MEMO 20.2
Some Reasons Why Einstein Needed The Concept Of Symmetry For His Special Theory

1.  So that he could do away with asymmetric ether measurements and postulate that all motions are relative (Chapter 20C).

2.  So that he could claim that inertial reference frames, as in Galileo’s Relativity, evidenced the symmetry of equivalent and reciprocal relative translational motion (Chapter 20C, 23 & 24).

3.  So that he could claim that one inertial observer is as good as another for measuring the velocity of light in an inertial frame (Chapters 20F & 21A).

4.  So that he could claim that the symmetry of simultaneity (identical times) was the proper starting point for a definition of time (Chapter 25).

5.  So that he could claim that all clocks on an inertial reference frame must be synchronized to show a symmetrical and mathematical simultaneous time (Chapter 25).

6.  So that he could then refer to and describe non-simultaneous times as asymmetric (Chapters 26 & 28).

7.  So that he could postulate that the velocity of light was the same in two opposite directions (Chapters 25 & 27).

8.  So that he could claim that coordinate measurements between inertial frames could not be made simultaneously (symmetrically) (Chapters 26 & 28).

9.  So that he could rationalize that the symmetry of Galileo’s Relativity applies not only to mechanics but also to light (Chapter 24).

10. So that he could finally claim that all physical laws of nature (including light) must be constituted so that they are algebraically the same (symmetrical and algebraically covariant) for every inertial observer (Chapters 20E, 20F & 20G).