thumb|200px|[[Arthur Stanley Eddington (1882–1944)]]
In astrophysics, the Eddington number, , is the number of protons in the observable universe. 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 and the foundations of physics.
History
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. 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, 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<sub>Edd</sub> point to a value of about . 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.
See also
- Eddington–Dirac number
- The Sand Reckoner
- Universe