1  What Is Physics?

If you want to know what physics is, there are two main answers.

• There is the traditional subject matter of physics.
• There is a style of doing physics. This is also known as thinking like a physicist.

The traditional subject matter of physics includes

– Symmetry and conservation; continuity of flow; conservation of energy, momentum, angular momentum; translational and rotational symmetry; Galilean relativity; conservation of charge; gauge invariance; – Laws of motion: time, position, velocity, acceleration; force and mass; – Waves: acoustics; optics; shear waves, surface acoustic waves, etc.; dispersion, nonlinearity, and shocks; – Thermodynamics statistical mechanics: microstate versus macrostate entropy i.e. P log (1/P); spontaneity and irreversibility; Brownian motion; why the sky is blue. – Atomic and sub-atomic structure: atomic physics; physical basis of chemistry; nuclear physics; radioactivity and transmutation; elementary particles and fields. – Relativity: Geometry and trigonometry in flat spacetime; mass and rest-energy; GR and curvature of spacetime; quantitative cosmology; – Quantum mechanics: amplitude versus probability; complementarity and irreducible uncertainty; – Condensed matter physics: solid state physics; semiconductors; superconductivity and superfluidity;

The subject matter of physics is simple, in the following sense:

• Certain ultra-simplified parts chemistry are called atomic physics.
• Certain ultra-simplified parts of astronomy are called astrophysics.
• Certain ultra-simplified parts of biology are called biophysics.
• The fundamental equations of mechanics, electromagnetism, gravitation, thermodynamics, etc. are all rather simple and could all be written down on one or two pages.
• et cetera.

Students think physics is hard. This is not because of the subject matter per se, but because of how far we push it.

For example, Newton’s theory of gravitation is adequate for most purposes over a wide range of length scales. This range covers at least 17 orders of magnitude, from laboratory scale (the Cavendish experiment) to at least the size of a globular cluster. The Coulomb interaction has a range of at least 20 orders of magnitude, from the size of an atomic nucleus to the size of the ionosphere (and presumably much larger than that).