By Daniel Siegel
In December of 1923 a pierce of doggerel appeared in Punch, poking fun at Albert Einstein's newly famous theory of relativity:
There once was a lady named Bright,
Who traveled faster than light.
She set out one day,
In a relative way,
And returned on the previous night.
The piece was unsigned, but years later A.H. Reginald Buller stepped forward to claim authorship. He was a fellow of the Royal Society of Canada and came from a different field of science: he was editor of the seven-volume Researches in Fungi.
In the early years, experimental support for relativity theory was meager: full vindication of Albert Einstein's ideas was still to come. Relativity theory had drawn startling conclusions concerning the four most basic physical quantities-length, time, mass, and energy. In the course of the century, these results would receive direct and very striking experimental confirmation. The relativistic effects also became the basis for new technologies, such as Global Positioning Systems (GPS), whose continued functioning would verify these effects every day and every passing hour.
The disruption of time was the most fundamental conclusion. Both in Einstein's technical paper of 1905 and in Relativity Clear and Simple, the relativity of simultaneity formed the basis for all subsequent discussion. In particular, Einstein showed that moving clocks as compared with stationary clocks would run slow as a result of their motion.
As Einstein was philosophically committed to the idea that time was nothing more nor less than what you could measure with standardized clocks, he necessarily concluded that time itself passed more slowly in a moving frame of reference and the faster the motion of the reference frame, the slower the passage of time. Background, collage on the relativity of time; inset, Einstein's written notes on the theory of relativity; left, Einstein in 1905, the year his theory was published. This was called time dilation: time slows down, stretches out, dilates, in a moving reference frame. This was the most revolutionary conclusion of relativity theory. It was also, for a period of more than thirty years, completely unsupported by any direct experimental evidence.
Critics of relativity theory, of course, jumped on Einstein: Wasn't it ridiculous to make the claim--on the basis of no direct evidence whatsoever--that time itself could slow down? And wouldn't various paradoxes and absurdities result from this kind of elasticity of time? Would an astronaut who traveled in a rocket ship at high velocity age less than his twin who stayed at home? If time could slow down as a result of motion at high speed, would time reverse if one went fast enough?
Discussion of time dilation left the realm of the fanciful when it became possible to verify this effect in a direct manner.
This first occurred in 1941, when time dilation was detected in experiments on cosmic rays. The earth is continually bombarded by atomic particles from outer space. These swiftly moving particles are the "primary" cosmic rays. When the particles reach the top of the atmosphere, they collide with atomic nuclei. Subatomic debris is produced, constituting the "secondary" cosmic rays, which then travel downward toward the surface of the earth. In particular, particles called muons are produced in the upper atmosphere and move downward toward the surface.
Muons are highly unstable particles, having an average lifetime of about a millionth of a second. Given the short lifetimes of the muons and the long distances they have to travel to get down to the surface of the earth, one can calculate that, given the velocities at which they travel, very few of them should actually make it down to sea level. However, large numbers can be detected- many more than expected. It appears that, somehow, the moving muons have longer lifetimes than expected, so that they can travel longer distances than expected. This is exactly what would be expected on the basis of time dilation. The muons are traveling at velocities comparable to the velocity of light, and their internal "clocks" should slow down as a result--in accordance with Einstein's prediction--so that many more are able to reach the surface of the earth than would be otherwise expected. Precise experiments on muons gave results exactly in accord with Einstein's equation for time dilation, verifying the effect quite convincingly.
In the years after World War II, experiments on unstable elementary particles such as muons were carried out by using high-voltage particle accelerators to produce beams of the particles. The time dilation equation was again verified to high precision, and the experimental technologies used in particle physics have come to rely on time dilation for their successful day-to-day operation.
For those who are not particle physicists, verification of time dilation has become possible with the development of a device known as an atomic clock, which can measure time intervals to a precision of one part in a trillion.
Consider flying in an airplane at five hundred miles per hour. This produces minimal time dilation, and air travelers have not noticed their watches running slow as a result of this effect. Calculations on the basis of Einstein's equation for the time dilation, however, show that the expected effort is a slowing down by about one part in a trillion, which should be measurable by an atomic clock.
In 1971, a team of scientists who were experts in the use of atomic clocks set out to detect and measure time dilation and other relativistic effects. The research team was able to devise a cheap and effective plan, which received some support from the Office of Naval Research. We are told that the researchers purchased three around-the-world tickets on regularly scheduled commercial airliners-two tickets for the accompanying scientists and one for an array of four atomic clocks. The clock array had its own seat; it sat, belted in for safety, between its two caretakers. Before leaving on the trip, the clocks were synchronized with a master clock at the U.S. Naval Observatory. The four clocks then went around the world, following which they were compared again with their counterpart, which had stayed behind at the Naval Observatory. After correcting for the rotation of the earth and the variation of the force of gravity with altitude, it was found that the clocks that had been in motion in their journey around the earth had in fact slowed as compared with the clock at the Naval Observatory, and by exactly the amount predicted by the theory of relativity. The result was further confirmed in a second around-the-world flight in the opposite direction.
The effect of this exercise on the scientific community was more to demonstrate the capabilities of atomic clocks than to make any substantive change in the way scientists regarded the theory of relativity, but the result was nevertheless satisfying: it was the most direct possible realization of Einstein's thinking about measuring time with real, physical, standard clocks. The clocks had behaved exactly as Einstein had predicted they would.
These days, when atomic clocks are transported by air from Washington, D.C., to Boulder, Colorado, as part of the regular maintenance of the United States time standard, corrections are made for the time dilation based on a log of the flight with records of ground speed and elapsed time. Similarly, in the satellite-based GPS that represents the current state of the art in navigation, corrections are made for the effect of time dilation on the atomic clocks orbiting in satellites whose time signals form the basis for the system.
If time dilation were somehow turned off, not only would particle physics experiments shut down, but the most advanced navigational systems for both military and civilian needs would fail to operate. Aircraft and missiles, ships and submarines, trucks and trains, and hikers and hunters with their consumer-market GPS systems would literally lose their bearings. In the new millennium both science and technology have come to depend at every passing moment on the particulars of the behavior of time as specified in the theory of relativity.