Max Planck

“ For if, while engaged in body and mind in patient and often modest individual endeavor, one thought strengthens and supports us, it is this, that we in physics work, not for the day only and for immediate results, but, so to speak, for eternity. ”

“ Nothing short of the assumption of the general validity of the relativity principle in mechanics can justify the inclusion by physics of the Copernican cosmical system, since through it the independence of all processes upon the earth of the progressive motion of the earth is secured. If one were obliged to take account of this motion, I should have, e. g., to admit that the piece of chalk in my hand possesses an enormous kinetic energy, corresponding to a velocity of something like kilometers per second. ”

“ If, therefore, two observers move with uniform but different velocities, then each of the two with exactly the same right may assert that with respect to empty space he is at rest, and there are no physical methods of measurement enabling us to decide in favor of the one or the other. The principle of relativity in its generalized form is a very recent development. ”

“ Let us consider in any given medium any electromagnetic field. For any volume element of the medium, the law holds that is invariant in the passage from the one to the other observer. It follows from this that is invariant; i. e., the kinetic potential of a unit volume or the “space density of kinetic potential” is invariant. ”

“ It is my purpose to discuss with you today an hypothesis which represents a magnificent attempt to establish quite generally the dependency of the kinetic potential upon the velocities, and which is commonly designated as the principle of relativity. The gist of this principle is: it is in no wise possible to detect the motion of a body relative to empty space ”

“ For this purpose let us consider, returning to the previous example, the propagation, as it appears to the two observers and , of an instantaneous signal creating an infinitely thin light wave which, at the instant at which the observers meet, begins to spread out from the common origin of coordinates. For the observer the wave travels out as a spherical wave:
For the second observer the same wave also travels as a spherical wave with the same velocity:
for the first observer has no advantage over the second observer. ”

“ The revolution introduced by this principle into the physical conceptions of the world is only to be compared in extent and depth with that brought about by the introduction of the Copernican system of the universe. ”