The concept of time travel, jumping between different points in history, has captivated imaginations in countless science fiction stories. From H.G. Wells’ “The Time Machine” to the iconic “Back to the Future” and the enduring series “Doctor Who,” the allure of altering the past or glimpsing the future is undeniable. But how much of this fantastical idea aligns with the actual laws of physics? Can You Travel Time in reality?
While “Doctor Who” cleverly uses the Tardis, a vehicle bigger on the inside than the outside, to traverse the temporal landscape, the show prioritizes storytelling over scientific accuracy. But what does real-world physics have to say? Is building a time machine and embarking on a journey to witness historical events or meet future generations within the realm of possibility? The answer, according to current scientific understanding, is complex. Traveling to the future appears achievable, but venturing into the past presents formidable, potentially insurmountable, obstacles.
The Arrow of Time: Einstein’s Relativity and the Future
Albert Einstein’s theories of relativity revolutionized our understanding of space, time, gravity, and mass. A core tenet of relativity is that time isn’t a fixed, constant entity; its flow is relative to the observer.
“This is where can you travel time can come in and it is scientifically accurate and there are real-world repercussions from that,” explains astrophysicist Emma Osborne from the University of York.
One of the key consequences of relativity is time dilation. Time slows down as an object’s speed increases, with the effect becoming significant as speeds approach the speed of light. This gives rise to the famous “twin paradox,” where one twin travels at near-light speed while the other remains on Earth. Upon the traveling twin’s return, they would be younger than their Earth-bound sibling. The Kelly twins, Scott and Mark, provided a real-world example of this when Scott spent an extended period in space, although not at relativistic speeds.
Similarly, time slows down in strong gravitational fields, like those near black holes. “Your head is ageing quicker than your feet, because Earth’s gravity is stronger at your feet,” says Osborne.
This concept was central to the “Doctor Who” episode “World Enough and Time,” where the Doctor and his companions find themselves on a spaceship near a black hole, experiencing vastly different rates of time passage. The movie Interstellar also explores this effect of gravity on time.
These relativistic effects, although subtle in everyday life, have practical implications. The satellites used for GPS rely on incredibly precise timekeeping, and the effects of relativity need to be accounted for to ensure accuracy. “The clocks above click faster than the clocks on Earth”, and must be constantly readjusted, says Osborne. “If we didn’t, Google Maps would be wrong about 10km (six miles) a day.”
In essence, relativity suggests that future time travel is possible, though it does not mean we need to build a time machine. By traveling at near-light speeds or spending time in a strong gravitational field, one could experience a relatively short period of subjective time while decades or centuries pass elsewhere in the universe.
The Challenges of Backward Time Travel
While journeying into the future seems plausible according to relativity, traveling to the past presents considerably more complex and theoretical challenges.
“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 for backward time travel, but they involve extreme and speculative scenarios. One possibility is the creation of a closed time-like curve: a path through spacetime that loops back on itself. A person following this path would eventually return to their starting point in time and space. The first mathematical description of such a path was by Kurt Gödel in 1949.
However, closed time-like curves face significant obstacles. As Vedral puts it, “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, “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.”
Even if such curves could be created, they might lead to undesirable outcomes. “You would literally be repeating exactly the same thing over and over again,” says Vedral.
Another theoretical possibility involves “cosmic strings,” hypothetical objects that may have formed in the early universe. Physicist Richard Gott proposed in 1991 that two cosmic strings moving past each other in opposite directions could create closed time-like curves.
However, cosmic strings remain purely theoretical, with no observational evidence to support their existence. “We don’t have any reason to believe cosmic strings exist,” says Mack. Even if they exist, the probability of finding two moving in the required configuration is incredibly low.
Wormholes, theoretical tunnels connecting distant points in spacetime, are another concept that seemingly permits can you travel time in theory.
However, like closed time-like curves and cosmic strings, wormholes are hypothetical. “It’s been shown mathematically that they can exist, but whether they exist physically is something else,” says Osborne.
Even if wormholes exist, they are likely to be extremely short-lived and microscopically small. Their intense gravitational fields would cause them to collapse rapidly, and they would be too small for anything larger than a subatomic particle to pass through.
Stabilizing a wormhole and making it traversable would require enormous amounts of “negative energy,” a concept that, while theoretically possible on a tiny scale, is unlikely to be achievable in practice.
Vedral summarizes the situation: “It doesn’t sound like a very realistic proposal.”
Quantum Mechanics and Retrocausality
While relativity offers some tantalizing but ultimately problematic possibilities for backward time travel, quantum mechanics presents another intriguing perspective.
Quantum mechanics governs the behavior of the very small, such as atoms and subatomic particles. One of the key features of the quantum world is non-locality, where a change in the state of one particle can instantaneously influence another entangled particle, regardless of the distance separating them. Einstein famously called this “spooky action at a distance”.
Some physicists, uncomfortable with the implications of non-locality, have proposed alternative interpretations that involve retrocausality, where events in the future can influence events in the past.
However, as Adlam notes, “Retrocausality’s not quite the same thing as time travel.”
Even if retrocausality is real, it likely wouldn’t allow us to become time travelers. Our observations of non-locality have been limited to tiny numbers of particles, and scaling this up to macroscopic objects would be an immense challenge.
Furthermore, it may not even be possible to send a message into the past using retrocausality. According to Adlam, any attempt to influence the past would necessarily involve destroying all records of the event, making it impossible to utilize the information.
The Unfinished Puzzle
So, where does this leave us regarding can you travel time? Based on our current understanding of the universe, traveling to the future is possible, but traveling to the past appears highly improbable.
However, it’s crucial to remember that our current theories are incomplete. Relativity and quantum mechanics, while remarkably successful in their respective domains, are fundamentally incompatible. A deeper, unifying theory is needed, but despite decades of research, it remains elusive. “Until we have that theory, we cannot be sure,” says Shoshany.
In the meantime, perhaps we can take solace in the fact that, as you’ve read this article, you’ve already traveled several minutes into the future.
