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This is an archive of the entries that have appeared or will appear on the Wikipedia Physics Portal.

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January

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Plasma lamp, illustrating some of the more complex phenomena of a plasma, including filamentation. The colors are a result of relaxation of electrons in excited states to lower energy states after they have recombined with ions. These processes emit light in a spectrum characteristic of the gas being excited.

In physics and chemistry, plasma is a gas, in which a certain proportion of its particles are ionized. The presence of a non-negligible number of charge carriers makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma therefore has properties quite unlike those of solids, liquids, or gases and is considered to be a distinct state of matter. Plasma typically takes the form of neutral gas-like clouds, as seen, for example, in the case of stars. Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container; unlike gas, in the influence of a magnetic field, it may form structures such as filaments, beams and double layers (see section 3, below).

Plasma was first identified in Crookes tube, and so described by Sir William Crookes in 1879 (he called it "radiant matter").[1] The nature of the Crookes tube "cathode ray" matter was subsequently identified by British physicist Sir J.J. Thomson in 1897,[2] and dubbed "plasma" by Irving Langmuir in 1928,[3] perhaps because it reminded him of a blood plasma. Langmuir wrote, "Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons.[3]


February

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On top is corner detail in a photograph taken with a higher quality lens; bottom is a similar photograph taken with a wide angle lens showing severe visible chromatic aberration, the effect is pronounced on the right side of the building's roof, where a long blue streak is visible.

In optics, chromatic aberration (also called achromatism or chromatic distortion) is the failure of a lens to focus all colors to the same point. It occurs because lenses have a different refractive index for different wavelengths of light (the dispersion of the lens). The refractive index decreases with increasing wavelength.

Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis.

Since the focal length f of a lens is dependent on the refractive index n, different wavelengths of light will be focused on different positions. Chromatic aberration can be both longitudinal, in that different wavelengths are focused at a different distance from the lens; and transverse or lateral, in that different wavelengths are focused at different positions in the focal plane (because the magnification of the lens also varies with wavelength).


March

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Main article: Light cone
Light cone in 2D space plus a time dimension.

A Light cone is the path that a flash of light, emanating from a single event E (localized to a single point in space and a single moment in time) and traveling in all directions, would take through spacetime. Imagine the light confined to a two-dimensional plane, the light from the flash spreads out in a circle after the event E occurs—and when graphed the growing circle with the vertical axis of the graph representing time, the result is a cone, known as the future light cone (some animated diagrams depicting this concept can be seen here). The past light cone behaves like the future light cone in reverse, a circle that contracts in radius at the speed of light until it converges to a point at the exact position and time of the event E. In reality, there are three space dimensions, so the light would actually form an expanding or contracting sphere in 3D space rather than a circle in 2D, and the light cone would actually be a four-dimensional shape. However, the concept is easier to visualize with the number of spatial dimensions reduced from three to two.

Because signals and other causal influences cannot travel faster than light in relativity, the light cone plays an essential role in defining the concept of causality. For a given event E, the set of events that lie on or inside the past light cone of E would also be the set of all events that could send a signal that would have time to reach E and influence it in some way. For example, at a time ten years before E, if we consider the set of all events in the past light cone of E that occur at that time, the result is a sphere with a radius of ten light-years centered on the future position E will occur. So, any point on or inside the sphere could send a signal moving at the speed of light or slower that would have time to influence the event E, while points outside the sphere at that moment would not be able to have any causal influence on E. Likewise, the set of events that lie on or inside the future light cone of E would also be the set of events that could receive a signal sent out from the position and time of E. Events that lie neither in the past or future light cone of E cannot influence or be influenced by E in relativity.


April

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Main article: Introduction to Quantum mechanics
Left to right: Max Planck, Albert Einstein, Niels Bohr, Louis de Broglie, Max Born, Paul Dirac, Werner Heisenberg, Wolfgang Pauli, Erwin Schrödinger, Richard Feynman.

Introduction to Quantum mechanics is an introductory version of Quantum mechanics. It describes Quantum mechanics as the set of scientific principles describing the behavior of energy and matter on the atomic and subatomatic scale. Much like the universe on the large and very vast scale (i.e., general relativity), so the universe on the small scale (i.e., quantum mechanics) does not neatly conform to the rules of classical physics. As such, it presents a set of rules that is counterintuitive and difficult to understand for the human mind, as humans are accustomed to the world on a scale dominated by classical physics. In other words, as stated by Richard Feynman: quantum mechanics deals with "Nature as She is—absurd." (see biography below) [4]

Many elementary parts of the universe, such as photons (discrete units of light) have some behaviours which resemble a particle but other behaviours that resemble a wave. Radiators of photons such as neon lights have spectra, but the spectra are chopped up instead of being continuous. The energies carried by photons form a discontinuous and colour coded series. The energies, the colours, and the spectral intensities of electromagnetic radiation produced are all interconnected by laws. But the same laws ordain that the more closely one pins down one measure (such as the position of a particle) the more wildly another measure relating to the same thing (such as momentum) must fluctuate. Put another way, measuring position first and then measuring momentum is not the same as measuring momentum first and then measuring position. Even more disconcerting, particles can be created as twins and therefore as entangled entities -- which means that doing something that pins down one characteristic of one particle will determine something about its entangled twin even if it is millions and millions of miles away.

Around the turn of the twentieth century, it became clear that classical physics was unable to explain several phenomena. As Thomas Kuhn explains in his analysis of the philosophy of science, The Structure of Scientific Revolutions, understanding these limitations of classical physics led to a revolution in physics and resulted in a shift of the original scientific paradigm: the development of quantum mechanics in the early decades of the last century.[5]


May

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Main article: Introduction to Quantum mechanics
Left to right: Max Planck, Albert Einstein, Niels Bohr, Louis de Broglie, Max Born, Paul Dirac, Werner Heisenberg, Wolfgang Pauli, Erwin Schrödinger, Richard Feynman.

Introduction to Quantum mechanics is a simpified version of Quantum mechanics. It describes Quantum mechanics as the set of scientific principles describing the behavior of energy and matter on the atomic and subatomatic scale. Much like the universe on the large and very vast scale (i.e., general relativity), so the universe on the small scale (i.e., quantum mechanics) does not neatly conform to the rules of classical physics. As such, it presents a set of rules that is counterintuitive and difficult to understand for the human mind, as humans are accustomed to the world on a scale dominated by classical physics. In other words, quantum mechanics deals with "Nature as She is—absurd."[6]

Many elementary parts of the universe, such as photons (discrete units of light) have some behaviours which resemble a particle but other behaviours that resemble a wave. The energies carried by photons form a discontinuous and colour coded series. The energies, the colours, and the spectral intensities of electromagnetic radiation produced are all interconnected by laws. But the same laws ordain that the more closely one pins down one measure (such as the position of a particle) the more wildly another measure relating to the same thing (such as momentum) must fluctuate.

Around the turn of the twentieth century, it became clear that classical physics was unable to explain several phenomena. Understanding these limitations of classical physics led to a revolution in physics: the development of quantum mechanics in the early decades of the last century.

For the rest of the article see: Introduction to Quantum mechanics

Physicist Biography: Richard Feynman

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Main article: Richard Feynman

Richard Phillips Feynman (/ˈfaɪnmən/ FYEN-mən; May 11, 1918 – February 15, 1988) was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics (he proposed the parton model). For his contributions to the development of quantum electrodynamics, Feynman, jointly with Julian Schwinger and Sin-Itiro Tomonaga, received the Nobel Prize in Physics in 1965. He developed a widely used pictorial representation scheme for the mathematical expressions governing the behavior of subatomic particles, which later became known as Feynman diagrams. During his lifetime, Feynman became one of the best-known scientists in the world.

He assisted in the development of the atomic bomb and was a member of the panel that investigated the Space Shuttle Challenger disaster. In addition to his work in theoretical physics, Feynman has been credited with pioneering the field of quantum computing,[7] and introducing the concept of nanotechnology.[8] He held the Richard Chace Tolman professorship in theoretical physics at the California Institute of Technology.

Feynman was a keen popularizer of physics through both books and lectures, notably a 1959 talk on top-down nanotechnology called There's Plenty of Room at the Bottom and The Feynman Lectures on Physics. Feynman also became known through his semi-autobiographical books (Surely You're Joking, Mr. Feynman! and What Do You Care What Other People Think?) and books written about him, such as Tuva or Bust!

He was regarded as an eccentric and free spirit. He studied Maya hieroglyphs, was a prankster, juggler, safecracker, bongo player, and a proud amateur painter.

Feynman also had a deep interest in biology, and was a friend of the geneticist and microbiologist Esther Lederberg, who developed replica plating and discovered bacteriophage lambda.[9] They had several mutual physicist friends who, after beginning their careers in nuclear research, moved for moral reasons into genetics, among them Leó Szilárd, Guido Pontecorvo, and Aaron Novick.


June

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The subject of the month is Radio

Below are several introductions to Wikipedia articles related to radio: History of radio, Invention of radio, an article entitled Radio and the radio "Antenna". Also below, is a biography of an individual who contributed to this technology.

  • History of radio - is the history of technology that produced radio instruments that use radio waves. Within the timeline of radio, many people contributed theory and inventions in what became radio. Radio development began as "wireless telegraphy". Later radio history increasingly involves matters of programming and content. During its early development and long after wide use of the technology, disputes persisted as to who could claim sole credit for this obvious boon to mankind. Closely related, radio was developed along with two other key inventions, the telegraph and the telephone.
  • Invention of radio - Within the history of radio, several people were involved in the invention of radio and there were many key inventions in what became the modern systems of wireless.[10] Radio development began as "wireless telegraphy".[10] Closely related, radio was developed along with two other key inventions, the telegraph and the telephone.[10] During the early development of wireless technology and long after its wide use, disputes persisted as to who could claim credit for the invention of radio. The matter was important for economic, political and nationalistic reasons.

Selected biography: Jagadish Chandra Bose

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Main article: Jagadish Chandra Bose
Jagadish Chandra Bose in Royal Institution, London

Sir Jagadish Chandra Bose CSI CIE FRS (Bengali: জগদীশ চন্দ্র বসু Jôgodish Chôndro Boshu) (November 30, 1858 – November 23, 1937) born in a Bengali Hindu Kayasth family was a polymath: a physicist, biologist, botanist, archaeologist, and writer of science fiction. He pioneered the investigation of radio and microwave optics, made very significant contributions to plant science, and laid the foundations of experimental science in the Indian subcontinent. He is considered one of the fathers of radio science, and is also considered the father of Bengali science fiction. He was the first person from the Indian subcontinent to get a US patent, in 1904.


July

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Sunlight takes about 8 minutes, 19 seconds to reach Earth.
White light dispersed by a prism into the colors of the optical spectrum.
The theme of the month is "Light". The following articles discuss different aspects of light that have been researched and discovered over time.
  • Photon has several interrelated descriptions. One of these is that the photon is the basic unit of light, and all electromagnetic radiation. It is also a carrier of the "force" that causes the interaction between electrically charged particles in electromagnetism and electrodynamics. It also has a dual nature. This means it can travel and transfer energy from one point to another; propagating (traveling) through space and time as a wave. Or it can be viewed as a fundamental particle that is not known to be made up of smaller particles. Hence, it is considered to be one of the basic building blocks of the universe.

Physicist biographies

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Sir Arthur Schuster
John William Strutt

Sir Arthur Schuster (FRS) (12 September 1851 – 17 October 1934) was a versatile German-born British physicist known for his work in spectroscopy, electrochemistry, optics, X-radiography and the application of harmonic analysis to physics. He contributed enormously to making the University of Manchester a centre for the study of physics. (Click on the name to read the entire article)

John Strutt, 3rd Baron Rayleigh (OM) (12 November 1842 – 30 June 1919) was an English physicist who, with William Ramsay, discovered the element argon, an achievement for which he earned the Nobel Prize for Physics in 1904. He also discovered the phenomenon now called Rayleigh scattering, explaining why the sky is blue, and predicted the existence of the surface waves now known as Rayleigh waves. In 1910 Lord Rayleigh discovered that an electrical discharge in nitrogen gas produced "active nitrogen", an allotrope considered to be monatomic. The "whirling cloud of brilliant yellow light" produced by his apparatus reacted with quicksilver to produce explosive mercury nitride. (Click on the name to read the entire article)


August

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The flow of sand in an hourglass can be used to keep track of elapsed time.

Time is the theme for this month. A description of time, crafted by a Wikipedia editor, which is derived from reliable sources, is as follows:

Time is a one-dimensional quantity used to sequence events, to quantify the durations of events and the intervals between them, and (used together with space) to quantify and measure the motions of object[disambiguation needed]s. Time is quantified in comparative terms (such as longer, shorter, faster, quicker, slower) or in numerical terms using units (such as seconds, minutes, hours, days). Time has been a major subject of religion, philosophy, and science, but defining it in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.

2007 description of time

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Previously, in January of 2007, the opening lead for the article entitled Time read as follows:

There are two distinct views on the meaning of time. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. This is the realist view, to which Sir Isaac Newton [12] subscribed, in which time itself is something that can be measured.

A contrasting view is that time is part of the fundamental intellectual structure (together with space and number) within which we sequence events, quantify the duration of events and the intervals between them, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. This view is in the tradition of Gottfried Leibniz[13] and Immanuel Kant,[14][15] in which time, rather than being an objective thing to be measured, is part of the mental measuring system. The question, perhaps overly simplified and allowing for no middle ground, is thus: is time a "real thing" that is "all around us", or is it nothing more than a way of speaking about and measuring events?

Many fields avoid the problem of defining time itself by using operational definitions that specify the units of measurement that quantify time. Regularly recurring events and objects with apparent periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, and the swing of a pendulum.

Time has long been a major subject of science, philosophy and art. The measurement of time has also occupied scientists and technologists, and was a prime motivation in astronomy. Time is also a matter of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human lifespans. This article looks at some of the main philosophical and scientific issues relating to time.

For further reading see the main article: Time. Below are other articles related to time:
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Main article: Time in physics

Time in physics is largely defined by its measurement: time is what a clock reads.[16] One can measure time and treat it as a geometrical dimension, such as length, and perform mathematical operations on it. It is a scalar quantity and, like length, mass, and charge, is usually listed in most physics books as a fundamental quantity. Time can be combined mathematically with other fundamental quantities to derive other concepts such as motion, energy and fields. Timekeeping is a complex of technological and scientific issues, and part of the foundation of recordkeeping.

Main article: Orders of magnitude (time)

Time occurs in orders of magnitude from less than one billionth of a second (nanosecond), to billions of years (age of the universe) and beyond.

Main article: Spacetime

According to certain Euclidean space perceptions, the universe has three dimensions of space and one dimension of time. By combining space and time into a single manifold, physicists have significantly simplified a large number of physical theories, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels.

Main article: Planck epoch

In physical cosmology, the Planck epoch (or Planck era), named after Max Planck, is the earliest period of time in the history of the universe. At this point approximately, 13.7 billion years ago one could also say that it is the earliest moment in time, as the Planck time is perhaps the shortest possible interval of time, and the Planck epoch lasted only this brief instant.

Main article: System time

In computer science and computer programming, system time represents a computer system's notion of the passing of time. In this sense, time also includes the passing of days on the calendar. System time is measured by a system clock, which is typically implemented as a simple count of the number of ticks that have transpired since some arbitrary starting date, called the epoch. For example, Unix and POSIX-compliant systems encode system time as the number of seconds elapsed since the start of the epoch at 1 January 1970 00:00:00 UT. Windows NT counts the number of 100-nanosecond ticks since 1 January 1601 00:00:00 UT as reckoned in the proleptic Gregorian calendar, but returns the current time to the nearest millisecond.

Main article: Atomic clock

Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the frequency of television broadcasts, and in global navigation satellite systems such as GPS. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized, by using leap seconds, to UT1, which is based on actual rotations of the earth with respect to the solar time.

Template: Time measurement and standards

Wikipedia has a number of articles related to "Time". See our template entitled Time measurement and standards , and click on the link entitled "show".


September

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The theme for this month is unsolved problems in physics. Some of these problems are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. Other unsolved problems are the result of an inability to in create an experiment to test a proposed theory, or investigate a phenomenon in greater detail.


Hence, theoretical problems are part of quantum gravity, cosmology, general relativity, high energy physics (particle physics), nuclear physics, and others. Empirical phenomena lacking clear scientific explanation are part of cosmology and astronomy, high energy physics(particle physics), astronomy, astrophysics, condensed matter physics, and biological problems approached with physics. Click on the following link to read about this subject, which is supplied with references, and other types of information for further reading.
Main article: List of unsolved problems in physics


October

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History of Physics

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Galileo Galilei (1564–1642)

Elements of what became physics were drawn primarily from the fields of astronomy, optics, and mechanics, which were methodologically united through the study of geometry. These mathematical disciplines began in Antiquity with the Babylonians and with Hellenistic writers such as Archimedes and Ptolemy. Meanwhile, philosophy, including what was called “physics”, focused on explanatory (rather than descriptive) schemes, largely developed around the Aristotelian idea of the four types of “causes”. Descartes, like Galileo, was convinced of the importance of mathematical explanation, and he and his followers were key figures in the development of mathematics and geometry in the 17th century

History of science and technology in China

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The history of science and technology in China is both long and rich with many contributions to science and technology. In antiquity, independently of Greek philosophers and other civilizations, ancient Chinese philosophers made significant advances in science, technology, mathematics, and astronomy. The first recorded observations of comets, solar eclipses, and supernovae were made in China.[17] Traditional Chinese medicine, acupuncture and herbal medicine were also practiced. Among the earliest inventions were the abacus, the "shadow clock," and the first flying machines such as kites and Kongming lanterns. The four Great Inventions of ancient China: the compass, gunpowder, papermaking, and printing, were among the most important technological advances, only known in Europe by the end of the Middle Ages. The Tang dynasty (AD 618 - 906) in particular, was a time of great innovation. A good deal of exchange occurred between Western and Chinese discoveries up to the Qing dynasty.

The Jesuit China missions of the 16th and 17th centuries introduced Western science and astronomy, then undergoing its own revolution, to China, and knowledge of Chinese technology was brought to Europe.[18] Much of the early Western work in the history of science in China was done by Joseph Needham.

History of Indian science and technology

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The history of science and technology in India begins with prehistoric human activity at Mehrgarh, in present-day Pakistan, and continues through the Indus Valley Civilisation to early states and empires. The British colonial rule introduced western education in India. The British system of education, in its efforts to give rise to a native class of civil servants, exposed a number of Indians to foreign institutes of higher learning. Following independence science and technology in India has included automobile engineering, information technology, communications as well as space, polar, and nuclear sciences.


November

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The theme for this month is the recipients of the Nobel Prize in Physics in the new millennium, between 2010 and the year 2001.

Wilhelm Röntgen (1845–1923), the first recipient of the Nobel Prize in Physics.

The Nobel Prize in Physics (Swedish: Nobelpriset i fysik) is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901. The first Nobel Prize in Physics was awarded to Wilhelm Conrad Röntgen "in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays (or x-rays)." This award is administered by the Nobel Foundation. A maximum of three Nobel laureates and two different works may be selected for the Nobel Prize in Physics. The nomination and selection process for the prize in Physics is long and rigorous. This is a key reason it has grown in importance over the years to become the most important prize in Physics.


December

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First page from Einstein's manuscript explaining general relativity.
Einstein's scientific publications

Albert Einstein (1879–1955) was a renowned theoretical physicist of the 20th century who is best known for his theories of special relativity and general relativity. He also made important contributions to statistical mechanics, especially his treatment of Brownian motion, his resolution of the paradox of specific heats, and his connection of fluctuations and dissipation. Despite his reservations about its interpretation, Einstein also made seminal contributions to quantum mechanics and, indirectly, quantum field theory, primarily through his theoretical studies of the photon.

Main article: List of scientific publications by Albert Einstein


  1. ^ Crookes presented a lecture to the British Association for the Advancement of Science, in Sheffield, on Friday, 22 August 1879 [1] [2]
  2. ^ Announced in his evening lecture to the Royal Institution on Friday, 30th April 1897, and published in "J. J. Thomson 1897". Philosophical Magazine. 44: 293. 1897.
  3. ^ a b I. Langmuir (1928). "Oscillations in ionized gases". Proc. Natl. Acad. Sci. U.S. 14 (8): 627–37. Bibcode:1928PNAS...14..627L. doi:10.1073/pnas.14.8.627. PMC 1085653. PMID 16587379.
  4. ^ Richard P. Feynman, QED, p. 10
  5. ^ Kuhn, Thomas S. The Structure of Scientific Revolutions. Fourth ed. Chicago ;London: The University of Chicago Press, 2012. Print.
  6. ^ Richard P. Feynman, QED, p. 10
  7. ^ West, Jacob (July 2003). "The Quantum Computer" (PDF). Xootic. Retrieved 2009-09-20.
  8. ^ Edwards 2006, pp. 15–17.
  9. ^ "Esther M. Zimmer Lederberg Memorial Web Site".
  10. ^ a b c The Invention of Radio inventors.about.com/od/rstartinventions/a/radio.htm
  11. ^ Dictionary of Electronics By Rudolf F. Graf (1974). Page 467.
  12. ^ Newton's Views on Space, Time, and Motion - Stanford University http://plato.stanford.edu/entries/newton-stm/
  13. ^ Leibniz on Space, Time, and Indiscernibles - Against the Absolute Theory -- Internet Encyclopedia of Philosophy http://www.iep.utm.edu/l/leib-met.htm#H7
  14. ^ Critique of Pure Reason - Lecture notes of G. J. Mattey, UC Davis http://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTM
  15. ^ Kant's Transcendental Idealism - Internet Encyclopedia of Philosophy http://www.iep.utm.edu/k/kantmeta.htm#H4
  16. ^ Considine, Douglas M.; Considine, Glenn D. (1985). Process instruments and controls handbook (3 ed.). McGraw-Hill. p. 18-61. ISBN 0-070-12436-1.
  17. ^ Ancient Chinese Astronomy
  18. ^ Agustín Udías, p.53
  19. ^ Pais, pp. 93–100.
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