memory requires that the past be orderly—that is, have a low entropy. If the entropy is high, almost all “memories” would be random fluctuations, completely unrelated to what actually happened in the past.

Is the multiverse theory testable?
The idea that the universe stretches far beyond what we can see is not really a theory—it is a prediction made by certain theories of quantum mechanics and of gravity. Admittedly, it is a prediction that is hard to test. But all theories of physics force us to go beyond what we can directly see. For instance, our current best model for the origin of cosmic structure, the inflationary universe scenario, requires us to understand the conditions even before inflation.