| 21. | A four-velocity is thus the normalized future-directed timelike tangent vector to a world line, and is a contravariant vector.
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| 22. | In the short story " Life-Line ", author Robert A . Heinlein describes the world line of a person:
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| 23. | Two world lines that start out separately and then intersect, signify a " collision " or " encounter ".
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| 24. | The slope of the world line ( deviation from being vertical ) gives the relative velocity to the observer.
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| 25. | The basic mathematics is as follows : The theory of special relativity puts some constraints on possible world lines.
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| 26. | Analogously, the world lines of test particles in free fall are spacetime geodesics, the straightest possible lines in spacetime.
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| 27. | Gamow was also writing " My World Line : An Informal Autobiography ", which was published posthumously in 1970.
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| 28. | Importantly, the world line of a particle free from all external, non-gravitational force, is a particular type of geodesic.
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| 29. | In curved spacetime, the " shortest " world line is that geodesic that minimizes the curvature along the geodesic.
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| 30. | The orbit of the Earth is an ellipse in space alone, but its world line is a helix in spacetime.
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