Sir George Gabriel Stokes, 1st Baronet ( STOHKS; 13 August 1819 – 1 February 1903), was a British mathematician and physicist, born in County Sligo, Ireland, in the United Kingdom of Great Britain and Ireland. Stokes spent his entire career at the University of Cambridge, where he served as Lucasian Professor of Mathematics for 54 years—from 1849 until his death in 1903—the longest tenure held by any Lucasian Professor.
As a physicist, Stokes made seminal contributions to fluid mechanics, including the Navier–Stokes equations; and to optics, with notable works on polarisation and fluorescence. As a mathematician, he popularised Stokes' theorem in vector calculus and contributed to the theory of asymptotic expansions. Stokes, along with Felix Hoppe-Seyler, first demonstrated the oxygen transport function of haemoglobin, and showed colour changes produced by the aeration of haemoglobin solutions.
Stokes represented Cambridge University in the House of Commons from 1887 to 1892, sitting as a Conservative. He also served as President of the Royal Society from 1885 to 1890, and was briefly Master of Pembroke College, Cambridge. Stokes's extensive correspondence and his work as secretary of the Royal Society has led him to be referred to as a gatekeeper of Victorian science, with his contributions surpassing his own published papers.
George Gabriel Stokes was born on 13 August 1819 in Skreen, Ireland, the youngest son of the Reverend Gabriel Stokes, a clergyman in the Church of Ireland who served as Rector of Skreen, and Elizabeth Haughton, daughter of the Reverend John Haughton. His home life was strongly influenced by his father's evangelical Protestantism; three of his brothers entered the Church, of whom the most eminent was John Stokes, who became Archdeacon of Armagh. Alongside a lifelong commitment to his Protestant faith, his childhood in Skreen had a strong influence on his later decision to pursue fluid dynamics as a research area. His daughter, Isabella Humphreys, wrote that her father "told me that he was nearly carried away by one of these great waves when bathing as a boy off the coast of Sligo, and this first attracted his attention to waves."
John and George were always close, and George lived with John while attending school in Dublin. Of all his family, he was closest to his sister, Elizabeth. Their mother was remembered in the family as "beautiful but very stern." In 1837, after attending schools in Skreen, Dublin, and Bristol, Stokes matriculated at Pembroke College, Cambridge. In 1841, he graduated as Senior Wrangler and Smith's Prizeman, achievements that earned him election as a fellow of the college that year.
In accordance with the college statutes, Stokes had to resign from his fellowship when he married in 1857. Twelve years later, under new statutes, he was re-elected to the fellowship and retained that place until 1902, when he was elected Master of Pembroke College. He died the following year on 1 February at the age of 83, and was buried at Mill Road Cemetery in Cambridge. There is also a memorial to him in the north aisle at Westminster Abbey.
In 1849, Stokes was appointed Lucasian Professor of Mathematics at the University of Cambridge, a position he held until his death in 1903. On 1 June 1899, the golden jubilee of this appointment was celebrated there in a ceremony attended by numerous delegates from European and American universities. A commemorative gold medal was presented to Stokes by the chancellor of the university, and marble busts of Stokes by Hamo Thornycroft were formally offered to Pembroke College and to the university by Lord Kelvin. At 54 years, Stokes' tenure as Lucasian Professor was the longest in history.
Stokes, who was made a baronet in 1889, further served his university by representing it in parliament from 1887 to 1892 as one of the two members for the Cambridge University constituency. From 1885 to 1890, he was also president of the Royal Society, of which he had been one of the secretaries since 1854. As he was also Lucasian Professor at this time, he was the first person to hold all three positions simultaneously; Isaac Newton held the same three, although not at the same time.
Stokes was the oldest of the trio of natural philosophers, James Clerk Maxwell and Lord Kelvin being the other two, who especially contributed to the fame of the Cambridge school of mathematical physics in the middle of the 19th century.
Stokes' original work began about 1840, and is distinguished for its quantity and quality. The Royal Society's catalogue of scientific papers gives the titles of over a hundred memoirs by him published down to 1883. Some of these are only brief notes, others are short controversial or corrective statements, but many are long and elaborate treatises.
In scope, Stokes' work covered a wide range of physical inquiry but, as Marie Alfred Cornu remarked in his Rede Lecture of 1899, the greater part of it was concerned with waves and the transformations imposed on them during their passage through various media.
Stokes's first published papers, which appeared in 1842 and 1843, were on the steady motion of incompressible fluids and some cases of fluid motion. These were followed in 1845 by one on the friction of fluids in motion and the equilibrium and motion of elastic solids, and in 1850 by another on the effects of the internal friction of fluids on the motion of pendulums. To the theory of sound he made several contributions, including a discussion of the effect of wind on the intensity of sound and an explanation of how the intensity is influenced by the nature of the gas in which the sound is produced. These inquiries together put the science of fluid dynamics on a new footing, and provided a key not only to the explanation of many natural phenomena, such as the suspension of clouds in the air, and the subsidence of ripples and waves in water, but also to the solution of practical problems, such as the flow of water in rivers and channels, and the skin resistance of ships.
Stokes' work on fluid motion and viscosity led to his calculating the terminal velocity for a sphere falling in a viscous medium. This became known as Stokes' law. He derived an expression for the frictional force (also called drag force) exerted on spherical objects with very small Reynolds numbers.
Stokes' work is the basis of the falling sphere viscometer, in which the fluid is stationary in a vertical glass tube. A sphere of known size and density is allowed to descend through the liquid. If correctly selected, it reaches terminal velocity, which can be measured by the time it takes to pass two marks on the tube. Electronic sensing can be used for opaque fluids. Knowing the terminal velocity, the size and density of the sphere, and the density of the liquid, Stokes' law can be used to calculate the viscosity of the fluid. A series of steel ball bearings of different diameters is normally used in the classic experiment to improve the accuracy of the calculation. The school experiment uses glycerine as the fluid, and the technique is used industrially to check the viscosity of fluids used in processes.
The same theory explains why small water droplets (or ice crystals) can remain suspended in air (as clouds) until they grow to a critical size and start falling as rain (or snow and hail). Similar use of the equation can be made in the settlement of fine particles in water or other fluids.
The stokes, the CGS unit of kinematic viscosity, was named in recognition of his work.
Perhaps Stokes' best-known researches are those which deal with the wave theory of light. His optical work began at an early period in his scientific career. His first papers on the aberration of light appeared in 1845 and 1846, and were followed in 1848 by one on the theory of certain bands seen in the spectrum.
In 1849, Stokes published a long paper on the dynamical theory of diffraction, in which he showed that the plane of polarisation must be perpendicular to the direction of propagation. Two years later, he discussed the colours of thick plates.