Stargazing: Time dilation and the speed of light


Jane gets into a spacecraft and travels to a nearby star. From her point of view, she has taken most of her lifetime to get there and back. But when she returns, she finds that all of her relatives are dead and Apple no longer supports her version of the iPhone.

Astronomers can predict with absolute certainty that Jane would age more slowly than her counterparts on Earth. The effect seems to violate common sense because, well, it does violate common sense.

It is, in fact, common sense that is to blame. We assume that our neighborhood, planet Earth, is typical of the universe as a whole. That assumption, which forms the basis for our common sense, is manifestly false.

The world around us is dense with matter, but the universe is mostly empty. More significantly, we live in only a tiny part of that world relative to the universe as a whole ­ — the small and the slow.

So what gives in the larger, faster world?

What gives is Einstein’s Special Theory of Relativity, which has the reputation of being difficult to understand. There are good reasons that this is so.

Einstein’s theory is meant to explain the behavior of the world at all levels of velocity. For our day-to-day purposes, the older Newtonian physics work just fine, thank you very much, and we aren’t much interested in what happens on the level of the very large and the very fast.

However, if we want humans finally to travel to the stars, we’d better start getting interested. To do so, we need to shed some light on light. (And now, gentle reader, please prepare yourself for a long but necessary digression that will take up the rest of this week’s column.)

For most of human history, people, scientists included, assumed that light got where it was going instantaneously. Galileo tried to measure the speed of light with signal lanterns set at varying distances from each other. He failed because the lanterns were much too close to measure differences. Light can travel several times around the earth in one second. In our very small neck of the universe, light in effect takes no time at all to get from one place to another.

Things changed dramatically when astronomers started to use telescopes to study the universe at astronomical distances. As with most matters scientific, a revolution in our understanding of light occurred when astronomers were studying something else entirely.

Over the years, Giovanni Cassini had plotted the orbits of the four main moons of Jupiter. Periodically, the moons pass behind Jupiter and are eclipsed by it. Once he knew the time each moon took to travel around Jupiter, Cassini should have been able to predict when each moon should pass behind the planet. But mysteriously, the intervals between eclipses weren’t always the same.

Ole Roemer, one of Cassini’s assistants, studied Cassini’s data and noticed that when the Earth and Jupiter were far apart, the times between eclipses were longer. Six months later, when the Earth was on the other side of the sun and closer to Jupiter, the intervals got shorter.

Roemer realized that the light from Jupiter’s moons simply had to travel farther when the Earth and Jupiter were farther apart. That meant that the light took time to get where it was going!

Roemer noticed the difference because his “lanterns” were hundreds of millions of miles apart. From the data, Roemer was able to calculate the speed of light with amazing accuracy, his lasting contribution to science.

Scientists concluded that since light was traveling from one place to another, it had to be traveling through some medium. Sound needs the medium of air or water to go from its origin to somebody’s ear, for example. Our common sense experience of sound dictates that conclusion, but remember what I said about common sense.

Light’s medium came to be called the luminiferous aether. Everything, including our own planet, moved through the aether. For the purpose of living life on our planet, we entertain the pleasing fiction that we are stationary, but we are not. We are plowing through space at 30 miles per second as we orbit the sun.

Thus, from our point of view, the aether wind should be blowing in our faces.

This is much like a dog hanging its head out a car window. The wind might not be blowing at all, but from the standpoint of the dog, it’s going to seem like he’s stationary and the wind is plastering him in the face.

By 1887, Albert Michelson and Edward Morley had attempted to chart the flow of that aether wind. Using a complex set of mirrors and prisms, Michelson and Morley projected a beam of light and analyzed its return speed of light in different directions at various different times. By detecting small variations in the light’s speed, they thought they could detect the motion of the Earth through the aether. To their — and everybody’s ­ — surprise, they detected no difference in light speed. The speed of light seems to be constant both against and with the flow of the aether.

In fact, the constancy of the speed of light is intimately woven into the fabric of space and time, which astronomers refer to as one entity that they call Spacetime.

If the notion of Spacetime seems strange to you, try considering time as a fourth dimension. In order to exist, an object certainly has to have the first three dimensions: height, width, and breadth.

Now consider the same object without the fourth dimension — duration. An object without duration simply doesn’t exist at all. Time is therefore a necessary fourth dimension. As such, the forces of nature can change it in similar ways to the other three dimensions. Time can be stretched and bent in ways that are analogous to the ways that objects in space can be bent or stretched.

That fact and the fixity of the speed of light led to a revolution in our understanding of the universe we will discuss next week.

Tom Burns


Tom Burns is director of the Perkins Observatory in Delaware.

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