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General Relativity |
When Einstein developed the special theory of relativity, it
soon became obvious that there was a problem
with gravity.
Therefore, Einstein was forced to take another look at gravitation and to try to find a way to express gravity in a form compatible with relativity. He recognized that gravity was the only real force in nature that was proportional to the mass of the body experiencing that force. All other mass-proportional forces were recognized to be fictitious forces resulting from accelerated coordinate systems. And because gravity was mass-proportional, then gravity, like all fictitious forces, could be eliminated simply by a change in the coordinate system. Therefore, Einstein proposed his principle of equivalence, which asserted that gravitation and acceleration were equivalent – that there was absolutely no possible way to distinguish experimentally between an object at rest in a gravitational field and an object accelerating uniformly in free space.
One consequence of the principle of equivalence was the conclusion that a freely falling observer in a gravitational field must be in a locally inertial frame of reference. In other words, there is absolutely no way an observer in a freely falling laboratory can distinguish his situation from that of an identical laboratory floating freely in space. However, this equivalence is only local. On a large scale, it is easy to distinguish between the two situations. All one has to do is to look out of the window of the laboratory.
Because the special
theory of relativity depicted reality as a flat, four-dimensional Minkowski spacetime, Einstein
wondered what would happen if he relaxed the requirement that spacetime be flat. What would happen to the theory of
relativity if he generalized it to include curved spaces? After all, curved spaces look locally flat but
globally curved. Is this not like gravitation, which looks locally inertial but
globally gravitational?
Einstein knew that parallel lines on a spherical surface eventually intersected one another. If these parallel lines are interpreted as the worldlines of objects in a curved spacetime, then the objects originally at rest with respect to one another will eventually drift together and collide with one another, just as if some mysterious force like gravity were pulling them together. Furthermore, just like in the case gravity, neither object would feel the force. Each would perceive itself to be floating freely in space in a locally inertial frame of reference.
Therefore, Einstein generalized his theory of relativity to include curved spaces and assumed that curved space was equivalent to gravity. Since he knew that mass was the source of gravitation, he concluded that there must be some relationship between the distribution of mass and the curvature of space. Eventually he found that relationship and expressed it in the form of his gravitational field equation.
Therefore, the Einsteinian concept of gravitation is that mass curves space and curved space determines how objects move. This is in contrast to the Newtonian concept of gravitation that says mass causes forces and forces determine how objects move. The difference between the two theories can become quite dramatic when the gravitational fields are strong. Nevertheless, the correspondence principle requires that Einsteinian gravitation reduce to Newtonian gravitation whenever the gravitational fields are weak.
When Einstein first proposed the general theory of relativity in 1916, he suggested three experiments to test its validity. One was concerned with the precession of the orbit of mercury, another with the deflection of light as it passes near the sun, and a third with the gravitational redshift of light as it leaves a gravitating body. Additional tests of the general theory of relativity have been proposed in more recent years, including: gravitational lensing of distant galaxies, gravitational radiation from closely orbiting neutron stars, and observations of black holes. All of these tests, and others, have been performed and, without exception, they support the general theory of relativity. However, current observations suggests that a modified form of Einstein’s gravitational field equation must be used in order to explain the recent measurements of the accelerated expansion rate of the universe.
Questions:
Q1. Einstein developed General
Relativity because (A) the Newtonian view of space and time were inconsistent with
the observed properties of light, (B) Newtonian gravitation was philosophically
inconsistent with Special Relativity, (C) Newtonian gravitation was
experimentally inconsistent with Special Relativity, (D) Newtonian physics was
experimentally inconsistent with the properties of fast moving particles, (E)
Newtonian gravitation did not work for strong gravitational fields.
Q2. Which of the following asserts
that the laws of physics are the same in every inertial frame of reference and
is used as a foundation of special relativity? (A) Correspondence Principle,
(B) Equivalence Principle (C) Principle
of Relativity, (D) More than one of these. (E) None of these.
Q3. Which are true? (A) Einstein
proposed his general theory of relativity because Newtonian gravity propagated
information slower than the speed of light. (B) The principle of equivalence
asserts that uniform gravitation is perfectly equivalent to uniform
acceleration. (C) Einsteinian gravitation asserts
that mass causes forces and forces determine how
objects move. (D) Two of these. (E) Three of these.
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