Many among us hunger for humans to travel to the stars, but sadly, the stars may lie beyond our grasp.
Here’s a practical example to illustrate the difficulty.
Jane gets into a spacecraft and travels to a nearby star. From her perspective, she has taken most of her life to get there and back. But when she returns, she finds that her relatives are long dead, and Apple no longer supports her version of the iPhone.
Astronomers can confidently predict that Jane would age more slowly than her counterparts on Earth. The effect seems to violate common sense because it does violate common sense.
In fact, common sense is mostly to blame. We assume that our neighborhood, planet Earth, is typical of the universe as a whole. However, that assumption, which forms the basis for what we call “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 explains the behavior of matter 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 to travel to the stars, we’d better start getting interested. To do so, we need to shed some light on light.
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 tiny neck of the universe, light, in effect, takes no time at all to get from one place to another.
Things dramatically changed when astronomers started to use telescopes to study the universe at astronomical distances.
Over the years, Giovanni Cassini had plotted the orbits of the four main moons of Jupiter. Periodically, the moons pass behind Jupiter. Astronomers call such events eclipses.
Those eclipses should occur at precise intervals, but they didn’t.
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, the intervals got shorter when the Earth was on the other side of the sun and closer to Jupiter.
Roemer realized that the light from Jupiter’s moons 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 could calculate the speed of light with startling accuracy, his lasting (and mostly unappreciated) contribution to science.
Scientists concluded that since light was traveling from one place to another, it had to travel through some medium. Sound needs some medium, like air, 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. Earth must be plowing through the aether at 30 miles per second as we orbit the sun.
From our perspective, the aetheric wind should be blowing in our faces, much like a dog hanging its head out a car window. The dog thinks he’s stationary, and the wind is plastering him in the snout.
By 1887, Albert Michelson and Edward Morley had attempted to chart the flow of that aetheric 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 different times.
By measuring tiny variations in the light’s speed, they thought they could detect the motion of the Earth through the aether. To their surprise, the speed of light appeared to be constant against and with the flow of the aether.
At this point, some of you are probably thinking, “What’s with this light obsession? Why doesn’t he get to the point?”
But light is the point, gentle reader. The universe, not I, is obsessed with light and its velocity. The constancy of the speed of light is intimately woven into the fabric of space and time, which astronomers refer to as one entity called Spacetime.
If the concept of Spacetime seems strange, try considering time as a fourth dimension. For an object to exist, it has to possess the first three dimensions: height, width, and breadth.
Now consider the same object without the fourth dimension, duration. An object without duration doesn’t exist at all. Therefore, time is a necessary fourth dimension.
As such, time can be stretched and bent in ways that are analogous to the ways that objects in space can be bent or stretched. Velocity is one of the things that does the stretching.
To illustrate, let’s hop in the car. Imagine that you are driving down the highway. You have to speed up because Einstein is on the road. You want to run him over because you just flunked a test on cursed relativity.
It takes energy to accelerate, and thus you must add larger and larger quantities of energy for your car to go faster. The effect isn’t particularly noticeable in our low-velocity world. But as you approach the speed of light, the energy you need to accelerate becomes enormous.
It’s as if you were driving the car through a liquid that got thicker and denser as you moved until the liquid was as solid as a brick wall that no amount of energy would allow you to plow through.
The brick wall is the speed of light. If you could get to light speed, the energy required would become infinite, and only an infinite amount of energy could get you there. That’s more energy than you’ll find in the entire Universe, which possesses only a finite amount.
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.
Now we can finally get to the point. How then does the acceleration of an object with mass toward the speed of light affect time?
The fixity of light speed turns out to be a vital feature of the mathematical equations that describe how things work in Spacetime. It is thus fundamental to any description of how Spacetime functions.
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, for example, the International Space Station in orbit around Earth. Our clocks tick off the seconds at what we think is a fixed rate. But the clocks on the International Space Station, which is moving more rapidly than we are, tick more slowly. The astronauts aboard the Space Station age 0.014 seconds slower every year they are aboard the ISS. Their internal clocks are ticking more slowly as well.
As an 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 need infinite energy to do so. Moreover, time would stretch out to infinity. If the clock 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.
When that astronaut returned to Earth after what for her was a lifetime of travel, she would have aged normally from her perspective. However, 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. Who among us would make such a journey?
The stars may be beyond our grasp, but they are gloriously within our sight. And that, dear reader, is good enough for me.
Tom Burns is the former director of the Perkins Observatory in Delaware.