## Happy 95th Anniversary, Relativity 120 120

StartsWithABang (3485481) writes

*"It's hard to believe, but there are people alive today who remember a world where Newtonian gravity was the accepted theory of gravitation governing our Universe. 95 years ago today, the 1919 solar eclipse provided the data that would provide the test of the three key options for how light would respond to the presence of a gravitational field: would it not bend at all? Would it bend according to Newton's predictions if you took the "mass" of a photon to be E/c^2? Or would it bend according to the predictions of Einstein's wacky new idea? Celebrate the 95th anniversary of relativity's confirmation by reliving the story."*
## Re:Does mass matter? (Score:5, Informative)

primaryinsight is the gravitational field, ie: two bodies attract each other with a force proportional to the combined masses and the distance between them. That he invented calculus to prove it and wrote it all down in his "Principia" is why he is remembered. A photon is neither a hammer , nor an atom. Photons did not have mass so they were believed to be unaffected by gravity. Einstein came along and said mass and energy are two forms of the same thing and a photon would be affected by gravity. The experiment in TFA allowed the universe to make the final call.Trivia: Newton's Principa contains only two explicit assumptions, one of them was the assumption that "time is constant".

## Re:95 years but (Score:5, Informative)

## Re:Does mass matter? (Score:5, Informative)

Newton's primary insight is the gravitational field, ie: two bodies attract each other with a force proportional to the combined masses and the distance between them.

It's worth noting that this insight was not at all unique to Newton. There was, in fact, a major dispute [wikipedia.org] in the scientific community about who came up with this idea at the time, since Robert Hooke had already published on this notion. Other scientists had basically also postulated similar ideas in the decades before the

Principia.That he invented calculus to prove it and wrote it all down in his "Principia" is why he is remembered.

Yes -- Newton may have been the first to explicitly identify the specific inverse square relationship (rather than a general form relationship mentioned in the first quotation above), and he had the mathematical apparatus to prove how it all worked.

But it's also important to be clear that the idea of a "gravitational field" or an "unseen force acting at a distance" was a very spooky and strange notion to contemporary scientists in Newton's era. In fact, such ideas were commonly associated with occult ideas; they didn't fit in with the conception of a simple mechanistic universe. Thus, Newton's idea of some strange unseen "force" acting across vast distances would seem like invoking the power of God or angels or some mystical astrological "force" today.

Because of that, many scientists were initially very suspicious of Newton's methodology. Newton therefore wrote a clarification [wikipedia.org] as an appendix to the second edition of the

Principiaexplicitly saying he was NOT assuming the existence of unseen forces and fields. Instead, he claimed his model was valuable simply because the mathematics were an accurate model. (Some historians have argued that this was in fact the most important element of Newton's revolution in thought: he argued for the acceptance of a mathematical model as a scientific explanation, even if we can't explain the underlying causes of that model.) Of course, Newton was a pretty weird guy and believed in all sorts of things that modern science would think weird, so obviously he thought the unseen forces were real. But it's interesting that he worked so hard to distance himself from such ideas at the time -- to be in accord with science of the time, the "force" in his model was thus to be considered a mere mathematical contrivance, rather than how the universe actually worked.