The notion of traversing through time, leaping into the future or delving into the past, has captivated the imaginations of storytellers, scientists, and dreamers for generations. From classic novels to blockbuster movies, the allure of time travel is undeniable. But beyond the realm of fiction, How Can We Travel Through Time, and is it even truly possible? Let’s explore what physics tells us about this fascinating concept.
From the whimsical adventures of Doctor Who to the thought-provoking narratives of The Time Machine and Back to the Future, time travel stories have long explored the enticing prospects and inherent paradoxes of visiting bygone eras and venturing into what lies ahead. These narratives often play with our understanding of time, space, and causality, prompting us to ponder the very fabric of reality.
In Doctor Who, the iconic Tardis serves as a vehicle for temporal journeys, a remarkable craft capable of navigating the vast expanse of time and space. Famously, the Tardis possesses an interior dimension far exceeding its external appearance, a concept that playfully challenges our everyday understanding of physical dimensions.
Time: The Ultimate Guide, a comprehensive exploration marking the 60th anniversary of Doctor Who, delves into the profound questions surrounding time. This includes investigating the scientific underpinnings of time travel, examining the pivotal role of clocks in shaping human civilization, and even contemplating the mind-bending temporal implications of venturing near a black hole. Explore more from Time: The Ultimate Guide to deepen your understanding of this multifaceted concept.
While Doctor Who ingeniously employs time travel as a central plot device, the series intentionally avoids grounding the Tardis’s capabilities in established physics. This creative liberty aligns with the show’s fairy-tale essence, prioritizing imaginative storytelling over strict scientific realism. However, for those seeking to understand the real-world feasibility of time travel, we must turn to the realm of scientific inquiry.
Could we ever construct a time machine capable of transporting us to witness historical events or glimpse future civilizations? Addressing this question requires a fundamental grasp of how time itself operates – a subject that continues to puzzle physicists. Current scientific understanding suggests that traveling to the future is within the realm of possibility, while journeying into the past presents formidable challenges, bordering on the impossible.
Let’s begin with Albert Einstein’s groundbreaking theories of relativity, which revolutionized our understanding of space, time, gravity, and mass. A pivotal revelation of relativity is that time is not a universal constant; its flow is relative and can be altered depending on various conditions.
“This is where the scientifically accurate aspects of time travel emerge, and these have tangible real-world consequences,” explains Emma Osborne, an astrophysicist at the University of York.
For instance, time dilation, a consequence of relativity, dictates that time elapses more slowly for objects moving at high speeds. While this effect becomes noticeable only at velocities approaching the speed of light, it has been experimentally verified. This phenomenon gives rise to the famous “twin paradox”: if one twin embarks on a high-speed space journey while the other remains on Earth, the traveling twin will age more slowly. “Upon returning, the space-traveling twin will indeed be younger than their Earthbound sibling,” confirms Vlatko Vedral, a quantum physicist at the University of Oxford. Astronauts Scott and Mark Kelly provide a real-world example, with Scott experiencing subtle age differences after spending extended periods in space, albeit at speeds far below light speed.
Similarly, time is also affected by gravity. Time slows down in stronger gravitational fields. “Your head actually ages slightly faster than your feet because Earth’s gravitational pull is stronger at your feet,” Osborne points out.
Doctor Who cleverly incorporated this gravitational time dilation in the season 10 finale, “World Enough and Time.” In this episode, the Doctor and companions find themselves on a spaceship near a black hole. The ship’s proximity to the black hole causes time to pass at drastically different rates at the front and rear of the vessel. This temporal distortion allows Cybermen at the rear to evolve into a massive army within what appears to be mere minutes from the Doctor’s perspective at the front. The film Interstellar also prominently features the effects of gravity on time as a key plot element.
In our daily lives, these relativistic time effects are imperceptible. However, they are crucial considerations for technologies like the Global Positioning System (GPS). “Clocks on GPS satellites orbiting Earth run faster than clocks on the ground,” Osborne notes. These time discrepancies must be constantly corrected. “Without these adjustments, Google Maps would accumulate errors of approximately 10 kilometers (six miles) daily,” explains the European Space Agency, highlighting the practical implications of Einstein’s theories.
Relativity, therefore, confirms the possibility of future time travel. We don’t necessarily require a time machine in the conventional science fiction sense. Instead, achieving future time travel involves either traveling at near-light speeds or spending time in intense gravitational fields. Relativity equates these two scenarios in terms of their temporal effects. In either case, the traveler experiences a shorter subjective duration, while decades or even centuries could pass in the rest of the universe. If witnessing the distant future is your goal, these are the scientifically plausible paths.
However, venturing into the past presents a far more complex and uncertain picture.
“Whether it’s possible remains an open question,” states Barak Shoshany, a theoretical physicist at Brock University in Canada. “Our current scientific framework simply lacks the definitive knowledge, and possibly even the necessary theories, to provide a conclusive answer.”
Relativity offers some theoretical avenues for backward time travel, but these are highly speculative and fraught with challenges. “Physicists grapple with intricate concepts, attempting to manipulate space-time to enable travel to the past,” explains Katie Mack, a theoretical cosmologist at the Perimeter Institute for Theoretical Physics.
One such concept is the closed time-like curve, a theoretical path through space-time that forms a loop. Following such a path would, in theory, bring a traveler back to their starting point in both space and time. Logician Kurt Gödel mathematically described such paths in a 1949 study, and subsequent researchers have further explored these ideas.
However, closed time-like curves face significant hurdles.
“We lack any evidence of their existence in the universe,” Vedral emphasizes. “It remains purely theoretical, with no observational support.”
Furthermore, even if they exist, the means to create them are entirely unknown and likely beyond our technological capabilities. “Even with vastly advanced technology, intentionally creating closed time-like curves seems improbable,” suggests Emily Adlam, a philosopher at Chapman University.
Even hypothetically, Vedral cautions against the desirability of closed time-like curves for time travel. “You would be trapped in an endless loop, repeating the same events perpetually,” he explains.
Doctor Who alluded to a similar concept in the episode “Heaven Sent,” where the Doctor endures the same few hours repeatedly for billions of years. However, this scenario was achieved through repeated teleportation rather than a closed time-like curve.
In a related vein, physicist Richard Gott proposed in a 1991 study a theoretical scenario involving “cosmic strings,” hypothetical one-dimensional objects with immense density potentially formed in the early universe. His calculations suggested that two cosmic strings moving past each other in opposite directions could create closed time-like curves.
However, the existence of cosmic strings remains unconfirmed. “We have no compelling reasons to believe cosmic strings exist,” Mack notes. Even if they did, finding two conveniently aligned and moving in parallel would be extraordinarily improbable. “There’s no basis to assume such a scenario would naturally occur.”
Why is the Tardis a police box? The Tardis, Doctor Who’s time-traveling vessel, is famously disguised as a British police box due to a malfunction in its camouflage system, known as the chameleon circuit. Ironically, chameleons primarily use color changes for communication, not concealment.
Another theoretical possibility arising from relativity is the concept of wormholes. These hypothetical tunnels through space-time could potentially connect distant points, acting as shortcuts. “Wormholes are theoretically permissible within general relativity,” Vedral confirms.
However, wormholes also face substantial challenges. Firstly, their existence remains purely theoretical. “Mathematical models suggest they could exist, but their physical reality is uncertain,” Osborne points out.
Secondly, even if wormholes exist, they are predicted to be extremely short-lived and unstable. “Wormholes are often conceptualized as interconnected black holes,” Osborne explains. This implies incredibly intense gravitational forces that would likely cause a wormhole to collapse rapidly.
Furthermore, theoretical wormholes are predicted to be microscopically small, far too tiny for even a bacterium to pass through.
While theoretically solvable, overcoming these limitations to create traversable wormholes would require manipulating vast amounts of “negative energy,” a concept that remains highly speculative and potentially unattainable. While pockets of negative energy might exist at subatomic scales, “expanding these tiny regions of negative energy to a usable scale seems highly improbable,” Osborne concludes.
Vedral succinctly summarizes the situation: “It doesn’t appear to be a realistic prospect.”
WATCH: What We Get Wrong About Time
Leaving relativity aside, let’s consider the implications of quantum mechanics, the other fundamental theory governing the universe.
While relativity describes the macroscopic world of planets and galaxies, quantum mechanics governs the microscopic realm of atoms and subatomic particles. At these scales, physical phenomena often defy our everyday intuitions.
One such phenomenon is quantum non-locality. Changes to a quantum particle in one location can instantaneously affect another “entangled” particle, regardless of distance – a phenomenon Einstein famously termed “spooky action at a distance.” This effect has been experimentally verified numerous times, as recognized by Nobel Prize-winning research.
“Many physicists are uneasy with the implications of non-locality,” Adlam notes. Instantaneous influence seemingly violates the speed of light limit, a cornerstone of relativity.
In response, some physicists propose alternative interpretations of quantum mechanics that eliminate non-locality but introduce complexities related to time.
“Instead of instantaneous non-local effects, these interpretations suggest that effects propagate into the future and then potentially loop back to the past,” Adlam explains. “This could appear instantaneous but would involve a journey through time.”
This interpretation introduces “retrocausality,” where future events can influence the past, challenging our conventional understanding of cause and effect flowing linearly from past to future. In these quantum scenarios, information might travel forward and then backward in time.
However, this interpretation of quantum mechanics remains far from universally accepted. Many physicists find retrocausality as problematic, or even more so, than non-locality.
Even if retrocausality is real, it’s unlikely to offer a practical pathway to becoming time travelers in the science fiction sense. “Retrocausality is not equivalent to time travel as we imagine it,” Adlam clarifies.
Firstly, observations of non-locality involve minuscule numbers of particles. Scaling this up to macroscopic objects, even something as small as a piece of paper, would be an immense challenge.
Furthermore, even with retrocausality, sending messages to the past appears impossible. “The retrocausal effect is inherently concealed by its implementation,” Adlam explains.
Consider an experiment where Adam makes a measurement, but the outcome depends on a later measurement by Beth. Beth’s future experiment influences Adam’s past result. However, this only works if Beth’s experiment erases all records of Adam’s actions and observations.
“In a sense, a signal is sent to the past, but only by destroying all evidence of sending that signal,” Adlam elaborates. “This precludes any practical application as the necessary record destruction prevents any usable information transfer.”
In conclusion, based on our current scientific understanding, future time travel, while requiring extreme conditions, is theoretically consistent with relativity. However, past time travel faces significant theoretical and practical obstacles, potentially rendering it impossible.
The crucial caveat is that our current theories, relativity and quantum mechanics, are incomplete and incompatible in certain domains. A deeper, unified theory is needed, but remains elusive despite ongoing research. “Until we achieve such a unified theory, definitive answers remain beyond our grasp,” Shoshany concludes.
Ultimately, perhaps the most accessible form of time travel is the continuous journey into the future we all experience. In the time it took to read this article, you’ve already traveled several minutes into the future. You’re welcome to consider this your own personal time travel experience!
—
If you enjoyed this exploration, [sign up for The Essential List newsletter](https://cloud.email.bbc.com/SignUp10_08?&at_bbc_team=studios&at_medium=Onsite&at_objective=acquisition&at_ptr_name=bbc.com&at_link_origin=featuresarticle&at_campaign=essentiallist&at_campaign_type=owned) for a curated selection of insightful articles, videos, and must-read news delivered directly to your inbox twice weekly.*
For more captivating stories about science, technology, the environment, and health from the BBC, follow us on Facebook, X and Instagram.
