Understanding gravity by a physicist’s scientific way rather than HRD minister’s non-scientific way

This post has its origin in the recent statement by the Indian MHRD minister (this ministry controls and provides funds to most higher education institutions in India) that gravity was mentioned much before Newton in ancient Indian astronomical texts.

Now while politicians have their ways of making statements that walk the thin line between truth and misinformation (and irking us short-trigger scientists), this is perhaps an opportunity to bring out some important points about how science has progressed, that may be appreciated by everyone.

There are many distinct steps in the understanding of gravity. (I may be missing many) :

1. Noticing that all bodies fall to earth
2. Making measurements and calculations of motions of falling bodies
3. Making measurements and calculations of motions of astronomical bodies
4. Getting the idea that earth pulls the falling bodies to itself
5. Getting the idea that there is some force that makes the astronomical bodies move.
6. Getting the idea that earth may be going around the Sun.
7. Realizing that the heliocentric system gives simple rules for planetary motion (Kepler’s laws).

The first three are observations, the next three theoretical conjectures, while the last one involves some deeper insight. Of course all the above was known before Newton. It is quite likely that most of the 1-6 were known to (and done by) ancient Indian astronomers (though I am not an expert in history). I am not sure about 7.

But Newton’s work involves insights far beyond these.
8. The idea that the same law that governs bodies falling to the ground on earth governs planets going around the sun. 
9. Showing by astronomical calculations that this law is inverse square law (1/r^2).
10. The realization that the gravitational force of sun and planets acts as if the mass of each body is at its center.

When one says “Newton discovered gravity”, one is normally referring to 8 and 9 above. (Point 10 is rather underappreciated even in the scientific community, however, it induced Newton to write his famous treatise “Principia Mathematica …”. See the commentary by S. Chandrasekhar. )

Saying someone “mentioned gravity” earlier is trivializing the meaning of the phrase “Newton discovered gravity”, and indeed, of what the discovery means.

(I have restricted myself to pre-Einstein understanding, so no General Theory of Relativity here, which of course improved our understanding of gravity by one more leap.)

The article is taken from Amol Dighe’s facebook wall with his full agreement to publish in Afflatus. Amol Dighe is a Professor of Physics at Tata Institute of Fundamental Research (TIFR), Mumbai. 


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