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 6 5 two identical clocks, one located in field of potential U and the other in a field of potential , are related by the expression divided by 1 – U. A clock located on the Sun, he continued, would run more slowly than an identical one located on Earth. The optical lines emitted by the same substances on the Sun and on the Earth originate in natural mechanisms that, in a sense, can be thought of as identical clocks. Those coming from the Sun, therefore, must have a lower frequency than the terrestrial lines originating from the same matter. That is to say, the former must be shifted to the red. The experiment, he said, suggests that they are redshifted, but there are so many causes of disturbances in an atmosphere as complicated as the solar atmosphere that we cannot be absolutely sure if this effect is really the one predicted by the theory. Some experimenters think so. But really, we cannot affirm such a thing categorically. Future Development of the Theory We will conclude, Dr. Einstein said, by making some statements and predictions related to the future development of the theory. We are familiar with two kinds of forces, he said: electrical and gravitational that is, two fields, the electromagnetic field and the gravitational field. Each one of them, he added, has its own equations. These forces are essentially of the same nature, he said gravitational forces may also be electrical. The future development of the theory, therefore, cannot help but consist of establishing equations that will encompass both gravitational and electromagnetic phenomena. Weyl and Eddington have made attempts in this direction. I believe—the sage concluded—that none of the paths chosen is the true one, but I am certain that it will not be long until it is discovered. U′ 1 U′ –