The additional time it takes for light or radio signals to travel past a massive object due to the curvature of spacetime, as predicted by general relativity. The delay increases when the signal path passes closer to the massive object.
Named after physicist Irwin Shapiro who proposed the test in 1964, combined with 'delay' (Old French 'delaier'). Shapiro suggested using radar signals bounced off planets to test Einstein's theory, with the first successful measurement made in 1968 using Venus.
When planets pass behind the Sun from Earth's perspective, radio signals take an extra 200 microseconds to reach us—not much, but enough to prove that massive objects literally slow down time itself! This delay has become so precisely measurable that it's now used to map the mass distribution of objects in our solar system.
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