physis- φύσις), in everyday terms, is the scienceof matter[ R. P. Feynman, R. B. Leighton, M. Sands(1963), " The Feynman Lectures on Physics", ISBN 0-201-02116-1 Hard-cover. p.1-1 Feynman begins with the atomic hypothesis.] and its motion. It is the sciencethat seeks to understand very basic concepts such as force, energy, mass, and charge. More completely, it is the general analysis of nature, conducted in order to understand how the world around us and, more broadly, the universe, behaves. [H.D. Young & R.A. Freedman, "University Physics with Modern Physics": 11th Edition: International Edition (2004), Addison Wesley. Chapter 1, section 1.1, page 2 has this to say: "Physics is an "experimental" science. Physicists observe the phenomena of nature and try to find patterns and principles that relate these phenomena. These patterns are called physical theories or, when they are very well established and of broad use, physical laws or principles."] [Steve Holzner, "Physics for Dummies" (2006), Wiley. Chapter 1, page 7 says: "Physics is the study of your world and the world and universe around you." See [http://www.amazon.com/gp/reader/0764554336 Amazon Online Reader: Physics For Dummies (For Dummies(Math & Science))] , last viewed 24 Nov 2006.] Note that the term universeis defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. However, the term " universe" may be used in slightly different contextual senses, denoting such concepts as the cosmos, the worldor Nature.
In one form or another, physics is one of the oldest academic disciplines, perhaps the oldest through its inclusion of
astronomy. [Evidence exists that the earliest civilizations dating back to beyond 3000BC, such as the Sumerians, Ancient Egyptians, and the Indus Valley Civilization, all had a predictive knowledge and a very basic understanding of the motions of the Sun, Moon, and stars.] Over the last two millennia, physics has sometimes been synonymous with philosophy, chemistryand certain branches of mathematicsand biologybut it emerged as a modern science in the 16th century. [ Francis Bacon(1620), "Novum Organum" was critical in the development of scientific method.] Physics is now generally distinct from these other disciplines, even though its boundaries remain difficult to define rigorously.
Physics is significant and influential, in part because advances in its understanding have often translated into new technologies, but also because new ideas in physics often resonate with the other sciences, mathematics and philosophy. For example, advances in the understanding of
electromagnetismled directly to the development of new products that have transformed society (including television, computers and domestic appliances); advances in thermodynamicsled to the development of motorized transport; and advances in mechanicsinspired the development of the calculus, quantum chemistry, and the use of instruments like the electron microscopein microbiology.
Today, physics is both a broad and very deep subject that, in practical/fundamental terms, can be split into several subfields. It can also be divided into two conceptually different branches: theoretical and
experimental physics; the former dealing with the development of new theories, whilst the latter deals with the experimental testing of these new, or existing, theories. Despite many important discoveries during the last four centuries, many significant questions about nature still remain unanswered, and many areas of the subject are still highly active.
: "There is also a
list of basic physics topics" and a " list of basic science topics".
cope and goals
Physics is the discipline devoted to understanding
naturein a very general sense: the fundamental characteristic of physics is that it aims to gain knowledge, and hopefully understanding, of the general properties of the world around us. As an example, we can consider asking the following question on the nature of the Universeitself: how many dimensionsdo we need? Given that we know the Universe to consist of four dimensions (three space dimensions, and one time dimension), we can also ask why the universe picked those particular numbers: why not have four space dimensions? The fact that a choice was made out of a possibility of many means that questions like these fall under the scope of physics. Other general properties of nature include the existence of mass(as in Newton's laws of motion), charge(as in Maxwell's equations), and spin(in Quantum mechanics), amongst others.
However, whilst physics studies the general properties of nature, it will often also study the properties of certain objects within nature. Thus it is also physics whose job it is to describe what happens to, for example,
planets whose motionis affected by nearby stars. Generally, the study of the specific objects in nature are shared between the three sciences: biologyis roughly responsible for the living organisms, chemistryfor the study of the elements and molecules, and physics is given responsibility over all that remains (See the section Relation to mathematics and the other sciences for further information).The fact that physics is delegated all objects besides those covered by biologyand chemistrymeans that it is responsible for the study of a wide range objects and phenomena, from the smallest sub-atomic particles, to the largest galaxies. Included in this are the very most basic objects from which all other things are composed of, and therefore physics is sometimes said to be the "fundamental science".
Generalities aside, physics aims to describe the various phenomena in
naturein terms of simpler phenomena: that is, to find the mechanisms for why nature behaves the way it does. Thus, physics aims to both connect the things we see around us to a root cause, and then to try to connect these root causes together in the hope of finding an ultimate reason for why nature is as it is. For example, the ancient Chinese observed that certain rocks ( lodestone) were attracted to one another by some invisible force. This effect was later called magnetism, and was first rigorously studied in the 17th century. A little earlier than the Chinese, the ancient Greeks knew of other objects ( amber) that when rubbed with fur would cause a similar invisible attraction between the two. This was also first studied rigorously in the 17th century, and came to be called electricity. Thus, physics had come to understand two observations of nature in terms of some root cause ( electricityand magnetism). However, further work in the 19th century revealed that these two forces were just two different aspects of one force - electromagnetism. This process of "unifying" forces continues today (see section Current research for more information).
Physics uses the scientific method
Physics uses the
scientific method. That is, that the sole test of the validity of a physical theory be comparison with observation. Experiments and observations are to be collected and matched with the predictions of theories, thus verifying or falsifying the theory.
Those theories which are very well supported by data and which are especially simple and general have been called
scientific laws. Of course, all theories, including those called laws, can also be replaced by more accurate and more general statements, if a disagreement of theory with observed data were to be found. [ Some principles, such as Newton's laws of motion, are still generally called "laws" even though they are now known to be limiting cases of other theories.]
Data collection and theory development
There are many approaches to studying physics, and many different kinds of activities in physics. There are two main types of activities in physics; the collection of data and the development of theories.
The data in some subfields of physics is amenable to experiment. For example, condensed matter physics and nuclear physics benefit from the ability to perform experiments. Sometimes experiments are done to explore nature, and in other cases experiments are performed to produce data to compare with the predictions of theories.
Some other fields in physics like astrophysics and geophysics are primarily observational sciences because most their data has to be collected passively instead of through experimentation. Nevertheless, observational programs in these fields uses many of the same tools and technology that are used in the experimental subfields of physics. The accumulated body of knowledge in some area of physics through experiment and observation is known as phenomenology. Theoretical physics often uses quantitative approaches to develop the theories that attempt to explain the data. In this way, theoretical physics often relies heavily on tools from
mathematicsand computational technologies (particularly in the subfield known as computational physics). Theoretical physics often involves creating quantitative predictions of physical theories, and comparing these predictions quantitatively with data. Theoretical physics sometimes creates models of physical systems before data are available to test and validate these models.
These two main activities in physics, data collection and theory production and testing, draw on many different skills. This has lead to a lot of specialization in physics, and the introduction, development and use of tools from other fields. For example, theoretical physicists apply mathematics and numerical analysis and statistics and probability and computers and computer software in their work. Experimental physicists develop instruments and techniques for collecting data, drawing on engineering and computer technology and many other fields of technology. Often the tools from these other areas are not quite appropriate for the needs of physics, and need to be adapted or more advanced versions have to be produced.
The culture of physics research differs from the other sciences in the separation of
theoryfrom data collection through experimentand observation. Since the 20th century, most individual physicists have specialized in either theoretical physicsor experimental physics. The great Italian physicist Enrico Fermi(1901–1954), who made fundamental contributions to both theory and experimentation in nuclear physics, was a notable exception. In contrast, almost all the successful theorists in biologyand chemistry(e.g. American quantum chemist and biochemist Linus Pauling) have also been experimentalists, though this is changing as of late.
Although theory and experiment are usually performed by separate groups, they are strongly dependent on each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories. Likewise, ideas arising from theory often inspire new experiments. In the absence of experiment, theoretical research can go in the wrong direction; this is one of the criticisms that has been leveled against
M-theory, a popular theory in high-energy physics for which no practical experimental test has ever been devised.
Physics is quantitative
Physics is more quantitative than most other sciences. That is, many of the observations experimental results in physics are numerical measurements. Most of the theories in physics use
mathematicsto express their principles. Most of the predictions from these theories are numerical. This is because of the areas which physics has addressed are more amenable to quantitative approaches than other areas. Physical definitions, models and theories can often be expressed using mathematical relations, as early as 1638, when Galileopublished the law of falling bodies in his " Two New Sciences".
A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its
theoriesby comparing the predictions of its theories with data from observations or experiments, whereas mathematics is concerned with abstract logical patterns not limited by those observed in the real world (because the real world is limited in the number of dimensions and in many other ways it does not have to correspond to richer mathematical structures). The distinction, however, is not always clear-cut. There is a large area of research intermediate between physics and mathematics, known as mathematical physics.
Relation to mathematics and the other sciences
Physics relies on
mathematics["Philosophy is written in that great book which ever lies before our eyes. I mean the universe, but we cannot understand it if we do not first learn the language and grasp the symbols in which it is written. This book is written in the mathematical language, and the symbols are triangles, circles and other geometrical figures, without whose help it is humanly impossible to comprehend a single word of it, and without which one wanders in vain through a dark labyrinth." -- Galileo(1623), " The Assayer", as quoted by G. Toraldo Di Francia (1976), "The Investigation of the Physical World" ISBN 0-521-29925-X p.10] to provide the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories can be succinctly expressed using mathematical relations.
analytic solutions are not feasible, numerical analysisand simulations can be utilized. Thus, scientific computation is an integral part of physics, and the field of computational physicsis an active area of research.
Beyond the known universe, the field of
theoretical physicsalso deals with hypotheticalissues [Concepts which are denoted "hypothetical" can change with time. For example, the atomof nineteenth century physics was denigrated by some, including Ernst Mach's critique of Ludwig Boltzmann's formulation of statistical mechanics. By the end of World War II, the atomwas no longer deemed hypothetical.] , such as parallel universes, a multiverse, or whether the universe could have expanded as predominantly antimatterrather than matter.
In the "Assayer" (1622), Galileo noted that mathematics is the language in which Nature expresses laws, to be discovered by physicists. Physics is also intimately related to many other sciences ["
The Feynman Lectures on Physics" Volume I. Feynman, Leighton and Sands. ISBN 0-201-02115-3 See Chapter 3 : "The Relation of Physics to Other Sciences" for a general discussion. For the philosophical issue of whether other sciences can be "reduced" to physics, see reductionismand special sciences).] , as well as applied fields like engineering and medicine. The principles of physics find applications throughout the other natural sciences as they depend on the interactions of the fundamental constituents of the natural world. Some of the phenomena studied in physics, such as the phenomenon of conservation of energy, are common to "all" material systems. These are often referred to as laws of physics. Others, such as superconductivity, stem from these laws, but are not laws themselves because they only appear in some systems. Physics is often said to be the "fundamental science" (chemistry is sometimes included), because each of the other disciplines ( biology, chemistry, geology, material science, engineering, medicineetc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of collections of matter (such as gases and liquids formed of atoms and molecules) and the processes known as chemical reactions that result in the change of chemical substances. The structure, reactivity, and properties of a chemical compoundare determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics(called in this case quantum chemistry), thermodynamics, and electromagnetism.
Physics in many ways stemmed from
ancient Greek philosophy. From Thales' first attempt to characterize matter, to Democritus' deduction that matter ought to reduce to an invariant state, to the Ptolemaic astronomyof a crystalline firmamentupon which the stars rested, our view of the universe seemed static. By the twentieth century, this picture became less certain, and now a static universe is only one possibility in an array of possible universes. Aristotle's early observations in natural history, and natural philosophyusually did not involve much fact checking or detailed observation, which allowed errors to come to rest in our knowledge of the world. When closer investigation overturned this picture of the world, philosophers came to study other possible forms of reasoning. The use of a priori reasoning found a natural place in scientific methodas well as the use of experiments and a posteriori reasoning came to be used in Bayesian inference[Peter Godfrey-Smith (2003), Chapter 14 "Bayesianism and Modern Theories of Evidence" "Theory and Reality: an introduction to the philosophy of science" ISBN 0-226-30063-3 ] . By the 19th century physics was realized as a positive scienceand a distinct discipline separate from philosophy and the other sciences.
Study of the philosophical issues surrounding physics, the
philosophy of physicscan be encapsulated as empiricism, naturalism, and for some, realism[Peter Godfrey-Smith (2003), Chapter 15 "Empiricism, Naturalism, and Scientific Realism?" "Theory and Reality: an introduction to the philosophy of science" ISBN 0-226-30063-3 ] . The mathematical physicist Roger Penrosehas been called a Platonistby Stephen Hawking[Stephen Hawking and Roger Penrose (1996), "The Nature of Space and Time" ISBN 0-691-05084-8 p.4 "I think that Roger is a Platonist at heart but he must answer for himself."] , while Penrose continues to eschew quantum mechanicsas a final theory about reality [Roger Penrose, "The Road to Reality" ISBN 0-679-45443-8] . Ørsted(1811) noted that physicists readily make deductions about nature, based on their closer familiarity with experiments about nature ["The student of nature ... regards as his property the experiences which the mathematician can only borrow" -- H. C. Ørsted (1811) "Introduction to General Physics", "Selected Scientific Works of Hans Christian Ørsted" ISBN 0-691-04334-5 p.296 ] , whereas the mathematicians and philosophers must make do with fewer positive statements about nature.
There are certain statements, such as Newton's Third Law of Motion, ["You cannot touch without being touched.", the formulation ofPaul G. Hewitt (2002), "Conceptual Physics" 9th Ed. ISBN 0-321-05160-2 p.19] that can be generalized into the Principle of Equivalence. This principle is the logical basis for
general relativity, whose solutions give metrics for spacetime. The success of general relativity influenced Einstein to eschew quantum theory, to which he made seminal contributions, and to eventually believe that all physical theory ought to be independent of observation. ["Physics constitutes a logical system of thought which is in a state of evolution, and whose basis cannot be obtained through distillation by any inductive method from the experiences lived through, but which can only be attained by free invention." — Albert Einstein(1936), "Physics and Reality", summarized in his "Essays in Physics" (1950) New York, Philosophical Library p. 51] [" [The] general laws on which the structure of theoretical physics is based claim to be valid for any natural phenomenon whatsover. With them, it ought to be possible to arrive at the description, that is to say, the theory, of every natural process, including life, by means of pure deduction ... " -- Albert Einstein(1918, Max Planck's 60th birthday) "Principles of Research" in "Ideas and Opinions", ISBN 0-517-55601-4 (1954) p.226.]
(from the Greek, φύσις ("phúsis"), "nature" and φυσικός ("phusikós"), "natural")
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. Another mystery was the character of the universe, such as the form of the Earthand the behavior of celestial objects such as the Sunand the Moon. Several theories were proposed, the majority of which were disproved. These theories were largely couched in philosophical terms, and never verified by systematic experimental testing as is popular today. On the other hand, the commonly accepted works of Ptolemyand Aristotle are not always found to match everyday observations. There were exceptions and there are anachronisms: for example, Indian philosophers and astronomers gave many correct descriptions in atomismand astronomy, and the Greek thinker Archimedesderived many correct quantitative descriptions of mechanicsand hydrostatics.
The willingness to question previously held truths and search for new answers eventually resulted in a period of major scientific advancements, now known as the
Scientific Revolutionof the late 17th century. The precursors to the scientific revolution can be traced back to the important developments made in Indiaand Persia, including the elliptical model of planetary orbits based on the heliocentric solar systemof gravitationdeveloped by Indian mathematician-astronomer Aryabhata; the basic ideas of atomic theorydeveloped by Hinduand Jainaphilosophers; the theory of light being equivalent to energy particles developed by the Indian Buddhistscholars Dignāgaand Dharmakirti; the optical theory of lightdeveloped by Arab scientist Alhazen; the Astrolabeinvented by the Persian Mohammad al-Fazari; and the significant flaws in the Ptolemaic systempointed out by Persian scientist Nasir al-Din al-Tusi. As the influence of the Islamic Caliphate expanded to Europe, the works of Aristotle preserved by the Arabs, and the works of the Indians and Persians, became known in Europe by the 12th and 13th centuries.
Middle Agessaw the emergence of experimental physicswith the development of an early scientific methodemphasizing the role of experimentation and mathematics. Ibn al-Haytham(Alhazen, 965-1039) is considered a central figure in this shift in physics from a philosophical activity to an experimental one. In his " Book of Optics" (1021), he developed an early scientific methodin order to prove the intromission theory of vision and discredit the emission theory of vision previously supported by Euclidand Ptolemy.citation|first=Rüdiger|last=Thiele|year=2005|title=In Memoriam: Matthias Schramm, 1928–2005|journal=Historia Mathematica|volume=32|issue=3|date=August 2005|pages=271–274|doi=10.1016/j.hm.2005.05.002: quote|“Through a closer examination of Ibn al-Haytham's conceptions of mathematical models and of the role they play in his theory of sense perception, it becomes evident that he was the true founder of physics in the modern sense of the word; in fact he anticipated by six centuries the fertile ideas that were to mark the beginning of this new branch of science.”] [citation|first=Rüdiger|last=Thiele|year=2005|title=In Memoriam: Matthias Schramm|journal=Arabic Sciences and Philosophy|publisher= Cambridge University Press|volume=15|pages=329–331|doi=10.1017/S0957423905000214] cite journal |last=Gorini |first=Rosanna |title=Al-Haytham the man of experience. First steps in the science of vision |journal=Journal of the International Society for the History of Islamic Medicine |volume=2 |issue=4 |pages=53–55 |date=October 2003 |url=http://www.ishim.net/ishimj/4/10.pdf |format=pdf |accessdate=2008-09-25 |quote=According to the majority of the historians al-Haytham was the pioneer of the modern scientific method. With his book he changed the meaning of the term optics and established experiments as the norm of proof in the field. His investigations are based not on abstract theories, but on experimental evidences and his experiments were systematic and repeatable.] His most famous experiments involve his development and use of the camera obscurain order to test several hypotheses on light, such as light travelling in straight lines and whether different lights can mix in the air. [ A. I. Sabra(1989), "Ibn al-Haytham, The Optics of Ibn al-Haytham", London: The Warburg Institute, vol. I, pp. 90-91] This experimental tradition in optics established by Ibn al-Haytham continued among his successors in both the Islamic world, with the likes of Qutb al-Din al-Shirazi, Kamāl al-Dīn al-Fārisīand Taqi al-Din, and in Europe, with the likes of Robert Grosseteste, Roger Bacon, Witelo, John Pecham, Theodoric of Freiberg, Johannes Kepler, Willebrord Snellius, René Descartesand Christiaan Huygens.
The Scientific Revolution
The Scientific Revolution is held by most historians (e.g., Howard Margolis) to have begun in 1543, when the first printed copy of
Nicolaus Copernicus's "De Revolutionibus" (most of which had been written years prior but whose publication had been delayed) was brought from Nurembergto the astronomer who died soon after receiving the copy.
Further significant advances were made over the following century by
Galileo Galilei, Christiaan Huygens, Johannes Kepler, and Blaise Pascal. During the early 17th century, Galileo pioneered the use of experimentation to validate physical theories, which is the key idea in modern scientific method. Galileo formulated and successfully tested several results in dynamics, in particular the Law of Inertia. In 1687, Newton published the "Principia", 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 forceof gravity. Both theories agreed well with experiment. The Principia also included several theories in fluid dynamics. Classical mechanics was re-formulated and extended by Leonhard Euler, French mathematician Joseph-Louis Comte de Lagrange, Irish mathematical physicist William Rowan Hamilton, and others, who produced new results in mathematical physics. The law of universal gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.
After Newton defined
classical mechanics, the next great field of inquiry within physics was the nature of electricity. Observations in the 17th and 18th century by scientists such as Robert Boyle, Stephen Gray, and Benjamin Franklincreated a foundation for later work. These observations also established our basic understanding of electrical charge and current.
In 1821, the English physicist and chemist
Michael Faradayintegrated the study of magnetismwith the study of electricity. This was done by demonstrating that a moving magnetinduced an electric currentin a conductor. Faraday also formulated a physical conception of electromagnetic fields. James Clerk Maxwellbuilt upon this conception, in 1864, with an interlinked set of 20 equations that explained the interactions between electric and magnetic fields. These 20 equations were later reduced, using vector calculus, to a set of four equations by Oliver Heaviside.
In addition to other electromagnetic phenomena, Maxwell's equations also can be used to describe
light. Confirmation of this observation was made with the 1888 discovery of radioby Heinrich Hertzand in 1895 when Wilhelm Roentgendetected X rays. The ability to describe light in electromagnetic terms helped serve as a springboard for Albert Einstein's publication of the theory of special relativityin 1905. This theory combined classical mechanics with Maxwell's equations. The theory of special relativityunifies space and time into a single entity, spacetime. Relativity prescribes a different transformation between reference frames than classical mechanics; this necessitated the development of relativistic mechanics as a replacement for classical mechanics. In the regime of low (relative) velocities, the two theories agree. Einstein built further on the special theory by including gravity into his calculations, and published his theory of general relativityin 1915.
One part of the theory of general relativity is
Einstein's field equation. This describes how the "stress-energy tensor" creates curvature of spacetimeand forms the basis of general relativity. Further work on Einstein's field equation produced results which predicted the Big Bang, black holes, and the expanding universe. Einstein believed in a static universe and tried (and failed) to fix his equation to allow for this. However, by 1929 Edwin Hubble's astronomical observations suggested that the universe is expanding. Thus, the universe must have been smaller and therefore hotter in the past. In 1933 Karl Janskyat Bell Labs discovered the radio emission from the Milky Way, and thereby initiated the science of radio astronomy. By the 1940s, researchers like George Gamowproposed the " Big Bang" theory, [Alpher, Herman, and Gamow. "Nature" 162, 774 (1948).] evidence for which was discovered in 1964; [cite web|last=Wilson |first=Robert W. |authorlink=Robert Woodrow Wilson|date=1978 |url=http://nobelprize.org/physics/laureates/1978/wilson-lecture.pdf |title=The cosmic microwave background radiation |format=PDF |accessdate=2006-06-07 Wilson's Nobel Lecture.] Enrico Fermiand Fred Hoylewere 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.
From the late 17th century onwards,
thermodynamicswas developed by physicist and chemist Boyle, Young, and many others. In 1733, Bernoulli used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of statistical mechanics. In 1798, Thompson demonstrated the conversion of mechanical work into heat, and in 1847 Joule stated the law of conservation of energy, in the form of heat as well as mechanical energy. Ludwig Boltzmann, in the 19th century, is responsible for the modern form of statistical mechanics.
1900 to Present
In 1895, Röntgen discovered
X-rays, which turned out to be high-frequency electromagnetic radiation. Radioactivitywas discovered in 1896 by Henri Becquerel, and further studied by Marie Curie, Pierre Curie, and others. This initiated the field of nuclear physics.
In 1897, Joseph J. Thomson discovered the
electron, the elementary particle which carries electrical current in circuits. In 1904, he 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.)
These discoveries revealed that the assumption of many physicists that atoms were the basic unit of
matterwas flawed, and prompted further study into the structure of atoms.
Ernest Rutherforddeduced 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 Chadwick. The equivalence of mass and energy (Einstein, 1905) was spectacularly demonstrated during World War II, as research was conducted by each side into nuclear physics, for the purpose of creating a nuclear bomb. The German effort, led by Heisenberg, did not succeed, but the Allied Manhattan Projectreached its goal. In America, a team led by Fermi achieved the first man-made nuclear chain reactionin 1942, and in 1945 the world's first nuclear explosive was detonated at Trinity site, near Alamogordo, New Mexico.
Max Planckpublished his explanation [See, for example pp.116-117, Max Planck"A Survey of Physical Theory" ISBN 0-486-67867-9] of blackbody radiation. This equation assumed that radiators are quantized in nature, which proved to be the opening argument in the edifice that would become quantum mechanics.Beginning in 1900, Planck, Einstein, Niels Bohr, and others developed quantumtheories to explain various anomalous experimental results by introducing discrete energy levels. In 1925, Heisenberg and 1926, Schrödinger and Paul Diracformulated quantum mechanics, which explained the preceding heuristic 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. During the 1920s Erwin Schrödinger, Werner Heisenberg, and Max Bornwere able to formulate a consistent picture of the chemical behavior of matter, a complete theory of the electronic structure of the atom, as a byproduct of the quantum theory. Quantum field theorywas formulated in order to extend quantum mechanics to be consistent with special relativity. It was devised 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, and successfully explained the Lamb shift. Quantum field theory provided the framework for modern particle physics, which studies fundamental forces and elementary particles. Chen Ning Yangand Tsung-Dao Lee, in the 1950s, discovered an unexpected asymmetry[ [http://cwp.library.ucla.edu/Phase2/Wu,_Chien_Shiung@841234567.html C.S. Wu's contribution to the overthrow of the conservation of parity] ] in the decay of a subatomic particle. In 1954, Yang and Robert Mills then developed a class of gauge theories [Yang, Mills 1954 " Physical Review" 95, 631; Yang, Mills 1954 "Physical Review" 96, 191.] which provided the framework for understanding the nuclear forces. The theory for the strong nuclear forcewas first proposed by Murray Gell-Mann. The electroweak force, the unification of the weak nuclear forcewith electromagnetism, was proposed by Sheldon Lee Glashow, Abdus Salamand Steven Weinbergand confirmed in 1964 by James Watson Croninand Val Fitch. This led to the so-called Standard Modelof particle physics in the 1970s, which successfully describes all the elementary particles observed to date.
Quantum mechanics also provided the theoretical tools for
condensed matter physics, whose largest branch is solid state physics. It studies the physical behavior of solids and liquids, including phenomena such as crystal structures, semiconductivity, and superconductivity. The pioneers of condensed matter physics include Bloch, who created a quantum mechanical description of the behavior of electrons in crystal structures in 1928. The transistor was developed by physicists John Bardeen, Walter Houser Brattainand William Bradford Shockleyin 1947 at Bell Telephone Laboratories.
The two themes of the 20th century, general relativity and quantum mechanics, appear inconsistent with each other ["There is as yet no logically consistent and complete relativistic quantum theory."
V. B. Berestetskii, E. M. Lifshitz, L. P. Pitaevskii(1971, 1979) "Relativistic Quantum Theory, Volume 4, Part 1, Course of Theoretical Physics" ISBN 0080160255 p. 4] . General relativity describes the universeon the scale of planets and solar systems while quantum mechanics operates on sub-atomic scales. This challenge is being attacked by string theory, which treats spacetimeas composed, not of points, but of one-dimensional objects, strings. Strings have properties like a common string (e.g., tension and vibration). The theories yield promising, but not yet testable results. The search for experimental verification of string theory is in progress.
The United Nations declared the year 2005, the centenary of Einstein's
annus mirabilis, as the World Year of Physics.
Principles and concepts
The search for regularities in nature serves to motivate our search for principles of physics. Thus Kepler's discovery of the inscribed
Platonic solidmodel of the solar systemseemed to him his greatest achievement. Of course, Kepler's laws, which he derived over a period of twenty years, were the mathematical relations which Newton was able to incorporate into his system of the world. Matteris a mass nounwhich can refer to ensembles of atoms and molecules as well as their constituent subatomic particles. Einsteinbelieved that fields were more fundamental than particles, which illustrates that "matter" is not the simple topic it appears to be. Newtontreated matter as points endowed with mass. This of course allowed mechanics to be reduced to geometry, as illustrated in Galileo's " Two New Sciences". Feynmanstarted out the Feynman Lectures on Physicswith the atomic hypothesis, which he considered to be the most compact statement of physics, from which the science could be rebuilt, were we to lose all our knowledge but that.By modeling matter as collections of hard spheres, much like Galileo's bronze ball, with which the law of falling bodies was measured, it is possible to describe statistical mechanics. Statistical mechanics, and the assumption that gases can be modelled by the collisions of hard spheres, can be used to derive the laws of thermodynamics.
Liouville's theorem for statistical and Hamiltonian mechanics is a classical nineteenth century result which describes the behavior of the
phase space distribution function[See, for example, Roger Penrose "The Road to Reality", which surveys the manifolds, symmetry groups, fibre bundles and gauge connections beneath spacetime.] . Liouville's theorem has a suggestive formulation, the Poisson bracket, which encodes Hamilton's equationsof classical mechanics. The Poisson bracketis in form much like the commutatorof quantum mechanics. The laws of nature appear to follow the postulates of quantum mechanics, and the theories that follow these postulates are said to have been quantized.
The special theory of relativity enjoys a relationship with electromagnetism and mechanics; that is, the
principle of relativityand the principle of stationary actionin mechanics can be used to derive Maxwell's equations[Landau and Lifshitz (1951, 1962), "The Classical Theory of Fields", Library of Congress Card Number 62-9181, Chapters 1-4 (3rd edition is ISBN 0080160190)] , [Corson and Lorrain, "Electromagnetic Fields and Waves" ISBN 0716718235] , and "vice versa".
Relativity and quantum mechanics can describe the physics of the extremely small (
atoms, nuclei, fundamental particles), the extremely large (the Universe), and the extremely fast (relativity). But no complete theory yet exists. The Schrödinger pictureof quantum mechanics and the Heisenberg picturecan be connected by the Ehrenfest theorem, the analog of Liouville's theoremnoted above.
While physics deals with a wide variety of systems, there are certain theories that are used by all physicists. Each of these theories were experimentally tested numerous times and found correct as an approximation of Nature (within a certain domain of validity). For instance, the theory of
classical mechanicsaccurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These theories continue to be areas of active research; for instance, a remarkable aspect of classical mechanics known as chaos was discovered in the 20th century, three centuries after the original formulation of classical mechanics by Isaac Newton(1642–1727). These "central theories" are important tools for research into more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.
Major topics of physics
hidden |1=Topics in physics|bg1=#ccccff
2=Topics in physics
Contemporary research in physics is divided into several distinct fields that study different aspects of the material world.
Condensed matter physics, by most estimates the largest single field of physics, is concerned with how the properties of bulk matter, such as the ordinary solids and liquids we encounter in everyday life, arise from the properties and mutual interactions of the constituent atoms. The field of atomic, molecular, and optical physicsdeals with the behavior of individual atoms and molecules, and in particular the ways in which they absorb and emit light. The field of particle physics, also known as "high-energy physics", is concerned with the properties of submicroscopic particles much smaller than atoms, including the elementary particles from which all other units of matter are constructed. Finally, the field of astrophysicsapplies the laws of physics to explain celestialphenomena, ranging from the Sunand the other objects in the solar systemto the universe as a whole.
Since the 20th century, the individual fields of physics have become increasingly specialized, and nowadays it is not uncommon for physicists to work in a single field for their entire careers. "Universalists" like
Albert Einstein(1879–1955) and Lev Landau(1908–1968), who were comfortable working in multiple fields of physics, are now very rare.
Research in physics is progressing constantly on a large number of fronts, and is likely to do so for the foreseeable future.
In condensed matter physics, one of the big unsolved theoretical problem is the explanation for
high-temperature superconductivity. Strong efforts, largely experimental, are being put into making workable spintronicsand quantum computers.
In particle physics, the first pieces of experimental evidence for physics beyond the
Standard Modelhave begun to appear. Foremost amongst these are indications that neutrinos have non-zero mass. These experimental results appear to have solved the long-standing solar neutrino problemin solar physics. The physics of massive neutrinos is currently an area of active theoretical and experimental research. In the next several years, particle accelerators will begin probing energy scales in the TeVrange, in which experimentalists are hoping to find evidence [584 co-authors "Direct observation of the strange 'b' baryon " Fermilab-Pub-07/196-E, June 12, 2007http://arxiv.org/abs/0706.1690v2 finds a mass of 5.774 GeV for the ] for the Higgs bosonand supersymmetric particles.
Theoretical attempts to unify
quantum mechanicsand general relativityinto a single theory of quantum gravity, a program ongoing for over half a century, have not yet borne fruit. The current leading candidates are M-theory, superstring theoryand loop quantum gravity.
Many astronomical and cosmological phenomena have yet to be satisfactorily explained, including the existence of ultra-high energy cosmic rays, the
baryon asymmetry, the acceleration of the universe and the anomalous rotation rates of galaxies.
Although much progress has been made in high-energy,
quantum, and astronomical physics, many everyday phenomena, involving complexity, chaos, or turbulenceare still poorly understood. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics, such as the formation of sandpiles, nodes in trickling water, the shape of water droplets, mechanisms of surface tensioncatastrophes, or self-sorting in shaken heterogeneous collections are unsolved. These complex phenomena have received growing attention since the 1970s for several reasons, not least of which has been the availability of modern mathematical methods and computerswhich enabled complex systemsto be modeled in new ways. The interdisciplinary relevanceof complex physics has also increased, as exemplified by the study of turbulencein aerodynamicsor the observationof pattern formationin biological systems. In 1932, Horace Lambcorrectly prophesized:
"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."Fact|date=January 2007
Applications and Influence
Applied physicsis a general term for physics which is intended for a particular use; thus happinesscan come from a successful application of the science. "Applied" is distinguished from "pure" by a subtle combination of factors such as the motivation and attitude of researchers and the nature of the relationship to the technology or science that may be affected by the work. [ [http://www.stanford.edu/dept/app-physics/general/ Stanford Applied Physics Department Description] ] Applied Physics curriculum will usually contain a few classes from the applied disciplines, like chemistry, computer science, or electrical engineering. It usually differs from engineeringin that an applied physicist may not be designing something in particular, but rather is using physics or conducting physics research with the aim of developing new technologies or solving a problem. The approach is similar to that of applied mathematics. Applied physicists can also be interested the use of physics for scientific research. For instance, people working on accelerator physicsmight seek to build better particle detectors for research in theoretical physics. Physics is used heavily in engineering. Statics, a subfield of mechanics, is used in the building of bridges or other structures; the simple machines such as the leverand the ramphad to be discovered before they could be used; today, they can be taught to schoolchildren. The understanding and use of acousticswill result in better concert halls; similarly, the use of opticscreates better optical devices. An understanding of physics makes for more realistic flight simulators, video games, and movies, as well as in forensicinvestigations (what do we know and when do we know it; what did the subject know and when did the subject know it).
Because of its historical relationship to the development of
scientific method, physics reasoning can handle items which would ordinarily be mired in conundrums or uncertainty. For example, in the History of Earth#Origin, one can reasonably model Earth's mass, temperature, and rate of rotation, over time. From these values, the chemical composition of Earth at differing epochs can be posited. Even if a precise linear timeline might be problematic, qualitative statements can then be made about the history of Earth, which are still founded in the laws of physics.
There are many fields of physics which have strong applied branches, as well as many related and overlapping fields from other disciplines that are closely related to applied physics.
Acoustics, the study of sound waves, is used everywhere we wish to hear, for example in music, speech, and audible alarms.
Agrophysicsis the study of the physics in agronomy.
Biophysicsis the interface of biology and physics.
Chemical physicsstudies the structure and dynamics of ions, free radicals, polymers, clusters, and molecules, using both classical and quantum mechanical viewpoints.
Communications has used physics extensively, for example in the Bell Laboratories. The first communications satelliteappeared 300 years after Newton.
Econophysicsis the interface of economics and physics.
Engineering physicsgraduates specialists in optics, nanotechnology, control theory, aerodynamics, or solid-state physics.
Fluid mechanicsis the study of fluids (liquids and gases) at rest and in motion. The Navier-Stokes equationsare used in supercomputers to model Earth's weather.
Geophysicsis the physics of Earth.
Lasers and radarwere developed in the laboratories, used by the military, and now have extensive peacetime uses. Quantum electronicsincludes the study of lasers.
Materials scienceis the systematic study of the properties of materials.
Medical physicsincludes the standards for radiation exposure and infrastructure for radiology.
Nanotechnologystudies the creation of machines less than a micrometer in size.
Opticshas existed as a science for over 1000 years. Like acoustics, it has its own journals, practitioners, and university departments, as well as industries which utilize those graduates.
Optoelectronicscreates devices which use light rather than current.
Photovoltaics, or solar cells, promise to generate current from light.
Physics of computationmust rely upon the state of the art. For example, the accuracy of a computation depends on the ability to manufacture to tolerance, which was the undoing of Babbage's difference engine. Babbage's design worked when built with twentieth century technology.
Plasma physicsis the physics of an ionized gas.
Quantum chemistrymodels matter using quantum mechanics.
Quantum information scienceapplies qubitdevices.
Solid state physics, including the material properties of semiconductor devices and integrated circuits. Engineeringutilizes physics in service of technology rather than science.
Vehicle dynamicsis a form of kinematics.
Varieties of physics
Glossary of classical physics
List of basic physics topics
List of physics topics
* Perfection in physics and chemistry
Philosophy of physics
Physics (Aristotle)- an early book on physics, which attempted to analyze and define motion from a philosophical point of view
Unsolved problems in physics
* [http://www.tandf.co.uk/journals/titles/00107514.asp Contemporary Physics]
* [http://www.physics.org/sitesofmonth/sitesofmonth.asp Physics.org, Sites of the month]
* Aristotle's [http://etext.library.adelaide.edu.au/a/a8ph/ "Physics", trans. by R. P. Hardie and R. K. Gaye]
* [http://us.geocities.com/alex_stef/mylist.html Physics and Math Textbooks] Numerous online textbooks on Physics and Mathematics
* [http://math.ucr.edu/home/baez/physics/ Usenet Physics FAQ] . A
FAQcompiled by sci.physics and other physics newsgroups.
* [http://www.physics.org/ Physics.org] - Web portal run by the [http://www.iop.org/ Institute of Physics] .
* [http://scienceworld.wolfram.com/physics/ World of Physics] . An online encyclopedic dictionary of physics.
* [http://nobelprize.org/nobel_prizes/physics/ Website of the Nobel Prize in Physics] .
* [http://thephysics.net The Physics Network] Official Physics Network
* [http://www.physicstoday.org Physics Today] - Your daily physics news and research source
* [http://musr.physics.ubc.ca/~jess/hr/skept/ The Skeptic's Guide to Physics]
* [http://www.planetphysics.org PlanetPhysics] Online Physics
* [http://www.physics2005.org Physics 2005] : Website of the
World Year of Physics 2005
* [http://physicsweb.org Physicsweb.org]
* [http://www.tandf.co.uk/journals/titles/00107514.asp Contemporary Physics]
* [http://www.aip.org/index.html AIP.org] Website of the
American Institute of Physics
* [http://www.iop.org IOP.org] Website of the
Institute of Physics
* [http://www.aps.org APS.org] Website of the
American Physical Society
* [http://www.spsnational.org SPS National] Website of the
Society of Physics Students
* [http://www.physicsmathforums.com Physics & Math Forums]
* [http://www.royalsoc.ac.uk Royal Society] Website of the Royal Society. Although not exclusively a physics institution, it has a strong physical history.
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