Relativity: Concept of Space and Frame of reference

Relativity

In Newtonian mechanics, mass, time, length and space were treated as absolute. It is regarded that space existed without any reference to objects. Space was considered absolute and at rest.

In the wave theory of light, it is assumed that a medium called ether exists in space and pervaded all space in the universe. But as per Michelson and Morley experiment, the concept of ether and absolute space become meaningless. In Einstein's view, there is no absolute space and all motions are relative. The time, mass, length and space are interdependent and are not absolute. Einstein, in 1905, profounded the special theory of relativity. The special theory of relativity deals with objects and systems, which are either moving at a constant speed with respect to one another or at rest.

1.Concept of Space

In classical mechanics, motion in absolute space led to two useful results.

(i)  Fixed frame of reference by which the position or motion of any object in the universe could be measured.

(ii)    The geometrical form of an object remains the same irrespective of changes in position or state of motion of the object or observer.

Concept of time

According to classical mechanics,

(i)The time interval between two events has the same value for all observers irrespective of their motion.

(ii)  If two events are simultaneous for an observer, they are simultaneous for all observers, irrespective of their position or motion. This means simultaneity is absolute.

Concept of mass

In classical mechanics, the mass of the body is absolute and constant and independent of the motion of the body.

2. Frame of reference

A system of co-ordinate axes which defines the position of a particle in two or three dimensional space is called a frame of reference.

The simplest frame of reference is the Cartesian co-ordinate system in which the position of a particle is specified by three co-ordinates x,y and z. There are infinite number of reference frames in the universe. Our earth itself is a frame of reference. There are two types of frames of reference (i) inertial and (ii) non - inertial frames.

(i) Inertial (or) unaccelerated frames.

A frame of reference is said to be inertial, when the bodies in this frame obey Newton's law of intertia and other laws of Newtonian mechanics. In this frame, a body remains at rest or in continuous motion unless acted upon by an external force.

(ii) Non-inertial (or) accelerated frames

A frame of reference is said to be a non-intertial frame, when a body not acted upon by an external force, is accelerated. In this frame, Newton's laws are not valid.