How an observer moving in a strong gravitational field or at a high velocity perceives measurements of mass, lengths or the passage of time in a different way that an observer at rest would is the basic idea of relativity, developed by Albert Einstein in the early part of this century.
1. The laws of physics are the same for all inertial observers.
2. The speed of light is the same for all inertial observers, regardless of their motion relative to the source of the light.
1. Special relativity governs the properties of space-time in regions where the gravitational influence of matter is neglible. However, space-time around a massive object is warped by gravitation.
2. There is no way to tell locally the different between acceleration and gravity. This is called the Equivalence Principle.
When gravitational fields are strong or when objects travel at very high velocities, close to the speed of light, perception of the common properties of mass, length and time become more complicated than our everyday experience would imply.
Einstein's theory of general relativity was proposed in 1916 and evoked a sensation. It is important to note that Einstein did not prove that Newton was wrong in his understanding of ordinary space and time. Newtonian mechanics applies in weak gravitational fields (the normal condition) and when velocities are low relative to the speed of light. In normal circumstances, the increased accuracy in predicting, for example, orbits offered by Einstein's theory over Newton's is insignificant. In the case of the binary pulsar PSR1913+16, however, Einstein's theory is critical.
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