WhatsAppRedditShareEmailTweetPin0 Shares
One of the favorite themes of science fiction is time travel. H.G. Wells wrote «The Time Machine» back in 1895, but Gaspar, the creator of "Anacronopeta," wrote it in 1887, and it also features time travel. Charles Dickens wrote «A Christmas Carol» in 1843, in which, technically, a person travels both to the past and to the future. Our fascination with this concept has continued to this day.
The appeal of time travel may lie in the fact that there are circumstances under which science allows it to be possible, at least in theory. For example, we are all traveling into the future right now. This happens continuously, at a rate of one second per second!
But most of us, when we talk about time travel, have something completely different in mind, right? We're talking about going back in time and riding a dinosaur, or traveling to the distant future and riding a robot dinosaur. So is it possible? Let's find out!
Time dilation and clocks
Dr. Ana Alonso-Serrano explained that space and time are not absolute quantities. Mathematically, that is, on paper, we can account for the curvature of space and time, creating a closed loop, which is what time travel represents. But the problem lies in translating these phenomena from paper to reality. From a physical perspective, we need to better understand this issue before any of this becomes reality.
What we understand, and can even demonstrate right now, is the principle of time dilation. The idea behind time dilation is that time flows differently for different observers depending on several factors, including gravitational fields and relative motion. Simply put, as far as we on Earth understand, speed and gravity affect time. This is all part of the theory of relativity.
This is where the relative component of relativity comes into play. You've probably heard that if you're on a rocket traveling close to the speed of light, then in the five years that pass from your perspective, perhaps 36 years will have passed from Earth. This is relativity in action. You'll never feel like you're somehow jumping through time. You're simply moving fast. But relative to people on Earth, you're moving into the future.
A funny example of this, which we observed in the real world, is the comparison between twin brothers Scott and Mark Kelly. Scott spent a full year in orbit, traveling at 17,500 mph in a much lower gravity than his brother experienced on Earth. When Scott returned from orbit, Mark appeared to be 5 milliseconds older than his brother, thanks to the effect of time dilation.
Experiments with atomic clocks in the 1970s showed that the clocks carried on jets around the world differed from the clocks left on the ground after landing. The effect amounts to tiny fractions of a second and would never have any significance for a person during their lifetime, but it proves the truth of Einstein's theory. More modern and even more accurate clocks have confirmed this.
Due to their speed and altitude, GPS satellites require specially calibrated clocks. They lose approximately 38 microseconds every day and require constant adjustments, otherwise they will never function properly on Earth. Without correction, they will provide coordinates with an error of 10 kilometers every day.
Time flows differently depending on relative conditions, and if we could achieve incredibly high speeds, the effect would be much more pronounced. Based on the observations of those you left behind, you would be transported into the future. A hundred years might pass on Earth, but you might age only a few years or even months, depending on the speed you achieved.
What's stopping us from going into the future?
The main obstacle to achieving speeds comparable to time travel is mass. As speed increases, so does mass, which means more energy is required. This is why nothing can travel faster than light. Moving mass at the speed of light would require infinite energy, which is simply impossible. Light, having no mass, travels at the maximum possible speed.
Let's say you want to travel at a fraction of the speed of light. You'll still need a lot of energy. To double the speed, you need to quadruple the kinetic energy. To triple the speed, you need to increase the kinetic energy ninefold.
Mathematically, moving a 50-kilogram payload at 1% the speed of light would require 200 trillion joules of energy, equal to the average daily energy consumption of two million Americans. This is by no means impossible, but neither is it very practical or easy to achieve. Furthermore, the average space shuttle weighs 2 million kilograms, so energy consumption would need to be increased accordingly.
Another important point about this kind of time travel is that you're traveling somewhere. If you simply wanted to circle around space and return to Earth in the future, that would theoretically be possible, but you wouldn't be able to go back. Traveling to the future at near-light speed is a one-way trip.
Speed and time
Theoretically, traveling close to the speed of light is one way to get to the future. But many time travel theories focus on returning to the past. Is this even possible? One theory suggests it's possible if you travel faster than the speed of light, which, as we just explained, is impossible. However, these are just theoretical speculations, so let's just experiment.
To an observer on Earth, if you were to set off on a faster-than-light ship and then return, people on Earth would see the ship return, reverse, and then stop right where it started. Things get even more complicated from there. But none of this actually transports you back to yesterday. It simply ensures that you are always here, in space, and simultaneously traveling backward.
Others, such as Stephen Hawking, have argued that even if time travel to the past were possible, you could never travel to an era before the invention of a time-traveling machine. That's like saying you could never take the subway to a place where there's no subway station.
Time travel under the influence of gravity
You may have heard that time slows down as you approach a black hole. Even if you were at the event horizon of a black hole, time would stop completely. This has nothing to do with speed, but with gravity. It's the flip side of Einstein's theory of relativity. The stronger the gravity, the slower time moves.
Time flows more slowly closer to the center of the Earth and more quickly as we move away from it. This is partly why experiments with atomic clocks work. They are affected by Earth's gravity. Theoretically, if we understood all the physics and had the technology to harness it, we could use a black hole as a time machine. While minutes would pass within our time machine, years could potentially pass outside it. Of course, you just have to be careful not to get sucked inside.
This effect is fictionally depicted in the film Interstellar, where a planet nears a black hole and time is dramatically altered. This will likely never happen in reality, but, again, it's theoretically possible.
Forward versus backward
So, moving forward in time is possible in several ways. But going backward? That's a difficult question. The laws of thermodynamics discourage reverse time travel because the universe cannot return to its previous state. Things remain the same or become more chaotic over time; they don't become more orderly, especially not in the same state they were before.
Time travel opens up a world of paradoxes that are difficult to solve. For example, the grandfather paradox. If you went back in time and killed your own grandfather, thereby preventing the birth of one of your parents and, therefore, preventing your own birth, how would you even be able to go back in time?
One attempt to explain this is that you obviously didn't kill your grandfather, and if someone did, they never existed for us, and so we don't remember them enough to know they did it. So it's not a paradox, it's just that if you had, you would never have known it because you never existed, and no one ever knew enough to say otherwise. How's that for a conundrum?
Another theory, which researchers consider mathematically sound, suggests that any paradox you create will inevitably resolve itself out of necessity. Essentially, time corrects itself, no matter what.
In practice, most scientists believe that time travel by traditional means is simply impossible. However, there are unconventional methods.
Special cases for traveling back
Einstein once again reveals the possibility of fast time travel. Wormholes—passages that allow spacetime to warp and transport you to a place and time that already existed—are a theoretical possibility. It's highly theoretical, as no one has ever seen one before, and we have no evidence of their existence; science simply doesn't rule them out.
Wormholes are beloved in science fiction because they make time and space travel convenient. While the Einstein-Rosen bridge sounds good on paper, we haven't yet seen much evidence of its existence in real life. But space is vast, so you never know. More importantly, not everyone believes that even if they existed, they could be used for time travel.
If a wormhole could travel into the past, how would it work? One theory suggests a tube in spacetime connecting a black hole to a white hole. Mathematically, a white hole makes sense and is exactly what its name implies—the opposite of a black hole. While nothing can escape a black hole, nothing can remain in a white hole. It radiates energy just as a black hole sucks it in.
If a white hole is connected to a black hole, the time dilation effect at one end could mean that the moment you enter the hole and the moment you exit it will differ significantly. So much so that you could exit before you entered.
The main problem here is that to pass through this wormhole, it is necessary to overcome the event horizon of the black hole, and this is technically impossible.
Another potential route to the past is cosmic strings. These strings, which represent a "one-dimensional topological defect" created at the very beginning of time, could create closed timelike curves that, theoretically, would allow us to travel back in time.
The cosmic string is theoretically formed at the dawn of the universe. As time, space, and everything else expanded, tiny cracks or folds—imperfections, essentially—appeared in the fabric of reality itself. These are strings, and theoretically, they can do a lot. If they go back to the very beginning, and you can find such a string, you can use it to reach any point in time.
Of course, if such strings exist, they would be incredibly unstable. Extremely dense and possessing almost immeasurable energy, they would be like lasers, cutting through everything in their path, including time travelers and planets. To travel through time, one would have to learn to navigate such a string very carefully.
If two threads intersect, the time machine will be able to travel into the past by following them.
As you've noticed, the word "theoretically" appears a lot here. What works in equations doesn't always work in reality, and we're far from being able to test all of this in practice. For example, detecting a black hole with our current technology would take centuries. And then we'd still have to figure out how to navigate it.
Is time travel possible? Yes. We've even proven it by moving objects forward in time. But can we visit our ancestors or the future world and then return in time for dinner? It seems unlikely.
Other articles that may interest you