World Lines in General Relativity
The use of world lines in general relativity is basically the same as in special relativity, with the difference that spacetime can be curved. A metric exists and its dynamics are determined by the Einstein field equations and are dependent on the mass distribution in spacetime. Again the metric defines lightlike (null), spacelike and timelike curves. Also, in general relativity, world lines are timelike curves in spacetime, where timelike curves fall within the lightcone. However, a lightcone is not necessarily inclined at 45 degrees to the time axis. However, this is an artifact of the chosen coordinate system, and reflects the coordinate freedom (diffeomorphism invariance) of general relativity. Any timelike curve admits a comoving observer whose "time axis" corresponds to that curve, and, since no observer is privileged, we can always find a local coordinate system in which lightcones are inclined at 45 degrees to the time axis. See also for example Eddington-Finkelstein coordinates.
World lines of free-falling particles or objects (such as planets around the Sun or an astronaut in space) are called geodesics.
Read more about this topic: World Line
Famous quotes containing the words world, lines, general and/or relativity:
“To be in a world which is a hell, to be of that world and neither to believe in or guess at anything but that world is not merely hell but the only possible damnation: the act of a man damning himself. It may be—I hope it is—redemption to guess and perhaps perceive that the universe, the hell which we see for all its beauty, vastness, majesty, is only part of a whole which is quite unimaginable.”
—William Golding (b. 1911)
“Wittgenstein imagined that the philosopher was like a therapist whose task was to put problems finally to rest, and to cure us of being bewitched by them. So we are told to stop, to shut off lines of inquiry, not to find things puzzling nor to seek explanations. This is intellectual suicide.”
—Simon Blackburn (b. 1944)
“There is in general good reason to suppose that in several respects the gods could all benefit from instruction by us human beings. We humans are—more humane.”
—Friedrich Nietzsche (1844–1900)
“By an application of the theory of relativity to the taste of readers, to-day in Germany I am called a German man of science, and in England I am represented as a Swiss Jew. If I come to be regarded as a bête noire the descriptions will be reversed, and I shall become a Swiss Jew for the Germans and a German man of science for the English!”
—Albert Einstein (1879–1955)