"and I do not feign hypotheses” Newton

On May 9, 2012, at 12:33 PM, REK wrote:

To clarify, my question was not 'why do particles follow geodesics' but 'how can 'spacetime' 'cause' particles to follow geodesics’.

JS: I still don’t understand the question. Why “cause”. Geodesic equation comes from the ACTION principle just like all the laws of physics do. Action for a free test particle is INVARIANT integral mods  (generalized Newton’s 1st law).

REK: My point is that I don't think that 'spacetime' causes particles to follow geodesics; of course forces are involved, and I don't see why we need to tie those forces to a substantive 'spacetime’.  

JS: “Cause” here is inappropriate. No forces involved in geodesic. By definition geodesic motion is (real) force-free motion. I agree with Ruth, if I understand her words? -  that no forces needed for gravity. Newton’s “gravity force” is replaced by “substantive space time” i.e. the LOCAL GEOMETRODYNAMICAL FIELD e^I (tetrads)& S^I^J (spin connection) - as explained by Rovelli Ch 2 of “Quantum Gravity” on ontic par with EM field F = dA, weak field F^a, a = 1,2,3 and strong field F^b, b = 1,2,3,4,5,6,7,8

all obey generalized

DF = 0  (Faraday, no magnetic monopole)

D*F = *J (Ampere, Gauss)

D*J = 0 LOCAL CONSERVATION OF BOSON FIELD SOURCE CURRENT DENSITIES

D = d + (LOCAL GAUGE CONNECTION)/\

Hypotheses non fingo
From Wikipedia, the free encyclopedia
Hypotheses non fingo (Latin for "I feign no hypotheses", or "I contrive no hypotheses") is a famous phrase used by Isaac Newton in an essay General Scholium which was appended to the second (1713) edition of the Principia.
Here is a recent translation (published 1999) of the passage containing this famous remark:
I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy. In this philosophy particular propositions are inferred from the phenomena, and afterwards rendered general by induction. [1]
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Forces cause off-geodesic motion. (Newton’s 2nd law)


That's why I mentioned the Harvey Brown book--he argues that 'spacetime' is superfluous to the question.

Best
R