This book reviews and contrasts contemporary and historical perceptions
of time from scientific and intuitive human points of view. Ancient and
modern clocks, Augustinian ideas, the deterministic Newtonian universe,
biological clocks, deep time, thermodynamics, quantum mechanics, and
relativity all contribute to the perspective. The focus is on what can
be inferred from established technologies and science as opposed to
futuristic speculation
.
Chapter 1 describes clocks, including the cesium atomic clocks
establishing the current global time standard, a history of clock
development, biological clocks, phylogenetic trees, radioactive dating,
and astronomical methods to determine the age of the universe. Chapter 2
poses ancient questions about time not fully addressed by an
understanding of the technical nature of clocks. An early summary of
some of these questions as described by Augustine in the 3rd century CE
is followed by a description of how Newton, 1300 years later, introduced
a conception of time which provided some answers, such as the nature of
an infinitesimally short present. Implications concerning the reality of
events in the past, present, and future are also discussed. The
Newtonian picture is contrasted with the intuitive human one and the
possibilities of time travel and temporal recurrence are briefly
discussed. Chapter 3 introduces the second law of thermodynamics and
addresses how it is compatible with a time-reversible Newtonian
description of a universe, even though it appears to define an "arrow of
time." The nature of entropy and its relation to coarse graining and
emergence play a central role in the discussion. Chapter 4 discusses
ways in which quantum mechanics has altered the Newtonian perspective,
accounting for various interpretations of the meaning of quantum
mechanics with regard to time. Chapter 5 describes basic elements of
special relativity and their implications for the nature of time.
Examples of time dilation and the changing order of space, such as
separated events in different frames, are described. The examples are
chosen to avoid evocation of currently unattainable technologies. An
afterword in chapter 6 reviews questions raised by Augustine and
summarizes how the development of science since then has addressed them.
This book was originally developed for an interdisciplinary seminar for
beginning undergraduates at the University of Minnesota. It uses a small
amount of algebra, mainly in supplementary appendices, and does not
assume any prior knowledge of physics, chemistry, biology, or astronomy.
In contrast to many semipopular books on time, it avoids speculation
either about engineering (techno-optimism) or physical theory (strings,
loop quantum gravity, black hole entropy). Instead, it takes a more
grounded approach and describes what is currently known (and not known)
to help both students and the general reader make better sense of time.
