The concept of time travel, jumping forward or backward in time, has captivated the imagination of both science fiction enthusiasts and physicists alike. From H.G. Wells’ The Time Machine to Back to the Future and the enduring popularity of Doctor Who, the idea of manipulating time has been a rich source of storytelling. But beyond the realm of fiction, How Do You Time Travel, and is it truly possible according to the laws of physics?
While Doctor Who embraces the fantastical with the TARDIS, a time-and-space-bending vehicle famously “bigger on the inside,” the show doesn’t concern itself with real-world physics. However, the question of whether time travel is possible in reality requires a deeper understanding of how time itself works – a topic that continues to challenge physicists. Currently, scientific understanding suggests that traveling to the future is plausible, but traveling to the past presents significant, potentially insurmountable obstacles.
Let’s begin with Albert Einstein’s theories of relativity, which revolutionized our understanding of space, time, mass, and gravity. A fundamental consequence of relativity is that the passage of time is not constant. Time can speed up or slow down, depending on the observer’s relative motion or the strength of the gravitational field they are experiencing.
“This is where time travel can come in, and it is scientifically accurate, and there are real-world repercussions from that,” explains Emma Osborne, an astrophysicist at the University of York.
Time Dilation: Your Ticket to the Future
One of the key concepts is time dilation. Time passes more slowly for objects traveling at high speeds. The closer you get to the speed of light, the more significant this effect becomes. This phenomenon leads to the famous “twin paradox,” where one twin travels into space at near-light speed while the other remains on Earth. Upon the astronaut twin’s return, they will have aged less than their Earthbound sibling. As quantum physicist Vlatko Vedral from the University of Oxford notes, “If you travel and come back, you are really younger than the twin brother.” The reality of this was demonstrated when astronaut Scott Kelly spent months in space, aging ever so slightly less than his twin brother Mark on Earth.
Time also slows down in strong gravitational fields, such as those near a black hole. As Osborne explains, “Your head is ageing quicker than your feet, because Earth’s gravity is stronger at your feet.” This effect was dramatically portrayed in the Doctor Who episode “World Enough and Time” and the film Interstellar.
These relativistic effects, while subtle in everyday life, are crucial for the accuracy of technologies like the Global Positioning System (GPS). Osborne highlights that “the clocks above click faster than the clocks on Earth” and must be constantly adjusted. “If we didn’t, Google Maps would be wrong about 10km (six miles) a day.”
Therefore, relativity makes future time travel possible, even without a dedicated “time machine.” By traveling at near-light speeds or spending time in a strong gravitational field, one can experience a shorter passage of subjective time compared to the rest of the universe, effectively leaping into the future.
The Perplexing Past: Wormholes, Cosmic Strings, and Closed Time-like Curves
Traveling backwards in time presents a far greater challenge. “It may or may not be possible,” states Barak Shoshany, a theoretical physicist at Brock University. “What we have right now is just insufficient knowledge, possibly insufficient theories.”
Relativity offers some theoretical possibilities for backwards time travel, but they are highly speculative. Katie Mack, a theoretical cosmologist, explains that some physicists are “trying to find ways to rearrange space-time in order to make time travel to the past possible.”
One concept involves creating a closed time-like curve: a path through space and time that loops back on itself, allowing a traveler to return to their starting point in time and space. Logician Kurt Gödel published a mathematical description of such a path in 1949, but the practicality remains questionable.
Vedral points out that “We don’t know whether this exists anywhere in the Universe.” Even if such curves existed, Emily Adlam, a philosopher at Chapman University, suggests that “Even if we had much greater technological powers than we currently do, it seems unlikely that we would be able to create closed time-like curves on purpose.” Furthermore, Vedral cautions that “You would literally be repeating exactly the same thing over and over again.”
Another theoretical possibility involves cosmic strings. In a 1991 study, physicist Richard Gott proposed that two cosmic strings moving past each other in opposite directions could create closed time-like curves. However, cosmic strings are hypothetical phenomena that may have formed in the early universe, and none have ever been detected. Mack emphasizes, “We don’t have any reason to believe cosmic strings exist.”
Wormholes are another theoretical construct allowed by relativity. These are hypothetical tunnels through spacetime that could provide shortcuts between distant points. Vedral confirms that “Wormholes are theoretically possible in general relativity.”
However, the challenges associated with wormholes are significant. First, there is no evidence that wormholes exist. Osborne notes that “It’s been shown mathematically that they can exist, but whether they exist physically is something else.” Even if they do exist, wormholes are likely to be incredibly short-lived and microscopically tiny, collapsing under their own gravity.
Stabilizing a wormhole would require an enormous amount of “negative energy,” a concept that Osborne deems unlikely: “I don’t think that’s possible in any way.”
Quantum Mechanics and Retrocausality: A Glimmer of Hope?
What about quantum mechanics, the other great theory of the Universe? Quantum mechanics describes the behavior of particles at the subatomic level, where physics operates in ways that often defy our intuition.
One peculiar quantum phenomenon is non-locality, where a change in one particle’s state can instantaneously influence another entangled particle, regardless of the distance separating them. Einstein famously called this “spooky action at a distance.”
Adlam notes that “A lot of physicists are very unhappy with the possibility of non-locality.” Some physicists propose that these effects are not instantaneous but involve information traveling faster than light. To avoid this, alternative interpretations suggest that the effect travels into the future and then back into the past.
This concept introduces “retrocausality,” where future events can influence the past, challenging our conventional understanding of causality. However, this interpretation is not universally accepted, with many physicists finding retrocausality as unsettling as non-locality.
Even if retrocausality is real, it’s unlikely to enable practical time travel. Adlam clarifies that “Retrocausality’s not quite the same thing as time travel.” The observed instances of non-locality involve tiny numbers of particles, and scaling this up to a macroscopic level would be extremely difficult.
Furthermore, even in these scenarios, sending a message into the past seems impossible. According to Adlam, any attempt to send a signal to the past would necessarily involve destroying all records of the event, making it unusable.
The Unfinished Puzzle: Unifying Relativity and Quantum Mechanics
In conclusion, while traveling to the future appears possible based on our current understanding of relativity, traveling to the past remains highly speculative and faces significant theoretical challenges.
The key caveat is that our understanding of the Universe is incomplete. Relativity and quantum mechanics, while highly successful in their respective domains, are incompatible with each other. This suggests that a deeper, unified theory is needed. As Shoshany concludes, “Until we have that theory, we cannot be sure.”
Until then, perhaps the simplest form of time travel is the one we all experience: moving inexorably forward into the future, one moment at a time.
