Class 12 > Unit # 24: Relativity > General Theory of Relativity

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Postulates and Consequences of General Theory of Relativity

Video Lecture: General Theory of Relativity

Watch the complete video lecture explaining the postulates and key consequences of the general theory of relativity:

Postulates of General Theory of Relativity

Postulate I

The laws of physics may be expressed in equations having the same form in all frames of reference, regardless of their states of motion. Thus, the general theory covers uniform as well as accelerated motion. Hence it is able to describe gravitational phenomena.

Postulate II (Principle of Equivalence)

“There is no way for an observer in a closed laboratory to distinguish between the effects produced by a gravitational field and those produced by an acceleration of the laboratory.”

This postulate follows from the experimental observation that the inertial mass of a body is always exactly equal to its gravitational mass.

Consequences of General Theory of Relativity

1. Space-Time

Einstein's general theory of relativity states that time is a fourth dimension adding to the three dimensions of space. Einstein called this four-dimensional geometry space-time.

Fig: Four-dimensional space-time continuum curvature.

2. Red Shift

The general theory of relativity also predicted that light coming from a strong gravitational field would have its wavelength shifted toward longer wavelengths, called a red-shift.

Example: A white-hot star near a massive body still remains white at its surface, but to a distant observer, the light reaching them has a lower frequency, so it appears slightly red-shifted.

3. Black Holes

The theory also predicted that when gravity becomes great enough, it would produce objects called black holes. Black holes are objects whose gravity is so massive that light cannot escape from the surface at all. Since no light can escape, such objects appear black.

4. Gravitational Lensing

Light is bent as it passes through curved space-time. This can cause distant objects to appear distorted or magnified. Gravitational lensing has been used to study distant galaxies and other astronomical objects.

5. The Expansion of the Universe

General relativity predicts that the universe is expanding. This remarkable prediction has been fully confirmed by modern astronomical observations.

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