History of physics

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The growth of physics has brought not only fundamental changes in ideas about the material world, mathematics and philosophy, but also, through technology, a transformation of society. Physics is considered both a body of knowledge and the practice that makes and transmits it. The scientific revolution, beginning about year 1600, is a convenient boundary between ancient thought and classical physics. Year 1900 marks the beginnings of a more modern physics; today, the science shows no sign of completion, as more issues are raised, with questions rising from the age of the universe, to the nature of the vacuum, to the ultimate nature of the properties of subatomic particles. Partial theories are currently the best that physics has to offer, at the present time. The list of unsolved problems in physics is large; however,

"Outside the nucleus, we seem to know it all." -- Richard Feynman.


Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. Also a mystery was the character of the universe, such as the form of the Earth and the behavior of celestial objects such as the Sun and the Moon. Several theories were proposed; most of them were wrong, but this is part of the nature of the scientific enterprise, and even modern theories of quantum mechanics and relativity are merely considered "theories that haven't broken yet". Physical theories in antiquity were largely couched in philosophical terms, and rarely verified by systematic experimental testing.

Typically the behavior and nature of the world was explained by invoking the actions of gods. Around 200 BC, many Greek philosophers began to propose that the world could be understood as the result of natural processes. Many also challenged traditional ideas presented in mythology, such as the origin of the human species (anticipating the ideas of Charles Darwin), although this falls into the history of biology, not physics. The atomists attempted to characterize the nature of matter, which anticipated work in our present day.

Due to the absence of advanced experimental equipment such as telescopes and accurate time-keeping devices, experimental testing of many such ideas was impossible or impractical. There were exceptions and there are anachronisms: for example, the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and also hydrostatics when, so the story goes, he noticed that his own body displaced a volume of water while he was getting into a bath one day. Another remarkable example was that of Eratosthenes, who deduced that the Earth was a sphere, and accurately calculated its circumference using the shadows of vertical sticks to measure the angle between two widely separated points on the Earth's surface. Greek mathematicians also proposed calculating the volume of objects like spheres and cones by dividing them into very thin disks and adding up the volume of each disk - anticipating the invention of integral calculus by almost two millennia.

Modern knowledge of these early ideas in physics, and the extent to which they were experimentally tested, is sketchy. Almost all direct record of these ideas was lost when the Library of Alexandria was destroyed, around 400 AD. Perhaps the most remarkable idea we know of from this era was the deduction by Aristarchus of Samos 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 which also had their own accompanying planets (and possibly, lifeforms upon those planets).

The discovery of the Antikythera mechanism points to a detailed understanding of movements of these astronomical objects, as well as a use of gear-trains that pre-dates any other known civilization's use of gears.

An early version of the steam engine, Hero's aeolipile was only a curiosity which did not solve the problem of transforming its rotational energy into a more usable form, not even by gears. The Archimedes screw is still in use today, to lift water from rivers onto irrigated farmland. The simple machines were unremarked, with the exception (at least) of Archimedes' elegant proof of the law of the lever. Ramps were in use several millennia before Archimedes, to build the Pyramids.

Regrettably, this period of inquiry into the nature of the world was eventually stifled by a tendency to accept the ideas of eminent philosophers, rather than to question and test those ideas. Pythagoras himself is said to have tried to suppress knowledge of the existence of irrational numbers, discovered by his own school, because they did not fit his number mysticism. For one thousand years following the destruction of the Library of Alexandria, Ptolemy's (not to be confused with the Egyptian Ptolemies) model of an Earth-centred universe with planets moving in perfect circular orbits was accepted as absolute truth.

We should mention physics outside Europe at this stage to make this history more balanced.

Middle-Eastern contributions to physics

Civilization eclipsed by the Roman Empire, many Greek doctors began to practice medicine for the Roman elite, but sadly the physical sciences were not so well supported. Following the collapse of the Roman Empire, Europe saw a decline in interest in classical culture which some have called the Dark Ages, though modern scholars do not use this phrase, and almost all scientific research ground to a halt. The rise of Christianity saw the suppression and destruction of most classical Greek philosophy (along with Greek and Roman art, literature and religious iconography) as heretical and pagan. In the Middle East, however, many Greek natural philosophers were able to find support for their work, and scholars built upon previous work in astronomy and mathematics while developing such new fields as alchemy (chemistry). For example, the Muslim scholar al-Khwarizmi gave his name to what we now call an algorithm, and the word algebra is derived from al-jabr, the beginning of the name of one of his publications in which he developed a system of solving quadratic equations, thus beginning Algebra, without which modern physics could hardly exist.

Indian contributions to physics

Modern physics can hardly be imagined without a system of arithmetic in which simple calculation is easy enough to make large calulations even possible. The positional numeral system and the concept of zero were first developed in India and were adopted by the Islamic empire.

The Middle Ages

The monk Roger Bacon conducted experiments into optics, although much of it was similar to what had been done and was being done at the time by Arab scholars. He did make a major contribution to the development of science in medieval Europe by writing to the Pope to encourage the study of natural science in university courses and compiling several volumes recording the state of scientific knowledge in many fields at the time. He described the possible construction of a telescope, but there is no strong evidence of his having made one. He recorded the manner in which he conducted his experiments in precise detail so that others could reproduce and independently test his results - a cornerstone of the scientific method, and a continuation of the work of researchers like Al Battani.

The annexation of the last Muslim states on the Iberian Peninsula (1492) by the recently formed Hispanic monarchy, and the fall of Constantinople in hands of the Turks (1453) coincided with the dawn of the Renaissance. This "rebirth" of European culture was in part brought about by the re-discovery of those elements of ancient Greek, Indian, Chinese and Islamic culture preserved and further developed by Islam from the 8th to the 15th centuries, and translated by Christian Monks into Latin, such as the Almagest.

The scientific revolution

The scientific revolution can be viewed as a flowering of the Renaissance and the portal to modern civilization. It started with only a few researchers, evolving into an enterprise which continues to the present day. Starting with astronomy, the principles of natural philosophy crystallized into fundamental laws of physics which were enunciated and improved in the succeeding centuries. By the 19th century, the sciences had segmented into multiple fields with specialized researchers and the field of physics, although logically pre-eminent, no longer could claim sole ownership of the entire field of scientific research.

16th century

In the 16th century Nicholas Copernicus revived the heliocentric model of the solar system devised by Aristarchus (which survives 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.

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.

17th century

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.

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. Classical mechanics would be exhaustively extended by Joseph-Louis de Lagrange, William Rowan Hamilton, and others, who produced new formulations, principles, and results. The Law of Gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.

18th century

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 Franklin conducted his researches into the nature of electricity in 1752. In 1798, Benjamin Thompson demonstrated the conversion of unlimited mechanical work into heat; it would take the work of James Prescott Joule to demonstrate the conservation of energy in the next century.

19th century

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.

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 or enunciated in various forms by perhaps a dozen German, French, British and other scientists during the first half of the 19th Century.

The behavior of electricity and magnetism was studied by Michael Faraday, Georg Ohm, 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. A prediction of this theory was that light is an electromagnetic wave. A more subtle part of Maxwell's deduction was that the observed speed of light does not depend on the speed of the observer, a premonition of the development of special relativity by Albert Einstein.

With two installments in 1876 and 1878, Josiah Willard Gibbs develops much of the theoretical formalism for thermodynamics, and a decade later firmly lays the foundation for statistical mechanics — this Ludwig Boltzmann had independently invented.

In 1887 the Michelson-Morley experiment is conducted and it is interpreted as counter to the general held theory of the day, that the Earth was moving through a "luminiferous aether". The development of what later became Einstein's Special Theory of Relativity provided a complete explanation which did not require an aether, and was consistent with the results of the experiment. Albert Abraham Michelson and Edward Morley are not convinced of the non-existence of the aether. Morely goes on to conduct experiments with Dayton Miller.

In 1887, Nikola Tesla investigates X-rays using his own devices as well as Crookes tubes. In 1895, Wilhelm Conrad Röntgen observes and analyses X-rays, which turned out to be high-frequency electromagnetic radiation. Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by the Pierre and Marie Curie and others. This initiated the field of nuclear physics.

In 1897, J.J. Thomson studies the electron, the elementary particle which carries electrical current in circuits. He deduces that cathode rays existed and were negatively charged "particles", which he called "corpuscles".

20th century

The beginning of the 20th century brought the start of a revolution in physics. The long-held theories of Newton were shown not to be correct in all circumstances. Not only did quantum mechanics show that the laws of motion didn't hold on small scales, but even more disturbingly, general relativity showed that the fixed background of spacetime, on which both Newtonian mechanics and special relativity depended, could not exist.

In 1904, Thomson proposed the first model of the atom, known as the plum pudding model. (The existence of the atom had been proposed in 1808 by John Dalton.)

In 1905, Einstein formulated the theory of special relativity, unifying space and time into a single entity, spacetime. Relativity prescribes a different transformation between reference frames than classical mechanics, necessitating the development of relativistic mechanics as a replacement for classical mechanics. In the regime of low (relative) velocities, the two theories agree. In 1915, Einstein extended special relativity to explain gravity with the general theory of relativity, which replaces Newton's law of gravitation. In the regime of low masses and energies, the two theories agree. One principal result of general relativity is the gravitational collapse into black holes, which was anticipated two centuries earlier, but elucidated by Robert Oppenheimer. Oppenheimer would later direct the Manhattan Project at Los Alamos. Important exact solutions of Einstein's field equation were found by Karl Schwarzschild in 1915 and Roy Kerr only in 1963.

A variational principle is a principle in physics which is expressed in terms of the calculus of variations. According to Cornelius Lanczos, any physical law which can be expressed as a variational principle describes an expression which is self-adjoint1. These expressions are also called 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. On July 16, 1918, before a scientific organization in Goettingen, Klein read a paper written by Emmy Noether, because she was not allowed to present the paper before the scientific organization herself. In particular, in what is referred to in physics as Noether's theorem, this paper 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.

David Hilbert had derived the same equation as the Einstein equation for general relativity within a period of the same few weeks as Einstein, in November 1915. The chief difficulty, which concerned Hilbert, was that the conservation of energy did not hold for a region subject to a gravitational field. (Byers' commentaryTemplate:Fn notes that sometimes the objects which are needed to define conserved quantities are not tensors, but pseudotensors.Template:Fn) Noether's theorem remains right in line with current developments in physics to this day.

In 1911, Ernest Rutherford deduced from scattering experiments the existence of a compact atomic nucleus, with positively charged constituents dubbed protons. 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 by introducing discrete energy levels. In 1925, Werner Heisenberg and Erwin Schrödinger formulated quantum mechanics, which explained the preceding quantum theories. 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 condensed matter physics, which studies the physical behavior of solids and liquids, including phenomena such as crystal structures, semiconductivity, and superconductivity. 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.

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. By the 1940s, researchers like George Gamow proposed the Big Bang theoryTemplate:Fn, evidence for which was discovered in 1964Template:Fn; 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, with a well-accepted age of the universe.

During World War II, physics became a major source of government funding and research on all sides of the conflict. Its importance in the technologies of radar, rocketry, operations research, and anti-aircraft weapons was seen as paramount to the war efforts of both the Allied and Axis powers. Though physics had received some more funding after World War I, this was dwarfed by the amount it received only a few decades later.

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.

Both the Germans and the Americans pursued research in nuclear physics to assess the ability to create such a weapon in war. The German nuclear energy project, led by Heisenberg, was unsuccessful, but the Allied Manhattan Project reached its goal. In America, a team led by Fermi 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. In August 1945, the USA dropped two nuclear weapons on the Japaense cities of Hiroshima and Nagasaki, and official press reports gave (perhaps an unfair amount) of the credit to the physicists involved in the project. After the war, industrial 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."

The terms of this new relationship with the military would be harshly set when Oppenheimer had his security clearance revoked in a much publicized hearing during the height of the McCarthy era under suspicions of his loyalty.

Though the process had begun with the invention of the cyclotron by Ernest O. Lawrence in the 1930s, physics in the postwar period entered into a phase of what historians have called "Big Science", requiring massive machines, budgets, and laboratories in order to test their theories and move into new frontiers. The primary patron of physics became state governments, who recognized that the support of "basic" research could often lead to technologies useful to both military and industrial applications (it was not until the post-Cold War 1990s that the US Congress would fail to approve funding for a particle accelerator). Currently CERN still enjoys funding from the European community.

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. They formulated the theory of quantum electrodynamics, which describes the electromagnetic interaction.

Quantum field theory 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. The Standard Model, which was completed in the 1970s, successfully describes almost all elementary particles observed to date.

At the same time, Stephen Hawking had discovered the spectrum of radiation emanating during the collapse of matter into black holes; by 2004, even Hawking would admit that some Hawking radiation could escape a black hole.

Developments since 1990

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. Loop quantum gravity, string theory, and black hole thermodynamics all predicted quantized spacetime on the Planck scale.

Developments since 2000

A new experiment demonstrated that gravity propagates at approximately the speed of light, confirming one prediction of general relativity.


See also

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