Vassar Today

Relatively Speaking

By Morton A. Tavel

Einstein’s Special Theory of Relativity was a result of asking an innocent and, to anyone but a physicist, a seemingly uninteresting question: “If you move with a constant velocity, can you tell that you are moving?” In the late 1800s, the answer was believed to be, “Yes, you can!”

The reason you could detect your motion, the answer went, was that the universe was thought to be filled with a fixed “aether,” a mysterious substance that provided the mechanism by which light traveled and which could, therefore, provide a frame of reference against which to measure your own motion.

Unfortunately, all attempts to measure the Earth’s speed through the aether met with failure, because that speed always turned out to be zero. Now, this is a “failure” if you believe the aether is really there and that we move through it. Einstein, however, believed that measuring one’s constant speed should be fundamentally impossible, that such motion was in fact a “symmetry,” a property of one’s existence that was unmeasurable. Einstein realized that if variations in the motion of light through the aether were measurable, then that could provide a mechanism by which to measure your own motion. So he simply did away with that possibility by declaring that all observers must measure the same speed of light, regardless of their own motion.

Einstein was so sure that measuring your own speed was impossible, that he was willing to make a statement about light that seemed equally impossible. Special relativity is the combination of Einstein’s two beliefs: (1) It is impossible to measure one’s own constant velocity, and (2) The speed of light is absolute; it is the same for all observers.

Once you accept the absoluteness of light speed, an incredible series of consequences follows. First, the concept of “simultaneity” loses its absolute nature and becomes relative. Einstein showed that an observation by one observer that two separated events occur at the same time becomes, for another observer, moving at a different speed, an observation that they occur at different times. This relativity of simultaneity then becomes the reason for yet another seeming impossibility, the relativity of length. Einstein pointed out that a measurement of the length of a moving object requires the simultaneous observation of both ends of the object being measured and their positions on a ruler placed against those ends. Since two length measurers will disagree on the simultaneity of ruler placement, they will disagree on the length they measure.

Like a collapsing house of cards, the next “truth” to fall is the absoluteness of time duration. Since all clocks depend for their accuracy on a length (length of a pendulum, spring, crystal, etc.), the relativity of length produces a corresponding relativity of the time duration between ticks of any conceivable clock mechanism. By demonstrating that it is the clocks and rulers that fail the test of absoluteness, Einstein also challenged the conception that there is a metaphysical “time” and “space” that clocks and rulers are measuring. Rather, argued Einstein, there are only measurements made by clocks and rulers. It is from those measurements that we build an abstract edifice called space and time. But that edifice, albeit useful, is an illusion.

Above right: Professor of Physics Mort Tavel reveals that his “intellectual passion is explaining complex scientific ideas to a wide audience of non-science majors.” Tavel’s new book, Contemporary Physics and the Limits to Knowledge (Rutgers U. Press), is an outgrowth of the Relativity course he has taught since 1975.

As I go to great pains to point out in my lectures, our brains have genetically evolved to comprehend the reality we live in, what I call “mesoscopic” or middle-sized reality. Since we have never lived in a world of high-speed motion, we have never observed the relativity of length and time duration that Einstein’s theory predicts. To us, there is space and time, two separate and independent aspects of reality. What we must teach ourselves is that there are really only measurements of spatial separation and temporal duration and that space and time are our useful, but illusory, constructions.

Because of the way these two measurements correlate (lengths shrink, time durations stretch out), we are forced to conclude that a synthesis of these two measurements, call it spacetime length, is what is absolute. In some future reality, in which interstellar motion at speeds close to light speed are possible, the separate notions of space and time would become useless. Just as “up” and “down” are useless notions to an astronaut orbiting the Earth, so would separate space and time measurements be useless to an astronaut zipping through the void.

Finally I tell my students that if they do not understand relativity, it is not a result of inattentiveness to my lectures; rather it is a genetic disability.

Morton A. Tavel is one of the founders of the Science, Technology, and Society program, Professor of Physics Morton A. Tavel has been teaching at Vassar since 1967. In 1971, Tavel introduced his wine tasting course that has been taught every year since.