Question

Distinguish between general theory of relativity and special theory of relativity

Answer 1

Special relativity is the science of describing physics from the perspective of observers who do not accelerate. The basic principle is that for all these observers, the laws of physics should be the same, regardless of their velocity. At the time of its birth, special relativity was referred to as the theory of relativity.

General relativity was born out of the desire to extend the principle of relativity to observers in arbitrary motion. Einstein’s “happiest thought” amounted to the realization that such a theory must necessarily involve gravity, because an observer inside a closed chamber cannot distinguish between floating in empty space or freely falling in a gravitational field. Once “the general theory” (as it was called back then) was born, what was previously called the theory of relativity was now referred to as “the special theory”, as it was indeed a special case of the more general theory, restricted to uniform (nonaccelerating) motion.

In practice, special relativity is needed when dealing with things that move fast. (How fast… well, that depends on the accuracy required). Curiously, general relativity is not needed just because something accelerates; special relativity can deal with that, it just treats accelerating observers as second-class citizens. General relativity is, however, needed in the presence of strong gravitational fields.

A good example is spacecraft navigation, which is usually done by observing the properties (frequency shift and time of travel) or radio waves between the spacecraft and the ground. Special relativity is required to calculate the expected frequency shift correctly at the level of accuracy required, e.g., to insert a spacecraft in orbit around a planet, because the spacecraft’s velocity (several ten km/s) is large enough for relativistic corrections to matter.

When the radio wave passes near a massive object (especially if it passes near the Sun) general relativity is required to calculate its travel time correctly, incorporating the so-called Shapiro delay due to the presence of gravity.