Maybe it’s just the constant, looming sensation that someone has got hold of a sports almanac they shouldn’t have, but there’s been a bit of an explosion in time travel lately.
Whether it’s the time-heisting shenanigans of Avengers: Endgame, the red angel mystery in Star Trek: Discovery, the apocalyptic warnings of The Umbrella Academy, or the upcoming, Spike Lee produced See You Yesterday, time travel is our favourite retcon, the ultimate in foreshadowing, the best deus ex tempus machina.
Largely these stories use time travel as a plot device, rather than partaking in a serious exploration of how time travel could work, and audiences don’t really care about that so long as the internal logic of the story remains consistent. Which is why a lot of films have come up with some pretty ridiculous ideas for how time travel could work - but they’re nowhere near as weird as the ideas actual scientists have come up with.
And no, by ridiculous we don’t mean “Shaped like a phone box, or a DeLorean” because those time machines are works of genius. We also don’t mean “Shaped like a hot tub”, because there are limits.
Anyway, let's get cracking, exploring the ridiculous attempts to master time travel in both the real world and fictional stories...
REAL: going (much) faster than 88mph
You’re probably not surprised to learn that any effort to build a working time machine starts with ex-patent clerk and Doc Brown hair inspiration, Albert Einstein. If you’re on this website we’re also going to assume it’s not too much a leap to suggest you know the gist of his theories of relativity.
However, to quickly speed through the easy bits - the speed of light is a universal constant. If a DeLorean is travelling at 88mph, and you’re chasing after it in a VW camper van at 60mph, from your perspective the DeLorean will appear to be travelling at 28mph. If a light beam is travelling at 299,792,458 metres per second, and you chase after it in a spaceship travelling at 149,896,229 metres per second, the light beam will still appear to you to be travelling at 299,792,458 metres per second. Meanwhile, someone watching the light beam from back on Earth will see that you are travelling at 149,896,229 metres per second, and the light beam is still travelling at 299,792,458 metres per second.
The only way this makes sense is if time is moving differently for you in the spaceship than it is for the person back on Earth. As well as speed, gravity can have a similar effect on time, with strong sources of gravity slowing time down just as travelling at high speed does.
This isn’t just theoretical, it’s an observed effect. Atomic clocks flown aboard commercial airliners have been found to be slower than their identical, stationary counterparts and similar experiments have found differences in the speeds of clocks at different altitudes. While the time dilation may not be noticeable on a human scale, it builds up over time - to the point where it was recently calculated that the centre of the Earth is two and a half years younger than the outside of the planet.
Time dilation due to relativity is a phenomenon that gets more attention in books than TV or film, although we’ve seen it explored in Interstellar and the original Planet of the Apes. The trouble is that it’s one-way trip into the future, with little hope for inventing rock and roll or accidentally preventing your own birth. However, once you realise time is flexible, it opens up other possibilities.
NOT REAL: using the universal constant of love in the middle of a black hole
Interstellar did really well, they tried really hard, and they get points for depicting one of the most accurate images of a black hole years before we took our first photo of one. It gets so much right. Then they fly into the middle of a black hole and Cooper is able to send messages back in time by knocking books off a shelf because “Love is the one thing we’re capable of perceiving that transcends dimensions of time and space.” That’s a nice thought, but it’s not as backed up by scientific theory as the time dilation stuff.
NOT REAL: something, something wormholes (Donnie Darko)
We’ve seen this film a few times, we’ve read the lengthy explanation of what actually happened in it, and, well, we’re no strangers to complicated time travel logic but we’re still not 100% sure how it’s supposed to work. The gist seems to be a jet engine travels backwards in time and lands on Donnie Darko’s house, but the ghost rabbit from the future tells him to leave the house, and then he can see people’s “wormholes” that show where they’re going for some reason? And then he travels back in time so he can stay in bed and have the jet engine land on him to collapse the tangential timelines like a more depressing Captain America.
Still, it makes more sense than the spacetime rifts caused by quantum entanglement from power plants that generate power from the Earth’s tides while slowing the Earth’s rotation, as happened in Southland Tales.
REAL: wormhole to the centre of the Earth
Donnie Darko did get one thing right though. It’s the word “wormholes”. That’s it. Wormholes are one of the more easily understood theoretical forms of time travel.
There are numerous theories that describe how wormholes might be formed and a behave, and most of them only get so far before someone has to start talking about “exotic matter” to explain why the wormhole won’t simply collapse in on itself or destroy anything trying to pass through it.
However, if we allow for that we are awarded with an interstellar transit system that allows us to travel light years in a reasonable amount of time. Things really get interesting, when you put one end of your wormhole next to a powerful source of gravity, or find a way to move it at relativistic speeds.
If you’ve been paying attention, you’ll have twigged that this set up means that time is moving more slowly for one wormhole than the other. Say when the Earth was formed we put one end of our wormhole on the surface, and one at the centre of the planet at the bottom of a very long and superbly air-conditioned stairwell. By now the difference in altitude between those wormholes would mean they were two and a half years out of sync, so today if you went down our really long stairwell, and stepped through the wormhole, you would arrive back on the surface only a few days after the 2016 US Presidential election, and who doesn’t want to relive those glory days?
Time travel through wormholes is something that we’re a lot more familiar with, whether it’s the TARDIS plunging through the “vortex”, Bill & Ted’s phone booth riding through the “time circuits” or the various time travel episodes of Stargate. Of course, in reality this model has issues of its own. The first (and continually recurring) problem is that you can’t travel back in time further than the moment you built your time machine. Most fictional time machines will ignore this rule so they can go and punch Hitler, aside from possibly the only film to ever get time travel logic 100% right, Primer.
However, while wormholes are cool, plausible (by time machine standards) and maybe even possible thanks to observed properties of the Casimir Effect which arises from quantum fields and… shit, Avengers: Endgame might actually be the most scientifically accurate time machine in fiction.
NOT REAL: Hypnosis (Somewhere In Time)
In Somewhere In Time, Christopher Reeve is handed a pocket watch by an old lady, sparking a chain of events that lead him to believe he needs to travel back in time to 1912 and fall in love with her. He does this by hiring a hotel room and removing anything non-period from it, then hypnotising himself. To be clear, he’s not just hypnotising himself into believing he’s in 1912, he actually finds physical evidence in the present to suggest he had a physical existence in the past.
That won’t actually work.
REAL: The Cosmic String
However it still doesn’t sound as weird as an actual theory of time travel that involves the use of “Cosmic Strings”, which are a Marvel movie MacGuffin waiting to happen if ever we saw one.
These are hypothetical, one-dimensional formations of spacetime that could have formed from quantum fields not long after the Big Bang, producing extremely narrow, threadlike tubes of concentrated field energy… we’re just describing Infinity stones, aren’t we? Infinity stones only they’re made of string.
Now the weird thing about Cosmic String, is that if they’re in a loop, they exert gravity just like any other mass. But if you stretch it out in a line, it exerts no gravity whatsoever, even though a kilometre of Cosmic String will have as much mass as the entire planet. However, the straight Cosmic String will have an effect on light (that stuff nothing can move faster than) which is where things get really interesting.
An astrophysicist by the name of J. Richard Gott has theorised that if you had two lengths of Cosmic String, that were completely parallel and infinite in length (easy) and you had those strings fly apart from each other at high speeds, could theoretically fly a path around them that would allow you to move backwards in time.
NOT REAL: Flying around the sun really fast (Star Trek IV: The Voyage Home)
Star Trek has a whole heap of time machines at its disposal, from Time Crystals in Star Trek: Discovery (Which are a Real Thing but which Do Not Work Like That) to Tachyons (Tachyons are a Theoretical Thing that makes use of the loophole in Relativity that things can’t accelerate past the speed of light, but if they’re moving faster than light anyway it’s not an issue) to Chronitons (Not a Thing) and Red Matter (So extremely Not a Thing but we can’t wait for the franchise to come up with Matter for all the other colours).
But our favourite Star Trek time travel has to be in Star Trek IV: A Voyage Home, where they make use of a method (used accidentally in “Tomorrow is Yesterday” and deliberately in “Assignment Earth”) called The Slingshot Effect, which seems to involve flying at light speed around something with a lot of gravity.
REAL: The Tipler Cylinder
Flying really close around something very fast actually makes an awful lot of sense - providing the thing you fly around is long enough. Infinitely long. The Tipler Cylinder, proposed by Frank Tipler in 1974, consists of an infinitely long, rapidly spinning cylinder, will create a frame dragging effect (which shows how an object's rotation can affect its gravitational field). If you find a Tipler Cylinder, by flying around it in the opposite direction to the one it is spinning, you can travel backwards in time.
NOT REAL: Government Surveillance (Source Code, Déjà Vu)
We live in a panopticon, beneath a ceiling of constantly filming surveillance satellites, while our inner-most desires and most idle thoughts are being regularly scooped up by every search engine, social media platform and cookie-capturing website you ever visit.
Science fiction has suggested that this technology could be used to create a digital clone to bring you back from the dead (Black Mirror’s “Be Right Back”) or predict the future with unerring accuracy in a way that doesn’t at all provide unnecessary credence to the false-positive laden, racial profiling rich actual algorithm based crime-prediction methods (Hey there Person Of Interest). But as well as flawlessly predicting the future, total government surveillance can also give us a window into the past. In Déjà Vu, our surveillance satellites allow us to accurately view anywhere on Earth as it was four days ago. In Source Code, a soldier is able to relive the final moments of a man who dies in a terrorist attack.
In both films, the protagonists are explicitly told that they can observe and learn from the past, but can’t change it. In both films, they change it anyway.
REAL: The Space Doughnut
Aside from strings, another popular trick for physicists trying to show how a time machine might work is to simply tie spacetime itself in knots. Physicist Amos Ori, for instance, has demonstrated how spacetime formed into a doughnut shape would cause space and time to close back on themselves. Yes, this involves altering the shape of the very fabric of spacetime itself, but unlike wormholes, which inevitably end up relying on some form of “exotic matter”, or Cosmic Strings, which have to be literally infinite in length, the time doughnut can be constructed using boring old gravity.
Of course, none of these time machines are going to be built in anyone’s garage any time soon, although we have seen practical experiments that show time moving at different speeds, and recently, an experiment in a quantum computer that actually reversed time (for a particle at the subatomic level).
But for creators of science fiction, these ideas show that there are far more interesting things to do with time machines than a simple magic box with a button on it. These hypothetical time machines all come with restrictions, but time travel stories work best when those restrictions are laid bare, as we’ve seen with Endgame, with Primer and yes, even with Back To The Future, a film that it’s fun to mock but which works precisely because it lays its time travel rules out so clearly.
And who doesn’t want to see an infinitely long spinning cylinder time machine, a wormhole to the centre of the Earth, or spacetime twisted into the shape of a doughnut? At least it’s more imaginative than another phone box.