Also Read: “A Briefer History of Time” by Stephen Hawking

Hawking recounts the evolution of cosmology/physics from the time of the ancient Greek to the present, with emphasis on Newton’s law of gravitation, Einstein’s theory of general relativity, Planck’s quantum and Heisenberg’s uncertainty principles in quantum mechanics, Feynman’s “sum over histories” approach, and the string theory. He also examines the existence of the Big Bang, black holes, worm holes and time travel.

I like Hawking’s logical, clear, and succinct presentation, especially his explanation of General and Special Relativity, which enables me to appreciate the basic assumptions and logical reasoning inherent in the theory.

**The Fundamental Postulate** of Special Relativity: The laws of science should be the same for all freely-moving observers, regardless of their speed. The speed of light is the same for all observers; Observers moving at different speed would measure time differently.

**The Principle of Equivalence** of General Relativity: One can not distinguish acceleration due to a uniform gravitational field from acceleration due to other forces.

**Space-Time**

Space-time is not flat, but curved by the distribution of mass and energy in it. Bodies follow the nearest thing to a straight path in a curved space, which is called geodesic (the shortest or longest path between two points).

Space-time interacts with matter-energy, and they are interdependent on each other. The direction of time is the same as the direction of the increase of entropy.

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