Eddington number
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In astrophysics, the Eddington number, Template:Math, is the number of protons in the observable universe. Eddington originally calculated it as about Template:Val; current estimates make it approximately Template:Val.<ref name="Munafo"></ref>
The term is named for British astrophysicist Arthur Eddington, who in 1940 was the first to propose a value of Template:Math and to explain why this number might be important for physical cosmology and the foundations of physics.
HistoryEdit
Eddington argued that the value of the fine-structure constant, 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.<ref>Template:Cite book</ref> This led him in 1929 to conjecture that α was exactly 1/136.<ref>Template:Cite journal</ref> He devised a "proof" that Template:Nowrap, or about Template:Val. 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
During a course of lectures that he delivered in 1938 as Tarner Lecturer at Trinity College, Cambridge, 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.<ref>Template:Harvnb</ref>
Current estimates of NEdd point to a value of about Template:Val.<ref name="Munafo">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> These estimates assume that all matter can be taken to be hydrogen and require assumed values for the number and size of galaxies and stars in the universe.<ref>Template:Cite journal</ref>
Recent theoryEdit
The modern CODATA recommended value of α−1 is Template:Physconst
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 NEdd.Template:Cn
On possible roles for NEdd in contemporary cosmology, especially its connection with large number coincidences, see Template:Harvtxt (easier) and Template:Harvtxt (harder).