Book: The Evolution of Physics

Overview
Albert Einstein and Leopold Infeld’s 1938 book The Evolution of Physics traces the growth of fundamental ideas in physics from early mechanics to relativity and quantum theory, written for general readers without heavy mathematics. It focuses on how shifting concepts, mass, force, field, space, time, energy, and probability, reshape the picture of nature. The narrative emphasizes thought experiments, the tension between mechanical models and field concepts, and the continual refinement of theories to reconcile paradoxes exposed by experiment and logic.

Classical Foundations
The story begins with Galileo and Newton, who replaced qualitative explanations with quantitative laws of motion and universal gravitation. Nature is pictured as particles moving in absolute space and time, acted on by forces at a distance. This mechanical worldview achieves triumphs in celestial and terrestrial dynamics, but it carries unresolved puzzles: how can force act instantaneously across empty space, and what medium transmits influences? These questions set the stage for a transition from forces as mysterious pulls to fields as tangible, structured realities filling space.

Fields and Electromagnetism
Faraday’s lines of force and Maxwell’s equations transform electricity and magnetism into a unified field theory. The electromagnetic field gains independent status, with energy and momentum residing in space itself. The proposed ether, a material medium for waves, fails under scrutiny, especially after experiments like Michelson–Morley. The field view undermines action at a distance and invites a deeper rethinking of motion and measurement. The speed of light, emerging as a universal constant from Maxwell’s theory, collides with classical notions of absolute time and simultaneity, prompting a conceptual revolution.

Relativity: Space, Time, and Gravity
Special relativity reframes mechanics by imposing the invariance of light’s speed. Simultaneity becomes relative to the observer; space and time fuse into a coherent structure in which laws must hold for all inertial frames. Mass and energy are intertwined, and electromagnetism and mechanics are made compatible without invoking the ether. General relativity extends these ideas through the equivalence of inertial and gravitational effects: freely falling motion erases weight, hinting that gravity is not a force but the geometry of spacetime. Curved spacetime explains planetary orbits, the bending of light, and the shift of clocks in gravitational fields, while reducing to Newton’s theory in appropriate limits.

Thermodynamics and Statistical Thought
The book also highlights the conceptual leap from macroscopic laws of heat to the statistical behavior of countless microscopic constituents. Thermodynamics imposes constraints, entropy and irreversibility, while statistical mechanics links them to probability and fluctuations. This probabilistic turn foreshadows the later debates over the meaning of chance in fundamental physics and illustrates how laws can be exact in form yet statistical in content.

Quanta and the New Dilemma
The quantum emerges from the failure of classical physics to account for radiation and atomic phenomena. Energy arrives in discrete packets; light behaves as both wave and particle; matter shows wave-like interference. Heisenberg’s uncertainty principle limits the simultaneous precision of complementary quantities, and the quantum formalism assigns probabilities to outcomes rather than definite trajectories. While acknowledging its predictive power, the authors express dissatisfaction with a theory that replaces causal description with statistical rules, raising questions about completeness and the search for a deeper, more unified framework.

Method and Legacy
Across these episodes, the book presents physics as the evolution of organizing ideas guided by experiment and logic, not by accumulating formulas. Progress comes from discarding inadequate pictures and crafting new concepts that subsume old successes while explaining anomalies. The closing outlook stresses the unfinished character of the enterprise: relativity and quantum theory stand as towering achievements yet still resist unification. The evolution continues wherever clearer concepts can be forged to bring order to experience.