Empirical Earth · Gravity, Buoyancy & the Air

Gravity — what it is and how we measured it

The claim

“Gravity is unproven; things fall by density, or the disc just accelerates up at 9.8 m/s².”

What is measured

Everything falls toward Earth’s center at about 9.8 m/s² everywhere on the surface, toward a center that only exists for a sphere.

What would show this is wrong

a downward acceleration that changed with the direction you faced, or objects falling toward somewhere other than Earth’s center.

Sources

  1. H. Cavendish (1798), "Experiments to determine the Density of the Earth." Philosophical Transactions of the Royal Society — torsion-balance measurement of G. CODATA value G = 6.674×10⁻¹¹. Ph link
  2. The Schiehallion experiment (1774). N. Maskelyne, Royal Society — deflection of a plumb line by a mountain, used to estimate Earth's mean density. Schiehallion experiment link
  3. Tests of General Relativity. Eddington (1919, light bending); Pound & Rebka (1959, redshift, Phys. Rev. Lett.); Touboul et al. / MICROSCOPE (2017, equivalence principle, PRL); Abbott et al. link
  4. N. Ashby, "Relativity in the Global Positioning System." Living Reviews in Relativity 6, 1 (2003) — the ≈38 µs/day clock correction. Living Reviews (Ashby) link
  5. Gravity gradiometry and passive navigation. Submarines fix their position by matching the local gravity field, measured in milligals, against a pre-surveyed gravity map, a passive method tha link
  6. GRACE weighs Earth’s shifting mass from orbit. The Gravity Recovery and Climate Experiment (NASA and the German space agencies), GRACE 2002–2017 and GRACE-FO from 2018, flies two satellites link
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