Editing Shapiro time delay (section)

{{Short description|Time delay caused by space-time distortion near massive objects}}
{{General relativity sidebar}}
The '''Shapiro time delay''' effect, or '''gravitational time delay''' effect, is one of the four classic Solar System [[tests of general relativity]].  [[Radar]] signals passing near a massive object take slightly longer to travel to a target and longer to return than they would if the mass of the object were not present. The time delay is caused by [[time dilation]], which increases the time it takes light to travel a given distance from the perspective of an outside observer. In a 1964 article entitled ''Fourth Test of General Relativity'', [[Irwin I. Shapiro|Irwin Shapiro]] wrote:<ref name=Shapiro1964>{{cite journal | author=Irwin I. Shapiro | title=Fourth Test of General Relativity | journal=[[Physical Review Letters]] | date=1964 | volume=13 | pages=789–791 | doi=10.1103/PhysRevLett.13.789 | issue=26 | bibcode=1964PhRvL..13..789S}}</ref>

<blockquote>
Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased by almost {{val|2|e=−4}} sec when the radar pulses pass near the sun. Such a change, equivalent to 60 km in distance, could now be measured over the required path length to within about 5 to 10% with presently obtainable equipment.
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Throughout this article discussing the time delay, Shapiro uses ''c'' as the speed of light and calculates the time delay of the passage of light waves or rays over finite coordinate distance according to a [[Schwarzschild solution]] to the [[Einstein field equations]].