Next in my series of summaries of major theories in physics, I'd like to discuss Einstein's theory of special relativity. Under the rubric of "relativity," Einstein actually had two theories, those of special and general relativity. The former, being a "special" case, applies only to instances of constant velocity, where acceleration can be ignored. Einstein's later theory of general relativity incorporates acceleration, most prominently gravity; general relativity will be discussed in a later posting.
As I noted in a previous posting, special relativity was one of four major ideas Einstein published within a single year, 1905, the centennial of which is currently being celebrated as the World Year of Physics.
As with many other concepts discussed on this blog, I was introduced to special (and general) relativity by Brian Greene's book The Fabric of the Cosmos. As the term "relativity" implies, people's experiences of time and distance are not absolute, but relative (at least under certain conditions). Writes Greene:
The relativity of space and of time is a startling conclusion. I have known about it for more than twenty-five years, but even so, whenever I quietly sit and think it through, I am amazed. From the well-worn statement that the speed of light is constant, we conclude that space and time are in the eye of the beholder. Each of us carries our own clock... The same is true of distance. Each of us carries our own yardstick... (p. 47).
One of the things Greene initially does is get the reader to think of space and time -- which many people may think of as separate concepts -- in integrated fashion:
Newton thought that motion through time was totally separate from motion through space -- he thought these two kinds of motion had nothing to do with each other. But Einstein found that they are intimately linked. In fact, the revolutionary discovery of special relativity is this: When you look at something like a parked car, which from your viewpoint is stationary -- not moving through space, that is -- all of its motion is through time... But if the car speeds away, some of its motion through time is diverted into motion through space (p. 48).
In other words, with special relativity, motion through space -- at extremely high speeds -- makes time move along more slowly!
One research study that supports special relativity was done with objects very familiar to laypeople, namely clocks and an airplane. As Greene describes it:
In 1971, Joseph Hafele and Richard Keating flew state-of-the-art cesium-beam atomic clocks around the world on a commercial Pan Am jet. When they compared the clocks flown on the plane with identical clocks left stationary on the ground, they found that less time had elapsed on the moving clocks. The difference was tiny -- a few hundred billionths of a second -- but it was precisely in accord with Einstein's discoveries (p. 50).
This website provides further detail on the experiment (when the new page comes up, it may look as though there is only one screen of information, but scroll down for more).
Also, for those of you old enough to remember Pan Am airlines, here's an historical website on the company.
I'll have more on special relativity next week...
