Chapter One – Physical World

1.1 WHAT IS PHYSICS ?

Humans have always been curious about the world around
them. The night sky with its bright celestial objects has
fascinated humans since time immemorial. The regular
repetitions of the day and night, the annual cycle of seasons,
the eclipses, the tides, the volcanoes, the rainbow have always
been a source of wonder. The world has an astonishing variety
of materials and a bewildering diversity of life and behaviour.
The inquiring and imaginative human mind has responded
to the wonder and awe of nature in different ways. One kind
of response from the earliest times has been to observe the
physical environment carefully, look for any meaningful
patterns and relations in natural phenomena, and build and
use new tools to interact with nature. This human endeavour
led, in course of time, to modern science and technology.
The word Science originates from the Latin verb Scientia
meaning ‘to know’. The Sanskrit word Vijnan and the Arabic
word Ilm convey similar meaning, namely ‘knowledge’.
Science, in a broad sense, is as old as human species. The
early civilisations of Egypt, India, China, Greece, Mesopotamia
and many others made vital contributions to its progress.
From the sixteenth century onwards, great strides were made
in science in Europe. By the middle of the twentieth century,
science had become a truly international enterprise, with
many cultures and countries contributing to its rapid growth.
What is Science and what is the so-called Scientific
Method? Science is a systematic attempt to understand
natural phenomena in as much detail and depth as possible,
and use the knowledge so gained to predict, modify and
control phenomena. Science is exploring, experimenting and
predicting from what we see around us. The curiosity to learn
about the world, unravelling the secrets of nature is the first
step towards the discovery of science. The scientific method
involves several interconnected steps : Systematic
observations, controlled experiments, qualitative and

quantitative reasoning, mathematical
modelling, prediction and verification or
falsification of theories. Speculation and
conjecture also have a place in science; but
ultimately, a scientific theory, to be acceptable,
must be verified by relevant observations or
experiments. There is much philosophical
debate about the nature and method of science
that we need not discuss here.
The interplay of theory and observation (or
experiment) is basic to the progress of science.
Science is ever dynamic. There is no ‘final’
theory in science and no unquestioned
authority among scientists. As observations
improve in detail and precision or experiments
yield new results, theories must account for
them, if necessary, by introducing modifications.
Sometimes the modifications may not be drastic
and may lie within the framework of existing
theory. For example, when Johannes Kepler
(1571-1630) examined the extensive data on
planetary motion collected by Tycho Brahe
(1546-1601), the planetary circular orbits in
heliocentric theory (sun at the centre of the
solar system) imagined by Nicolas Copernicus
(1473–1543) had to be replaced by elliptical
orbits to fit the data better. Occasionally,
however, the existing theory is simply unable
to explain new observations. This causes a
major upheaval in science. In the beginning of
the twentieth century, it was realised that
Newtonian mechanics, till then a very
successful theory, could not explain some of the
most basic features of atomic phenomena.
Similarly, the then accepted wave picture of light
failed to explain the photoelectric effect properly.
This led to the development of a radically new
theory (Quantum Mechanics) to deal with atomic
and molecular phenomena.
Just as a new experiment may suggest an
alternative theoretical model, a theoretical
advance may suggest what to look for in some
experiments. The result of experiment of
scattering of alpha particles by gold foil, in 1911
by Ernest Rutherford (1871–1937) established
the nuclear model of the atom, which then
became the basis of the quantum theory of
hydrogen atom given in 1913 by Niels Bohr
(1885–1962). On the other hand, the concept of
antiparticle was first introduced theoretically by
Paul Dirac (1902–1984) in 1930 and confirmed
two years later by the experimental discovery of
positron (antielectron) by Carl Anderson.

Physics is a basic discipline in the category
of Natural Sciences, which also includes other
disciplines like Chemistry and Biology. The word
Physics comes from a Greek word meaning
nature. Its Sanskrit equivalent is Bhautiki that
is used to refer to the study of the physical world.
A precise definition of this discipline is neither
possible nor necessary. We can broadly describe
physics as a study of the basic laws of nature
and their manifestation in different natural
phenomena. The scope of physics is described
briefly in the next section. Here we remark on
two principal thrusts in physics : unification
and reduction.
In Physics, we attempt to explain diverse
physical phenomena in terms of a few concepts
and laws. The effort is to see the physical world
as manifestation of some universal laws in
different domains and conditions. For example,
the same law of gravitation (given by Newton)
describes the fall of an apple to the ground, the
motion of the moon around the earth and the
motion of planets around the sun. Similarly, the
basic laws of electromagnetism (Maxwell’s
equations) govern all electric and magnetic
phenomena. The attempts to unify fundamental
forces of nature (section 1.4) reflect this same
quest for unification.
A related effort is to derive the properties of a
bigger, more complex, system from the properties
and interactions of its constituent simpler parts.
This approach is called reductionism and is
at the heart of physics. For example, the subject
of thermodynamics, developed in the nineteenth
century, deals with bulk systems in terms of
macroscopic quantities such as temperature,
internal energy, entropy, etc. Subsequently, the
subjects of kinetic theory and statistical
mechanics interpreted these quantities in terms
of the properties of the molecular constituents
of the bulk system. In particular, the
temperature was seen to be related to the average
kinetic energy of molecules of the system.

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