Concept Summary      Chapter 7

In the last chapter we saw how Newtonians were forced to assume that space and time are absolute -- that each event has one objective spatial and temporal location. In this chapter we have seen that part of Einstein's discovery involved a philosophical insight, that Newtonianism involved this assumption and that scientific knowledge of space and time would require empirical tests.

It seems paradoxical that we cannot safely assume and know upon reflection alone that time flows uniformly on throughout the universe as our common sense dictates. It never occurs to me that if I leave my home at 8:00 and arrive at my office at 8:30 that I am assuming that it is not then 9:00 at home. In the eighteenth century, in spite of the tremendous practical success of Newtonian physics, the British philosopher David Hume showed that the application of another major concept, causality, was also paradoxical. No matter how successful a science, such as Newtonian physics, had been in predicting the conjunction of events, it could not be known with certainty that one event actually caused the other.

For the Newtonian philosopher and physicist Immanuel Kant, this was a disturbing epistemological challenge. As the sophists had challenged the ancient Greeks centuries before, Hume was challenging all Newtonians to demonstrate how this new science provided a durable knowledge, to show how the "truths" of this new science would not be overthrown by future experiences. Kant concluded that this was possible only through a radical inversion of our normal thinking about objectivity. Rather than seeing such fundamental concepts as causality, space, and time (and the basic principles of mathematics used to describe applications of these concepts) as reflections or representations of a known objective reality external to the human mind, such concepts must now be seen as fundamental categories of understanding or conceptual "filters" by which external reality must be experienced. For Kant, Hume may have shown that it is impossible to be certain that Newtonian concepts mirror the things-in-themselves (the real universe independent of human perceivers and knowers), but we can be certain nevertheless in knowing that all future experience will conform to these concepts, because these concepts make perception possible; they are fundamental concepts that we "impose" upon the world.

Kant's influence was enormous, but he prepared the way for Einstein, and much of modern thinking, most by being wrong. By attempting to demonstrate that the fundamental concepts of Newtonianism were irrefutable, he drew attention, as did Newton in his attempt to rationalize the absoluteness of space and time, to the fragileness and anthropocentrism of these assumptions. If reality could not be shown to conform to common-sense human assumptions, then the door was wide open for much creative thought on other possibilities.

Einstein recognized that one could not assume that two widely separated, initially synchronized clocks continued to keep the same time. He recognized that time must be tested by measuring and comparing what time each clock records. To compare such clocks, one must take into account the speed of light and the relative motions of the reference frames of each clock. Einstein showed that when this is actually done, because the speed of light had been discovered to be the same regardless of its direction and the speed of its source, our intuitive sense that time is something that just clicks along independently of moving objects will be violated when we compare initially synchronized clocks. Simultaneity is relative to a reference frame, and time dilation, the slowing of time relative to another reference frame, is a fact of life. If my home and office were separated by many light-years, not only could I not assume the times to be the same, but my home (depending on the distance and the speed of my travel) could be many thousands of years in the future when I arrive at work.

Although the success of Einstein's theories does not imply that everything is relative or that scientists create reality, it does show that an honest empiricism, one that tests fundamental assumptions, has brought the observer into twentieth-century physics. The pane of glass that separated the observer from reality in Newtonian physics has been shattered. To some extent what is real does depend on us. Einstein's theories also set the stage for the great paradox of twentieth-century science. We must be careful about what we assume to be objective; what seems to be obviously an objective property of reality, may be a subjective projection of a merely human point of view, one applicable to only a limited range of experience. Nothing seems to me more certainly objective and independent of my wishes than the thought that it is the same time at my home now as at my office regardless of the distance between the two. Kant may have shown why we have this sense of certainty in terms of the way the human mind works, but Einstein has shown that reality need not obey our sense of certainty or the workings of the human mind.