Scope and Excitement of Physics

Physics is conveniently pictured as spanning two domains. The macroscopic domain covers phenomena at laboratory, terrestrial and astronomical scales; the microscopic domain covers atomic, molecular and nuclear phenomena (with a mesoscopic domain of a few tens to hundreds of atoms as an active research frontier in between).

Classical physics deals mainly with the macroscopic domain and is organised into a handful of sub-disciplines: Mechanics (built on Newton's laws of motion and gravitation) handles the motion and equilibrium of particles and bodies — from a rocket's propulsion to a loaded beam bending; Electrodynamics (its laws given by Coulomb, Oersted, Ampere and Faraday, and unified by Maxwell) covers electric and magnetic phenomena — a current-carrying wire in a magnetic field, an antenna, radio-wave propagation; Optics deals with light — telescopes, microscopes, the colours of thin films; Thermodynamics, unlike mechanics, does not track individual motions but the macroscopic equilibrium of a system — its internal energy, temperature and entropy, and how these change through work and heat transfer, as in the efficiency of a heat engine or refrigerator.

The microscopic domain — the structure of matter at the scale of atoms and nuclei, and its interaction with probes like electrons and photons — cannot be handled by classical physics at all; quantum theory is the framework used there.

(See Fig. 1.1: a schematic of Rutherford's alpha-scattering experiment — alpha particles from a polonium source strike a thin gold foil, and the scattering angle of the deflected particles, picked up by a microscope viewing a fluorescent screen, is what revealed the nuclear model of the atom. It illustrates, concretely, how theory and experiment drive each other forward in physics.)

The sheer range of physics is staggering. At one extreme it probes lengths of $10^{-14}$ m or smaller (electrons, protons); at the other, astronomical scales of order $10^{26}$ m (galaxies, the observable universe) — a span of roughly $10^{40}$. Dividing length by the speed of light gives a comparable range of time scales, from about $10^{-22}$ s to $10^{18}$ s, and masses range from about $10^{-30}$ kg (an electron) to $10^{55}$ kg (the observable universe), with everyday terrestrial phenomena sitting somewhere in the middle.

What makes physics exciting differs from person to person: some are drawn to the elegance of a few basic laws explaining an enormous range of phenomena; others to the thrill of designing a clever experiment that tests or overturns a theory; others still to the ingenuity needed to turn physical laws into useful devices (applied physics). Three shifts in outlook underlie physics's rapid historical progress: recognising that quantitative measurement, not just qualitative reasoning, is essential, because nature's laws are expressible as precise mathematical equations; recognising that the basic laws are universal — the same laws hold in very different contexts; and adopting the strategy of approximation — isolating the essential feature of a phenomenon first, and adding corrections afterwards. The classic illustration is a stone and a feather dropped together: in ordinary air they land at very different times because of air resistance, but drop them down an evacuated tube and they fall together, revealing the essential law — that acceleration due to gravity does not depend on mass. Only once that basic law is established does it make sense to add back air resistance as a correction. …