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''See [[Theory of Relativity]]'' for a comprehensive treatment of this topic.''

''See [[Theory of Relativity]]'' for a comprehensive treatment of this topic.''

The '''General Theory of Relativity''' is an extension of [[Special theory of relativity|special relativity]], dealing with curved coordinate systems, accelerating frames of reference, curvilinear motion, and curvature of spacetime itself.  It could be said that general relativity is to special relativity as vector calculus is to vector algebra.  General relativity is best known for its formulation of gravity as a [[fictitious force]] arising from the curvature of spacetime.  In fact, "general relativity" and "Einstein's formulation of gravity" are nearly synonymous in many people's minds.  That's because they are.

The '''General Theory of Relativity''' is an extension of [[Special theory of relativity|special relativity]], dealing with curved coordinate systems, accelerating frames of reference, curvilinear motion, and curvature of spacetime itself.  It could be said that general relativity is to special relativity as vector calculus is to vector algebra.  General relativity is best known for its formulation of gravity as a [[fictitious force]] arising from the curvature of spacetime.  In fact, "general relativity" and "Einstein's formulation of gravity" are nearly synonymous in many people's minds.

The general theory of relativity was first published by Marcel Grossman in 1913 and [[David Hilbert]] and [[Albert Einstein]] in 1916.

The general theory of relativity was first published by Marcel Grossman in 1913 and [[David Hilbert]] and [[Albert Einstein]] in 1916.

However, "even though General Relativity has passed many tests, most physicists don’t believe it is ultimately correct because it conflicts with [[quantum mechanics]]." [http://newsdesk.org/2010/08/conservapedia-calls-theory-of-relativity-a-liberal-conspiracy/] Hopefully, within the next few years, a 'theory of everything' will be found, that will combine general relativity and quantum mechanics into one, thus ending the conflict.

However, "even though General Relativity has passed many tests, most physicists don’t believe it is ultimately correct because it conflicts with [[quantum mechanics]]." [http://newsdesk.org/2010/08/conservapedia-calls-theory-of-relativity-a-liberal-conspiracy/]

General relativity, like [[quantum mechanics]] (the other of the two theories comprising "modern physics") both have reputations for being notoriously complicated and difficult to understand.  In fact, in the early decades of the 20^th century, general relativity had a sort of cult status in this regard.  General relativity and quantum mechanics are both advanced college-level and postgraduate level topics.  Hence this article can't possibly give a comprehensive explation of general relativity at the expert level, because it is written by conservapedia.  But we will attempt to give a rough outline, for lay people, of the general relativistic formulation of gravity.

General relativity, like [[quantum mechanics]] (the other of the two theories comprising "modern physics") both have reputations for being notoriously complicated and difficult to understand.  In fact, in the early decades of the 20^th century, general relativity had a sort of cult status in this regard.  General relativity and quantum mechanics are both advanced college-level and postgraduate level topics.  Hence this article can't possibly give a comprehensive explation of general relativity at the expert level.  But we will attempt to give a rough outline, for lay people, of the general relativistic formulation of gravity.

In the [[weak field approximation]], where velocities of moving objects are low and gravitational fields are not very severe, the theory of general relativity is said to ''reduce to'' the law of universal gravitation. That is to say, under those circumstances the equations of general relativity are mathematically equivalent to the equations of Newtonian gravitation.

In the [[weak field approximation]], where velocities of moving objects are low and gravitational fields are not very severe, the theory of general relativity is said to ''reduce to'' the law of universal gravitation. That is to say, under those circumstances the equations of general relativity are mathematically equivalent to the equations of Newtonian gravitation.

Modern science does not say that Newtonian (classical) gravity is wrong.  It is obviously very very very very very very very very very very very very very very very very nearly correct.  In the weak field approximation, such as one finds in our solar system, the differences between general relativity and Newtonian gravity are miniscule.  It takes very sensitive tests to show the difference.  The history of those tests is a fascinating subject, and will be covered near the end of this article.  But in all tests conducted so far, where there are discrepancies between the predictions of general relativity and Newtonian gravity (or other competing theories for that matter), experimental results have shown general relativity to be a better description.

Modern science does not say that Newtonian (classical) gravity is wrong.  It is obviously very very nearly correct.  In the weak field approximation, such as one finds in our solar system, the differences between general relativity and Newtonian gravity are miniscule.  It takes very sensitive tests to show the difference.  The history of those tests is a fascinating subject, and will be covered near the end of this article.  But in all tests conducted so far, where there are discrepancies between the predictions of general relativity and Newtonian gravity (or other competing theories for that matter), experimental results have shown general relativity to be a better description.

Outside of the solar system, one can find stronger gravitational fields, and other phenomena, such as quasars and neutron stars, that permit even more definitive tests.  General relativity appears to pass those tests as well.

Outside of the solar system, one can find stronger gravitational fields, and other phenomena, such as quasars and neutron stars, that permit even more definitive tests.  General relativity appears to pass those tests as well.

The diagram shows some "events"—my house, now; my house, 5 minutes from now; and my neighbor's house now and 5 minutes from now.  The diagonal line depicts my neighbor walking from his house to mine, arriving 5 minutes from now.  That line is called his ''world line''.  The line going straight up in my house is my own world line (I'm sitting at home.)

The diagram shows some "events"—my house, now; my house, 5 minutes from now; and my neighbor's house now and 5 minutes from now.  The diagonal line depicts my neighbor walking from his house to mine, arriving 5 minutes from now.  That line is called his ''world line''.  The line going straight up in my house is my own world line (I'm sitting at home.)

A car is driving down the street, from left to right.  Figure 2 shows the@� $e four events and two world lines, but with different calibration—the car's own coordinate system.  The car is driving 500 feet per minute, but in the opposite direction.  The event of my neighbor's arrival at my house is now at x=-2500.  It's way behind the car, though the car was directly in front of my house at t=0.

A car is driving down the street, from left to right.  Figure 2 shows the same four events and two world lines, but with different calibration—the car's own coordinate system.  The car is driving 500 feet per minute, but in the opposite direction.  The event of my neighbor's arrival at my house is now at x=-2500.  It's way behind the car, though the car was directly in front of my house at t=0.

Because the car's frame of reference is in motion, the calibration lines in figure 2 are not perpendicular.  The formerly vertical lines are now slanted.  But there is something very important to notice about the two coordinate systems:  They are ''flat''<ref>The words "flat" and "curved" used in this article are the same terms used by differential topology experts.</ref>.  The flatness comes from the fact that the calibration lines are straight and parallel.  The boxes created by the lines are parallelograms.  But note that the lines don't have to be perpendicular, and the boxes don't have to be rectangles.  Straight parallel lines and parallelograms are all that is required.

Because the car's frame of reference is in motion, the calibration lines in figure 2 are not perpendicular.  The formerly vertical lines are now slanted.  But there is something very important to notice about the two coordinate systems:  They are ''flat''<ref>The words "flat" and "curved" used in this article are the same terms used by differential topology experts.</ref>.  The flatness comes from the fact that the calibration lines are straight and parallel.  The boxes created by the lines are parallelograms.  But note that the lines don't have to be perpendicular, and the boxes don't have to be rectangles.  Straight parallel lines and parallelograms are all that is required.