Is time travel possible?

 

In our minds, we all engage in a form of time travel. Reflecting on memories, dreaming of the future, or intensely wishing to reshape past events for different outcomes—all of these are various forms of time travel. But is everything we imagine possible in reality? In this article, we will explore how feasible real-time travel is and whether it is possible at all.

We frequently see time travel in science fictions. Even before modern literature, ancient folklore from various cultures often included aspects of time travel. For example, in Japanese folklore, a fisherman named Urashima Taro saves a turtle’s life in the deep sea. In gratitude, the turtle takes him to an underwater palace. When Urashima returns to the surface after a few days, he finds that many years have passed on Earth. This centuries-old tale resembles Einstein’s theory of relativity.

We know that since H.G. Wells published his science fiction novel The Time Machine in 1895, scientists have begun exploring the possibility of time travel. Literature touched on time travel even before Wells. American author Washington Irving’s story Rip Van Winkle, published in 1819, tells of a man who sleeps for twenty years, effectively traveling twenty years into the future. In 1843, Charles Dickens wrote about time travel in A Christmas Carol. However, H.G. Wells’s The Time Machine sparked scientific curiosity about real possibilities for time travel, inspiring many science fiction stories, novels, and movies about cosmic time travel in the 20th century.

The scientific exploration of time travel possibilities began with Einstein in the early decades of the 20th century. Let’s see which theories in physics support the concept of time travel.

Theories Supporting Time Travel

Albert Einstein's theories of relativity—the Special Theory and the General Theory—both, in some way, support the concept of time travel. According to the Special Theory of Relativity, time depends on the observer’s speed. If someone moves at an extremely high speed (close to the speed of light), their experience of time will slow down significantly. This phenomenon is known as time dilation. In time dilation, time stretches—meaning if we measure time on Earth in a stationary state, one second is just one second. But as the observer’s speed increases, one second no longer equals one second. It takes a much longer period to complete a single second. For instance, if an astronaut travels close to the speed of light for a year and returns to Earth, they will find that people on Earth have aged many years compared to them. Because their time passed slowly, their one year becomes equivalent to many years on Earth. When the astronaut returns, they would find Earth in their future.

In the General Theory of Relativity, when gravity and acceleration were included, it became evident that time no longer had a separate existence. Time combined with space to form the unified concept of space-time. When gravity increases, the curvature of space-time increases, meaning time stretches in such areas. This phenomenon is known as gravitational time dilation. In regions with strong gravitational fields, time passes more slowly. Near a black hole, where the gravitational field is exceptionally strong, time moves so slowly that, compared to a distant star, it seems as if time on that distant star is racing ahead—so much so that, from the black hole’s perspective, the distant star has already moved into the far future.

The Wormhole Theory

The first idea of a "scientific bridge" between the past and the future through the curvature of space-time came from Einstein himself, alongside his collaborator Nathan Rosen, in a 1935 research paper. As a solution to the equations of General Relativity, they proposed a tunnel-like structure through which different parts of space-time could connect. Due to gravitational time dilation, these connected parts of space-time could represent either the past or the future. This structure later became known as the "Einstein-Rosen Bridge."

In the 1950s, American physicist John Wheeler coined the term "wormhole," drawing inspiration from how earthworms create tunnels. A wormhole is essentially another name for an Einstein-Rosen Bridge. Wheeler theoretically demonstrated that tiny wormholes could exist at the quantum level. However, little further research was conducted on wormholes for some time.

In 1988, Caltech physicist Kip Thorne (who won the Nobel Prize in Physics in 2017) and his student Michael Morris presented detailed theories on the use of wormholes for time travel. Their model differed in some ways from the Einstein-Rosen Bridge. They suggested that traveling through a wormhole would require a special type of material with negative energy density—meaning the average energy per unit volume would be less than zero. While theoretically possible, creating negative energy is challenging. Mathematically, however, negative energy density is necessary to keep a wormhole’s “mouth” open. Otherwise, the gravitational force would crush and close the tunnel. Negative energy density creates a repulsive force, or “anti-gravity,” that could hold the wormhole’s mouth open.

Although difficult to achieve, negative energy density is not entirely impossible. In 1948, Dutch physicist Hendrik Casimir discovered a unique electromagnetic effect, which became known as the Casimir Effect. If two thin metal plates with no charge are placed close to each other in a vacuum, there should be no attraction between them. But unexpectedly, an attractive force is generated, believed to result from tiny fluctuations in the vacuum field. Further research revealed that the Casimir Effect can create regions with negative energy density.

However, even if the wormhole’s mouth remains open, traveling through it from the past to the future is not so simple. To travel through such a wormhole, one would need to move at nearly the speed of light from one mouth to the other, which is far beyond human capability with our current technology.

Cosmic Strings

Another theory that supports time travel is the hypothetical concept of cosmic strings. Scientists speculate that during the early stages of the universe, as its temperature decreased, certain phase transitions occurred (like a liquid solidifying), which may have produced cosmic strings. These strings are considered “flaws” in space-time, with an extremely high density. Due to their immense gravitational force, these cosmic strings can warp space-time to such an extent that the past and future come very close to each other. Princeton University professor John Richard Gott termed this phenomenon “closed time like curves.” In such a scenario, two cosmic strings could cross paths, creating a point where a “time-link” is established, and the curvature of space-time connects the past and future. However, this remains purely theoretical.

Quantum Mechanics

Quantum mechanics also offers some intriguing ideas supporting the possibility of time travel. The concept of quantum entanglement suggests that two quantum particles can be mysteriously linked, regardless of their distance, position, or even time. This has led to the idea of parallel universes with parallel timelines—meaning what is the present in one universe could be the past or future in another. Additionally, in the emerging field of quantum gravity (which combines quantum mechanics and general relativity), extremely complex changes in space-time might theoretically allow time travel on a very tiny scale.

The Difference Between Theory and Reality

The practical application of time travel theories encounters various challenges, the most significant being paradoxes arising from logical inconsistencies. The “Grandfather Paradox” is one of the most famous: if someone travels to the past and kills their grandfather before their father is born, then logically, their father could never be born. Without their father, they themselves couldn’t exist, so how did they travel back in time to begin with?

American theoretical physicist Joseph Polchinski illustrated another paradox, now known as “Polchinski’s Paradox.” He imagined a billiard ball being sent through a wormhole into the past, only to collide with itself, preventing it from entering the wormhole in the first place. But if it couldn’t enter the wormhole, then how did it reach the past to create the collision?

Stephen Hawking proposed that such paradoxes imply time travel is impossible on a macroscopic level, even if theoretically plausible on a microscopic level. Essentially, even if particles could undergo limited time travel, it would be impossible for a person to alter history by traveling into the past.

Beyond paradoxes, time travel theories face significant practical challenges. To experience time travel through the warping of space-time, a traveller would need to reach speeds close to that of light. However, reaching such speeds and sustaining them requires energy and technology beyond our current means. According to Einstein's special theory of relativity, as an object approaches the speed of light, its relative mass becomes infinite, necessitating infinite energy—a feat unattainable by any material object.

Even if cosmic strings were possible, a time traveller would have to navigate extreme gravitational fields. Traveling near such intense gravitational zones, like those around black holes, is beyond the capacity of any spacecraft or person to survive intact.

In conclusion, genuine time travel is practically impossible within our universe's current understanding and technological limits.

Time Travel in Alternate Dimensions

Is time-travel possible in a different-dimensional universe? Supporters of string theory and M-theory have mathematically shown that in an eleven-dimensional universe, extra dimensions might be compacted in such a way that time travel could be feasible. However, as no empirical evidence for string theory or M-theory exists yet, time travel in these realms remains a theoretical possibility.

The concept of the multiverse or parallel universes also hints at time travel potential. In such universes, standard physical laws may not apply as they do in a single universe. Here, entirely new theories and equations would be necessary, potentially allowing time travel across different universes. However, these ideas remain speculative.

Einstein’s addition of time as a fourth dimension in his theory of relativity inspired further ideas, such as the fifth dimension proposed by physicists Theodor Kaluza and Oskar Klein in 1926. Their Kaluza-Klein theory sought to unify gravity and electromagnetism within a five-dimensional framework. If this theory holds in practice, time travel might indeed become possible.

In 1949, Austrian physicist Kurt Gödel provided a complete solution to Einstein’s field equations, proposing a rotating universe where some closed sections of space-time bend in a loop, theoretically allowing the future to connect with the past. This closed-loop concept suggested time travel in a rotating universe with specific space-time curvature. However, practical application would require speeds near the speed of light and extreme gravitational fields, conditions under which a traveller could not survive.

Is Time Travel Possible?

To assess time travel's feasibility, we can examine it across three dimensions: logical possibility, physical possibility, and practical feasibility.

1. Logical Possibility: This aspect is satisfied if there is no paradox or logical inconsistency. However, time travel is rife with paradoxes, such as the grandfather paradox and Polchinski’s paradox. Thus, logically, time travel appears impossible.
2. Physical Possibility: This considers the possibility within known physical laws. Certain theories, including relativity and some extensions like Kaluza-Klein theory, suggest that time travel is physically possible, so it cannot be ruled out entirely from a physical perspective.
3. Practical Feasibility: Practically, however, time travel remains impossible for now. It requires near-light speeds or survival in extreme gravitational fields—conditions beyond human capacity to endure or attain. Thus, time travel, as we understand it in our physical world, is practically unattainable.

In conclusion, while some advanced theories provide a glimpse into time travel's theoretical possibility, practical time travel remains a distant dream.

References:

1. Stephen Hawking, A Brief History of Time, 2nd Edition, Bantam Press, America, 1996.
2. Adrian Bardon, A Brief History of the Philosophy of Time, 2nd Edition, Oxford University Press, 2024.
3. Sarah Scoles, "Is Time Travel Possible?" Scientific American, April 26, 2023.
4. James Gleick, Time Travel: A History, Pantheon Books, New York, 2016.
5. Brian Clegg, Build Your Own Time Machine, Duckworth Overlook, London, 2011.