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 Sclentia meaning to know". The Sanskrit word Vijanan 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 llrst 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 I 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.

This is the text from the NCERT Book “Physics for Class XI”.