Albert Einstein developed two theories, the special theory of relativity and the general theory of relativity, between 1905 and 1915. These theories challenged classical mechanics and had bizarre consequences, such as time dilation and length contraction. The general theory of relativity explains gravity as curves in spacetime and predicts phenomena such as the bending of light and the precession of planetary orbits. It took decades to confirm these theories, but they were ultimately validated.
There is no “theory of relativity” – just two theories that occasionally fall under the same designator – the special theory of relativity and the general theory of relativity. Both were devised by famed physicist Albert Einstein between 1905 and 1915. Both of these theories upended Newton’s classical mechanics, which had endured centuries earlier.
The special theory of relativity is a theory of the structure of spacetime, which states that 1) the laws of physics are the same for all observers in uniform motion relative to each other and 2) the speed of light in a vacuum is the same for all observers, regardless of their motion of the motion of the light source. This leads to bizarre consequences. For example, imagine you are moving on a train going 100 mph next to another train moving in the same direction at 100 mph. From your point of view, the train next to you seems almost motionless. But imagine you are on a train moving at the speed of light, while standing next to a beam of light. Instead of appearing to be stationary, the beam of light moves in front of you at the speed of light. The light is so strange.
Several of the consequences that follow from the special theory of relativity are 1) time dilation (clocks moving near the speed of light run slower than stationary clocks), 2) length contraction (objects moving near the speed of light appear contracted in the direction of motion), 3) relativity of simultaneity (things that appear to happen at the same time to stationary observer A may appear to happen at different times to moving observer B), 4) mass-energy equivalence (mass and energy are essentially the same thing and transmutable into each other).
The general theory of relativity, conceived a few years after the special theory of relativity, is a theory of how gravity works. It was initially based on the equivalence principle, the idea that accelerating and standing still in a gravitational field of a given strength are physically identical. Since no special force is required to create inertial effects in an accelerating object, Einstein proposed that we should think in the same way as gravity, giving up the classical notion of gravitational force and instead conceiving of gravity as curves in spacetime. This explains phenomena such as why light bends in the presence of a gravitational field even though it lacks mass.
The general theory of relativity provides explanations and makes predictions that would have seemed ludicrous in the worldview of classical mechanics. Apart from the bending of light in the presence of a gravitational field, predicts that time passes more slowly in the presence of a gravitational field, the precession (change) of planetary orbits due to torque exerted by the gravitational field of the Sun, dragging occurs of the frame, whereby massive rotating bodies “pull” the inertial frame of the surrounding spacetime with them and that the universe is expanding, in some cases faster than the speed of light, because it is space itself that is expanding , not the objects inside it.
Unraveling the implications of these theories took decades and is still ongoing today. Einstein’s theories were so far-sighted that it took decades to test and confirm them with great precision. In retrospect, Einstein’s ideas were almost completely validated.
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