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Feb24-12, 01:08 PM
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Quote Quote by robphy View Post
While geodesics are nicely-behaved curves, there are physically important non-geodesic worldlines of piecewise-inertial particles and worldlines of accelerated particles.

In the proofs of some theorems involving causal structure, there are "causal curves" whose tangents can be timelike or null.
A causal curve might play a role in a proof. So might any curve. But no physical entity can follow curve that is part timelike and part null.