2 Tests of general relativity
Einstein became a public celebrity when Arthur Eddington and colleagues measured the deflection of light by the Sun during the solar eclipse of 1919 and found that their results agreed with the predictions of general relativity. Measurements of the deflection (top) - plotted as (1 + γ)/2, where γ is related to the amount of spatial curvature generated by mass - have become more accurate since 1919 and have converged on the prediction of general relativity: (1 + γ)/2 = 1. The same is true for measurements of the Shapiro time delay (bottom). "Optical" denotes measurements made during solar eclipses (shown in red), with the arrows pointing to values well off the chart; "radio" denotes interferometric measurements of radio-wave deflection (blue); while Hipparcos was an optical-astrometry satellite. The left-most data point is the measurement made by Eddington in 1919, while the arrow just above it refers to the value obtained by his compatriot Andrew Crommelin. The best deflection measurements (green) are accurate to 2 parts in 104 and were obtained with Very Long Baseline Radio Interferometry (VLBI; see Shapiro et al. in further reading). A recent measurement of the Shapiro time delay by the Cassini spacecraft, which was on its way to Saturn, was accurate to 1 part in 105 (see Bertotti et al. in further reading).