Update 13/08: This is the first of three posts on time travel. We'll be exploring time travel to the future in this post, and time travel to the past in the next two posts which should appear in the following weeks. So stay tuned!
Exploring the fourth dimension
For as long as we have strolled the Earth, the idea of time travel (the ability to travel to any point in the past or future) has sparkled our curiosity and imagination. Countless science fiction writers have explored the possibilities and paradoxes of time travel.
The term 'time machine' for instance was first coined by H. G. Wells in his 1895 novel The Time Machine which told the story of an English scientist and gentleman inventor who built a Victorian-looking time travel device to explore the fourth dimension. Nearly one century later, Steven Spielberg produced the movie Back to the Future, where a young Michael J. Fox traveled back in time to re-unite his parents and safe his own existence.
But will science fiction ever turn in science fact? Will scientists ever build a working time machine? Will we become masters of time and find shortcuts to the future? And what would happen if you could open a portal to the past to kill your grandmother as a little girl? Finally, and perhaps most interestingly, where would you go?
In this trilogy of posts, I will provide some answers to these tricky questions. In this first post, we'll start by exploring the idea of time travel to the future. And I bet you'll be surprised to learn "it is not a dream, it is a simple feat" (to paraphrase Nikola Tesla), one that everyone can understand! That's right, after reading this post you'll know everything you need to embark on your own voyage to the future! Now how cool is that?!
But things will get even cooler, as I hope to deal with the thornier issues of time travel to the past in the next two posts. In any case, before we can handle any of this, we first need to better understand the very concept of time itself.
The river of time
Time is like a majestic river, flowing relentlessly from the past to the future, and we are all carried along by its current at a speed of one second per second. In that sense, we are all time travelers, leaving the past for ever behind us, and moving into the unknown future. Whether we like it or not, we are all subject to time's arrow; we are born young and grow old, remembering the past but not the future.
But you might wonder: couldn't the river of time, just like any other river, flow at different speeds in different locations? Couldn't we use the rapids in time's river to speed up the process and travel to the future a little faster than usual?
It turns out we can! And the answer was given to us by the brilliant Albert Einstein more than a century ago, when he revolutionized the scientific landscape with his theory of special relativity.
Special relativity in a nutshell
The secret to time travel boils down to one of Einstein's greatest discoveries — that the speed of light is a fundamental constant of nature that will never change in magnitude. To appreciate the greatness of this discovery, imagine standing on a platform while a train rides past you, and let's say that from your point of view the train is moving at a speed of 100 km/h. Now imagine that instead of standing on the platform, you are sitting on another train, moving parallel to the train in question at exactly the same speed. Looking out of the window, it will seem as if the other train was standing still.
What this example illustrates is that the speed of an object is relative and not absolute. It depends on the way you are moving relative to the object. Given this, you might think the same would apply to a light ray, which travels at the humongous speed of 299 792 km/s! If only you could find a way to travel at the speed of light, surely, the ray of light would seem to stand perfectly still.
But here's the shocking thing. Even if you could chase the ray of light at light's speed, the ray would still move away from you at 299 792 km/s. No matter how fast you move, and in whichever direction you fancy, the speed of light will always be constant!
Clearly, for an idea as revolutionary as this to hold sway, something had to give way. And that something was our common everyday conception of space and time. Before Einstein, space and time were thought to be absolute and universal; they were the same for each and everyone of us. Scientists like Newton for instance believed there was something like a cosmic clock so that everyone could agree on the same time, no matter where they were in the Universe.
But after Einstein, space and time became relative and 'personal', allowing my space and my time to be different from your space and your time depending on our relative motion. By having our own set of clocks and rulers, we might measure time and space differently, in such a way that we would always agree on the same constant speed of light. What's more, in Einstein's view, space and time were no longer separate, independent entities; they had become mixed up to the point of merging into one. In the words of Einstein's teacher, Hermann Minkowski: "Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality."
That union is what scientists have since called spacetime — the four-dimensional fabric resulting from intertwining the three dimensions of space (length, depth and height) with the one dimension of time. (An artistic depiction is shown above.)
So far, so good, but how does any of this help us in building our time machine? For this, we need to consider the idea of a light clock. A light clock is a funny little device with two parallel mirrors, facing each other, and a light source at the bottom. By switching on the light source, a photon emerges and travels vertically upwards to the upper mirror, where it is reflected downwards to the lower mirror to be reflected once again, ultimately bouncing back and forth between both mirrors. Now imagine that whenever the photon hits the upper mirror, you would hear a "tick". And whenever it hits the lower mirror, you would hear a "tack". Sure enough, after pulling the switch, the device would go "tick ... tack ... tick ... tack"; you would have made a working light clock.
But now consider moving the light clock horizontally at a certain speed. As you can see on the picture above, the photon would have to travel a greater distance between both mirrors, since it is now traveling both vertically and horizontally. But remember that the speed of light is constant. Hence, the photon will have to travel a longer time between both mirrors, and in this case, the clock would go "tick ......... tack ......... tick ......... tack." Time would be running slower!
The idea of a light clock illustrates that the flow of time is not absolute but relative: moving objects age more slowly relative to objects standing still. The faster you go, the slower time will tick away.
So in order to transport yourself to the future, all you need to do is travel very, very fast. The best way would be to step in a rocket ship, whisk across space at lightning speed, make a couple of circles around Jupiter, and finally head back to planet Earth.
While you are flying at such gargantuan speeds, the clocks in your rocket ship, including your own biological clock, will tick slower as compared to the clocks left on Earth. If your friends on Earth could see you aboard your rocket ship, they would see you doing everything in slow motion.
So when you finally return to Earth, you might have aged a year, while your friends here on Earth might have aged a hundred years. Basically, you would have travelled 99 years into the future.
Believe it or not, but time travel to the future has already been done in real life, albeit on a very small scale. In 1971, physicists Joe Hafele and Richard Keating performed an experiment with two perfectly synchronized atomic clocks. One clock was loaded into an airplane which made a couple of cycles (eastward) around the world at high speed. When they compared the atomic clock with the one left on the ground, it was out by 59 nanoseconds.
Now what use is that, you might think! After all, traveling a mere 59 nanoseconds into the future is not all that impressive. But it does prove that time travel to the future is theoretically possible. One day, future technologies might make it a practical possibility, enabling the human species to fast-forward to the future by traveling close to the speed of light.
Besides these technological challenges, there is still one more catch, because time travel is a one-way ticket. You're free to explore the fourth dimension into the future, but once arrived, there's no way back to the past. At least, that is how the story went during Einstein’s life.
Anno 2014, scientists are breaking new ground, and uncovering novel ways to travel to the past. Most of them rely on the idea of creating a so-called wormhole in the four-dimensional spacetime fabric, but that is a whole other story. More on this in the second part of the time travel trilogy! I hope to see you there!