Robert McNees
When Einstein proposed general relativity he laid out three tests: the precession of Mercury's perihelion, deflection of light by the sun, and gravitational redshift.
Irwin Shapiro proposed a fourth test #OTD in 1964: the gravitational time delay of light.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13.789
Robert McNees
It's a simple idea: The round-trip time of a radio signal bounced off a distant object increases a little if it passes through the gravitational field of a massive object along the way.
Image: Simon Tyran, https://en.wikipedia.org/wiki/File:Shapiro_delay.gif
Robert McNees
Shapiro estimated that signals bouncing off Venus or Mercury at superior conjunction, when they are on the opposite side of the sun from Earth, would experience a delay of about 200 microseconds due to the Sun's gravitational field.
This was a measurable effect at the time!
Robert McNees
Shapiro et al tested the idea in 1966-67, first with Venus and then with Mercury, at MIT's Lincoln Laboratory Haystack radar site.
The results agreed with the predictions of general relativity to well within the expected uncertainties. #TeamRadar
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.20.1265
Robert McNees
The result has since been refined by measuring the effect with distant space probes.
In 2003, Bertotti, Iess, and Tortoran used a signal sent to and from the Cassini probe to confirm the predictions of general relativity with an accuracy of 20 parts per million.
bergmayer
@mcnees Is there a non-technical explanation out there about Mercury's perihelion? Planetary orbits seem like just the kind of thing that should be explainable by Newton alone
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