Shapiro time delay

The extra travel time of a signal passing near a massive body, caused by spacetime curvature. It is a measurable prediction of general relativity.

General relativity: Einstein’s theory of gravity, in which matter and energy shape spacetime and free-falling bodies follow its geometry. The curved spacetime of general relativity makes signals take longer paths in time near mass.
Shapiro time-delay experiment: A 1960s radar-ranging test that measured delays in signals passing near the Sun. Irwin Shapiro proposed this test and used planetary radar observations to detect the predicted delay.
Irwin Shapiro: An American physicist who proposed and measured the relativistic radar time delay now bearing his name. His 1964 proposal turned the predicted delay into a practical solar-system test.
Deep Space Network: NASA’s global system of antennas for communicating with interplanetary spacecraft. Its timed radio links can be used to measure propagation delays near the Sun.
Geometric time delay: A travel-time difference caused by differing path lengths, often in gravitational lensing. It arises from path geometry, unlike the Shapiro contribution from spacetime curvature.
Gravitational time dilation: The difference in elapsed time between clocks at different gravitational potentials. It helps distinguish local clock-rate effects from the total propagation delay measured between separated clocks.
Cassini–Huygens: A NASA–ESA mission that explored Saturn and its moons from 2004 to 2017. Radio links with Cassini passing near the Sun tested the delay with high precision.
Radar ranging: A method of estimating distance by measuring the round-trip travel time of radio signals. Shapiro adapted this established timing method to detect an additional gravitational contribution.
Spacecraft navigation: The determination and control of a spacecraft’s position, velocity, and trajectory. Accurate signal timing must account for relativistic delay when tracking distant spacecraft.
Doppler effect: A change in observed frequency caused by relative motion between a source and an observer. Frequency shifts can accompany ranging measurements but are distinct from the propagation-time delay.

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