Allow me to ask. Are there any error in the following passage ?

In the vacuum, it's said that many phenomena occurs. In it, there will be "substance" that is able to consider to be rest frame. Even Galileo's principle of relativity will be ideal, imaginary, convenient, partly and temporary. Much less, accelerated motion will be only relative in territory of numerical formulas (ignores change of momentum ; a vector quantity). The equivalence principle will be the same.

【Equivalence principle】 A elevator is accelerating to the above. At the center of (inside of) the elevator, a light source shines. Spherical waves of light reach floor earlier than roof. Is this the same in a elevator that is at a stand-still in gravitational field ? (scene like this is shown in a book in Japanese).

Mathematical expression of relativity of accelerated motion (without the rest frame) seems to be possible only between two rigid bodies (includes a case that the one is the universe). How do scholars grasp this ?

On every accelerated motion, there is its own cause (without exception). If an accelerated motion is relative, what about (the position of) this cause ?