The Philosophical Significance of the One-Way Speed of Light

Abstract

In the 5th century B.C., Zeno of Elea argued that motion is impossible. Although few philosophers have accepted Zeno's conclusion-the most notable exception being F. H. Bradley-it must be conceded, I believe, that Zeno did raise some profound questions. Subsequent discussions of these foundational problems have substantially deepened our understanding of space, time, and motion.1 If we admit that motion does occur, and that it constitutes a pervasive and fundamental feature of the universe, then we will immediately recognize that the concept of velocity plays a key role in our description and understanding of nature. The main purpose of the present paper is to discuss certain foundational questions about the concept of velocity, and its role in our knowledge of the physical world. I shall try to show why philosophers have reason to be vitally concerned with what appears at first blush to be a purely physical question (and a rather simple one at that): can we-and if so, how-measure the speed of light on a one-way trip? If classical physics were correct, we could readily give an affirmative answer to the question about the possibility of ascertaining the one-way speed of light, and methods which would be feasible for doing so could easily be devised. I shall explain below how this could be done. But classical physics is not acceptable; it has been superseded by Einstein's special theory of relativity. This theory, first enunciated in 1905,2 has