Physics: The Laws of Nature

Physics is about finding general rules to explain and predict and hence in some cases to control and manipulate phenomena observed in nature. It thus underlies other sciences such as chemistry and biology, and often makes use of sophisticated mathematics.

Sections on this page: Ancient and Mediaeval PhysicsPrinciples of MechanicsHeat, Thermodynamics and EntropyElectricity and MagnetismLight, Radio Waves and X-RaysAtoms, and Subatomic ParticlesRelativity and CosmologyInformation and Computing.


Ancient and Mediaeval Physics

From very early times it was recognised that there seemed to be a distinct difference between the phenomena observed on Earth and those in Heaven, that is above the clouds. On earth there was chaos but in heaven there was regularity, although that regularity was in some degree related to events on earth, for instance in the sun's course among the stars corresponding to the annual cycle of the seasons, at least in temperate regions. In heaven the motions of the celestial bodies were smooth and cyclic whereas on earth motions always came to a stop and generally required effort to maintain. On earth unsupported objects fell to the ground whereas the celestial bodies showed no sign of falling. It was only with the publication of Newton's Principia in 1687 that both realms, earth and heaven, were at last subsumed within a single system of mechanics.

The beginnings of gropings towards understanding of such general rules were based on a loose mixture of observation and imaginative speculation. Among the earliest to publish ideas on physics were a group of philosophers from Miletus: Thales & & (c.−625 - c.−545), who predicted an eclipse of the sun (−586). Anaximander & (c.−610 - c.−545). Anaximenes & (d.−528). Most of their speculation was eventually shown to be wrong, but sometimes it was right. Even at present there are speculative theories, such as the "multiverse" theory and "string theory" which attempt to provide a deeper explanation than that currently accepted. They may eventually be proved true, if they can be verified by systematic testing or observations, or they may be disproved and we will need to seek alternative ideas, or they may prove unprovable, in which case they will remain metaphysical speculations, i.e. "beyond physics".

It is often possible to find in early mystical writings apparent "insights" into modern physics, but most of these are contentious interpretations by enthusiasts for these mystics. Some however were genuine insights, but were ahead of their time and not followed up or developed into a systematic theory. Examples include early ideas on atoms and the way they combine (in Indian and Greek thinkers) and formulations (in Chinese) of the law of action and reaction, or of the law of inertia. It also seems that Aristarchus & & & & & of Samos (c.−310 - c.−230), anticipated the Copernican theory, that the Earth was a Planet that traveled around the Sun once a year, and rotated on its axis once a day (accounting for the seasons and the cycle of day and night), and that the stars were other, very distant suns.

Geometry, the study of the way points, lines, planes, and other shapes relate to each other, and the measurement or calculation of their lengths, areas and volumes, was the first part of physics to be developed substantially. The Elements by Euclid & & of Alexandria (c.−325 - −265), which systematised the results obtained by several generations of Greek geometers, is probably the greatest achievement of ancient thought. Archimedes & (c.−287 - −212) and others also developed methods for calculating lengths, areas and volumes by successive approximation that anticipated the methods of integral calculus. Geometry has become so well developed, and generalised to include more abstract systems, analogous to the traditional subject, that it is now regarded as a branch of mathematics rather than physics.

Genuine physical insights from the Greeks, apart from their geometry, included: Archimedes' principle of hydrostatics (discovered, so the story goes, when he noticed that his own body displaced a volume of water sufficient to allow him to float, while he was getting into his bath), and his statement of the principles of leverage. The Archimedes screw is still in use today, to lift water from rivers onto irrigated farmland. The discovery of the Antikythera mechanism & & & (video of working model) (−125±25) points to a detailed understanding of movements of the astronomical objects, as well as a use of gear-trains that is comparable to 17th century clocks. The aeolipiles & & made or described by Ctesibius (c.−285 - −222), Vitruvius (c.−70 - +15) and Hero (or Heron) of Alexandria (c.+10 - +70), an early form of steam engine (video of model), was only a curiosity which did not solve the problem of transforming its rotational energy into a more usable form. Ramps were in use several millennia before Archimedes, to build the Pyramids.

Early theories of vision, in so far as they existed in explicit form, seem to have thought of it as working in a manner analogous to touch; by rays of sight emanating from the eyes to detect objects. The first to explain vision in terms of light rays being reflected from objects into the eye seems to have been the Persian thinker Abu Ali al-Hasan ibn al-Haytham & & & (c. 965 - 1040), also known as Alhazen, who has also been called "the first scientist", since he based his conclusions concerning the behaviour of light upon experimental observation. He described the "camera obscura", which he may have invented. He explained the refraction of light on the supposition that light moved more slowly in a denser medium, but did not formulate an accurate law of refraction. His major work on optics, Kitab-at-Manazir, was translated into Latin in 1572. Roger Bacon (c.1250 in Oxford} conducted research into optics, although much of it was similar to what had been done by Alhazen.

In the 14th century, some scholars, such as Jean Buridan and Nicolas Oresme, started to question the received wisdom of Aristotle's mechanics. Buridan developed the theory of impetus, a step towards the modern concept of inertia. Oresme showed that the reasons proposed by Aristotle against the movement of the Earth were not valid and adduced the argument of simplicity for the theory that the Earth moves, and not the heavens. In this he is clearer than Copernicus.

In the 16th century Nicolaus Copernicus revived Aristarchus' heliocentric model of the Solar System in Europe (which survived primarily in a passing mention in The Sand Reckoner of Archimedes). When this model was published at the end of his life, it was with a preface by Andreas Osiander that piously represented it as only a mathematical convenience for calculating the positions of planets, and not an account of the true nature of the planetary orbits.


Principles of Mechanics

In the early 17th century Johannes Kepler formulated a model of the Solar System based upon the five Platonic solids, in an attempt to explain why the orbits of the Planets had the relative sizes they did. His access to extremely accurate astronomical observations by Tycho Brahe enabled him to determine that his model was inconsistent with the observed orbits. After a heroic seven-year effort to more accurately model the motion of the Planet Mars (during which he laid the foundations of modern integral calculus) he concluded that the Planets follow not circular orbits, but elliptical orbits with the Sun at one focus of the ellipse. This breakthrough overturned a millennium of dogma based on Ptolemy's idea of "perfect" circular orbits for the "perfect" heavenly bodies. Kepler then went on to formulate his three laws of planetary motion. He also proposed the first known model of planetary motion in which a force emanating from the Sun deflects the Planets from their "natural" motion, causing them to follow curved orbits.

Also important were developments of mechanical devices. Otto von Guericke constructed the first air pump in 1650 and demonstrated the physics of the vacuum and atmospheric pressure using the Magdeburg hemispheres. In 1656 the Dutch physicist and astronomer, Christian Huygens invented a mechanical clock using a pendulum that swung through an elliptical arc, powered by a falling counterweight, to usher in the era of accurate timekeeping.

The first quantitative estimate of the speed of light was made in 1676 by Ole Rømer, by timing the motions of Jupiter's Satellite Io with a telescope.

During the early 17th century, Galileo Galilei pioneered the use of experiment to validate physical theories, which is the key idea in the scientific method. Galileo's use of experiment, and the insistence of Galileo and Kepler that observational results must always take precedence over theoretical results (in which they followed the precepts of Aristotle if not his practice), brushed away the acceptance of dogma, and gave birth to an era where scientific ideas were openly discussed and rigorously tested. Galileo formulated and successfully tested several results in dynamics, including the correct law of accelerated motion, the parabolic trajectory, the relativity of unaccelerated motion, and an early form of the Law of Inertia.

In 1687, Isaac Newton published the Principia Mathematica, detailing two comprehensive and successful physical theories: Newton's laws of motion, from which arise classical mechanics; and Newton's Law of Gravitation, which describes the fundamental force of gravity. Both theories agreed well with experiment. The Law of Gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.

In about 1788, Joseph Louis Lagrange elaborated an important new formulation of mechanics using the calculus of variations, the principle of least action and the Euler-Lagrange equations.

Classical mechanics was given a new formulation by William Rowan Hamilton, in 1833 with the introduction of what is now called the Hamiltonian, which a century later gave an entry to wave mechanical formulation of quantum mechanics.


Heat, Thermodynamics and Entropy

From the 18th century onwards, thermodynamic concepts were developed by Robert Boyle, Thomas Young, and many others, concurrently with the development of the steam engine, onward into the next century. In 1733, Daniel Bernoulli used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of statistical mechanics. Benjamin Thompson demonstrated the conversion of unlimited mechanical work into heat.

In 1847 James Prescott Joule stated the law of conservation of energy, in the form of heat as well as mechanical energy. However, the principle of conservation of energy had been suggested in various forms by perhaps a dozen German, French, British and other scientists during the first half of the 19th century. About the same time, entropy and the second law of thermodynamics were first clearly described in the work of Rudolf Clausius. In 1875 Ludwig Boltzmann made the important connection between the number of possible states that a system could occupy and its entropy. With two installments in 1876 and 1878, Josiah Willard Gibbs developed much of the theoretical formalism for thermodynamics, and a decade later firmly laid the foundation for statistical mechanics - much of which Ludwig Boltzmann had independently invented. In 1881 Gibbs also was very influential in moving much of the notation of physics from Hamilton's quaternions to vectors.


Electricity and Magnetism

In England William Gilbert (1544-1603) studied magnetism and published a seminal work, De Magnete (1600), in which he thoroughly presented his numerous experimental results.

Otto von Guericke 1650s, he turned his interests to static electricity, and he invented a mechanical device consisting of a sphere of sulfur that could be turned on a crank and repeatedly charged and discharged to produce electric sparks.

In 1746 an important step in the development of electricity was taken in the invention of the Leyden jar, a capacitor, that could store and discharge electrical charge in a controlled way. Benjamin Franklin effectively used them (together with von Guericke's generator) in his researches into the nature of electricity in 1752.

In a letter to the Royal Society in 1800, Alessandro Volta described his invention of the electric battery, thus providing for the first time the means to generate a constant electric current, and opening up a new field of physics for investigation.

The behavior of electricity and magnetism was studied by Michael Faraday, Georg Ohm, Hans Christian Ørsted, and others. Faraday, who began his career in Chemistry working under Humphry Davy at the Royal Institution, demonstrated that electrostatic phenomena, the action of the newly discovered electric pile or battery, electrochemical phenomena, and lightning were all different manifestations of electrical phenomena. Faraday further discovered in 1821 that electricity can cause rotational mechanical motion, and in 1831 discovered the principle of electromagnetic induction, by which means mechanical motion is converted into electricity. Thus it was Faraday who laid the foundations for both the electric motor and the electric generator.

In 1855, James Clerk Maxwell unified the two phenomena into a single theory of electromagnetism, described by Maxwell's equations. The discovery of the Hall effect in 1879 gave the first direct evidence that the carrier of electricity was negatively charged.


Light, Radio Waves and X-rays

A prediction of Maxwell's theory was that light is an electromagnetic wave.

Dimensional analysis was used for the first time in 1878 by Lord Rayleigh who was trying to understand why the sky is blue.

In 1887 the Michelson-Morley experiment was conducted and it was interpreted as counter to the generally held theory of the day, that the Earth was moving through a "luminiferous aether". Albert Abraham Michelson and Edward Morley were not fully convinced of the non-existence of the aether. Morley conducted further experiments with Dayton Miller with improved interferometers, again giving null results.

In 1887, Nikola Tesla investigated X-rays using his own devices as well as Crookes tubes. In 1895, Wilhelm Conrad Röntgen observed and analysed X-rays, which turned out to be high-frequency electromagnetic radiation.


Atoms and Subatomic Particles

The Particle Physics Timeline provides a detailed account of this topic.

Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by Pierre and Marie Curie and others. This initiated the field of nuclear physics.

In 1897, J.J. Thomson and Philipp Lenard studied cathode rays. Thomson deduced that were negatively charged particles, which he called "corpuscles", which came to be called electrons. Lenard showed that the particles ejected in the photoelectric effect were the same as those in cathode rays, and that their energy was independent of the intensity of the light, but was greater for short wavelengths of the incident light.

In 1904, Thomson proposed the first model of the atom, known as the plum pudding model. The existence of atoms of different weights had been proposed in 1808 by John Dalton to explain the law of multiple proportions. The convergence of various estimates of Avogadro's number lent decisive evidence for atomic theory.

In 1911, Ernest Rutherford deduced from scattering experiments the existence of a compact atomic nucleus, with positively charged constituents dubbed protons. The first quantum mechanical model of the atom, the Bohr model, was published in 1913 by Niels Bohr. Sir W. H. Bragg and his son Sir William Lawrence Bragg, also in 1913, began to unravel the arrangement of atoms in crystalline matter by the use of x-ray diffraction. Neutrons, the neutral nuclear constituents, were discovered in 1932 by James Chadwick.

Beginning in 1900, Max Planck, Albert Einstein, Niels Bohr, and others developed quantum theories to explain various anomalous experimental results,e.g. the photoelectric effect and the black body spectrum, by introducing discrete energy levels and in 1925 Wolfgang Pauli elucidated the Pauli exclusion principle and introduced the notion of quantized spin and fermions. In that year Erwin Schrödinger formulated wave mechanics, which provided a consistent mathematical method for describing a wide variety of physical situations such as the particle in a box and the quantum harmonic oscillator which he solved for the first time. Werner Heisenberg described, also in 1925, an alternative mathematical method, called matrix mechanics, which proved to be equivalent to wave mechanics. In quantum mechanics, the outcomes of physical measurements are inherently probabilistic. The theory describes the calculation of these probabilities. It successfully describes the behavior of matter at small distance scales.

Quantum mechanics also provided the theoretical tools for understanding condensed matter physics, which studies the physical behavior of solids and liquids, including phenomena such as electrical conductivity in crystal structures. The pioneers of condensed matter physics include Felix Bloch, who created a quantum mechanical description of the behavior of electrons in crystal structures in 1928. Much of the behavior of solids was elucidated within a few years with the discovery of the Fermi surface which was based on the idea of the Pauli exclusion principle applied to systems having many electrons. The understanding of the transport properties in semiconductors as described in William Shockley's Electrons and holes in semiconductors, with applications to transistor electronics enabled the electronic revolution of the twentieth century through the development of the ubiquitous, ultracheap transistor.

In 1934, the Italian physicist Enrico Fermi had discovered strange results when bombarding uranium with neutrons, which he believed at first to have created transuranic elements. In 1939, it was discovered by the chemist Otto Hahn and the physicist Lise Meitner that what was actually happening was the process of nuclear fission, whereby the nucleus of uranium was actually being split into two pieces, releasing a considerable amount of energy in the process. At this point it became clear to a number of scientists independently that this process could potentially be harnessed to produce massive amount of energy, either as a civilian power source or as a weapon. Leó Szilárd actually filed a patent on the idea of a nuclear chain reaction in 1934. In America, a team led by Fermi and Szilárd achieved the first man-made nuclear chain reaction in 1942 in the world's first nuclear reactor, and in 1945 the world's first nuclear explosive was detonated at Trinity Site, north of Alamogordo, New Mexico. After the war, central governments would become the primary sponsors of physics. The scientific leader of the Allied project, theoretical physicist Robert Oppenheimer, noted the change of the imagined role of the physicist when he noted in a speech that: "In some sort of crude sense, which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin, and this is a knowledge which they cannot lose." Though the process had begun with the invention of the cyclotron by Ernest O. Lawrence in the 1930s, nuclear physics in the postwar period entered into a phase of what historians have called "Big Science", requiring costly huge accelerators and particle detectors, and large collaborative laboratories to test open new frontiers. The primary patron of physics became central governments, who recognized that the support of "basic" research could sometimes lead to technologies useful to both military and industrial applications. Toward the end of the twentieth century, a European collaboration of 20 nations, CERN, became the largest particle physics laboratory in the world.

Another "big science" was the science of ionized gases, plasma, which had begun with Crookes tubes late in the 19th century. Large international collaborations over the last half of the twentieth century embarked on a long range effort to produce commercial amounts of electricity through fusion power, which remains a distant goal.

The discovery of nuclear magnetic resonance in 1946 led to many new methods for examining the structures of molecules and became a very widely used tool in analytical chemistry, and it gave rise to an important medical imaging technique, magnetic resonance imaging.

Superconductivity, discovered in 1911 by Kamerlingh Onnes, was shown to be a quantum effect and was satisfactorily explained in 1957 by Bardeen, Cooper, and Schrieffer. A family of high temperature superconductors, based on cuprate perovskite, were discovered in 1986, and their understanding remains one of the major outstanding challenges for condensed matter theorists.

Quantum field theory was formulated in order to extend quantum mechanics to be consistent with special relativity. It achieved its modern form in the late 1940s with work by Richard Feynman, Julian Schwinger, Sin-Itiro Tomonaga, and Freeman Dyson. This provided the framework for modern particle physics, which studies fundamental forces and elementary particles. In 1954, Yang Chen Ning and Robert Mills developed a class of gauge theories, which provided the framework for the Standard Model. This was largely completed in the 1970s and successfully describes almost all elementary particles observed to date.


Relativity and Cosmology

The Lorentz transformations, the fundamental equations of special relativity, were published in 1897 and 1900 and also by Joseph Larmor and by Hendrik Lorentz in 1899 and 1904. They both showed that Maxwell's equations were invariant under the transformations. In 1905, Einstein formulated the theory of special relativity.

In 1915, Einstein extended special relativity to describe gravity with the general theory of relativity. One principal result of general relativity is the gravitational collapse into black holes, which was anticipated two centuries earlier, but elucidated by Robert Oppenheimer. Important exact solutions of Einstein's field equation were found by Karl Schwarzschild in 1915 and Roy Kerr only in 1963.

According to Cornelius Lanczos, any physical law which can be expressed as a variational principle describes an expression which is self-adjoint[1] or Hermitian. Thus such an expression describes an invariant under a Hermitian transformation. Felix Klein's Erlangen program attempted to identify such invariants under a group of transformations. Noether's theorem identified the conditions under which the Poincaré group of transformations (what is now called a gauge group) for general relativity define conservation laws. The relationship of these invariants (the symmetries under a group of transformations) and what are now called conserved currents, depends on a variational principle, or action principle. Noether's papers made the requirements for the conservation laws precise. Noether's theorem remains right in line with current developments in physics to this day.

In 1928 Paul Dirac produced a relativistic formulation built on Heinsberg's matrix mechanics, and predicted the existence of the positron and founded quantum electrodynamics.

In 1929, Edwin Hubble published his discovery that the speed at which Galaxies recede positively correlates with their distance. This is the basis for understanding that the Universe is expanding. Thus, the Universe must have been smaller and therefore hotter in the past. In 1933 Karl Jansky at Bell Labs discovered the radio emission from the Milky Way, and thereby starting the science of radio astronomy. By the 1940s, researchers like George Gamow proposed the Big Bang theory[2], evidence for which was discovered in 1964[3]; Enrico Fermi and Fred Hoyle were among the doubters in the 1940s and 1950s. Hoyle had dubbed Gamow's theory the Big Bang in order to debunk it. Today, it is one of the principal results of cosmology.

In 1974 Stephen Hawking discovered the spectrum of radiation emanating during the collapse of matter into black holes. These mysterious objects became objects of intense interest to astrophysicists and even the general public in the latter part of the twentieth century.

Attempts to unify quantum mechanics and general relativity made significant progress during the 1990s. At the close of the century, a theory of everything was still not in hand, but some of its characteristics were taking shape. String theory, loop quantum gravity and black hole thermodynamics all predicted quantized Spacetime on the Planck scale.

A number of new efforts to understand the physical world arose in the last half of the twentiety century that generated widespread interest: fractals and scaling, self-organized criticality, complexity and chaos, power laws and noise, networks, non-equilibrium thermodynamics, sandpiles, nanotechnology, cellular automata and the anthropic principle were only a few of these important topics.


Information and Computing

Further understanding of the physics of metals, semiconductors and insulators led a team of three men at Bell labs, William Shockley, Walter Brattain and John Bardeen in 1947 to the first transistor and then to many important variations, especially the bipolar junction transistor. Further developments in the fabrication and miniaturization of integrated circuits in the years to come produced fast, compact computers that came to revolutionize the way physics was done--simulations and complex mathematical calculations became possible that were undreamed of even a few decades previous.

Starting in 1960 the military establishment of the United States began using atomic clocks to construct the global positioning system which in 1984 achieved its full configuration of 24 Satellites in low Earth orbits. This came to have many important civilian and scientific uses as well.

Mathematics for Natural Philosophers

An important device, the vernier, which allows the accurate mechanical measurement of angles and distances was invented by a Frenchman, Pierre Vernier in 1631. It is in widespread use in scientific laboratories and machine shops to this day. Also important for the advancement of natural philosophy were the mathematical techniques of logarithms and slide rules.

Branches of physics
Acoustics studies the production and properties of sound.
Astronomy: The study of the motion, the structure, and the physical and chemical properties of material objects and energy sources situated outside the boundaries of the earth's atmosphere.
Astrophysics: the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature and chemical composition) of astronomical objects such as stars, galaxies, and the interstellar medium, as well as their interactions.
Atomic physics examines the structure, properties, and behavior of matter on the small scale.
Biophysics applies the tools and techniques of physics to the study of living things and the life processes.
Cosmology: A branch of astronomy focused on the study of the origin, structure, and evolution in space and time of the physical universe.
Cryogenics is the study of extremely low temperatures.
Dynamics: A chapter of mechanics dealing with the behavior of material objects under the action of external forces.
Electrostatics: The study of the behavior of electric charges and the fields they create in their surrounding space.
Electrodynamics: The study of the relations between electrical, magnetic, and mechanical phenomena.
Field Theory: A classical or quantum mechanical theoretical study of fields, based on the knowledge of the field equations, or of the commutation rules satisfied by the field operators.
Fluid Mechanics: The study of the properties and behavior of matter in fluid (gas or liquid) state.
Geometry: The study of the way points, lines, planes and other surfaces and volumes are related.
Geophysics is the study of earth and its atmosphere and waters by means of the principles of physics.
Health physics involves the protection of people work with are near radiation.
High Energy Physics (also known as Particle Physics): The study of the structure, properties and interactions of elementary particles.
Hydrostatics / Hydrodynamics: The study of the mechanical behavior of fluids and of solid bodies immersed in fluids, which are in static equilibrium or in motion relative to them.
Magnetism: The study of magnetic properties of matter and the fields created in the surrounding space.
Mathematical physics is the study of mathematical systems that stands for physical phenomena.
Mechanics deals with the behavior of objects and material systems in terms of their position in space, under the action of external forces which may be equal of different from zero.
Molecular physics examines the structure, properties, and behavior of molecules.
Nuclear Physics: The study of the properties of atomic nuclei, nuc;ear recations, and the forces responsible for the stability or the disintegration of atomic nuclei.
Optics is the study of the nature and behavior of light.
Particle physics, also called high energy physics, analyses the behavior and properties of elementary particles.
Plasma physics is concerned with the study of highly ionized gases- that is, gases that have been separated into positively and negatively charged particles.
Quantum Electrodynamics: A quantum theory of the interaction of radiation with electrically charged particles, in particular with atoms and their constituent electrons.
Quantum Mechanics: A theory of matter based on the idea that material particles may be described as waves, and waves may be described as particles.
Quantum physics includes various areas of study based on quantum theory, which deals with matter and electromagnetic radiation, and the interactions between them.
Solid-state physics, also called condensed-matter physics, examines the physical properties of solid materials.
Statics: A chapter of mechanics dealing with the equilibrium of external forces acting on material objects.
Statistical Mechanics: The discipline that attempts to relate the properties of macroscopic systems to their atomic and molecular constituents.
Surface Physics: The study of the structure of solid surfaces. The study of physical and chemical processes occurring at the interface between solid objects and the gas or liquid environments surrounding them.
Theoretical physics: attempts to understand the world by making a model of reality, used for rationalizing, explaining, and predicting physical phenomena through a “physical theory”.
Thermodynamics is the study of heat and other forms of energy, and of the conversion of energy from one form to another.
Wave Mechanics: See Quantum Mechanics.