James Clerk Maxwell (1831-79) discovered the equations (James Clerk Maxwell equations date published (1865) that describe the behaviour of electromagnetic radiation. Ernst Mach (1838-1916) made studies of mechanics and thermodynamics that led to a reassessment of Newtonian concepts. George Francis Fitzgerald (1851-1901) and Hendrik Antoon Lorentz (1853-1928) described how the properties of a moving body (such as its length) are affected by its motion. Gauss and his pupil Georg Friedrich Bernhard Riemann (1826-1866) and Nikolai Ivanovich Lobachevsky (1792-1856) pioneered non-Euclidean geometry.
In 1887 Albert Abraham Michelson (1852-1931) carried out an experiment with Edward Williams Morley (1838-1923) to establish the presence or absence of an ether, the medium through which light was supposed to travel. The experiment attempted to detect the motion of Earth relative to the ether by measuring the speed of light in two directions at right angles to each other. No such motion was found.
The Michelson-Morley experiment is sometimes regarded as having provided the impetus for Albert Einstein (1879-1955) to develop his Special Theory of Relativity (1905), which says that the speed of light in a vacuum is constant throughout the Universe and is independent of the motion of the observer and of the source of light. Special relativity deals with the dynamic relationships between objects moving at constant speeds in straight lines. It does not deal with acceleration or gravity, the word ‘special’ is used here to mean ‘restricted’.
According to special relativity no object can reach the speed of light because its mass would become infinite, its length would become zero and time would stand still. Special relativity also says that mass and energy can be interchanged in line with the equation E=mc2, where ‘c’ is the speed of light.
In 1908 Hermann Minkowski (1864-1909) provided an important insight into the significance of special relativity when he implied that space and time were not separate entities but components of a four-dimensional (space-time) non-Euclidean geometry. His space-time model was the foundation for all subsequent developments and allowed Einstein to move on to his General Theory of Relativity (1915) which predicts that gravitation changes the geometry of space and time, causing it to become curved.
Einstein believed that the universe was static, but his general relativity theory was predicting either an expanding or a contracting universe. To make his equations consistent with a static universe he added a term called the Cosmological Constant to counteract the dynamic effects of gravity which in a universe of matter would cause the static universe to collapse.
Willem de Sitter (1872-1934) derived a model in which there is no matter or radiation but expansion is driven by the cosmological constant. Although this model is physically unrealistic, it introduced the idea that the real Universe might be expanding. It can be said that whereas the Einstein universe had ‘matter without motion’, the de Sitter universe had ‘motion without matter’. If particles of matter are introduced into the de-Sitter model, they appear to recede from each other exponentially fast as space-time expands. This seemed to be unrealistic. In the expanding Universe as we see it now, distances increase proportionally. In 1922 Alexander Alexandrovich Friedmann (1888-1925) published solutions to Einstein’s equations that showed the Universe need not be static. Friedmann had found a family of solutions, a set of cosmological mathematical models. Assuming space-time to be curved and picturing the surface of a soap bubble as an analogy, in some variations the curvature expands forever and in others it expands to a certain size until it collapses back on itself as gravity overcomes the expansion. There are more complicated models but in all of them there is a period during the expansion of the Universe when its recessional velocity is proportional to distance (Hubble’s Law).
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