Between Newton and Einstein, James Clerk Maxwell Stands Alone
In his Principia Mathematica (1687), Sir Isaac Newton gave the foundations of classical mechanics and gravity. Newton showed that his laws explained Kepler’s Laws of planetary motion where the planet orbits are elliptical rather than circular.
In “A Dynamical Theory of the Electromagnetic Field” (1865), Maxwell produced the first complete set of equations governing electricity and magnetism, expressed in terms of fields. In his electromagnetic wave theory he calculated the propagation speed in air using only the available data on its electric and magnetic properties. Comparing the velocity of the waves with the velocity of light he came to this remarkable conclusion: This velocity is so nearly that of light that it seems we have strong reason to conclude that light itself (including radiant heat and other radiations if any) is an electromagnetic disturbance in the form of waves. Maxwell’s electromagnetic theory, and the resulting equations, were consequently the greatest advance in scientific knowledge since Newton’s Principia.
According the Newton, space and time are completely separate so all observers agree about time and distance irrespective of where they are or how they are moving. Thus an observer moving towards a source of light will see the speed of light increase. Albert Einstein fundamentally disagreed with this on the grounds that the measured speed of light in free space is always the same. His revolutionary new theory on Special Relativity, proposed that the scales of distance and time should vary with the velocity of the observer. He used Maxwell’s equations as justification by showing that Special Relativity made the speed of electromagnetic waves including light invariable, as observed. Einstein later published his theory of General Relativity (1915) which was formulated as a field theory. Maxwell had originated the idea of representing physical phenomena with a field theory governed by mathematical equations, and in tribute to this Einstein later said: This change in the conception of reality is the most profound and the most fruitful that physics has experienced since the time of Newton.
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Communications
In the early nineteenth century, despite many individual advances in knowledge, there was no inkling of a comprehensive theory of electricity and magnetism. In developing this, Maxwell pointed the way to the existence of the spectrum of electromagnetic radiation. Defining fields as a tension in the medium, he stated his belief in a new concept  that energies resides in fields as well as bodies. This pointed the way to the application of electromagnetic radiation for such presentday uses as radio, television, radar, microwaves and thermal imaging.
Thermodynamics
Maxwell made fundamental contributions to the development of thermodynamics. He was also a founder of the kinetic theory of gases. This theory provided the new subject of statistical physics, linking thermodynamics and mechanics, and is still widely used as a model for rarefied gases and plasmas.
