# Explain Einstein's theory of relativity in simple terms.

## Question:

Explain Einstein's theory of relativity in simple terms.

## The theory of relativity

The human intuition is geared to deal with stationary and slow-moving objects. The response time of our nervous system is of the order of about 0.1second. This sets a limit on the speeds which we can intuitively cognize.

Newtonian mechanics typically deals with objects possessing inertia or mass. Only when the inertia is small as in the microscopic world is it possible to accelerate particles to extremely high velocities.

It was with the advent of the electromagnetic theory that extremely fast-moving entities entered the realm of physics. The electric and magnetic field strengths can propagate quite rapidly through free space. Immediately it was noticed that Maxwell's electromagnetic field theory and Newtonian particle mechanics were not on the same page regarding the transformation equations from one inertial frame of reference to another.

Einstein realized that the validity of Newtonian mechanics was restricted to slow-moving objects. The electromagnetic theory of Maxwell had the requisite mathematical structure for dealing with rapid changes and also correctly approximates to the low-velocity limit. Therefore he proposed that we need to abandon our preconceived notions about space and time since these have evolved from a study of slow-moving objects. When the realm of the rapid is incorporated into physics it is seen that length and time intervals are no longer absolute. These kinematical quantities depend on the velocity of the object. Similarly, dynamical quantities like mass and momentum are also functions of velocity. If mass, length, and time are not invariant then what are the absolute quantities? The special theory of relativity answers this question. There are two postulates on which the theory is built:

The postulate of relativity states that the laws of physics should have the same mathematical structure in all inertial frames. The correct transformation equations from one frame to the other are the Lorentz transformations. The second postulate is called the postulate of the constancy of the speed of light. It states that the speed of light is the same in all inertial frames.

Based on these, Einstein showed that the absolute kinematical quantity is the space-time interval and absolute dynamical quantity is the norm of the energy-momentum four-vector or alternatively the rest mass. Space and time are no longer regarded as independent entities but as two aspects of a higher dimensional entity called space-time. Similarly, energy and momentum are no longer independent but are the two aspects of the energy-momentum four-vector. Thus relativity supplants the Newtonian notions of absolute space and absolute time with absolute space-time.

Later Einstein generalized his theory to the case of accelerated frames of reference. This is called the General Theory of Relativity(GTR). This maintains that gravitation is not a force like the conventional electromagnetic force or the nuclear force, but a consequence of the geometric structure of space-time. The geometry is determined by the mass and energy present in that particular region of space-time. The more the mass present the more curved is the fabric of space-time. Thus the mass of the sun is responsible for the curvature of the space-time in the locality of the solar system. The planets are simply following the great circles or geodesics in this curved geometry.