Describe the two fundamental postulates underlying Einstein's special theory of relativity.

Answer: The two fundamental postulates of the special theory of relativity are: 1. Principle of Relativity: It states that the laws of physics are the same for all observers in inertial frames of reference. For example, a ball dropped inside a moving train would appear to fall straight down to observers inside the train, and in a parabolic path to an observer outside on a platform, but the laws of physics governing the ball's motion would apply equally to all observers. 2. Constancy of the Speed of Light: It states that the speed of light in a vacuum always has a constant value, which is independent of the relative motions of the light source and the observer. For example, a stationary observer observing a car moving at high speed with its headlights on would measure the light emitted from the headlights to travel at the same speed as the moving observer inside the car.