Sir Isaac Newton (1642-1727)
English physicist and mathematician, considered by many to be the greatest physicist of all time. His most famous contributions to physics are his law of gravitation and laws of motion. He also invented calculus, and made important discoveries in the field of optics (for example, the discovery that white light may be split into the colours of the rainbow by a prism). The SI unit of force is named after him.

André-Marie Ampère (1775-1836)
French physicist most famous for investigating the magnetic fields produced by current-carrying wires. His work extended that of the Danish physicist Hans Oersted, who discovered in 1819 that a compass needle was deflected by a current-carrying wire. He also invented the solenoid. Today, the law that governs the magnetic fields produced by electric currents is called Ampère's Law, and the SI unit of electric current is named in his honour.

Carl Friederich Gauss (1777-1855)
German mathematician who is most famous for his discoveries in pure mathematics. Indeed, he has been dubbed the 'prince of mathematics'. However, he also made a number of important contributions to physics. He invented the magnetometer and with the German physicist Wilhelm Weber measured the intensity of magnetic forces. He also took Coulomb's famous inverse-square law for the electric field of a point charge and generalized it to an arbitrary charged distribution. This more general law is now known as Gauss's Law.

Michael Faraday (1791-1867)
English physicist who was one of the greatest experimentalists in the history of physics. This is remarkable as he had no formal training. Instead he learned about physics and chemistry by working as an assistant to Sir Humphrey Davy.
     Faraday made many important contributions to the study of electricity and magnetism, including the discovery of electromagnetic induction (now known as Faraday's Law), the invention of the electric motor, and the laws of electrolysis. The SI unit of capacitance is named after him.

James Clerk Maxwell (1831-1879)
Scottish mathematician and physicist who, in the 1860s, took the laws of electricity and magnetism that had been discovered over the previous century or so, and united them into one theory called electromagnetism. This theory is neatly summarized in 4 simple equations known as Maxwell's equations. One consequence of this was the demonstration that light is an electromagnetic wave.
     Maxwell also developed the kinetic theory of gases, deriving the distribution of molecular speeds in a gas at a given temperature.

James Prescott Joule (1818-1889)
English physicist who made many meticulous experiments that demonstrated that heat and work are equivalent. Although he was not the first to do this, it was his demonstration that eventually came to be accepted. The SI unit of work is named in his honour.

William Thomson, Lord Kelvin (1824-1907)
British physicist who published many important papers on the conservation and dissipation of energy.
     Kelvin also made contributions to other branches of physics (such as fluid mechanics), and was in charge of laying the first successful transatlantic cable in 1866.
     The SI unit of absolute temperature is named after him.

Ludwig Boltzmann (1844-1906)
Austrian physicist who founded the branch of physics known as statistical mechanics, which involves describing large numbers of atoms using averages. He showed that the entropy of a system was a measure of how disordered it is, and that the amount of disorder in the Universe tends to increase.

Albert Einstein (1879-1955)
In 1905, Einstein published a paper on what he called the Special Theory of Relativity, which correctly describes the motion of particles travelling at speeds close to the speed of light. The theory is based upon the simple postulates that the laws of physics are the same for all inertial (i.e. non-accelerating) observers, and that the speed of light is the same for all inertial observers (regardless of their motion relative to the source of the light). This theory includes the famous formula E = mc2. He subsequently developed the General Theory of Relativity, which is effectively a theory of gravitation.
     Einstein also contributed to the development of quantum theory. In 1905 he published a paper explaining the photoelectric effect, by postulating that light consists of particles (now known as photons). For this work, Einstein received the 1921 Nobel Prize for Physics.

Neils Bohr (1885-1962)
Danish physicist who, in 1913, developed a successful quantum theoretical model of the hydrogen atom. It was an extension of Rutherford's model of the atom, in which the electron orbits the nucleus. In particular, Bohr's model correctly predicted the frequencies of the spectral lines that had been observed by such men as Lyman, Balmer and Paschen. Bohr received the 1922 Nobel Prize for Physics for this work.
     By the late 1920s, Bohr was very much regarded as an elder statesman of quantum theory. Many of the young physicists who made important discoveries in the early days of quantum mechanics studied under him at Copenhagen.

Louis de Broglie (1892-1987)
French physicist who, in 1923, proposed that all particles have wave-like properties (just as Einstein had shown that light has particle-like properties). He came up with this theory while working on his PhD. As it was such a radical idea, the examiners wrote to Einstein to ask his opinion. Einstein realized what a brilliant idea it was, and de Broglie got his PhD.
     De Broglie's theory was instrumental in the development of quantum mechanics a few years later. For his work, de Broglie received the 1929 Nobel Prize for Physics.
     The wave-like nature of electrons was confirmed experimentally in the late 1920s when George Thomson and Clinton Davisson independently discovered electron diffraction. For this work they shared the 1937 Prize.

Werner Heisenberg (1901-1976)
German physicist who, in 1925, created quantum mechanics. One important aspect of Heisenberg's theory was that it only dealt with properties of a system that can in theory be measured (for example, the frequency of the radiation emitted by a hydrogen atom). He said we cannot assign a position in space at a given time to the electron, nor can we follow an electron in its orbit. This means we cannot assume the orbits postulated by Bohr actually exist.
     Mechanical quantities such as position and velocity cannot be represented by ordinary numbers, but instead must be represented by matrices. As a result, Heisenberg's version of quantum mechanics is sometimes called matrix mechanics.
     The following year, the Austrian physicist Wolfgang Pauli showed that Heisenberg's theory correctly predicted the hydrogen spectrum.
     In 1927 Heisenberg published his famous Uncertainty Principle, which states one cannot measure the position and momentum of a particle with arbitrary precision.
     Heisenberg received the 1932 Nobel Prize for Physics for his work on quantum mechanics.

Erwin Schrödinger (1887-1961)
Austrian physicist who, in 1926, created a version of quantum mechanics that involved waves, rather than the somewhat abstract matrices of Heisenberg's theory. Schrödinger's theory also correctly predicted the hydrogen spectrum. In the same year, Schrödinger showed that his theory (sometimes called wave mechanics) is equivalent to Heisenberg's matrix mechanics.
     Schrödinger shared the 1933 Nobel Prize for Physics with Paul Dirac for his work on quantum mechanics.

Paul Dirac (1902-1984)
English physicist who created a version of quantum mechanics very similar to Heisenberg's at about the same time. He also made early contributions to quantum electrodynamics (the study of the interaction of charged particles with electromagnetic fields). However, he is probably best known for his equation for the electron that encompassed both quantum mechanics and special relativity, which led to the discovery of antimatter.
     Dirac shared the 1933 Nobel Prize for Physics with Erwin Schrödinger for this work.

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