The Mechanics of Time Travel Explained

From Victorian-era fantasies of leaping across millennia to modern tales of teenagers hopping through time in phone booths, the concept of time travel often evokes our wildest imaginations about traversing the fourth dimension. However, you don’t necessarily require a time machine or a complex wormhole to journey through the years.

As you’ve likely observed, we are all perpetually traveling through time. Fundamentally, time represents the rate of change in the universe, and whether we like it or not, change is inevitable. We grow older, planets orbit the sun, and everything gradually decays.

We quantify time using seconds, minutes, hours, and years, but this doesn’t imply that time moves at a steady pace. According to Einstein’s theory of relativity, time is not absolute. Similar to how water in a river speeds up or slows down based on the width of its channel, time flows at varying rates depending on the context. In essence, time is relative.

What drives these variations in our irreversible journey from birth to death? The answer lies in the intricate connection between time and space. Humans navigate the three spatial dimensions—length, width, and depth. Time enters the equation as the essential fourth dimension. Time and space are inseparable; one cannot exist without the other. Together, they form the space-time continuum. Every event in the universe inherently involves both space and time.

In this article, we’ll explore the practical, everyday ways time travel manifests in our universe, alongside some of the more speculative theories about traversing the fourth dimension.

Traveling Into the Future

To move through time faster than others, you’ll need to manipulate space-time. Global positioning satellites achieve this daily, gaining an extra fraction of a second over time. In orbit, time moves quicker because satellites are farther from Earth’s mass. On the surface, the planet’s gravitational pull slightly slows time.

This phenomenon is known as gravitational time dilation. Einstein’s theory of general relativity explains gravity as a curvature in space-time, a concept astronomers observe when studying light near massive objects. For example, massive stars can bend light beams, an effect termed the gravitational lensing effect.

How does this relate to time? Recall that every event in the universe involves both space and time. Gravity affects not only space but also time.  

While tiny changes in time’s flow are imperceptible, an object with immense mass, like the supermassive black hole Sagittarius A at our galaxy’s center, creates significant effects. With the mass of 4 million suns compressed into a single point, or singularity [source: NASA], orbiting this black hole (without falling in) would slow time to half the rate on Earth. After five years of travel, a decade would have passed on Earth due to time dilation [source: Davies].

Speed also influences how we perceive time. As you approach the speed of light, time slows down. For example, a clock on a fast-moving train ticks slower than a stationary one. While humans wouldn’t notice the difference, the moving clock would lag by billionths of a second. If the train reached 99.999% of light speed, one year onboard would equal 223 years at the station [source: Davies].

In this scenario, our hypothetical traveler would effectively journey into the future. But what about traveling backward? Could even the most advanced starship reverse the flow of time?

Journeying Into the Past

We’ve confirmed that traveling into the future is not only possible but also a regular occurrence. Experiments have validated this, and it’s a cornerstone of Einstein’s theory of relativity. Reaching the future is inevitable; the only uncertainty is how quickly you’ll get there. But what about venturing into the past? The night sky holds clues to this mystery.

The Milky Way spans approximately 100,000 light-years, meaning light from its farthest stars takes tens of thousands of years to reach Earth. Observing this light is akin to peering into the past. When scientists study the cosmic microwave background radiation, they’re looking over 10 billion years into the universe’s early stages. But can we go even further?

Einstein’s theory doesn’t explicitly rule out traveling to the past, but the idea of pressing a button and returning to yesterday contradicts the law of causality, which governs cause and effect. In our universe, events unfold in a one-way sequence, where causes always precede effects. Imagine a scenario where a murder victim dies from a gunshot before the trigger is pulled—it defies logic. For this reason, many scientists reject the possibility of backward time travel.

Some scientists, inspired by Einstein’s theory of special relativity, suggest that faster-than-light travel could enable journeys into the past. If time slows as an object approaches the speed of light, could surpassing it reverse time? However, as an object nears light speed, its mass becomes infinite, making further acceleration impossible. Warp speed technology might bypass this limit by distorting space-time, but the energy required would be astronomical and far beyond current capabilities.

What if time travel to the past and future relies less on hypothetical propulsion systems and more on natural cosmic structures? The answer might lie in black holes.

Exploring Black Holes and Kerr Rings

Orbit a black hole for an extended period, and gravitational time dilation could propel you into the future. But what if you ventured directly into its core? Most scientists believe a black hole would destroy you, but a unique type, the Kerr black hole or Kerr ring, might offer a different outcome.

In 1963, New Zealand mathematician Roy Kerr introduced the first realistic model of a rotating black hole. His theory revolves around neutron stars, which are dense remnants of collapsed stars, as massive as the sun but as compact as Manhattan [source: Kaku]. Kerr suggested that if dying stars collapsed into a spinning ring of neutron stars, their centrifugal force would prevent the formation of a singularity. Without a singularity, the black hole might be traversable, avoiding infinite gravitational forces at its core.

If Kerr black holes exist, scientists theorize that passing through them could lead to a white hole. Imagine this as the opposite of a black hole—instead of pulling matter in, it expels everything outward, potentially into another time or even a different universe.

While Kerr black holes remain theoretical, they could provide daring time travelers with a one-way journey to the past or future. Even if an advanced civilization could harness this method, there’s no guarantee where or when a naturally occurring Kerr black hole might take you.

Exploring Wormholes

Theoretical Kerr black holes aren’t the sole cosmic shortcuts to the past or future. Popularized by works like "Star Trek: Deep Space Nine" and "Donnie Darko," the equally speculative Einstein-Rosen bridge offers another possibility. You might recognize this concept better as a wormhole.

Einstein’s general theory of relativity supports the idea of wormholes, as it explains how mass warps space-time. To visualize this, imagine two people holding a taut bedsheet. Placing a baseball on the sheet causes it to sag in the middle. If a marble is placed near the edge, it rolls toward the baseball due to the curvature. This illustrates how mass distorts space-time.

In this analogy, space is represented as a two-dimensional plane instead of a four-dimensional one. If the sheet is folded, creating a gap between the top and bottom layers, placing a baseball on the top layer causes a curve. Adding an equal mass on the bottom layer directly below the baseball would cause the two masses to meet. This mirrors how wormholes might theoretically form.

In the universe, massive objects exerting pressure on space-time could theoretically create tunnels connecting different points. These tunnels might bridge separate times, allowing travel between them. However, unknown physical or quantum properties could prevent wormholes from forming. Even if they exist, they might be highly unstable.

Astrophysicist Stephen Hawking suggested that wormholes might exist within the quantum foam, the universe’s smallest scale. In this realm, tiny tunnels flicker in and out of existence, briefly connecting different places and times like a dynamic game of "Chutes and Ladders."

These microscopic wormholes might be too fleeting and small for human travel, but could we someday learn to capture, stabilize, and expand them? Hawking believed it’s possible, though with a caveat. Artificially extending a tunnel through folded space-time could create a radiation feedback loop, potentially destroying the wormhole, much like audio feedback can damage a speaker.

Exploring Cosmic Strings

Beyond black holes and wormholes, there’s another theoretical method for time travel involving cosmic phenomena. Physicist J. Richard Gott proposed the concept of cosmic strings in 1991. These hypothetical, string-like structures are thought to have formed in the early universe, offering another potential pathway through time.

These strings might extend across the entire universe, thinner than an atom yet under extreme tension. As a result, they would exert a significant gravitational pull on nearby objects, allowing anything attached to them to move at extraordinary speeds and experience time dilation. By bringing two cosmic strings close together or positioning one near a black hole, it might be possible to distort space-time sufficiently to form a closed timelike curve.

By harnessing the gravitational forces of two cosmic strings (or a string and a black hole), a spacecraft could theoretically journey into the past. This would involve maneuvering in a loop around the cosmic strings.

However, cosmic strings remain highly speculative. Gott noted that to travel back just one year, a loop of string containing half the mass-energy of an entire galaxy would be required. This means converting half the galaxy’s atoms into energy to power such a time machine. Additionally, as with any time machine, you couldn’t travel back to a time before the machine was created.

And, of course, there’s the issue of time paradoxes.

Exploring Time Travel Paradoxes

As previously mentioned, traveling to the past becomes complicated when causality comes into play. In our universe, cause always precedes effect, which disrupts even the most well-planned time-traveling schemes.

For example, if you traveled back 200 years, you’d arrive in a time before your birth. Pause and consider that. In the timeline, the effect (you) would exist before the cause (your birth).

To grasp this better, consider the famous grandfather paradox. Imagine you’re a time-traveling assassin, and your target is your own grandfather. You travel through a wormhole and encounter your grandfather as a young man. You aim your laser blaster, but what happens when you pull the trigger?

Think about it. You haven’t been born yet, nor has your father. If you kill your grandfather in the past, he’ll never have a son, and that son will never have you. You’d never exist to become a time-traveling assassin, erasing the entire sequence of events. This is known as an inconsistent causal loop.

On the flip side, we must consider the concept of a consistent causal loop. This theoretical model of time travel, while equally intriguing, avoids paradoxes. Physicist Paul Davies describes it like this: A math professor travels to the future and steals a revolutionary math theorem. The professor then shares this theorem with a talented student, who later becomes the very individual from whom the theorem was originally stolen.

Another model is the post-selected model of time travel, which involves altered probabilities near any paradoxical event [source: Sanders]. Imagine you’re the time-traveling assassin again. In this model, your grandfather becomes nearly invincible. You can attempt to pull the trigger, but the laser might fail, or a random event like a bird’s interference could prevent the paradox from occurring.

Alternatively, quantum theory suggests that the past or future you visit might exist in a parallel universe. Think of it as a separate playground: you can create or destroy anything there without affecting your original reality. If the past you visit is in a different timeline, eliminating your grandfather has no impact on your own existence. However, this could mean each time jump lands you in a new parallel universe, making it impossible to return to your original timeline.

Feeling puzzled yet? Welcome to the complex world of time travel.