Why does time pass more slowly in space than on Earth?

On Space, Time, and Spacetime

by Tom Burns

      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. What gives?

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 and massiveness. We live in only a tiny part of that world relative to the universe as a whole—the small and the slow. 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.)

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 Domenico 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. Common sense dictates this. Sound needs the medium of air or water to go from its origin to somebody’s ear, for example.

This 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 A. Michelson and Edward W. 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 though 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.

Now at this point, some of you are probably thinking, “Wasn’t this scribbling supposed to be about time? What’s with this light obsession? Why doesn’t he just get to the point?”

But light IS the point, gentle reader. The universe, not I, is obsessed with light and its velocity.

It is the key number affecting everything the universe does and, by extension, what we do. Let’s say you are driving down the highway and you have to speed up because Einstein is in the road, and you want to run him over because you just flunked a test on cursed relativity.

As you accelerate, your mass increases, i.e., you get heavier. The effect isn’t noticeable in our low-velocity world. But as you approach the speed of light, you get much heavier, so much so that it takes an enormous amount of energy to keep going faster.

If you could get to the speed of light, your mass would become infinite. You would weigh more than the entire Universe. It would take an infinite amount of energy to reach the speed of light. Both infinite mass and infinite energy are absurd on the face of it.

Thus, any object with mass cannot travel as fast as the speed of light. Light can do it because it has no mass.

Physics has its God particle. If any number could be called a God number, the speed of light is it. That magic number is woven into the fabric of space and time itself.

Where were we? Oh, yes. Now we can finally get to the point. How then does acceleration of an object with mass toward the speed of light affect time?

The fixity of light speed turns out to be a key feature of the mathematical equations that describe the way things work in Spacetime. It is thus fundamental to any description of how Spacetime functions.

At low masses and speeds, nothing much seems to happen, but something does. If you hop into your car and drive to visit your Aunt Earline in Albuquerque, you are traveling at a very low velocity of only, say, 65 miles per hour. Both you and she will think you took exactly the same amount of time to get there, but in fact, although neither she nor you can detect it, from her point of view, you took a fraction of a second longer to get there than you thought you did.

At very high masses and velocities, Spacetime is a very different thing, and thus the Universe is a very different place. All of that happens because the speed of light always remains the same.

The revolutionary implications of the invariability of the speed of light came to fruition in Einstein’s Special Theory of Relativity.  He demonstrated mathematically that for an object in motion, time slows down. It, in effect, stretches out. That effect has been verified many times over the years with careful experimentation. It’s real.

On Earth, we are moving very slowly with respect to, say, the International Space Station in orbit around Earth. Our clocks tick off the seconds at what we think of as a fixed rate. But the clocks on the International Space Station, which is moving more rapidly than we are, tick more slowly. In fact, the astronauts aboard the Space Station age 0.014 seconds more slowly for every year the are aboard the ISS. Their internal clocks are ticking more slowly as well.

As our astronaut approaches the speed of light, the effect intensifies dramatically. If she is moving rapidly enough with respect to Earth, her clock would seem to slow down to a crawl if an observer on Earth had a camera pointed at it. To the astronaut, the clock would tick normally.

Our astronaut could never actually achieve light speed, of course. Her spacecraft would reach infinite mass. From the observer’s point of view, it would be bigger than the universe that surrounds it, which is silly. Time also stretches out to infinity. If the clocked ticked, the tock would never occur.

If the astronaut could observe events on Earth, she would see events happening in rapid motion as if the video of those events had been dramatically speeded up.

Thus, when and if that astronaut returned to Earth after what for her was a lifetime of travel, she would have aged normally from her point of view, but all her relatives would have long since passed away. She, the legendary space traveler, would perhaps be greeted by her great-great-grandchildren, like some latter-day Rip Van Winkle, a quaint representative of an earlier, more primitive age.

Tom Burns is Director of Perkins Observatory in Delaware, Ohio, as well as a columnist and Part-Time Professor of English at Ohio Wesleyan University.