GENERAL RELATIVITY: True motion is the opposite of free fall!!

 In the everyday description of the world, rest is the absence of velocity.

With Galilean and special relativity, rest became inertial motion, since no inertially moving observer can
distinguish its own motion from rest: nothing disturbs him (the rapid protons crossing the galaxy as cosmic rays are at rest).

 With the inclusion of gravity, we are led to an even more general definition of rest.

⊳ Every observer and every body in free fall can rightly claim to be at rest.

 If any body moving inertially is to be considered at rest, then any body in free fall must
also be.




The cosmonauts circling the Earth or the passengers in parabolic aeroplane flights,make the same observation: it is impossibleto distinguish anything happening in free fall from what would happen at rest.This
impossibility is called the principle of equivalence; it is one of the starting points of general
relativity. It leads to the most precise – and final – definition of rest that we will
encounter: rest is free fall. Rest is lack of disturbance; so is free fall!!!

(Christoph Schiller, 2012, Motion Mountain)