Talk:Nordtvedt effect

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"Thus far, the results have failed to find any evidence of the Nordtvedt effect, demonstrating that if it exists, the effect is exceedingly weak."

Can we quantify that? How weak must it be (or if you prefer, how strong can it be) before conflicting with experiment? RJFJR 19:06, 21 April 2007 (UTC)
It can be quantified, but it's a bit messy. Directly, the difference in acceleration between the Earth and the Moon in the Sun's gravity is no more than about 1 part in 10^13. But the whole idea is that the difference is due to the gravitational self-energy, which is a tiny fraction of the Earth's total mass - about 1 part in 10^9. So when you divide out to get the "Nordtvedt parameter" (η) you get a limit - gravitational self-energy falls differently from other mass by at most (about) one part in 10^3. There have been some tests with pulsars (one is in progress right now), where the self-energy is 10-15% of their mass, but things get more complicated when you're in this strong-field regime. Exact numbers and discussion are available in section 4.3.1 of this review. Anne (talk) 00:36, 5 January 2018 (UTC)