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“”The simplicity of nature is not to be measured by that of our conceptions. Infinitely varied in its effects, nature is simply only in its causes, and its economy consists in producing a great number of phenomena, often very complicated, by means of a small number of general laws.
|—Pierre-Simon de Laplace|
Physics is the study of space, time, matter, energy and their interactions, ranging in size (and specialty) from the smallest subatomic particles to the entire Universe; it is therefore the most basic of all natural sciences.
Laws and theories of physics are most often expressed in the language of mathematics. In many cases, breakthroughs in physics are made possible by using already existing mathematical techniques. An example is the implementation of linear algebra in the matrix formulation of quantum mechanics, or matrix mechanics for short. However, it is worth noting that the development of physics often necessitates that of mathematics. A typical example is classical mechanics and calculus.
Core developments will be discussed below.
 Early physics
Aristotle believed that everything was made up of one of the five elements: earth, fire, air, water, or magic: the "quintessence", or "fifth essence". He also thought the heavenly bodies were perfect and unchanging. His ideas were simply accepted. It is interesting to note that in Eastern philosophy, the Universe is considered to comprise of the five elements: metals, wood, water, fire and earth. Again, there is not much empirical evidence to support this claim. At this stage, research in the natural sciences in general and physics in particular were conducted by philosophers, who could not be bothered to verify their claims with careful observations or experiments.
Things improved dramatically with the arrival of Archimedes of Syracuse. Starting from empirical observations and experiments, he discovered the principle of buoyancy in hydrostatics, the law of lever and introduced the concept of the center of mass of a body. Archimedes also built an planetarium which clearly favors the heliocentric theory.
 Seventeenth century physics: Warming up
Galileo Galilei argues that bodies fall at the same rate regardless of mass, if air resistance is negligible. He proved that if air resistance can be ignored, falling bodies thrown at an angle will follow a parabolic trajectory. While telescopes are not unheard of at this time, Galileo built superior ones, using which he obtained empirical evidence in favor of the heliocentric theory, discovered the (Galilean) moons of Jupiter, sunspots, that the Moon is full of craters, among other imperfections, contrary to Aristotle's teachings. He formulated the law of the pendulum, but failed to design a clock that was usable at sea. Galileo is one of the very first scientists as we recognize them today. He fully understood the necessity of experiments and observation and made use of them whenever possible in his investigations. Réne Descartes introduced coordinate geometry, of fundamental importance is both physics as well as mathematics, and correctly stated that inertia is the tendency of a massive body to travel in a straight line at constant speed. This later became what we now call Newton's first law of motion.
Ole Rømer[wp] observed that from Earth, the moons of Jupiter appear to be regularly eclipsed. However, the time between these eclipse same; in fact they vary according to the position of Earth in its orbit about the Sun. He concluded that the speed of light must therefore be finite. Using the best estimate of the radius of the Earth available at the time, he calculated the speed of light to be 225,000km/s. Although this value is not very accurate, it was the first time that the speed of light was shown to be finite.
 Eighteenth century physics: Age of Newton
Sir Isaac Newton dominated this era. In his masterpiece the Philosophiae Naturalis Principia Mathematica, or Principia for short, he laid down the basics of calculus, the laws of motion and gravitation and investigated some of their consequences, including, but not limited to, Kepler's laws of orbital motion and Newton's shell theorem. Newton and Gottfried Wilhelm Leibniz independently developed calculus, which underlies much of physics. While physics has its roots in astronomy and a number of important results have been published before Newton, e.g. Archimedes' principle, it was Newton who constructed physics as a mathematical formalism. His Opticks describes his famous prism experiment, discusses the laws of optics and articulates his corpuscular theory of light. Christian Huygens favored a wave theory of light and introduced Huygens' principle: a wavefront is tangent to all the circular wavelets that generate it. He also studied the behavior of pendulums, formulating a sophisticated theory of oscillatory motion and inventing a pendulum clock in the process.
Later in the century, Leonard Euler, Daniel Bernoulli, Jean le Rond d'Alembert, Joseph-Louis Lagrange, Pierre-Simon de Laplace, William Rowan Hamilton, among others, continued the development of calculus and brought classical mechanics to great new heights. Charles Augustin de Coulomb laid down his eponymous inverse square law of electrostatics. It is interesting to note that Sir Henry Cavendish had himself discovered the same thing using similar experimental apparatus, namely a torsion balance but did not publish his findings till they were unearthed by Maxwell in the next century. However, Cavendish did publicize his determination of Newton's gravitational constant to a high degree of accuracy using a torsion balance and deduction that the Earth's core must consist of a very dense material. Benjamin Franklin performed his kite experiment, which demonstrated that lightning is an electrical phenomenon. Franklin then invented the lightning rod. The Vatican thought this was heretic because it interferes with the will of God. Franklin also put forth the convention of positive and negative charges.
Sadi Carnot explored the utility of heat to do work. He introduced a theoretical construct known as the Carnot engine and thus founded the science of thermodynamics.
 Nineteenth century physics: Heat, electricity and magnetism, and chaos
Laplace demonstrated the stability of the Solar System and expounded upon the nebula hypothesis for its origin. He showed that it is almost certain that all bodies the Solar System formed from the same cloud of gas. He also reintroduced the concept of a black hole, based on the escape velocity formula in Newton's theory of gravity.
Thomas Young performed the double-slit experiment, which lent credence to the wave theory of light. He also gave a rough estimate of the size of atoms, which remained hypothetical at this time. Jean-Baptiste Fourier formulated his analytical theory of heat diffusion using experiments and observations. The mathematical technique he discovered during his investigation, Fourier analysis, is of great value in theoretical physics. Fourier also predicted the phenomenon of global warming.
Benjamin Thomson (Count Rumford) and James Joule demonstrated by experiment that heat is a form of motion. William Thomson (Lord Kelvin), Rudolf Clausius, Hermann von Helmholtz, and others furthered the work of Carnot and established the laws of thermodynamics. Using these, Lord Kelvin estimated the age of the Earth to between 50 to 500 million years. Now that thermodynamics is established, engineers began designing ever more efficient heat engines and refrigerators.
Felix Savart and Jean-Baptiste Biot established that magnetism also obeys an inverse square law. Lord Kelvin, von Helmholtz, Denis Poisson, and others conducted further investigations of electricity and explored magnetism. Hans Christian Oersted demonstrated that a changing electric current induces a magnetic field. Andre-Marie Ampere initiated the study of electrodynamics. Michael Faraday showed that a changing magnetic field induces generates a magnetic field and built the first electric motor. James Clerk Maxwell did for electromagnetism what Newton had done for gravitation by building it into a coherent theory, now called Maxwell's equations. Maxwell's equations summarizes everything there is to know about classical electromagnetism and predicted the existence of electromagnetic waves, propagating at precisely the speed of light c. He could scarcely avoid the conclusion that light is itself an example of an electromagnetic wave. Maxwell's equations imply that the velocity of light does not depend on the velocity of the source or the observer, provided they are not accelerating. This seems to contradict Newtonian mechanics, in which the speed measured depends on the frame of reference. This puzzle would not be resolved till the next century. Shortly after Maxwell's death, Heinrich Hertz conducted a series of experiments in which he generated a low-frequency electromagnetic wave, now called radio waves. Like light, it can be reflected, refracted, diffracted and polarized. In doing so, he constructed a primitive radio antenna as well as forerunners of satellite dishes. But more importantly, his experiments verified Maxwell's electromagnetic theory of light.
With Ludwig Boltzmann, Maxwell derived the first ever statistical law of physics, the Boltzmann-Maxwell distribution of molecular speeds in an ideal gas, from Newton's laws of motion.  Boltzmann then went on to develop the kinetic theory of gases and statistical mechanics. Perhaps his greatest contribution is the statistical interpretation of the second law of thermodynamics. A system tends to be in a state of maximal entropy because such a state is the most likely. He also showed that the entropy of a given system is directly proportional to its thermodynamic probability; the constant of proportionality is known as Boltzmann's constant, in his honor, and is ubiquitous in statistical mechanics.
Gustav Kirchoff, perhaps best known for his laws of circuitry, used the laws of optics and thermodynamics to show that the radiation emitted by a black body is a function only of the temperature of that black body and its wavelength. He then challenged his colleagues to determine such at function. William Strutt (Lord Rayleigh) and Sir James Jeans deduced the Rayleigh-Jeans law in response. But it only works for short wavelengths and disturbingly fails for longer ones, predicting that even a mildly heated body is a source an infinite amount radiation. This became known as the ultraviolet catastrophe.
Henri Becquerel discovered a mind-boggling natural phenomenon which apparently violated the conservation of energy dubbed radioactivity by Marie Currie in an experiment involving a salt of uranium. Curie and her husband went on to isolate and identify two new elements, polonium and radium, both of which radioactive.
At this point, the basics of classical physics have all been established. Using Newton's laws of motion and gravitation, Maxwell's equations of electromagnetism and the laws of thermodynamics, one could explain pretty much everything in the known world. Despite its encouraging success, nineteenth-century physics encountered a number of major problems that it was unable to explain: (1) the fact that c is independent frames of reference, (2) the ultraviolet catastrophe, as mentioned above, (3) whether atoms are real or merely theoretical constructs, (4) radioactivity and (5) the photoelectric effect. Solutions to these problems took physicists to places where no one has gone before.
In the final decade of this century, Henri Poincare investigated the three-body problem and came to a surprising conclusion. In general, it has no solutions. He also recognized the existence of dynamical systems with extreme sensitivity to small changes to the initial conditions. Any changes to the initial conditions, no matter how small, will in time lead to entirely different behavior. Such systems are called chaotic. Unfortunately, while the discovery of that Newton's laws predict chaos all along is quite remarkable, it was been overshadowed by the fanfare associated with what happened right at the start of the next century.
 Twentieth century physics: Mainly relativity and quantum mechanics
Max Planck discovered that if he treated light as if it was made up of discrete particles, then he could resolve the ultraviolet catastrophe. During the process, he formulated Planck's law of blackbody radiation. Albert Einstein then entered the stage in the most spectacular manner possible. For his doctoral thesis, Einstein gave a mathematical description of Brownian motion, the kind of movement observed when tiny particles, such as pollen, is suspended in a liquid, thereby demonstrating the reality of atoms. Using Planck's quantum hypothesis, Einstein explained the photoelectric effect in full detail. Postulating that the speed of light is the same for all observers and that the laws of physics remain the same in all inertial reference frames, he developed the special theory of relativity, which reduces to classical mechanics if the velocities involved are 10% the speed of light or less. Fundamental special relativistic effects are time dilation for moving bodies, length contraction in the direction of motion, and the loss of simultaneity of clocks moving at different speeds. As an afterthought, he derived what is probably the most famous equation in science, , which gives the energy equivalent of a massive body at rest. This impressive creative outburst took place in 1905, a great year for Einstein as well as physics.
Ernest Rutherford discovered the law of radioactive decay: the rate of decay is directly proportional to the amount present. His gold foil experiment revealed that most of the atom is actually empty space and that the electrons are moving about the nucleus much like the way the Earth and other planets orbit the Sun. Unfortunately, his planetary model seems fatally flawed; Maxwell's equations predict that accelerating charged particles emit electromagnetic radiation. If this was true, electrons will continuously lose energy as they spiral towards the nucleus; atoms would collapse in tens of nanoseconds. Niels Bohr saw a way out. His atomic model took into account the quantization of energy. Electrons can only absorb or emit discrete amounts of energy; those at the ground state have no energy to emit and will not collide with the nucleus. Bohr's model successfully reproduces the spectrum of hydrogen and hydrogen-like atoms, those with just one electron, but falters for more complex ones. The cavalry soon arrived. In the 1920s, a group of physicists, perhaps some of the most brilliant in history, developed the modern theory of quantum mechanics. Louis de Broglie and Albert Einstein pointed out the importance of wave-particle.[note 1] Werner Heisenberg enunciated the crucial uncertainty principle, which differentiates the quantum world from what we are used to. It states that certain pairs of dynamical variables, such as position and momentum, are such that the more precisely one is measured, the less precisely the other one is known. There is no way around it; uncertainty is built into the fabric of nature. Paul Dirac succeeded in unifying special relativity and quantum mechanics for the first time with his relativistic wave equation for the electron.
As is often the case is science, his success in formulating the special theory of relativity pointed Einstein to the next big problem. While special relativity proclaims that no causal influence can travel faster than the speed of light, Newton's theory of gravity implicitly assumes that gravitational interactions are instantaneous. Einstein realized that (if air resistance is non-existent or negligible) falling bodies can consider themselves to be at rest and the ground is accelerating towards them; in other words, gravitation and acceleration are equivalent. Einstein considered this to be the happiest moment of his life. His opinion is quite reasonable, since principle of equivalence lies at the core of his theory of gravity, general relativity, nowadays thought to be his magnum opus. Moreover, it is a crucial insight in to the nature of the Universe. Special relativity stipulates that all inertial frames are equivalent, as far as the laws of physics are concerned. With the principle of equivalence, the basic laws of physics hold in all frames of reference, inertial or not. Einstein quickly recognized that in his new framework, spacetime itself is curved, which means the conventional Euclidean geometry cannot be used. Fortunately, the necessary mathematics, differential geometry, has already been introduced six decades before by Bernhard Riemann. After ten years of hard work, Einstein finally published his field equations for gravity in 1916. John Wheeler eloquently summarized the thus, "Mass [and energy] tells space [strictly, spacetime] how to curve. Space tells mass how to move."According to general relativity, the influence of gravity travels at exactly the speed of light. Indeed, a jiggling massive body emits gravitational waves, or radiation. Unfortunately, since gravity is so feeble, detecting gravitational waves is a Herculean task. Just a few months later, Karl Schwarzschild obtained the first non-trivial exact solution[note 2] to Einstein's field equations. Schwarzschild's solution describes a spherically symmetric body so compact that not even light on it surface can escape its gravitational pull. Wheeler later gave this object the name "black hole". Einstein showed that general relativity correctly accounts for the perihelion of Mercury, something Newton's theory of gravity cannot explain. Since spacetime is curved, the path of light near a massive body must be also curved. Sir Arthur Eddington verified this prediction by astronomical observations in 1919.
Quantum mechanics then made some very strange predictions about how the universe works at a very small scale. Due to the inherent "spookiness" of quantum mechanics, it has been a favorite target of pseudoscientists (to be fair, even Einstein wasn't too chuffed about its use of probabilities). The disagreements over the 'real' interpretation of quantum mechanics, and the lack of an underlying theory linking quantum mechanics and general relativity is a fertile area for people who like to point out that because science cannot answer everything, all science is wrong and therefore [insert orbiting teapot equivalent here] must be true. Einstein and Niels Bohr had one of the most famous exchanges of letters in science history, the Bohr-Einstein debates[wp]. Einstein would propose a thought experiment to demonstrate a weakness in quantum mechanics, and Bohr would explain where Einstein was mistaken. Bohr won that exchange hands down. Bohr was especially pleased when he was able to use Einstein's Theory of Relativity to make a point.
Interrupted by World War II, the development of quantum field theory resumed at full pace in the postwar years. The first major result was quantum electrodynamics, independently developed by Sin-Itiro Tomogana, Julian Schwinger and Richard Feynman, one of the most accurate theories of physics ever and a crown jewel of science. Physicists then developed quantum chromodynamics (it has nothing to do with color!) to describe the strong nuclear force, electroweak theory, which addresses the unification between the electromagnetic and weak nuclear forces and finally the Standard Model of particle physics, which encompasses all three non-gravitational interactions. Interest in general relativity returned, and important results concerning cosmic expansions and black holes were built upon. Also of great interest are superfluidity and superconductivity (and the unborn Superconducting Supercollider). As they currently stand, the Standard Model and general relativity contain almost everything physicists know for certain (within the limits of experimental uncertainty, pun intended) about how the Universe works. Not too surprisingly, contemporary physics has very little to say about the behavior and characteristics of neutrinos, dark matter, and dark energy; the latter two comprise the overwhelming majority of our Universe. So do expect some ground-breaking results in the twenty-first century.
In the 1960s, physicists rediscovered Poincare's results chaos theory via computer simulations. Since, this latest branch of classical mechanics received considerable attention, not least because of its relevance to weather forecasting. Since the behavior of a weather system depends sensitively on its initial condition, which cannot be measured with absolute accuracy, long-term forecasting is all but impossible.
 Twenty-first century physics
A major problem, certain one that has attracted the most attention, is to formulate a quantum theory of gravity. Physicists have tried to do this by unifying general relativity with quantum mechanics. Unfortunately, these two immensely successful theories as we know them are fundamentally incompatible. A calculation involving both yield nonsensical answers, such as infinities. No one has figured out how to get around this. One of the many proposals is string theory. It essentially "tames" the infinities by spreading out interactions in spacetime. An appealing aspect of string theory is that it also accounts for the other three fundamental forces of nature. It is thus called a "theory of everything" because it says that all four of the known fundamental forces (gravitational, weak nuclear, strong nuclear, and electromagnetic) are a manifestation of one underlying mechanism. The problem with string theory is, however, that while there is no evidence against it, neither is there any strong evidence for it - more or less all evidence which is consistent with string theory is also consistent with numerous other theories. Therefore, it should be strictly speaking called the "string hypothesis" until it gives some predictions that are supported by empirical observations or experiments, but on which other theories on which it is based are either silent or give incorrect results. In the meantime, one has every right to remain skeptical.
- ↑ Richard Feynman stated very simply that the wave-particle duality was the mystery of quantum mechanics and nothing could be said about it other than it had to be taken by faith. Of course, it has been verified experimentally for a gazillion times but we don't have anything by way of explanation. If a reactionary Christian reads this, (s)he will probably salivate: "See, the physicists base something on faith!" But it is a very different kind of faith, especially regarding the numerous experiments.
- ↑ Of course, a vacuum is an obvious solution; spacetime is perfectly flat.
- ↑ Explosition du système du monde, as quoted in Gribbin J. Science, A History, 1543-2001. London: Allen Lane; 2002.
- ↑ http://www.merriam-webster.com/dictionary/physics
- ↑ Giancoli D. Physics: Principles with Applications. 6th ed. Upper Saddle River, NJ: Pearson Education; 2005.
- ↑ Kakalios J. The Physics of Superheroes. New York: Gotham Books; 2005.
- ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 5.14 5.15 5.16 Gribbin J. Science, A History, 1543-2001. London: Allen Lane; 2002.
- ↑ Archimedes, Heath T.L (trans. and ed.). The Works of Archimedes, Edited in Modern Notation with Introductory Chapters. Mineola, NY: Dover Publications, Inc.; 1897.
- ↑ Geymonat M. The Great Archimedes. Waco, Tex.: Baylor University Press; 2010.
- ↑ Hand L, Finch J. Analytical Mechanics. Cambridge: Cambridge University Press; 1998. Just look inside a textbook on mechanics and you will see.
- ↑ 9.0 9.1 9.2 9.3 Baigrie B. Electricity and Magnetism: A Historical Perspective. Westport, Conn. [u.a.]: Greenwood Press; 2007.
- ↑ 10.0 10.1 10.2 10.3 Lewis J. Heat and Thermodynamics: A Historical Perspective. Westport, CT: Greenwood Press; 2007
- ↑ Gribbin J. In Search of Schrödinger's Cat: Quantum Physics and Reality. Toronto: Bantam Books; 1984.
- ↑ 12.0 12.1 Schroeder D. An Introduction to Thermal Physics. San Francisco, CA: Addison Wesley; 2000.
- ↑ Planck M, Masius M (trans). The Theory of Heat Radiation. New York: Dover Publications, Inc.; 1959
- ↑ 14.0 14.1 Hand L, Finch J. Analytical Mechanics. Cambridge: Cambridge University Press; 1998.
- ↑ 15.0 15.1 Greene B. The Elegant Universe. New York, NY: W. W. Norton and Company, Inc.; 1999.
- ↑ Gribbin J, Gribbin M. The Universe: A Biography. London: Penguin books; 2006.
- ↑ McEvoy J, Zarate O, Appignanesi R. Introducing Stephen Hawking.
- ↑ Incredible but true!
- ↑ Leonard to Sheldon (according to memory): "You have to invent 11 dimensions to make the math work!".