The Nature of Time,
Lead: Andrew Case & Mark Knights
Summary by Robert G. Crooks:
Aristotle may have regarded time as cyclical in that, when the heavenly bodies returned to the positions they held at the beginning of the world, time would start over, like replaying a CD.
In about the 4th century A.D., St. Augustine sought to linearize time, relating it to the crucifixion of Christ, an event not to be repeated. He also introduced some ideas that time might be subjective, and not merely physical or tied to the "heavenly bodies."
In the 17th century, Isaac Newton stoutly maintained in England that time flows uniformly of its own accord, independently of human minds and all material objects, like a long broad river which would never change course or reverse. But over in Germany, Newton's archrival, Baron Wilhelm von Leibniz, regarded time simply as the order of succession of phenomena. Could this have been a subtle put-down to Newton?
Then, in the 18th century, the German philosopher Immanuel Kant picked up Augustine's notion that time is a feature of the way men's minds visualize the external world and is not a characteristic of reality itself. That was still an interesting idea, but didn't prevent Newton's concepts from prevailing until 1905. In that year, when Albert Einstein announced his Theory of Special Relativity, all fundamental physical concepts including time had to be reexamined. For most terrestrial purposes, Newton's laws continued to work very well. Scientists continued their efforts to establish repeatable standards, including the adoption of the rate of oscillation of the cesium atom as the definition of the "atomic second." Apparently, the cesium atom has very stable habits.
Since then, the "General Conference of Weights and Measures" has ordained that "one second" of time shall henceforth be the interval required for 9,192,631,770 oscillations of the cesium atom. That seems to be a pretty accurate definition of "one second." And, of course, there will continue to be sixty seconds in a minute, and sixty minutes in an hour. So, what's left to discuss about "time?"
Well, Einstein's propositions in 1905, and again in 1915, have created some real waves in Newton's smoothly flowing river of time. Moreover, some later scientists have proposed concepts of time, which might have taxed even the imagination of Einstein.
According to special relativity, each inertial system in the universe has its own time parameter. So, the intuitive concepts of absolute time and absolute simultaneity, as espoused by Newton and others, do not exist in special relativity. And, with the advent of space travel and the forecasting of the trajectories of asteroids, relativistic considerations have become important to our lives as well as to our intellects. Even the possibility of "negative time" can no longer be disregarded, because the laws of physical mechanics operate equally well if "-t" is substituted for "t".
With regard to going backward in time, Einstein's Theory of General Relativity, announced in 1915, has provided a clue. He showed that both space and time are curved, and that the curvature of "space-time" can become extreme in the neighborhood of very massive objects. This showing opened the door to the concept of tunnels connecting distant regions of space-time. Possibly one could even use a so-called "worm hole" to travel into one's own past!
Another tongue-in-cheek suggestion for "time travel" was that of Richard Gott in 1991. The implementation required the use of "cosmic strings", thin strands of energy millions of light-years long, predicted by some theories of particle physics but not yet observed in the universe. Half the mass-energy of a galaxy would be required for the implementation!
Although Einstein's theories are not psychological in nature, Stephen Hawking has, to a limited extent, revisited subjective concepts such as those of Kant. For evaluation of the direction of time, he has proposed three distinct "arrows:"
(1) thermodynamic, (2) psychological, and (3) cosmological.
Hawking's "thermodynamic" arrow seems to draw upon the "information theory" developed by Claude Shannon in the late nineteen-forties, primarily for communications purposes. As a criterion for distinguishing useful "information" from predictable and useless signals, Shannon borrowed from thermodynamics the term "entropy." To him, entropy is a measure of spontaneity or degree of meaningful freedom, as distinguished from rote "organization." It also happened that some of Shannon's probability concepts took mathematical forms similar to the concepts defining thermodynamic entropy. But Hawking treats Shannon's "entropy" as if it really were rooted in physical thermodynamics.
In proposing his "psychological" arrow, Hawking once again departs from his usual high standard of rigor. But turning to his "cosmological" arrow, Hawking is undoubtedly right that we wouldn't be here if the universe were not expanding at its present rate, plus or minus some negligibly-small figure. And we certainly won't be here if and when the universe enters a contracting phase.
April 24, 2000