Time travel, the concept of moving through time to different points, has captivated the minds of storytellers and scientists for generations. From the adventures of Doctor Who to the thought-provoking scenarios in films like Back to the Future and The Time Machine, we are drawn to the possibility of visiting different eras, both past and future. But beyond fiction, what does physics say about our ability to journey through time?
While the whimsical time-traveling adventures of Doctor Who entertain us, they often stray far from real-world physics. The show uses the Tardis, a fantastical vehicle defying spatial dimensions, famously “bigger on the inside,” to navigate through time. However, to understand the real potential of time travel, we must turn to the established laws of the universe and explore what science deems possible, or impossible.
Time Travel to the Future: A Relativistic Reality
According to Albert Einstein’s groundbreaking theories of relativity, time is not a fixed constant but rather a flexible dimension that can be manipulated. A core principle of relativity is that time is relative; its passage can vary depending on speed and gravity. This variance is not just theoretical; it has tangible, measurable effects.
“Time travel, in a sense, is already happening and is scientifically proven with real-world implications,” explains astrophysicist Emma Osborne from the University of York.
One well-known consequence of relativity is time dilation due to speed. As an object approaches the speed of light, time slows down for it relative to a stationary observer. The famous “twin paradox” illustrates this: if one twin journeys into space at near-light speed while the other remains on Earth, the space-traveling twin will age slower. Quantum physicist Vlatko Vedral from the University of Oxford notes, “Upon returning from a high-speed journey, the traveling twin would indeed be younger than their Earth-bound sibling.” This phenomenon was even observed in a real-world scenario with astronauts Scott and Mark Kelly, where Scott’s time in space, although not at relativistic speeds, resulted in subtle age differences.
Similarly, gravity also affects time. The stronger the gravitational field, the slower time passes. “Even on Earth, your head ages slightly faster than your feet because gravity is weaker further from the Earth’s center,” Osborne points out.
The science fiction show Doctor Who cleverly used gravitational time dilation in the episode “World Enough and Time.” The Doctor encounters a spaceship near a black hole, where time moves at different rates from front to back of the ship due to the immense gravity. This allowed a small group of Cybermen at the rear to evolve rapidly, creating a vast army in what seemed like minutes to those at the front. This concept also played a crucial role in the film Interstellar.
According to Albert Einstein’s theories of relativity, time compression is achievable by traveling at high speeds relative to a stationary observer (Credit: Getty Images)
While these relativistic effects are minuscule in our daily lives, they are critical for technologies like Global Positioning Systems (GPS). “Clocks on GPS satellites in orbit run faster than clocks on Earth,” Osborne clarifies. Without constant adjustments accounting for relativity, “Google Maps would quickly become inaccurate by approximately 10 kilometers (six miles) per day.”
Relativity, therefore, confirms that future time travel is possible. We don’t need a complex time machine; we simply need to travel at a significant fraction of light speed or reside in a strong gravitational field. In essence, both scenarios achieve the same outcome: experiencing less subjective time while the universe outside progresses rapidly. If your goal is to witness the distant future, these are the scientifically validated methods.
Time Travel to the Past: A Theoretical Minefield
Journeying into the past presents a much greater challenge, bordering on impossibility, according to our current understanding of physics.
“Past time travel may or may not be possible. Our current scientific understanding and theories are insufficient to definitively answer this,” states theoretical physicist Barak Shoshany from Brock University.
Relativity opens some highly theoretical avenues for backward time travel, but these are fraught with complexities and uncertainties. “Physicists grapple with the intricacies of manipulating space-time to enable travel to the past,” explains theoretical cosmologist Katie Mack from the Perimeter Institute for Theoretical Physics.
One theoretical construct is the closed time-like curve, a loop in space-time that, if traversed, would bring a traveler back to their starting point in time and space. Mathematical descriptions of such curves emerged as early as Kurt Gödel’s 1949 study, with subsequent physicists exploring similar concepts.
However, closed time-like curves face significant hurdles.
“We lack any evidence of their existence in the Universe. It remains purely theoretical with no observational backing,” Vedral emphasizes.
Furthermore, even with advanced technology, creating a closed time-like curve seems improbable. Philosopher Emily Adlam from Chapman University suggests, “Even with technologies far beyond our current capabilities, deliberately creating closed time-like curves appears highly unlikely.”
Even if such curves were achievable, Vedral raises a concerning paradox, “You would be trapped in an infinite loop, repeating the same events endlessly.”
Doctor Who touched upon a similar concept in the episode “Heaven Sent,” where the Doctor is trapped in a time loop, reliving the same hours for billions of years, though this scenario involved teleportation rather than a true closed time-like curve.
Another theoretical concept involves cosmic strings. In a 1991 study, physicist Richard Gott proposed that two cosmic strings moving in opposite directions could create closed time-like curves.
Cosmic strings are hypothetical, extremely dense objects thought to have possibly formed in the early universe. However, “There’s no evidence to suggest cosmic strings exist,” Mack clarifies. Even if they did, finding a pair moving in the required configuration would be astronomically improbable.
Why is the Tardis a police box?
The Tardis, Doctor Who’s time machine, is famously disguised as a police box due to a malfunction in its camouflage system, known in the show as the chameleon circuit. Ironically, chameleons primarily use color changes for communication, not camouflage.
Wormholes offer another theoretical possibility for time travel within the framework of relativity. Wormholes are essentially tunnels through space-time, potentially connecting vastly distant points, acting as shortcuts. “General relativity does allow for the theoretical possibility of wormholes,” Vedral confirms.
The Tardis from Doctor Who, stuck as a police box due to a faulty chameleon circuit, symbolizes the blend of science fiction with time travel concepts.
However, wormholes, like closed time-like curves and cosmic strings, face significant challenges. First, their existence is purely theoretical. “Mathematical models suggest wormholes can exist, but physical evidence is lacking,” Osborne notes.
Second, even if they exist, wormholes are predicted to be incredibly unstable and short-lived. “Wormholes are often visualized as linked black holes,” Osborne explains, implying immense gravitational forces that would cause them to collapse rapidly.
Furthermore, realistic wormholes would likely be microscopically small, far too tiny for even atoms to pass through, let alone humans.
Theoretically, these limitations could be overcome with the hypothetical concept of “negative energy,” a form of energy with negative mass. While tiny pockets of negative energy might exist at subatomic scales, “expanding these microscopic regions of negative energy to create a traversable wormhole seems practically impossible,” Osborne concludes.
Vedral succinctly summarizes the challenges: “Wormhole-based time travel doesn’t appear to be a realistic prospect.”
Quantum Mechanics and Retrocausality: A Glimmer of Hope?
Shifting from relativity to quantum mechanics, the physics of the very small, introduces another layer of complexity and potential, albeit highly speculative, avenues for time travel.
Quantum mechanics governs the bizarre behavior of particles at the subatomic level, revealing phenomena that often defy our everyday intuition.
One such phenomenon is quantum non-locality, where entangled particles can instantaneously influence each other regardless of distance – what Einstein famously termed “spooky action at a distance.” This non-locality has been experimentally verified numerous times, as recognized by Nobel Prize-winning research, according to Adlam.
“Many physicists are uncomfortable with non-locality because it implies information transfer faster than light, seemingly violating a fundamental principle of relativity,” Adlam explains.
Some physicists have proposed alternative interpretations of quantum mechanics to eliminate non-locality. These interpretations often introduce the concept of retrocausality, where effects can precede their causes in time.
“Instead of instantaneous action at a distance, retrocausality suggests that effects travel into the future and then propagate back to the past, creating the illusion of instantaneity,” Adlam clarifies. In this view, events in the future could influence events in the past.
This concept of retrocausality, while intriguing, clashes with our intuitive understanding of time as a linear progression from past to present to future. In these quantum interpretations, information might be taking a detour through the future to influence the past.
However, it’s crucial to note that retrocausality interpretations of quantum mechanics are not universally accepted. Many physicists find retrocausality as problematic, if not more so, than non-locality.
Quantum entanglement, a phenomenon where linked particles can instantaneously affect each other across distances, is often described as ‘spooky’ and challenges our classical understanding of causality (Credit: Getty Images)
Even if retrocausality is real, it’s unlikely to provide a practical method for human time travel. “Retrocausality is not equivalent to time travel as we imagine it,” Adlam emphasizes.
Current observations of non-locality and potential retrocausality are limited to microscopic scales with very few particles. Scaling these effects up to macroscopic objects like humans presents insurmountable challenges.
Furthermore, even within these quantum scenarios, sending a message to the past appears impossible. “Retrocausality, if it exists, is inherently hidden by its implementation,” Adlam explains.
Consider an experiment where Adam’s measurement outcome depends on Beth’s future measurement. Beth’s future action influences Adam’s past result, but only if Beth’s experiment erases all records of Adam’s initial measurement.
“A signal is sent to the past, but only by destroying all evidence of that signal being sent and received. This prevents any practical application for communication or time travel,” Adlam concludes.
The Unwritten Future of Time Travel
Based on our current scientific understanding, future time travel appears theoretically possible, while past time travel remains firmly in the realm of speculation and potentially impossible.
However, our theories of the universe are still incomplete. Relativity and quantum mechanics, while powerful in their respective domains, are fundamentally incompatible. This incompatibility suggests a deeper, unifying theory is needed, but remains elusive despite decades of research. “Until we achieve a more complete theory, we cannot definitively rule out possibilities we haven’t yet conceived,” Shoshany concludes.
Ultimately, in the time it took to read this article, you have already journeyed several minutes into the future. Perhaps, for now, that is the most accessible form of time travel we possess.
—
If you enjoyed this article, sign up for The Essential List newsletter for a curated selection of features, videos, and must-read news delivered to your inbox twice weekly.
Explore more science, technology, environment, and health stories from the BBC by following us on Facebook, X, and Instagram.
