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Reference.org
Geodesics in general relativity
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<p>In <a href="/facts/General_relativity/jVsoIg8X">general relativity</a>, a geodesic generalizes the notion of a "straight line" to curved <a href="/facts/Spacetime/uRIN1x4b">spacetime</a>. Importantly, the <a href="/facts/World_line/rluamMcf">world line</a> of a particle free from all external, non-gravitational forces is a particular type of geodesic. In other words, a freely moving or falling particle always moves along a geodesic. </p><p>In general relativity, gravity can be regarded as not a force but a consequence of a <a href="/facts/Curved_spacetime/93NBB86g">curved spacetime</a> geometry where the source of curvature is the <a href="/facts/Stress%25E2%2580%2593energy_tensor/wWKrzjfI">stress–energy tensor</a> (representing matter, for instance). Thus, for example, the path of a planet orbiting a star is the projection of a geodesic of the curved four-dimensional (4-D) spacetime geometry around the star onto three-dimensional (3-D) space. </p>