Special Relativity | Sep 2005

FMBR Editorial: Sep, 2005

Special Relativity

William C. Gough

A hundred years ago Albert Einstein published a series of papers that changed science as we know it. I became acquainted and fascinated with Einstein's work while in high school. The atomic bombs had been dropped upon Japan, and World War II had quickly ended. We had entered the atomic age. My graduation speech was titled Atomic Energy Is Here for Good. My father gave me Einstein's small book, The Meaning of Relativity. On rare occasions at Princeton University I would see Einstein walking in town in his informal dress without socks. In Graduate School I worked on the nuclear fusion power project. So let's review some of the implications of Einstein's ideas about "relativity" and the scientific evidence that supports his theories.

Einstein's special theory of relativity has at its essence one simple statement: The laws of physics are the same in all uniformly moving reference frames. This means that the laws of physics on Earth remain the same when we ride on a speeding train, airplane, or spaceship, or land on the moon or another planet. Hence, in physical reality you will always get exactly the same results from any scientific experiment regardless of its state of motion. The principle of relativity implies that motion itself is undetectable and therefore meaningless. How many of us think about the motion of our planet Earth which is traveling at about 67,000 miles per hour through space? To Einstein all that matters is relative motion.

Although the idea behind relativity theory appears simple, the consequences are not. Among the laws of physics are Maxwell's equations of electromagnetism. These laws predict that all electromagnetic waves travel at the speed of light. Therefore, electromagnetic waves must travel at the speed of light in all reference frames and must be the same speed for all observers regardless of their states of motion. What Einstein's relativity also shows is that time and space are actually aspects of the same underlying physical reality. Newton's world in which time and space were separate quantities is no longer valid. The speed of light effectively becomes a conversion factor between units of space and time. This is the reason that physicists now use the term spacetime. There is no such thing as a universal "present." Time and space are not absolute and the order of events can depend upon one's frame of reference -- hence simultaneity is relative. In everyday life, we do not observe the differences in travel time between objects since the speed of light is so fast -- 186,000 miles per second. However, for astronomers observing distant galaxies the light takes from million to billions of years to reach us. They only can observe the past.

Most people have heard of the thought experiment known as the twin paradox. There are two twins, one twin takes a spaceship that travels close to the speed of light and goes to a distant location in our galaxy, the other remains on Earth. When the space traveler returns he is many years younger than his Earth bound twin. According to relativity theory this is possible since both time and space are different in different reference frames. Einstein's prediction of time dilation and length contraction was experimentally tested in the 1960's using subatomic particles. There exists a steady stream of subatomic particles called muons that come downward through the Earth's atmosphere at speeds approaching the speed of light. The muons are formed when cosmic rays slam into atoms high in the Earth's atmosphere. Muons are radioactive and act like clocks. Their rate of radioactive decay has been accurately measured.

Special detectors designed to sense muons traveling close to the speed of light were located at both the top of the 6,300 foot high Mount Washington in New Hampshire and at sea level. If Einstein is wrong the muons should self destruct and very few would survive to reach sea level. However, if Einstein's relativity theory regarding time dilation and length contraction is true, then many of these particles that are traveling at close to the speed of light (0.995c) would survive. The data was clear. The muons' lifetime confirmed that time dilation occurs, and in fact the results precisely fit Einstein's predictive equation. For the near-speed-of-light muons the height of Mount Washington is contracted -- the 6,300 foot mountain becomes only 700 feet high!

But can we check Einstein's theory of time dilation using something larger than a subatomic particle? There is no way that we can propel a regular clock at a speed close to that of light. However we do have clocks that are accurate to a few billionths of a second. These "atomic clocks" are used to set the world time standards. In the 1970's scientists placed atomic clocks on commercial airplanes and flew them around the Earth. The objective was to test Einstein's prediction of relativistic time-dilation by comparing the time readings of the "flying" clocks to atomic clocks that had remained on the ground. The experiment showed that less time had elapsed on the "flying" clock by some 300 billionths of a second. The relative motion of the airplanes to the Earth had resulted in time-dilation!

The confirmation of Einstein's special relativity theory demonstrated that the world we live in is stranger than either Newton or we had thought, -- and we haven't even discussed Einstein's theory of general relativity!

William C. Gough, FMBR Chairman of the Board


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