Behind this transducer lay the various theories formulated in Physics over the last century. The following text presents a brief historical record of the context in which these ideas were brought to light, as well as a short analysis of their content.

Albert Einstein, 1905

In 1905, the German physicist Albert Einstein (1879 – 1955), still little known at that time, published a number of papers clearly worthy of being awarded the Physics Nobel Prize. Eventually, Einstein was awarded the prestigious Prize some years later (in 1921) for one of those papers – the one he had written on the photoelectric effect. Also viewed as somewhat revolutionary at the time was his theory of relativity. In that same year, Einstein published in Annalen der Physik, one of the most important scientific journals at the time, the famous equation E = mc2 (where E stands for the energy, m for the inertial mass and c for the speed of light in void).

In order to celebrate Einstein’s annus mirabilis, 2005 was proclaimed the International Year of Physics.

Equivalence between mass and energy

Though apparently simple, the expression E = mc2 influences our perception of the physics world in a significant manner. It means that mass and energy are equivalent. According to Einstein (1907): “As far as inertia is concerned, mass (m) is equivalent to an energetic content (…) mc2. This conclusion is extremely important because it means that both the inertial mass and energy of a physics system are equivalent factors”. Although this equivalence between mass and energy was already known, thanks to some previous experiments carried out before, Einstein was truly the main responsible for its generalization.

As you know in an energy reaction it is not possible to extract all the energetic content of mass (for example, from a tonne of wood). So the efficiency associated to each product or energy source was obtained from tabulated data in international bibliographic sources.

Last but not least, it is important to add that the energy involved in human and animal labour (in this specific case, the horse’s – for historical reasons) derives from the usual definitions of these basic concepts.