While studying the conflict between the action/reaction principle and Lorentz ether theory, he tried to determine whether the center of gravity still moves with a uniform velocity when electromagnetic fields are included. Like others before, Poincaré (1900) discovered a relation between mass and electromagnetic energy. So it was Hermann Minkowski who worked out the consequences of this notion in 1907. Poincaré expressed a lack of interest in a four-dimensional reformulation of his new mechanics in 1907, because in his opinion the translation of physics into the language of four-dimensional geometry would entail too much effort for limited profit. In 1895 Lorentz had introduced an auxiliary quantity (without physical interpretation) called "local time" t ′ = t − v x / c 2 as a fourth imaginary coordinate, and he used an early form of four-vectors. At the same time Dutch theorist Hendrik Lorentz was developing Maxwell's theory into a theory of the motion of charged particles ("electrons" or "ions"), and their interaction with radiation. Poincaré's work at the Bureau des Longitudes on establishing international time zones led him to consider how clocks at rest on the Earth, which would be moving at different speeds relative to absolute space (or the "luminiferous aether"), could be synchronised.