Is time travel possible?

The idea of time travel has been rooted in human imagination for years and traveled through the channels of science fiction, theoretical physics, and popular imagination. The idea of stepping inside a time machine, only to be whirled either to a very distant past or an unknown future, comes as something that provokes both thrills and endless questions. But beyond movie plots and the pages of novels, can time travel actually be possible? Let's dive into the science, theories, and debates surrounding this captivating topic.

Any discussion about time travel must trace back to Albert Einstein's theory of relativity. Einstein faced a challenge in changing people's way of thinking about space and time with his special theory of relativity in 1905. It introduced the concept of time as relative, not constant, dependent on how fast one travels. In the theory, he postulated that time would get slower or faster depending on velocity. This phenomenon, time dilation, has already been experimentally proven. For example, on the ISS, time flows a bit slower for astronauts than on Earth because they are orbiting the planet at very high speed. Actually, this effect is called "time travel to the future," though on an incredibly small scale.

Einstein's general theory of relativity goes further by relating gravity and time. It says that enormous objects such as planets and stars bend space-time, creating what we experience as their gravity. Wherever the gravitational field is stronger, time goes slower. This subsequently provided scientists with the idea of speculating whether it could be possible for time travel near super-massive objects, like black holes. Theoretically, if you were orbiting close to a black hole without falling in, time would appear to pass slower for you relative to an observer far from it—once more allowing for a sort of one-way journey into the future.

But what of traveling into the past? Here things get far trickier. One of the most popular ideas put forward is that of "wormholes"—hypothetical tunnels through space-time, which could connect very distant points in time and space. Think of a wormhole like a tunnel through the universe. If you went in one end, it would, in theory, take you to quite a different time and place. While predicted by general relativity, wormholes are still purely theoretical. Even if they did exist, stabilizing them and making them sufficiently large for a human to travel through would require "exotic matter" possessing negative energy—something that has not been discovered.

Another theory that leans toward time travel is through closed time-like curves. These are trajectories in space-time that general relativity predicts will overlap with themselves, meaning that an object would be able to reach its own past. Again, however, the practicalities of the production of such a curve or survival from its conditions are beyond our current technological capabilities.

Even if time travel were possible, it would create so many paradoxes that the human mind cannot justify them. The most famous is that known as the "grandfather paradox." Imagine you time-traveled back to a point where you caused your grandfather never to meet your grandmother. If they never met, one of your parents wouldn't have been born and, therefore, neither would you. But if you had never been born, how could you have traveled back in time in the first place? A clear example of such paradoxes is that which makes past–directed time travel, at best, very unlikely or at worst, extremely cumbersome in its logic.

Scientists have thrown in solutions to such paradoxes. One of them is the "many-worlds interpretation": every time a time traveler goes back in time and does something that changes the present, he creates a parallel universe. In other words, instead of changing your timeline, you would be creating another completely different reality. This idea is often used in science fiction but stays just speculation in the real world.

Even if time travel is still largely hypothetical, its study has dramatically contributed to pushing the boundaries of physics and stimulating the human imagination. Another department of science—quantum mechanics, concerned with the behavior of particles at very small scales—offers some strange possibilities: according to several theories, particles can "jump" from one point in time to another. This would point out that time travel is possible at a quantum level. This is, however, only theoretically scalable to macroscopic objects like humans.

After all, while we are still very far from creating a time machine, the concept of time travel serves as a bridge between fantasy and the most advanced scientific knowledge, stimulating us to reflect on the nature of time itself, of the structure of the universe, and on what lies beyond the threshold of our present knowledge. Although the wildest dreams about hopping between centuries are not yet supported by science, as Einstein himself once said, "Imagination is more important than knowledge." Perhaps one day our knowledge will catch up with our dreams.