Is time the fourth dimension?
• Time is often thought of as the fourth dimension. • Time plays a key role as a dimension in mathematical formulations of physical laws and theories such as general relativity and string theory. • The qualitative behavior of time as the fourth dimension is debatable. Viewing time as the fourth dimension is appealing for a number of reasons. The first is that we naturally have experience with time coordinates. When we tell someone we will meet them for coffee at 3 P.M., we are specifying a point in time. However, to increase the odds that the meeting actually occurs, we also need to specify a place. So, establishing the meeting uniquely requires three spatial coordinates and one time coordinate. You might say, “Meet me at 3 P.M. on the fifth-floor terrace of the building on the northwest corner of 3rd Street and 4th Avenue,” for example. Of course, it is possible for time to change independently of the spatial coordinates—all you have to do is sit relatively still and your time coordin
