First mention of the Arrow of Time was by Sir Arthur Eddington. He was the noted British astrophysicist who in May, 1919 conducted an expedition to an island off the coast of West Africa where he photographed a solar eclipse showing bending of light and providing the first confirmation of Albert Einstein’s Theory of Relativity.
Eddington used the graphic term Arrow of Time in 1927 to illustrate the apparent directionality of time. His approach was phenomenological. Rather than speculating on a physical basis for time, he described human perception of time and linked it to entropy and Sadi Carnot’s Second Law of Thermodynamics (1824).
Because there are vastly more disordered than ordered states in the distribution of matter and energy in the universe, it is highly more likely (verging on certainty) that any change will be from the more ordered pattern to the less ordered pattern, i.e. toward an inevitable rise in entropy. This is the sole reason there is a perceived passage of time. From this fact all time-related phenomena inside and outside the realm of human perception can be derived.
We perceive events as occurring in time and space, but these domains are strikingly different. In time, events proceed along a temporal timeline, along which we move. Or, alternately, we are stationary and temporality is like a wind that blows past us. Everything exists along a one-dimensional line. We directly experience only the present. We experience the past indirectly through memory and the future indirectly through anticipation. Space, in contrast, is three-dimensional. It is laid out before us, its contents simultaneous rather than sequential as in time.
Theoretical physicists of the past two centuries have created a great many thought experiments to facilitate understanding of their ideas. Albert Einstein and Erwin Schrödinger indulged in this expedient, and it worked for them.
One of the all-time great thought experiments was promulgated by James Clerk Maxwell in connection with his claim that the Second Law of Thermodynamics has only statistical certainty. (It is not so much a law as an observation.)
Maxwell imagined a demon, which surfaced in his 1872 book on thermodynamics, titledTheory of Heat。考虑一个封闭的容器装满气体摩尔cules, evenly distributed and moving at various speeds. The container is divided internally by an insulated wall with a door that can be opened and closed by a hypothetical being known as Maxwell’s demon. Having an inquiring mind or for whatever motivation, the creature opens the door only as needed to allow faster moving molecules to pass to one sector of the container and slower moving molecules to migrate to the other side.
In time, temperatures of the two isolated sections diverge, one becoming hotter and the other colder. Would this not violate the Second Law of Thermodynamics or, seen another way, reverse the direction of time’s arrow?
As with many such proofs, this one has a simple answer. The energy expended and dissipated by Maxwell’s demon in processing the information, not to mention actually opening and shutting the door, would be more than enough to counterbalance the loss of entropy in his enterprise. The Second Law of Thermodynamics and the arrow of time don’t change.
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