# Twice Newton

by SimonB
Tags: newton
 P: 8 Hi I am reading a popular physics book. It discusses the test of Einsteins theory by Eddington at the eclipse. "The deviation of the light was double that predicted by Newton's physics" Why does classical physics predict any deviation of light by gravity, did light have a mass in classical physics and if so how was it estimated (to allow for the deviation to be predicted)??? Many thanks in advance Simon
 Emeritus Sci Advisor PF Gold P: 8,989 He meant Newton's law of gravity combined with the idea that light consists of particles with energy equal to Plancks constant times the frequency, and that you can define a "mass" of such a particle by setting $h\nu=mc^2$. If you use Newton's law of gravity with this mass, you get the wrong result by a factor of two.
PF Gold
P: 1,128
 Quote by Fredrik He meant Newton's law of gravity combined with the idea that light consists of particles with energy equal to Plancks constant times the frequency, and that you can define a "mass" of such a particle by setting $h\nu=mc^2$. If you use Newton's law of gravity with this mass, you get the wrong result by a factor of two.
You don't have to assign any mass to light to predict its Newtonian acceleration; you just assume that light in a beam accelerates in the same way as anything else.

Einstein's theory introduces an extra factor of (1+v2/c2) into the coordinate acceleration in this case, which means that the deflection is doubled for light and similarly increased for anything else moving at relativistic speeds.

P: 8

## Twice Newton

Thank you, very clear (now!)
 P: 5,634 Another way to think about curvature of light: half is classical (per Newtons laws) and the other half is relativistic due to the curvature of space and time itself. Had Eddington's experiment been conducted a few years earlier, Einstein's career might have suffered a major blow since he originally predicted the classical degree of curvature and only later when working general relativity discovered an additional amount. Dr Kaku just mentioned this on a 2 hour TV show now airing "EINSTEIN"....HISTORY CHANNEL I believe.
Emeritus