The Shapiro time delay, also known as the Shapiro effect, is a phenomenon predicted by the theory of general relativity that describes how light signals take longer to travel through a gravitational field compared to traveling in a vacuum. When light passes near a massive object, such as a planet or a star, its path is bent due to the curvature of spacetime caused by the object's gravity.
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Extra propagation time of a light or gravitational-wave signal through a gravitational field compared with an appropriate flat-space reference. In a weak static field with Newtonian potential of a point mass , the leading delay along an approximately straight ray is . The local speed of light remains ; the integrated geometry changes the relation between emission and reception times.