The concept of journeying through time, both forward and backward, has captivated the imaginations of science fiction writers and physicists for generations. From classic novels to blockbuster movies, the allure of time travel is undeniable. But does science offer any real possibility of visiting bygone eras or catching a glimpse of the future?
Stories like Doctor Who, The Time Machine, and Back to the Future have masterfully played with the paradoxes and potential of time travel, sparking our curiosity about manipulating time itself. In Doctor Who, the iconic Tardis allows for adventures across all of time and space, famously being much larger on the inside – a whimsical element that exists purely in the realm of fiction.
To commemorate the 60th anniversary of Doctor Who, discussions around time and its mysteries are more relevant than ever. Exploring the science behind time travel, alongside the impact of clocks on society and the mind-bending effects of black holes on time, reveals a fascinating intersection of science and imagination. While Doctor Who embraces time travel with a fantastical approach, the real question remains: could humanity ever develop a time machine to explore history or witness future events?
To address this, we must delve into our current understanding of time – a concept that still puzzles physicists. Currently, scientific consensus suggests that traveling to the future is plausible, while journeying into the past faces significant, possibly insurmountable, obstacles.
Let’s begin with Albert Einstein’s groundbreaking theories of relativity, which revolutionized our understanding of space, time, gravity, and mass. A fundamental aspect of relativity is the realization that time is not a constant; its flow is relative and can be altered. Time can speed up or slow down depending on various conditions, a phenomenon known as time dilation.
Image depicting a person in a hat seemingly stepping through a portal, visualizing the concept of time travel.
“This is where the genuine scientific basis for time travel emerges, and it has tangible, real-world implications,” explains Emma Osborne, an astrophysicist at the University of York.
One consequence of relativity is that time slows down for objects moving at high speeds. While noticeable effects require speeds approaching the speed of light, the principle is clear. This leads to the famous “twin paradox”: if one twin journeys into space at near-light speed while the other remains on Earth, the spacefaring twin will age more slowly. “Upon returning, the traveling twin would be demonstrably younger than their Earth-bound sibling,” confirms Vlatko Vedral, a quantum physicist at the University of Oxford, referencing real-world examples like astronaut Scott Kelly’s time in space compared to his twin brother Mark.
Similarly, gravity also affects time. Time elapses more slowly in stronger gravitational fields, such as near a black hole. “In fact, due to Earth’s gravity being slightly stronger at your feet than your head, your feet are technically aging marginally slower,” Osborne points out.
Doctor Who cleverly incorporated this concept in the season 10 finale, “World Enough and Time.” The Doctor and companions find themselves on a spaceship near a black hole, where time moves at different rates depending on proximity to the gravitational source. This allowed Cybermen at the rear of the ship, where time moved faster, to evolve into a massive army in what seemed like minutes from the Doctor’s perspective. The movie Interstellar also prominently features the effects of gravity on time as a key plot element.
Image illustrating Einstein’s theory of relativity with a visual representation of time compression at high speeds, emphasizing the concept of relative time.
While these relativistic effects are imperceptible in everyday life, they are crucial for technologies like the Global Positioning System (GPS). “Clocks on GPS satellites in orbit tick faster than clocks on Earth” and require constant adjustments, Osborne notes. “Without these corrections, GPS navigation would accumulate errors of approximately 10 kilometers (six miles) daily,” as highlighted by the European Space Agency, demonstrating the practical validation of Einstein’s theories.
Relativity, therefore, confirms the possibility of time travel into the future. No elaborate time machine is strictly necessary; achieving near-light speed travel or spending time in a strong gravitational field are the mechanisms. In essence, these are equivalent in relativistic terms. By undergoing either, an individual would experience a comparatively shorter subjective duration while decades or even centuries pass in the wider universe. If witnessing the distant future is the goal, these are the scientifically supported pathways.
However, venturing into the past presents a far greater challenge.
“Whether it’s truly possible remains an open question,” states Barak Shoshany, a theoretical physicist at Brock University. “Our current understanding and theories are simply inadequate to definitively answer.”
Relativity tentatively offers some theoretical avenues for backward time travel, but they are highly speculative. “Physicists grapple extensively with manipulating space-time to theoretically permit travel to the past,” explains Katie Mack, a theoretical cosmologist at the Perimeter Institute for Theoretical Physics.
One theoretical construct involves creating a closed time-like curve – a 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 research has explored similar concepts.
Yet, this approach faces significant hurdles.
“We lack any evidence of these structures existing anywhere in the universe,” Vedral points out. “It remains purely theoretical, devoid of observational support.”
Furthermore, the feasibility of creating such a structure is questionable. “Even with vastly superior technology, intentionally creating closed time-like curves seems improbable,” suggests philosopher Emily Adlam from Chapman University.
Even if such curves were achievable, Vedral questions the desirability: “You would be trapped in an endless repetition loop, reliving the same moments continuously.”
Doctor Who touched upon a similar scenario in the episode “Heaven Sent,” where the Doctor endures the same few hours for billions of years, though this was due to a teleporter loop rather than a closed time-like curve.
Another theoretical proposal emerged in a 1991 study by physicist Richard Gott, describing “cosmic strings” – hypothetical, incredibly dense objects possibly formed in the early universe. Gott’s 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 themselves is unconfirmed. “We have no compelling reason to believe cosmic strings exist,” Mack clarifies. Even if they did, finding two conveniently aligned and moving in parallel would be astronomically improbable. “There’s no basis to expect such a scenario.”
Image of the Doctor Who Tardis in its iconic police box form, a visual representation of time travel in popular culture and a nod to the fictional aspects of the concept.
Wormholes present another intriguing, albeit highly speculative, possibility allowed by relativity. These theoretical tunnels through space-time could act as shortcuts between distant points. “Wormholes are theoretically permissible within the framework of general relativity,” Vedral confirms.
However, like closed time-like curves and cosmic strings, wormholes face substantial challenges. Firstly, their existence remains unproven. “Mathematical models suggest their possibility, but their physical reality is uncertain,” Osborne emphasizes.
Secondly, even if wormholes exist, they are predicted to be incredibly unstable and short-lived. “Wormholes are often conceptualized as interconnected black holes,” Osborne explains. This implies immense gravitational forces that would likely cause a wormhole to collapse instantly.
Furthermore, realistic wormholes would likely be microscopically small, far too tiny for even a bacterium to pass through.
While theoretical solutions exist to stabilize and enlarge wormholes, they require vast quantities of “negative energy” – a concept that might exist on subatomic scales. “Expanding these minute pockets of negative energy to a usable scale seems physically impossible,” Osborne concludes.
Vedral succinctly summarizes: “Wormholes as practical time machines appear highly unrealistic.”
Transitioning from relativity to quantum mechanics, the second pillar of modern physics, reveals further complexities.
While relativity governs large-scale phenomena like galaxies and humans, quantum mechanics describes the bizarre world of subatomic particles. At this scale, physical laws behave in ways that defy everyday intuition.
One such quantum phenomenon is non-locality, exemplified by “entanglement.” A change in a quantum particle’s state can instantaneously affect another entangled particle, regardless of distance – a phenomenon Einstein famously termed “spooky action at a distance.” This has been experimentally verified numerous times, as recognized by Nobel Prize-winning research, Adlam notes.
“Many physicists are deeply uncomfortable with the implications of non-locality,” Adlam states. The instantaneous nature of the effect suggests information transfer faster than the speed of light, which contradicts fundamental physics.
To resolve this, some physicists propose alternative interpretations that challenge our conventional understanding of time.
“Instead of instantaneous non-local effects, these interpretations suggest that effects might propagate into the future, and then, in a sense, loop back to influence the past,” Adlam explains. “This would appear instantaneous but would involve a journey through time.”
This introduces the concept of “retrocausality” – future events influencing the past. This contradicts our intuitive linear perception of time flowing from past to present to future. In these quantum scenarios, information might travel forward in time and then back again.
However, this interpretation remains controversial and is not universally accepted within the physics community. Many physicists find retrocausality as problematic, if not more so, than non-locality.
Image depicting entangled particles, illustrating the “spooky action at a distance” in quantum physics and the non-intuitive nature of quantum phenomena.
Even if retrocausality is real, it is unlikely to enable human time travel. “Retrocausality is distinct from time travel as commonly imagined,” Adlam clarifies.
Firstly, observations of non-locality have been limited to microscopic scales. Scaling this up to macroscopic objects like humans or even small objects presents immense challenges.
Furthermore, even in these quantum systems, sending a message to the past appears impossible. “Retrocausality, in these interpretations, is inherently limited in a way that prevents practical exploitation,” Adlam explains.
Consider an experiment where Adam makes a measurement, but the outcome depends on a later measurement by Beth. Beth’s future action influences Adam’s past result. However, this only works if Beth’s experiment erases all records of Adam’s initial measurement.
“A signal is, in a sense, sent to the past, but only by destroying all evidence of that signal being sent or received,” Adlam summarizes. “This prevents any practical application, as the act of sending the signal inherently destroys the ability to utilize it.”
In conclusion, based on our current scientific understanding, traveling to the future appears possible through relativistic effects. However, journeying into the past faces formidable theoretical and practical barriers, potentially rendering it impossible.
The crucial caveat remains that our current theories of the universe – relativity and quantum mechanics – are incomplete and incompatible. A deeper, unified theory is needed, but despite decades of research, it remains elusive. “Until we achieve that unified theory, definitive answers about time travel remain beyond our grasp,” Shoshany concludes.
Of course, in a very real sense, you have already traveled into the future by reading this article – several minutes, in fact. You’re welcome to ponder the implications of that particular journey through time.
