Physics

• 1. Introduction

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  Physics is the scientific study of matter, its fundamental constituents, and its motion and behavior through space and time, along with the related ideas of energy and force. It is also one of the oldest academic disciplines. For much of the past two millennia, it was not even a separate subject. Physics, chemistry, biology, and parts of mathematics all lived together under a single banner called natural philosophy. So how did a field this sprawling pull itself apart from the rest of knowledge? And why do physicists call it the fundamental science, the one that everything else must answer to? The answers run from ancient stargazers worshipping planets they believed were gods, through a 4th century BC treatise that held sway for two thousand years, to a handful of stubborn experimental results that would upset the entire field in the first decades of the 20th century.

• 2. From Star Gods To Natural Cause

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  Early civilizations dating before 3000 BCE, including the Sumerians, the ancient Egyptians, and the Indus Valley Civilization, already tracked the motions of the Sun, Moon, and stars. The stars and planets were believed to represent gods, and they were often worshipped. The explanations attached to those positions were usually unscientific and lacked evidence. Yet the watching itself mattered, because the stars were found to trace great circles across the sky.

  According to Asger Aaboe, the origins of Western astronomy lie in Mesopotamia, and every Western effort in the exact sciences descends from late Babylonian astronomy. Egyptian astronomers left monuments showing knowledge of the constellations and the motions of celestial bodies. The Greek poet Homer wrote of various celestial objects in his Iliad and Odyssey. Later Greek astronomers supplied names, still used today, for most constellations visible from the Northern Hemisphere.

  Natural philosophy itself began in Greece during the Archaic period, roughly 650 BCE to 480 BCE. Pre-Socratic philosophers such as Thales rejected non-naturalistic explanations and insisted that every event had a natural cause. They proposed ideas to be checked by reason and observation. One of those guesses, atomism, was proposed by Leucippus and his pupil Democritus, and it was found to be correct about 2000 years later.

• 3. Aristotle's Four Elements

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  Aristotle, who lived from 384 to 322 BCE and studied under Plato, wrote a substantial treatise titled Physics in the 4th century BC. His version of physics stayed influential for about two millennia. His method blended limited observation with logical, deductive argument, but it did not test those deductions through experiment. The framework was very imperfect, and it is entirely superseded today.

  Aristotle explained motion and gravity through the theory of four elements: air, fire, water, and earth. Each element, he held, had its own natural place determined by density. Fire belonged at the top, then air, then water, then earth at the bottom. A flame on the ground climbs into the air because it is trying to return to where it belongs. When a small amount of one element strays into another's place, the less abundant element moves back toward home.

  Heavier objects, in Aristotle's laws of motion, fall faster, with speed proportional to weight and inversely related to the density of whatever the object falls through. For violent motion, meaning motion caused by an applied force, the object moves only as fast as the force applied to it. Studying motion and its causes led him to the idea of a prime mover, the ultimate source of all motion, which he set out in Book 8 of his treatise.

• 4. Keepers Of The Flame

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  The Western Roman Empire fell to invaders and internal decay in the fifth century, and intellectual life in western Europe declined with it. The Eastern Roman Empire, usually known as the Byzantine Empire, held off its attackers and kept advancing fields of learning, physics among them. In the sixth century, John Philoponus challenged the dominant Aristotelian approach, though much of his work centered on Christian theology. In that same century, Isidore of Miletus assembled an important compilation of the works of Archimedes, preserved in the Archimedes Palimpsest.

  Islamic scholarship inherited Aristotelian physics from the Greeks and, during the Islamic Golden Age, carried it further. The most notable advances came in optics and vision, drawn from scientists such as Ibn Sahl, Al-Kindi, Ibn al-Haytham, Al-Farisi, and Avicenna. In his Book of Optics, also known as Kitab al-Manazir, Ibn al-Haytham proposed light rays as an alternative to the ancient Greek idea of visual rays. Like Ptolemy, he ran controlled experiments, verifying the laws of refraction and reflection, though he still lacked the concept of image formation.

• 5. The Break Into A Science

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  Physics became a separate science when early modern Europeans turned to experimental and quantitative methods to find what we now call the laws of physics. The geocentric model of the Solar System gave way to the heliocentric Copernican model. Johannes Kepler worked out the laws governing planetary motion between 1609 and 1619. Galileo did pioneering work on telescopes and observational astronomy across the 16th and 17th centuries.

  Isaac Newton discovered and unified the laws of motion and universal gravitation that now carry his name. Newton, and separately Gottfried Wilhelm Leibniz, developed calculus, the mathematical study of continuous change, and Newton used it to solve physical problems. As energy needs rose during the Industrial Revolution, research delivered the laws of thermodynamics, chemistry, and electromagnetics. By the end of the 19th century, theories of thermodynamics, mechanics, and electromagnetics matched a wide range of observations and together formed what would later be called classical physics.

  A few experimental results refused to fit. Classical electromagnetism assumed a luminiferous aether to carry waves, yet that medium could not be detected. The intensity of light from hot glowing blackbody objects did not match what thermodynamics and electromagnetism predicted. The way illuminated metals emitted electrons also broke from prediction. These failures looked insignificant against the big picture.

• 6. The Modern Earthquake

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  Modern physics opened in the early 20th century with Max Planck's quantum theory and Albert Einstein's theory of relativity. Both arose from places where classical mechanics gave wrong answers. Classical mechanics said the speed of light should depend on the observer's motion, which clashed with the constant speed predicted by Maxwell's equations. Einstein's special relativity resolved this, replacing classical mechanics for fast-moving bodies and allowing a constant speed of light. Planck addressed black-body radiation by proposing that material oscillators can be excited only in discrete steps proportional to their frequency.

  That insight, together with the photoelectric effect and a theory predicting discrete energy levels for electron orbitals, produced quantum mechanics, which improves on classical physics at very small scales. Werner Heisenberg, Erwin Schrodinger, and Paul Dirac pioneered the new mechanics. From this early work grew the Standard Model of particle physics. The model accounts for the 12 known particles of matter, the quarks and leptons, interacting through the strong, weak, and electromagnetic forces by exchanging gauge bosons: gluons, W and Z bosons, and photons.

  In July 2012, CERN, the European laboratory for particle physics, announced the detection of a particle consistent with the Higgs boson. With that, all fundamental particles predicted by the Standard Model, and no others, appear to exist. Physics beyond the Standard Model, including theories such as supersymmetry, remains an active area of research.

• 7. How Theory Meets Experiment

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  Physicists use the scientific method to test whether a physical theory holds. They compare a theory's implications against the conclusions drawn from related experiments and observations, aiming for a logical, unbiased, and repeatable check. A scientific law, like Newton's law of universal gravitation, is a concise verbal or mathematical statement of a fundamental relation.

  Theorists build mathematical models that agree with existing experiments and predict future ones, while experimentalists devise and perform experiments to test those predictions and explore new phenomena. The two pursuits develop separately yet depend on each other. Progress often comes when experimental results defy existing theories, or when a new theory generates testable predictions that inspire fresh experiments and equipment. Those who work at this interplay are called phenomenologists. Beyond the known universe, theoretical physics also takes up hypothetical questions such as parallel universes, a multiverse, and higher dimensions.

  Experimental physicists in basic research use equipment such as particle accelerators and lasers, while those in applied research often work in industry, developing technologies such as magnetic resonance imaging and transistors. Feynman noted that experimentalists may seek out areas not yet well explored by theorists. The reach is wide: physics spans elementary particles such as quarks, neutrinos, and electrons all the way up to the largest superclusters of galaxies.

• 8. The Fundamental Science

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  The ancient Chinese observed that certain rocks, lodestone and magnetite, attracted one another through an invisible force later called magnetism, first studied rigorously in the 17th century. The ancient Greeks knew that amber rubbed with fur produced a similar invisible attraction, an effect later named electricity and also first studied rigorously in the 17th century. Work in the 19th century revealed that these were two aspects of a single force, electromagnetism. That unifying continues, and electromagnetism and the weak nuclear force are now seen as aspects of the electroweak interaction.

  Because all branches of natural science, including chemistry, astronomy, geology, and biology, are constrained by its laws, physics is called the fundamental science. Chemistry, by contrast, is often called the central science for its role linking the physical sciences. Condensed matter physics is the largest field of contemporary physics, and Philip Anderson apparently coined the term in 1967 when he renamed his research group, previously called solid-state theory. In 1978, the American Physical Society renamed its Division of Solid State Physics as the Division of Condensed Matter Physics.

  Many everyday phenomena involving complexity, chaos, or turbulence remain poorly understood, from the formation of sandpiles to the shape of water droplets. In the 1932 Annual Review of Fluid Mechanics, Horace Lamb captured the puzzle: 'I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic.'