The concept of journeying through time, both forward and backward, has captivated the minds of science fiction authors and physicists alike. But is it truly feasible to travel back in time, or is it merely a fantasy?
Stories like Doctor Who, with its iconic Tardis, have explored the alluring possibilities and perplexing paradoxes of time travel. While Doctor Who doesn’t attempt to ground its time travel mechanics in real-world physics, the question remains: could we ever construct a real time machine?
To answer this, we must delve into the intricacies of time itself. While traveling to the future seems plausible, the prospect of traveling to the past presents immense challenges, bordering on the seemingly impossible.
The Future is (Relatively) Easy
Albert Einstein’s theories of relativity revolutionized our understanding of space, time, mass, and gravity. A pivotal revelation is that the passage of time is not constant; it can accelerate or decelerate depending on various conditions.
Illustration depicting the concept of time travel, showcasing someone seemingly stepping into a different era.
“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 slows down as you approach the speed of light. The famous “twin paradox” illustrates this: an astronaut twin traveling at near-light speed will age slower than their twin on Earth. Vlatko Vedral, a quantum physicist at the University of Oxford, notes, “If you travel and come back, you are really younger than the twin brother.” This effect was demonstrated by twins Scott and Mark Kelly, with Scott’s time in space resulting in a slight age difference.
Similarly, time passes more slowly in strong gravitational fields, like those near black holes. As Osborne points out, “Your head is ageing quicker than your feet, because Earth’s gravity is stronger at your feet.” This principle was used in Doctor Who, and also features in the movie Interstellar.
These relativistic effects, while minute in our daily lives, are crucial for technologies like GPS satellites. Osborne explains, “The clocks above click faster than the clocks on Earth,” and must be constantly readjusted. “If we didn’t, Google Maps would be wrong about 10km (six miles) a day.”
Therefore, relativity suggests that future time travel is possible. By traveling at near-light speeds or spending time in intense gravitational fields, one can experience a shorter subjective time, while the rest of the universe advances by decades or even centuries.
The Perplexing Past
Traveling backwards in time presents a far greater hurdle. “It may or may not be possible,” says Barak Shoshany, a theoretical physicist at Brock University. “What we have right now is just insufficient knowledge, possibly insufficient theories.”
Relativity offers some theoretical avenues, but they are highly speculative. Katie Mack, a theoretical cosmologist at the Perimeter Institute, notes that scientists “tie themselves up in knots trying to find ways to rearrange space-time in order to make time travel to the past possible.”
One such avenue is the creation of a closed time-like curve, a path through space-time that loops back on itself. Kurt Gödel published a mathematical description of this concept in 1949, but its feasibility remains questionable.
A portrait of Albert Einstein, whose theories of relativity laid the groundwork for understanding time dilation and its potential implications for time travel.
Vedral emphasizes, “We don’t know whether this exists anywhere in the Universe… This is really purely theoretical, there’s no evidence.” Emily Adlam, a philosopher at Chapman University, adds that even with advanced technology, creating such curves “seems unlikely.” Furthermore, Vedral argues that repeating the same experience endlessly, as a closed time-like curve would imply, is undesirable.
Another theoretical possibility involves cosmic strings, hypothetical objects that may have formed in the early universe. Richard Gott proposed in 1991 that two cosmic strings moving past each other could create closed time-like curves. However, Mack points out that “We don’t have any reason to believe cosmic strings exist,” and even if they do, finding two moving in parallel would be improbable.
Wormholes, theoretical tunnels through space-time, are another potential avenue for backward time travel. “Wormholes are theoretically possible in general relativity,” says Vedral. However, their existence is unproven, and even if they exist, they are likely short-lived and microscopically tiny.
Osborne explains that wormholes would likely “collapse under its own gravity” due to their intense gravitational fields. Stabilizing them would require “negative energy,” which is exceedingly rare and only observed on the smallest scales. Vedral concludes that this prospect “doesn’t sound like a very realistic proposal.”
Quantum Quandaries and Retrocausality
Quantum mechanics, which governs the behavior of subatomic particles, introduces even more perplexing possibilities. One such phenomenon is non-locality, where a change in one particle’s state instantaneously affects another entangled particle, regardless of distance.
Adlam notes that many physicists are uncomfortable with non-locality because it implies faster-than-light communication. Some propose alternative interpretations involving retrocausality, where future events influence past events.
“Instead of having an instantaneous non-local effect, you would just send your effect into the future, and then at some point it would turn around and go back into the past,” Adlam explains. However, this interpretation is not universally accepted, with many physicists finding retrocausality equally unsettling.
An artistic representation of quantum entanglement, illustrating the interconnectedness of particles and the potential for non-local interactions.
Even if retrocausality is real, it’s unlikely to enable time travel. Adlam states that “Retrocausality’s not quite the same thing as time travel,” because observations have only been done on a small number of particles. Furthermore, using retrocausality to send messages into the past requires destroying all records of the signal being sent.
The Verdict: Future Possible, Past Unlikely (for now)
Based on our current understanding, traveling to the future appears achievable, while traveling to the past remains highly improbable. However, our current theories are incomplete. Relativity and quantum mechanics are incompatible, suggesting the need for a deeper, unifying theory. Shoshany concludes, “Until we have that theory, we cannot be sure.”
Of course, as you’ve read this, you’ve already traveled into the future.
