Physics (from the
Greek, φυσικός (''phusikos''), "natural", and φύσις (''phusis''), "nature") is the
science of
Nature in the broadest sense.
Physicists study the behavior and properties of
matter in a wide variety of contexts, ranging from the sub-nuclear particles from which all ordinary matter is made (
particle physics) to the behavior of the material
Universe as a whole (
cosmology).
Some of the properties studied in physics are common to
all material systems, such as the
conservation of energy. Such properties are often referred to as laws of physics. Physics is sometimes said to be the "fundamental science", because each of the other
natural sciences (
biology,
chemistry,
geology, etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of
molecules and the
chemicals that they form in the bulk. The properties of a chemical are determined by the properties of the underlying molecules, which are accurately described by areas of physics such as
quantum mechanics,
thermodynamics, and
electromagnetism.
Physics is also closely related to
mathematics. Physical
theories are almost invariably expressed using mathematical relations, and the mathematics involved is generally more complicated than in the other sciences. The difference between physics and mathematics is that physics is ultimately concerned with descriptions of the material world, whereas mathematics is concerned with abstract patterns that need not have any bearing on it. However, the distinction is not always clear-cut. There is a large area of research intermediate between physics and mathematics, known as
mathematical physics, devoted to developing the mathematical structure of physical theories.
Overview of physics research
Theoretical and experimental physics
The culture of physics research differs from the other sciences in the separation of
theory and
experiment. Since the
20th century, most individual physicists have specialized in either
theoretical physics or
experimental physics, and in the twentieth century, very few physicists have been successful in both forms of research
1.
In contrast, almost all the successful theorists in
biology and
chemistry have also been experimentalists.
Roughly speaking, theorists seek to develop theories that can describe and interpret existing experimental results and successfully predict future results, while experimentalists devise and perform experiments to explore new phenomena and test theoretical predictions. Although theory and experiment are developed separately, 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 have been levelled against
M-theory, a popular theory in high-energy physics for which no practical experimental test has ever been devised.
Central theories
While physics deals with a wide variety of systems, there are certain theories that are used by all physicists. Each of these theories is believed to be basically correct, within a certain domain of validity. For instance, the theory of
classical mechanics accurately 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. These "central theories" are important tools for research into more specialized topics, and any student of physics, regardless of his or her specialization, is expected to be well-versed in them.
Major fields of 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 physics deals 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
astrophysics applies the laws of physics to explain
astronomical phenomena, ranging from the
Sun and the other objects in the
solar system to the universe as a whole.
There are many areas of research that mix physics with other disciplines. For example, the wide-ranging field of
biophysics is devoted to the role that physical principles play in biological systems, and the field of
quantum chemistry studies how the theory of quantum mechanics gives rise to the chemical behavior of atoms and molecules. Some of these are listed below.
Acoustics -
Astronomy -
Biophysics -
Computational physics -
Electronics -
Engineering -
Geophysics -
Materials science -
Mathematical physics -
Medical physics -
Physical chemistry -
Physics of computation -
Quantum chemistry -
Quantum information science -
Vehicle dynamics
Fringe theories
Cold fusion -
Dynamic theory of gravity -
Luminiferous aether -
Steady state theory -
Wave Structure Matter
History
Main article: History of physics. See also Famous physicists and Nobel Prize in Physics.
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 which were wrong. These theories were largely couched in
philosophical terms, and never verified by systematic experimental testing as is popular today. There were exceptions and there are
anachronisms: for example, the Greek thinker
Archimedes derived many correct quantitative descriptions of
mechanics and
hydrostatics.
The works of
Ptolemy (Astronomy) and
Aristotle (Physics) were also found to not always match everyday observations. An example of this is an
arrow flying through the air after leaving a bow contradicts with Aristotle's assertion that the natural state of all objects is at rest.
[[Isaac Newton]]
The willingness to question previously held truths and search for new answers resulted in a period of major scientific advancements, now known as the
Scientific Revolution. Its origins can be found in the European re-discovery of Aristotle in the twelfth and thirteenth centuries. This period culminated with the publication of the
Philosophiae Naturalis Principia Mathematica in 1687 by
Isaac Newton (dates disputed).
The Scientific Revolution is held by most historians (e.g., Howard Margolis) to have begun in 1543, when there was brought to the Polish astronomer
Nicolaus Copernicus the first printed copy of the book
De Revolutionibus he had written about a dozen years earlier. The thesis of this book is that the Earth moves around the Sun.
Other significant scientific advances were made during this time 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 the
scientific method. Galileo formulated and successfully tested several results in
dynamics, in particular the Law of
Inertia. In 1687,
Newton published the
Principia Mathematica, detailing two comprehensive and successful physical theories:
Newton's laws of motion, from which arise
classical mechanics; and
Newton's Law of Gravitation, which describes the fundamental force of
gravity. Both theories agreed well with experiment. The Principia also included several theories in fluid dynamics. Classical mechanics was extended by
Leonhard Euler, Joseph-Louis de Lagrange,
William Rowan Hamilton, and others, who produced new results and new formulations of the theory. 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 Franklin created a foundation for later work. These observations also established our basic understanding of electrical charge and current.
[[James Clerk Maxwell]]
In 1821,
Michael Faraday integrated the study of
magnetism with the study of electricity. This was done by demonstrating that a moving
magnet induced an electric current in a
conductor. Faraday also formulated a physical conception of
electromagnetic fields.
James Clerk Maxwell built upon this conception, in 1864, with an interlinked set of 20 equations that explained the interactions between
electric and
magnetic field. These 20 equations were later reduced, using
vector calculus, to a set of
four equations by
Oliver Heaviside.
[[Albert Einstein in 1905]]
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
radio by Heinrich Hertz and in 1895 when Wilhelm Roentgen detected X rays. The ability to describe light in electromagnetic terms helped serve as a springboard for
Albert Einsteins publication of his theory of special relativity. This theory combined classical mechanics with Maxwells equations.
The theory of
special relativity unifies 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 relativity in 1915.
One part of the theory of general relativity is
Einstein's field equation. This describes how the
stress-energy tensor creates curvature of
spacetime and 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 argued that astronomical observations demonstrate that the universe is expanding.
From the
18th century onwards,
thermodynamics was developed by
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.
In 1895, Roentgen discovered
X-rays, which turned out to be high-frequency electromagnetic radiation. Radioactivity was 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.)
Henri Becquerel accidentally discovered radioactivity in 1896. The next year
Thomson discovered the
electron. These discoveries revealed that the assumption of many physicists that atoms were the basic unit of
matter was flawed, and prompted further study into the structure of
atoms.
In 1911, 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
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 Project reached its goal. In America, a team led by
Fermi achieved the first man-made
nuclear chain reaction in 1942, and in 1945 the world's first
nuclear explosive was detonated at Trinity site, near Alamogordo,
New Mexico.
In 1900,
Max Planck published his explanation 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
quantum theories to explain various anomalous experimental results by introducing discrete energy levels. In 1925,
Heisenberg and 1926,
Schrödinger and
Paul Dirac formulated
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 Born were 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.
[[Richard Feynman]]
Quantum field theory was 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 Yang and
Tsung-Dao Lee, in the
1950s, discovered an unexpected asymmetry in the decay of a
subatomic particle. In 1954, Yang and Robert Mills then developed a class of
gauge theories, which provided the framework for understanding the nuclear forces. The theory for the strong nuclear force was first proposed by
Murray Gell-Mann. The
electroweak force, the unification of the
weak nuclear force with electromagnetism, was proposed by
Sheldon Lee Glashow,
Abdus Salam and
Steven Weinberg and confirmed in 1964 by
James Watson Cronin and Val Fitch. This led to the so-called
Standard Model of 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 Brattain and William Bradford Shockley in 1947 at Bell Telephone Laboratories.
The two themes of the
20th century, general relativity and quantum mechanics, appear inconsistent with each other. General relativity describes the
universe on 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
spacetime as 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 have declared the year 2005, the centenary of Einstein's annus mirabilis, as the World Year of Physics.
Future directions
Main article: unsolved problems in physics.
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, the biggest unsolved theoretical problem is the explanation for
high-temperature superconductivity. Strong efforts, largely experimental, are being put into making workable
spintronics and
quantum computers.
In particle physics, the first pieces of experimental evidence for physics beyond the
Standard Model have 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 problem in 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
TeV range, in which experimentalists are hoping to find evidence for the
Higgs boson and
supersymmetric particles.
Theoretical attempts to unify
quantum mechanics and
general relativity into 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 theory and
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
turbulence are still poorly understood. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics, like the formation of sandpiles, nodes in trickling
water, the shape of water droplets, mechanisms of
surface tension catastrophes, 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 computers which enabled complex systems to be modelled in new ways. The
interdisciplinary relevance of complex physics has also increased, as exemplified by the study of
turbulence in
aerodynamics or the
observation of
pattern formation in
biological systems. In 1932,
Horace Lamb correctly 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.
Notes
Suggested readings
- Richard Feynman, The Character of Physical Law, Random House (Modern Library), 1994, hardcover, 192 pages, Order: ISBN 0679601279
- Feynman, Leighton, Sands, The Feynman Lectures on Physics, Addison-Wesley 1970, 3 volumes, paperback, Order: ISBN 0201021153. Hardcover commemorative edition, 1989, Order: ISBN 0201500647
- Lev Davidovich Landau, et. al., Course of Theoretical Physics, Butterworth-Heinemann, 1976, 10 volumes, paperback, Order: ISBN 0750628960
- Roger Penrose, The Road to Reality: A complete guide to the laws of the universe, Knopf, 2004, Order: ISBN 0-679-45443-8, LoC QC20.P366 2005
- Jearl Walker, The Flying Circus of Physics, Wiley, 1977, paperback, 312 pages, Order: ISBN 047102984X
- Anthony Leggett, The Problems of Physics, Oxford University Press, 1988, Order: ISBN 0192891863
- Paul A. Tipler & Ralph A. Llewellyn, Modern Physics, Fourth edition, W H Freeman & Co, 2002, hardcover, 700 pages, Order: ISBN 0716743450
Basic Physics
- Paul Hewitt, Conceptual Physics with Practicing Physics Workbook (9th Edition), Addison Wesley Publishing Company, 2001, hardcover, 790 pages, Order: ISBN 0321052021. A non-mathematical introduction to physics.
- Douglas C. Giancoli, Physics: Principles with Applications, 6/E, Prentice Hall, 2005, 1040 pages, ISBN: 0130606200. This is an algebra-based physics textbook.
- Jerry D. Wilson & Anthony J. Buffa, College Physics (5th edition), Prentice Hall, 2002, 2 volumes, 1040 pages, Order: ISBN 0130676446. This is a calculus-based physics textbook.
See also
External links
Physics for students
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