The great cosmic discovery - IV

Prof H Nandakumar Sarma *

The discovery of microwave background radiation, neutron star, pulsar; trying to understand the cosmic forces, the discovery of anisotropy of cosmic microwave background radiation, the discovery of Higgs boson ( God’s particle ), the detection of gravitational wave and black hole etc are some of the exciting moments in the understanding of the universe.

In this series, we trace some of the great cosmic discovery made from the beginning of the 20th century to the present day.

Some of the profound problems of nature which puzzled human beings for many generations are the motion and distribution of starts, internal constitution of star, the life and death of star, the sources of stellar energy etc. It was Arthur Stanley Eddington, son of a Somerset Quaker, who gave light to some of the above problems of nature. Eddington entered Trinity College, Cambridge in autumn of 1902.

Before entering Cambridge, Eddington was fortunate to have three exceptionally gifted teachers who imparted him keen interest in natural history, a love of good literature and a splendid foundation in mathematics. At Cambridge, the distinguished teacher R. A. Herman, E.T. Whittaker and A. N. Whitehead stressed Eddington both logic and the elegance of mathematical reasoning.

After winning the coveted position of first Wrangler in 1904, he gained his degree in 1905. In 1907, he was elected to the fellowship in Trinity College and in the same year he was appointed as the chief assistant at the Royal Greenwich Observatory, where he remained until 1913. There he obtained a thorough training in practical astronomy and theoretical investigation that placed him in the forefront of astronomical research.

In 1913 he returned to Cambridge as the Plumian Professor and moved to the Observatory house where he remained until his death in 1944. During the first world war he completed single handed the transit observation for the Zodiacal Catalogue. Eddington’s great pioneer work began in 1916 , when he attacked the equation of equilibrium at the very centre of a star taking into account three forces - gravitation, gas pressure and radiation pressure. The matter under stellar conditions are highly ionized and Eddington incorporated this in his theory of stellar equilibrium.

Eddington found that the force of radiation rose with the mass of the star and with increasing rapidity as the mass exceeded that of our sun. He concluded that relatively few stars would exceed ten times the mass of our sun. He also calculated the diameters of several red giant stars and dwarf stars. The great densities in white dwarf stars which he predicted were confirmed by W.S. Adam of Mount Wilson Observatory. Eddington also analysed the systematic motion of the stars and hypothesized two stars streams.

His contribution came to an end with the publication in 1914 of his first book “Stellar Movements and the Structure of the Universe”. Eddington’s investigations may be said to have founded the subject of stellar dynamics. Eddington’s most significant contribution to the physical sciences - his founding the modern theoretical astrophysics, creating the structure, the constitution and evolution of stars were culminated with the publication of ‘Internal Constitution of Stars’ in 1926.

From the mathematical calculations he observed that there is a limit to the luminosity of a star that can support a given mass. The limit is known as Eddington limit and plays an important roles in these days in the investigations to X-ray sources, luminosity of accretion discs around black hole

What is the source of energy of our sun and other stars? It was a great puzzle in 1920s. Among Eddington’s predictions, that of the source of stellar energy is perhaps the most spectacular. He suggested that the burning of hydrogen into helium as the most likely source of stellar energy.

On August 24, 1920 at the British Association meeting in Cardiff, Eddington said ‘’. ... A star is drawing some vast reservoir of energy by means unknown to us. This reservoir can scarcely be other than atomic energy which, it is known, exists abundantly is all matter; we sometimes dream that man will one day learn how to release it and use for his service. The store is well-high inexhaustible, if only it could be trapped. There is sufficient in the sun to maintain its output of heat for 15 billion years .... If, indeed, the subatomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfilment our dream of controlling their latest power for the well being of the human race or for its suicide ….”

To his critics he said “It has for example, been objected that the temperature of the stars is not great enough for the transmutation of hydrogen into helium - so ruling out one possible source of energy. But helium exists, and it is not much use for the critics to urge that the stars are not hot enough for its formation unless he is prepared to show us a hotter place.”

In 1938, came the famous proton-proton cycle and carbon cycle of Hans Bethe elegantly solved some of the problems of stellar energy, confirming some of the predictions made by Eddington who was the most distinguished theoretical astrophysicist of his time was also a great expositor and exponent of the Einstein’s general theory of relativity.

After founding the special theory of relativity in 1905, Einstein worked for ten years to bring Newtonian theory of Gravitation in conformity with the principles of relativity (in particular no signal be propagated with a velocity exceeding that of light).

Einstein communicated his papers to the Berlin Academy of Sciences in 1915. During the war years de Sitter sent copies of Einstein’s paper to Eddington who was the Secretary of the Royal Astronomical Society at that time. The papers were read at the December 1917 meeting of Royal Astronomical Society.

Einstein’s prediction could be verified by measuring the apparent displacement of stars due to the bending of light rays in passing near the sun. There were three possibilities. There might be no deflection at all; that is, light might not be subject to gravitation.

There might be a half deflection, signifying that light was subject to gravitation, as Newton had suggested Or, there might be a full deflection, confirming Einstein’s instead of Newton’s. Experiments during total solar eclipse could provide verification of Einstein’s prediction! Eddington wrote
‘: In a superstitious age a natural philosopher wishing to perform an important experiment would consult an astrologer to ascertain an auspicious moment for the trial with better reasons, an astronomer today consulting the stars would announce that the most favourable day of the year for weighing light is May 29 But by strange good fortune an eclipse did happen on May 29, 1919 “

Eddington prompted the astronomer Royal Sir Frank Dyson to provide the support for the total solar eclipse. On this, two expeditions were organised A.C.D. Crommelin and C. Davidson went to Sobral in Brazil; E.T. Cottingham and Eddington wentto Isle of Principe in West Africa. The Scientific results of the expedition were reported to a joint meeting of Royal Society and Royal Astronomical Society on November 6, 1919, with Sir J.J. Thomson, the President of the Royal Society, in the chair.

J J. Thomson remarked from the chair
‘: If his theory is right, it makes us an entirely new view of gravitation .... it has survived to very severe tests in connection with the perihelion of mercury and the present eclipse - there it is the result of one of the highest achievements of human thought. The weakest point in the theory is the great difficulty in expressing it “

The view at that time and for a long time was that the general theory of relativity was too difficult to understand. A might soon arose that only three persons in the world understood general relativity. After the dinner conversation at Trinity College, Cambridge, at which Subrahmanyan Chandrasekhar was also present, Ludwing Silberstein came to Eddington and said “Professor Eddington, you must be one of the three persons in the world who understands general relativity”.

On Eddington’s demurring to this statement Silberstein responded ‘Don’t be modest; Eddington’ and Eddington replied that ‘ On the contrary, I am trying to think who the third person is’. Subrahmanyan Chandrasekhar was the third person.

The result from the Africa expedition provided the first confirmation of Einstein’s theory that gravity will bend the path of light when it passes near a massive star. Eddington lectured on relativity at Cambridge, giving a beautiful mathematical treatment of the topic. He used these lectures as a basic for his book ‘Mathematical Theory of Relativity’ which was published in 1923. Einstein said that this work was “ ….. the finest presentation of the subject in any language”

Eddington was fascinated with very large number and fundamental constant of nature – the gravitational constant, velocity of light, the mass and charge of electron, the Planck’s constant etc. He calculated mathematically the total number of protons and electrons in the universe and came out about ten followed by seventy nine zeros!

The most distinguished astrophysicists of his time died on November 22, 1944.