L E C T U R E S A T U N I V E R S I T Y O F B U E N O S A I R E S 9 5 3 In other words, while impulse (quantity of movement) and kinetic energy appear as in- dependent in mechanics, as I have already said, the first being represented by a vector and the second by a scalar, in electrodynamics energy is represented by a tensor (symmetrical and of second order), which contains impulse as its constituting element. We can say, he continued, that regardless of the type of energy, there is a tensor of that nature, and that the equation established for force also serves when there are no electrical charges, in which case the tensor in question will pertain exclusively to ordinary matter. Immediately thereafter he deduced the type of tensor for matter at rest. (The reader may not be aware of the essential significance of what we have just said, for which reason we will express in simple words the content that concerns the stated relation- ships between impulse and energy. We have said that in classical mechanics, these obey independent principles. Through Maxwell’s equations and the postulate of special relativity we deduce that if a body moves with a velocity and absorbs electromagnetic energy E that originates in systems at rest with it, its energy increases with no change of velocity, in the amount given by the quotient between E and the square root mentioned on several occa- sions. That is, everything happens as if the mass of the body had experienced through ab- sorption an increase equal to E: , where c represents the speed of light. This is interpreted by saying that the inert mass of a body is not a constant, but rather variable in terms of changes in its energy. The law of conservation of mass is thus identified with that of con- servation of energy. The fact that, despite variations in energy, it is not possible to prove variations in mass in ordinary chemical reactions should not be surprising, since, to give one example, in com- bining oxygen with hydrogen, for each gram of water that forms, about 3300 calories are liberated that is, around 1300 million ergs. In order to obtain the loss of mass this produces, we have to divide by the speed of light expressed in centimeters per second, that is, by 30,000 million, which gives a number equal to that obtained by dividing 1.5 by 10,000 mil- lion, a number that is far beyond the reach of our best powers of observation. In radioactive transformations, the variables of energy, and consequently those of mass, are far larger, and even when they are closer to our possibilities of observation, they still elude them. The or- igin of the heat of the Sun might be expressed on the basis of this principle, by conceding— as Prout’s hypothesis assumes—that all substances have been formed from hydrogen.) At this point, Dr. Einstein said, I will conclude my discussion of special relativity, as various consequences we have not considered are easily accessible with what we already know. On General Relativity – Some Considerations – The Principle of Equivalence The principle of special relativity—Dr. Einstein repeated—establishes that in their phys- ical aspect, natural processes are described by laws in the same way in all systems that move relatively with rectilinear, uniform motion. In other words, that kind of movement of a body cannot be determined through internal experiments, no matter what they may be. In the presence of the simplicity communicated by that principle to the laws of phenomena and the nature with which these are described, a desire to generalize it is born, he added. The generalization might consist of assuming the identity of natural laws, that is, their c2