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In astrophysics
Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline, James Keeler, said, astrophysics "seeks to ascertain the ...
, the Eddington number, , is the number of proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
s in the observable universe
The observable universe is a Ball (mathematics), spherical region of the universe consisting of all matter that can be observation, observed from Earth; the electromagnetic radiation from these astronomical object, objects has had time to reach t ...
. Eddington originally calculated it as about ; current estimates make it approximately .
The term is named for British astrophysicist Arthur Eddington, who in 1940 was the first to propose a value of and to explain why this number might be important for physical cosmology
Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fu ...
and the foundations of physics
Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...
.
History
Eddington argued that the value of thefine-structure constant
In physics, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by (the Alpha, Greek letter ''alpha''), is a Dimensionless physical constant, fundamental physical constant that quantifies the strength of the el ...
, ''α'', could be obtained by pure deduction. He related ''α'' to the Eddington number, which was his estimate of the number of protons in the universe. This led him in 1929 to conjecture that ''α'' was exactly 1/136. He devised a "proof" that , or about . Other physicists did not adopt this conjecture and did not accept his argument. It even led to a major journal publishing a joke article making fun of the idea.
During a course of lectures that he delivered in 1938 as Tarner Lecturer at Trinity College, Cambridge
Trinity College is a Colleges of the University of Cambridge, constituent college of the University of Cambridge. Founded in 1546 by King Henry VIII, Trinity is one of the largest Cambridge colleges, with the largest financial endowment of any ...
, Eddington averred that:
This large number was soon named the "Eddington number".
Shortly thereafter, improved measurements of ''α'' yielded values closer to 1/137, whereupon Eddington changed his "proof" to show that ''α'' had to be exactly 1/137.
Current estimates of ''N''Edd point to a value of about . These estimates assume that all matter can be taken to be hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and require assumed values for the number and size of galaxies and star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
s in the universe.
Recent theory
The modern CODATA recommended value of ''α'' is Consequently, no reliable source maintains any longer that ''α'' is the reciprocal of an integer, nor does anyone take seriously a mathematical relationship between ''α'' and ''N''Edd. On possible roles for ''N''Edd in contemporary cosmology, especially its connection with large number coincidences, see (easier) and (harder).See also
* Eddington–Dirac number * '' The Sand Reckoner'' *Universe
The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...
References
Bibliography
* * * * * * * * * * * {{Large numbers Large integers Proton Astrophysics Physical cosmology