35.5 Einstein's Postulates
The inferences drawn alone were elevated to the status of postulates as they could not be deduced from some other principles.
Postulate I
The Principle of Relativity holds good for the whole of Physics, i.e. the laws of Physics should be invariant under transformations from one to another inertial frame in relative uniform translational motion with respect to each other.
Postulate II
The Speed of light in any direction in vacuum in any inertial frame of reference is the same universal constant C = 3 ´ 10^{8} m/s.
Based on these postulates and assumptions of isotropy and homogeneity of space and time. Einstein deduced the coordinate transformations known as Lorentz translations given by,
Note
1) The time is not 'absolute' in Einstein's theory; i.e, concept of simultaneity is relative to the frame
of reference concerned and is not absolute.
2) The geometrical representation of Equation (3) in terms of Space time diagrams can be done
by regarding time as the fourth dimension. The physical world is therefore Four  dimensional
Space time continuum.
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