The 2,107th Meeting of the Society

October 1, 1999

The Nature of Time in Physics and Human Experience

Robert A. Nelson

President, Satellite Engineering Research Corporation

About the Lecture

The physical and philosophical aspects of time are discussed and compared. The characteristics of scientific theories and physical laws are examined and illustrated by the concept of time. The operational significance of time in physics as interpreted by the Newtonian theory of gravitation and the general theory of relativity is discussed. This theoretical aspect of time is compared to its role in human experience, including the physical, biological, and social origins of time awareness.

About the Speaker

Robert A. Nelson is president of Satellite Engineering Research Corporation, an engineering consulting firm in Bethesda, Maryland. He holds the degree of Ph.D. in physics from the University of Maryland and is a licensed Professional Engineer. He is also a Lecturer in the Department of Aerospace Engineering at the University of Maryland. He is co-author of the textbook Satellite Communication Systems Engineering (Prentice-Hall, 1993) and is Technical Editor of Via Satellite magazine. His publications include a variety of technical papers and reports on the application of relativity to high precision time transfer systems.


President Garavelli called the 2107th meeting to order at 8:30 p.m. on October 1, 1999. The Recording Secretary read the minutes of the 2105th meeting and they were approved. [The 2106th meeting was the Joseph Henry lecture and minutes were not recorded by tradition.] The speaker for the 2107th meeting was Robert A. Nelson of the Satellite Engineering Research Corporation. The title of his presentation was “The Nature of Time in Physics and Human Experience.” In way of an overview, Mr. Nelson discussed and compared the physical and philosophical aspects of time. He also examined and illustrated the characteristics of scientific theories and physical laws using the concept of time — including the significance of time in physics as interpreted by the Newtonian theory of gravitation and the general theory of relativity. The theoretical aspects of time were compared to its role in human experience, including the physical, biological, and social origins of time awareness. In 1894, Albert A. Michelson remarked that in physics there were no more fundamental discoveries to be made. Quoting Lord Kelvin, he continued, “An eminent physicist remarked that the future truths of physical science are to be looked for in the sixth place of decimals.” However, only six years later, in a 1900 lecture, Lord Kelvin himself noted “two clouds on the horizon” of theoretical physics. He was referring to the problems of black body radiation and the null result of the Michelson-Morley experiment, which foreshadowed the coming revolution in physics with the birth of quantum mechanics and relativity. Our speaker went on to describe Newton's concept of absolute, mathematical time as universal with space described by Euclidean geometry. In Newtonian physics, there are two invariants - independent of the frame of reference. One invariant is spatial distance, the other is time. According to Newton, the natural motion of a body is a straight line unless some external force alters the motion. His theory of gravitation is represented by three second-order differential equations of motion, one for each direction of space. Alternately, in Einstein's relativity theory, there are two kinds of time. The so-called proper time as the reading on a clock, and in contrast, coordinate time as one of four space-time coordinates adopted in a convenient four-dimensional coordinate system that represents the order of events. The proper or clock time is related to coordinate time through the four-dimensional space-time interval, which is a generalization of the Pythagorean theorem and contains both the increments of spatial distance and the increment of time. However, the coordinate differentials are multiplied by functions called the metric coefficients, which are determined by the field equations from the distribution of matter. By appropriate adjustment in epoch and rate, a clock can be made to be a coordinate clock, so that its proper time is locally equal to coordinate time. The theory is based on the mathematics of Riemannian geometry and tensor analysis. The geometrical invariance of four-dimensional space-time interval embodies the fundamental premise of general relativity, that the laws of physics should have the same mathematical form in every frame of reference. In the measurement of length and time, time is the only fundamental quantity and length is derived. Therefore, clocks and light signals are the only fundamental instruments of measurement and there is no such thing as a rigid ruler. We are aware of time only through the process of change. As Newton said, “Relative, apparent, or common time, is some sensible and external measure of duration by the means of motion.” The rising and setting of the sun and the moon are our most immediate external measures of the progress of time. Next, are the seasons as reflected in the changing aspect of the night sky. These periodic phenomena became the bases of calendars created to plan farming, order commerce, and formalize religious activities. Sundials were the primary means of keeping the time of day, and the variable hour they indicated continued in general use until about the fourteenth century. At about this period, the first weight driven, mechanical clocks were developed. Finally, the development of atomic clocks in our own century permitted another definition of time. In 1967, the Thirteenth General Conference on Weights and Measures defined the second as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two ground states of the 133Cs atom.” Our speaker closed his presentation by looking at social origins of time awareness. He stated that it is gratuitous to assume that a Hopi who knows only the Hopi language and the cultural ideas of his own society has the same notions, often supposed to be intuitions, of time and space that we have, and that are generally assumed to be universal. In particular, the Hopi has no general notion or intuition of time as a smooth flowing continuum in which everything in the universe proceeds at an equal rate. The Hopi language contains no words, grammatical forms, constructions or expressions that refer directly to what we call “time,” or to past, present, or future, or to enduring or lasting. At the same time, the Hopi language is capable of accounting for and describing correctly, in a pragmatic sense, all observable phenomena of the universe. Thus, the Hopi language and culture conceals a metaphysics, such as our so-called naive view of space and time does, or as the relativity theory does, yet it is a different metaphysics from either. Mr. Nelson then closed his presentation and kindly answered questions from the floor. President Garavelli thereupon thanked Mr. Nelson for the society, announced the next meeting and made the usual parking announcement. He then adjourned the 2107th meeting to the Social Hour at 9:32 p.m. Attendance: 43 Temperature: 12.7°C Weather: clear Respectfully submitted, Bill Spargo Recording Secretary