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Geodesics
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Free-falling bodies follow straightest paths in spacetime.
The straightest possible paths in curved spacetime; free-falling objects and light follow them. They maximise proper time for timelike paths.
Orbits of planets are geodesics.
Not straight lines in Euclidean sense.
In more detail
Geodesics are the paths that freely falling objects follow in curved spacetime. Planets orbit the Sun along geodesics, which appear as curved paths in space.
Real-world uses
- Satellites follow geodesics, adjusted only by small corrections.
- Light paths near massive objects follow null geodesics.
- Engineers plan satellite orbits as geodesics in curved spacetime, which is the straightest possible path.
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Frequently asked questions
Why do planets orbit?
They follow geodesics in the curved spacetime created by the Sun.
Is a geodesic always a straight line?
In curved spacetime it is the locally straightest path, which may look curved from outside.
Why do orbits look like curves?
A geodesic in curved spacetime appears curved from a flat viewpoint, which is how gravity acts on orbiting bodies.
Related topics in Relativity
GPS and relativity · Curved spacetime · Einstein field equations · Schwarzschild metric · Black hole event horizon · Hawking radiation