“Make physics as simple as possible but not simpler”- Albert Einstein

In 1905 Albert Einstein published a paper with a title “On the electrodynamics of moving bodies”. This work of Einstein gave a totally different notion of Physical world. It changed the foundation of physics and also our understanding of nature.

In the following we briefly discuss the fundamental concepts of this theory.

**Frame of reference:** A place from where observation is made is called frame of reference. In physics a frame of reference is represented by a co-ordinate system with respect to which spatial co-ordinate of any point in space is determined.

**Frames of reference are of two types:**

(1) Inertial frame of reference: Two frames are said to be inertial if they are at rest or moving with constant velocity w.r.t each other.

In figure shown S’ is moving with constant speed v w.r.t S. Therefore Sand S’ are inertial frames.

Non inertial frame of reference: A frame of reference is said to be non- inertial if it is either accelerating or rotating.

**Postulates of STR:**

STR deals with the inertial frames of reference and it is based on the following two postulates.

**First Postulate:** “All inertial frames are equivalent”. Therefore laws of physics must be invariant in all inertial frame frames. What this postulate implies is that, if an experiment is done in a laboratory and the same experiment is repeated in a van moving with constant velocity then result of experiment in the two cases will be same.

**Second postulate:** “Speed of light in vacuum has same value in all inertial frame and speed of light is the ultimate speed in vacuum”. What this postulate implies that,

• Speed of any object or anything cannot be greater than speed of light in vacuum.

• Magnitude of velocity of light is same in all inertial frames, but components of velocity of light changes. If velocity of photon w.r.t to S frame is v=c then velocity of photon w.r.t S’ frame is v’=c (i.e. same as in S frame).

• Velocity of any object (for example electron) can be greater than speed of light in

Author: Neetu Choudhary