We all know that in Newton's view of absolute space and time, time and space are unaffected by matter and motion. This idea of "absolute" is fundamental to his theory.
Clearly, within this view, time travel lacks a theoretical basis and is nothing more than a fantasy.
But everything changed with the introduction of Einstein's Special Theory of Relativity. In this theory, time and space are no longer absolute concepts; instead, they are closely tied to the choice of the reference frame. In particular, time slows down for objects in motion, a phenomenon known as time dilation, which has been confirmed through numerous physical experiments. The results brought by Special Relativity opened up the first theoretical possibility for time travel: traveling into the future.
Time Travel into the Future
According to Special Relativity, if someone wants to travel to the future, they would need a spaceship that can travel at a speed close to the speed of light. The farther into the future they want to go, the faster the spaceship needs to move. For example, if a person wanted to travel 20 years (their time on the spaceship) into the future, and see what Earth looks like 20,000 years from now, all they would need to do is fly at 99.99995% of the speed of light for 10 years and then return at the same speed. After 20 years of travel, when they return to Earth, they would find that 20,000 years have passed, and they can witness a future human society—assuming humans still exist by that time. Imagine how excited future historians and archaeologists would be to welcome such a traveler!
Future Historians and Modern Society
In fact, not only would future historians and archaeologists be thrilled to meet such a time traveler, but people from the same era as this traveler would surely be eager to hear all about the world they saw in the future. Unfortunately, while Special Relativity opens the door for future time travel, it doesn't offer the same theoretical foundation for traveling back to the past. If we attempt to extend the math of Special Relativity, the only way to reverse time would involve superluminal (faster-than-light) motion, which, according to the theory, is impossible. Special Relativity sets a speed limit at the speed of light, and no known physical process can push an object— including a human—into superluminal speeds. Therefore, while time travel into the future is possible in the framework of Special Relativity, time travel back to the past remains a fantasy. This is far from the freedom we have in space, where we can move freely in any direction. Interestingly, traveling to the future doesn't necessarily require a time machine. We could achieve similar results by freezing a traveler for many years and then thawing them out. Thus, if time machines were to exist, their true value wouldn't lie in traveling to the future, but rather in traveling back to the past.
The Search for Time Travel to the Past
So where do we look to find a path to the past?
Ten years after the publication of Special Relativity, Einstein introduced his General Theory of Relativity. In General Relativity, time and space are not only closely related to the reference frame, but are also influenced by the distribution and motion of matter. One of the key differences with Special Relativity is that our concept of the "future" is no longer absolute—it can be affected by the movement of matter. At different times and places, the "future" may point in different directions. This is a fascinating result, as it suggests that spacetime behaves like a fluid, being dragged and stretched by the movement of matter. The direction of time itself may even change due to this drag.
Closed Timelike Curves: A Theoretical Possibility
Since the direction of time can be dragged by the movement of matter, is it possible that a certain distribution of matter could drag the direction of time so significantly that it turns the future into the past? Could it even create a closed curve in time, where different time directions connect, forming a loop? If such a closed curve exists, it would be a time machine. A spaceship traveling along this curve would experience normal passage of time but would eventually return to its starting point in both space and time. Imagine you embark on a 10-year journey along this curve; when you finish, you would not only return to where you started, but you'd also encounter your past self, about to set off on the journey! Physicists refer to this incredible curve as a "closed timelike curve," and it serves as the theoretical equivalent of a time machine in General Relativity. If closed timelike curves exist, then time travel would have a theoretical possibility.
Existence of Closed Timelike Curves in General Relativity
So, do closed timelike curves really exist in General Relativity? More specifically, is there a distribution and movement of matter that could make these curves possible? Physicists have conducted extensive research on this question, but the answer remains elusive. The idea of time travel has fascinated us for ages, and though much of it may seem like science fiction, with advances in our understanding of space, time, and the universe, who knows what might be possible in the future?
While we're still far from building time machines, the fascinating theories surrounding time travel challenge our understanding of the universe. As science continues to evolve, who knows what surprises the future holds? Let's keep exploring, and maybe someday we'll find a way to go back and revisit history—or even see the future!
