Max Planck

“ As the momentum located in a particular area can only change due to external effects, i.e. according to the theory of direct action only by processes at the surface of that space, the amount of change in unit time is a surface integral which can be described as the total momentum flux in the interior of that space. A major difference, however, to the energy flux is the fact that the energy is a scalar, yet the momentum is a vector. Therefore, the incoming energy in that space is expressed by a single surface integral, and the energy flux is a vector. ”

“ But I consider the independence from the reference frame not as a solid physical result, but more as a hypothesis, which I consider as promising but not to be established yet. However, we just need to consider whether these relationships actually exist in nature. This we can only learn by experiment, and hopefully the time is not far off when we experience it. ”

“ While the energy from the very outset represented a universal physical expression, the momentum was so far a special mechanical expression, the reaction principle a special mechanical theorem, and therefore the extension recognized as necessary was perceived as a revolution of principle, by which the previous relatively simple and uniform concept of momentum becomes considerably more complicated in nature. ”