The concept of journeying through time, both forwards and backwards, has captivated the imaginations of science fiction writers and physicists alike for generations. From classic novels like “The Time Machine” to iconic movies such as “Back to the Future” and the long-running TV series “Doctor Who,” the allure of time travel and its inherent paradoxes continues to fascinate. But beyond the realm of fiction, is time travel possible in reality?
In popular culture, “Doctor Who” stands out as a prominent example of time travel narratives. The Doctor traverses time in the TARDIS, a remarkable vehicle capable of navigating through both time and space. Famously, the TARDIS possesses the whimsical property of being larger on the inside than its external dimensions suggest, a charming disregard for conventional physics.
While “Doctor Who” embraces time travel as a central plot device, it deliberately avoids grounding the TARDIS’s capabilities in actual scientific principles. This fantastical approach aligns with the show’s fairy-tale essence, prioritizing imaginative storytelling over strict scientific accuracy.
However, when we turn to the real world, the question of time travel becomes a matter of serious scientific inquiry. Could we ever construct a time machine to journey into the distant past or leap into the far future? Addressing this question necessitates a deep dive into our understanding of time itself – a concept that continues to puzzle physicists. Currently, scientific consensus suggests that future time travel is plausible, while past time travel remains either exceptionally challenging or fundamentally impossible.
Time Travel to the Future: Einstein’s Relativity and Time Dilation
Let’s begin with Albert Einstein’s groundbreaking theories of relativity, which revolutionized our understanding of space, time, gravity, and mass. A fundamental consequence of relativity is the realization that time is not absolute; its passage is relative and can vary depending on different conditions.
“This is where the genuine science of time travel emerges, supported by real-world evidence and implications,” explains Emma Osborne, an astrophysicist at the University of York.
One of the most striking predictions of relativity is time dilation: time slows down for objects moving at high speeds. This effect becomes noticeable as speeds approach the speed of light. The classic example is the 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 would genuinely be younger than their Earthbound sibling,” confirms Vlatko Vedral, a quantum physicist at the University of Oxford. This isn’t just theoretical; astronaut Scott Kelly experienced a version of this when spending months in space, although not at relativistic speeds.
Similarly, gravity also affects time. Time passes more slowly in stronger gravitational fields, such as near a black hole. “Your head ages slightly faster than your feet due to the subtle difference in Earth’s gravitational pull,” Osborne points out.
The science fiction series “Doctor Who” cleverly incorporated this concept in the season 10 finale, “World Enough and Time.” The Doctor and his companions find themselves on a spaceship near a black hole, where time dilation causes time to pass at different rates at the front and rear of the ship. This temporal distortion allows Cybermen at the rear to evolve into a massive army in what seems like minutes from the Doctor’s perspective. The film “Interstellar” also prominently features the effects of gravity on time as a key plot element.
In everyday life, these relativistic effects are imperceptible. However, they are crucial for the functioning of Global Positioning System (GPS) satellites. “Clocks on GPS satellites tick faster than clocks on Earth” and require constant adjustments, Osborne notes. “Without these corrections, Google Maps would accumulate errors of about 10km (six miles) daily.”
Relativity, therefore, confirms the possibility of future time travel. No elaborate time machine is strictly necessary. Traveling at near-light speeds or spending time in a strong gravitational field will result in experiencing less time subjectively compared to the rest of the universe. If your goal is to witness the future centuries ahead, these are the scientifically validated methods.
The Challenges of Time Travel to the Past
In stark contrast to future time travel, journeying into the past presents formidable challenges.
“Its feasibility remains an open question,” states Barak Shoshany, a theoretical physicist at Brock University. “Our current scientific understanding and theories are simply inadequate to definitively answer whether backward time travel is possible.”
Relativity offers some theoretical avenues for past time travel, but they are highly speculative. “Physicists grapple with complex concepts of space-time manipulation to explore the potential for time travel to the past,” explains Katie Mack, a theoretical cosmologist at the Perimeter Institute for Theoretical Physics.
One such concept involves creating a closed time-like curve, a loop in space-time that returns to its starting point in both space and time. Theoretically, someone traversing this path would eventually arrive back at their initial time and location. Logician Kurt Gödel mathematically described such a path in a 1949 study, and subsequent research has explored similar ideas.
However, closed time-like curves face significant hurdles.
“We lack any evidence of their existence in the universe,” Vedral emphasizes. “It remains a purely theoretical construct without observational support.”
Furthermore, even if they exist, creating them seems technologically improbable. “Even with vastly advanced technology, intentionally generating closed time-like curves appears highly unlikely,” suggests Emily Adlam, a philosopher at Chapman University.
Even hypothetically creating one might be undesirable, according to Vedral. “You would be trapped in an endless loop, reliving the same moments repeatedly,” he explains.
“Doctor Who” touched upon a similar scenario in the episode “Heaven Sent,” where the Doctor endures the same few hours for billions of years, albeit through repeated teleportation rather than a closed time-like curve.
Another theoretical possibility emerged from physicist Richard Gott’s 1991 study, involving hypothetical “cosmic strings” – one-dimensional objects with immense density potentially 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 remains unconfirmed. “There’s no compelling reason to believe cosmic strings exist,” Mack points out. Even if they did, finding two conveniently aligned and moving in parallel would be astronomically improbable.
Wormholes: Space-time Shortcuts and Time Machines?
Another intriguing concept from relativity is wormholes, theoretical tunnels through space-time that could connect vastly distant points or even different times. Imagine folding space-time like paper and punching a tunnel through it to create a shortcut. “Wormholes are theoretically permissible within the framework of general relativity,” Vedral confirms.
However, the existence of wormholes remains purely hypothetical. “While mathematical models suggest their possibility, their physical reality is uncertain,” Osborne clarifies.
Even if wormholes exist, they are predicted to be extremely unstable and short-lived. “Wormholes are often visualized as connections between two black holes,” Osborne explains. This implies immense gravitational forces that would likely cause a wormhole to collapse instantly.
Furthermore, any naturally occurring wormholes would likely be microscopically small, far too tiny for even a bacterium to pass through.
Theoretically, these limitations could be overcome, but it would require enormous quantities of “negative energy,” a concept that might exist at subatomic scales. While energy fields must have positive overall energy, tiny pockets of negative energy might exist within them, Osborne explains. “Expanding these minute pockets of negative energy is what would be needed, but it seems fundamentally impossible.”
Vedral concludes, “It doesn’t strike me as a remotely realistic proposition.”
Quantum Physics and Retrocausality: A Glimmer of Hope for Past Time Travel?
Shifting from relativity to quantum mechanics, the physics governing the microscopic world of atoms and subatomic particles, introduces further complexities and potential paradoxes regarding time travel.
Quantum mechanics reveals phenomena that challenge our classical intuitions. One such phenomenon is non-locality, where changes to one entangled particle instantaneously affect another, even across vast distances – what Einstein famously termed “spooky action at a distance.” This 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 effects seemingly violate the cosmic speed limit – nothing should travel faster than light.
Some physicists propose alternative interpretations of quantum experiments to eliminate non-locality. These interpretations often involve retrocausality, where effects can travel backward in time.
“Instead of instantaneous non-local influence, an effect could propagate into the future, then reverse direction and travel into the past,” Adlam explains. “This would mimic instantaneous action but involve a detour through time.”
This retrocausal interpretation suggests that events in the future could influence the past, challenging our intuitive understanding of causality as flowing linearly from past to present to future. In these quantum scenarios, information might be traveling to the future and then looping back to the past.
However, it’s crucial to note that this retrocausal interpretation is not universally accepted within the physics community. Many quantum physicists find retrocausality as problematic, if not more so, than non-locality.
Even if retrocausality is real, it’s unlikely to offer a pathway to becoming a time-traveling Time Lord. “Retrocausality is distinct from time travel as we imagine it,” Adlam clarifies.
Current observations of non-locality involve minuscule numbers of particles. Scaling this up to macroscopic objects like humans or even small objects like a piece of paper would present insurmountable challenges.
Furthermore, even in these quantum experiments, sending a message to the past seems impossible. “Retrocausality, if it exists, is inherently concealed by its implementation,” Adlam explains.
Consider an experiment where Adam’s measurement outcome depends on Beth’s later measurement. Beth’s future experiment dictates 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 concludes. “Practical exploitation is impossible because successful signaling necessitates obliterating the records of that success.”
The Verdict: Future Time Travel Possible, Past Time Travel Highly Improbable
Based on our current scientific understanding, future time travel appears theoretically achievable through relativistic effects like time dilation. However, past time travel faces immense hurdles and might be fundamentally impossible based on our current theories.
The caveat remains that our current theories, relativity and quantum mechanics, are incomplete and incompatible in some respects. A deeper, unified theory is needed, but remains elusive despite decades of research. “Until we possess that complete theory, definitive conclusions are premature,” Shoshany cautions.
Ultimately, while the possibility of past time travel remains in the realm of science fiction, we can take solace in the fact that simply by reading this article, you have already journeyed several minutes into the future. You are, in a very real sense, a time traveler yourself!
