The interval between two events is a physical fact about them, not dependent upon the particular circumstances of the observer. There are two forms of the theory of relativity, the special and the general. The former is in general only approximate, but is exact at great distances from gravitating matter. When the special theory can be applied, the interval can be calculated when we know the distance in space and the distance in time between the two events, estimated by any observer. If the distance in space is greater than the distance that light would have traveled in the time, the separation is space-like. Then the following construction gives the interval between the two events: Draw a line =AB= as long as the distance that light would travel in the time; round =A= describe a circle whose radius is the distance in space between the two events; through =B= draw =BC= perpendicular to =AB=, meeting the circle in =C=. Then =BC= is the length of the interval between the two events. [Illustration] When the distance is time-like, use the same figure, but let =AC= be now the distance that light would travel in the time, while =AB= is the distance in space between the two events. The interval between them is now the time that light would take to travel the distance =BC=. Although =AB= and =AC= are different for different observers, =BC= is the same length for all observers, subject to corrections made by the general theory. It represents the one interval in “space-time” which replaces the two intervals in space and time of the older physics. So far, this notion of interval may appear somewhat mysterious, but as we proceed it will grow less so, and its reason in the nature of things will gradually emerge. CHAPTER V: SPACE-TIME Everybody who has ever heard of relativity knows the phrase “space-time,” and knows that the correct thing is to use this phrase when formerly we should have said “space _and_ time.” But very few people who are not mathematicians have any clear idea of what is meant by this change of phraseology. Before dealing further with the special theory of relativity, I want to try to convey to the reader what is involved in the new phrase “space-time,” because that is, from a philosophical and imaginative point of view, perhaps the most important of all the novelties that Einstein has introduced.