Hi Eckard,
"a) Does his observer look from above as uttered in 1921 or is he identical with the receiver as explained in 1905?"
a. In a coordinate free system with no privileged frame of reference, "above" and "below" have no meaning. Observers have equal claim to their own valid measurement values independent of other inertial systems.
b) Doesn't the Lorentz gamma clearly belong to a two-way speed?
b. To a two way *velocity.* A common textbook problem is to describe the speed of an object with a relativistic mass that is twice its rest mass. Calculation will show the object's instantaneous speed as about 86.6% the speed of light. If you take the gamma factor, 1/sqrt(1 - v^2/c^2), to represent the decomposition of speed from velocity, you see how a variable velocity inverse to the constant speed compresses the geometry from the POV of an observer at rest relative to the moving object. One wouldn't know that without a two-way geometric correction (Lorentz transform), because otherwise the observer would interpret the moving object to be only half its rest length.
"c) What does the velocity c of light refer to? I agree on that experimental evidence precludes its dependence on the velocity of the emitter as well as on the velocity of a hypothetical medium. However, I twice substantiated my argument that it cannot depend on the velocity of the receiver either."
The velocity of c varies relative to a medium. The instantaneous vacuum speed of c is constant. There are still ether theories floating about -- however, we know to a reasonable certainty if an ether exists, it is far more rigid than we can measure and is not useful for any physics we yet know. Or at least, that I know.
"d) What is wrong with my definition of c? I wonder if Einstein anticipated it and answered this question."
Because all motion is relative, any dependence of the value of c on emitter or receiver would imply the non-relativity of motion, and a privileged rest frame.
"e) Is the distinction between past and future really just an illusion?"
In relativity, an arbitrary distinction between past and future -- if not an illusion -- is still just that, arbitrary. Even the advanced solution to the Schrodinger wave equation tells us that. We arbitrarily discard that solution just for the reason you give below:
"This was and still is my primary concern. Otherwise I would be ready to believe almost anything. Evidence must be based on logically consistent notions."
The Schrodinger equation is logically consistent. (I dealt with this in my essay two contests ago.) One observer's past is another's future. Because we are so hung up on what is real for a specific laboratory observer, we forget the metaphysical realism that logically results from a system in which there is no privileged observer frame.
"For instance, Michelson's null-result does not confirm SR but it precludes a light-carrying medium relative to which the earth was thought to move."
Exactly. Negative results are often more important than results that only validate our theoretical predictions.
All best,
Tom
