Everyone time travels. You’re time traveling right now, really slowly into the future while instantaneously into the past through “the tenuous filaments of memory” (Clegg 97). Time travel time travel, the type of time travel that skips over chunks of existence to get a person to the good stuff, the type of time travel used in science fiction going back to H. G. Wells’ The Time Machine (1898), is a little more difficult. But not actually impossible, according to modern scientific theory. Albert Einstein’s ideas support a form of travel to the future using the physics of light and the expanse of space. Warping spacetime with black holes, wormholes or cosmic strings is theoretically possible, too.
This cool stuff is complicated to explain. Visual aids help. The cottage industry of distilling the hard sciences to science fiction fans has been led by PBS and Carl Sagan, Neil deGrasse Tyson, and Brian Greene throughout the years. James Kakalios examined the crazy but real science in comic books, so even the most ridiculous entertainment can provide just enough visual context to explain astrophysics, quantum mechanics and the like. As well the goofy pseudo-science of the CW Arrowverse shows The Flash and Legends of Tomorrow.
Everything in the Arrowverse is “quantum” this or “tachyon” that. Or speed force. Resident mechanical engineer Cisco Ramon whips up buzzword tech in a few hours that would take years, decades or centuries of R&D. Carlos Valdes delivers committed line readings of abstract gobbledygook that prove curiously persuasive. At a vaster scale, the Arrowverse‘s timeline is (or at least was) an enforced outcome of an “Oculus” apparatus residing at the end of time, i.e. a very pretty light effect in a dark room.
Despite these built-in, magical plot devices, and a high quota of painstakingly constructed, emo-laden scenarios, the writers of these ultimately fun superhero assemblages reliant on time travel for their narratives do hit on the occasional real theory, sometimes through direct reference, sometimes accidentally. While not so helpful in terms of the means of traveling through time, the events concocted for these shows can certainly be used to explain the foundational concepts and the theoretical by-products of the science of time travel.
On Four Dimensions and World Lines
First, for a start, consider a Team Flash pizza party.
The members of Team Flash need to know its location in a three dimensional coordinate system: the West Residence. They also need to know when the pizza party will take place. Maybe how long it will last, too. All this information helps them find this pizza party, and arrive at it on time. Basically, this is Space-Time. When we talk about 3D space, we really mean 4D, because duration is always a factor. A pizza party has a duration. The Earth’s path around the Sun has a duration. A lifetime has a duration. So duration is important.
A few years before the scientific establishment codified the term, H. G. Wells used “the fourth dimension” in The Time Machine (1898). In the story’s opening lines the character called the Time Traveller introduces the concept of the non-existence of “an instantaneous cube”. Objects must be measurable in four directions, length, breadth, thickness and duration, to exist.
Then the Time Traveller presents a hand-size prototype of the titular device. Pressing a lever on his “glittering metallic framework” causes the prototype to swing around, become indistinct, and vanish. The Time Traveller explains that he has sent the prototype into the future (or the past, he’s not entirely sure, but probably the future). He doesn’t fully explain where it went, though. When a guest considers why it should “still be here” but it’s not, the Time Traveller sweeps his hand over the now empty space where the machine sat and compares it to “the spoke of a wheel spinning, or a bullet flying through the air”. The machine is going too fast to be seen. Of course, it would hurt to stick your hand in the spokes of a spinning wheel, invisible or not, but we happily accept these types of erroneous analogies to natural phenomena in our entertainment.
A guy running at super speed or a timeship flying through a “Temporal Zone”? Pretty much the same. Our CW characters are conveniantly accessing a transitional threshold to travel from Point A to Point B, a threshold the writers’ use to quickly, and theatrically, address, on one level at least, any complications from ever-changing geographical points in space caused by the Earth spinning on its axis and orbiting the Sun. Think about it. Teleporting backwards or forwards in time by six months without changing position would leave you hanging out in space, the planet by then on the other side of the Sun. Time traveling just one hour to the future or the past would put you roughly 1,000 miles away from your starting position, due to the continuous rotation of the Earth at 1,000 mi/hr. You can look it up. So these interstitial realms, initially the products of comic book writers’ intuitions and mostly unexplainable by real science, solve for discrepencies in all four dimensions, while also operating like old-timey lobbies in movie theaters helping the viewer transition from reality to fantasy, affectively alleviating a sense of disbelief.
The means of travel in these fictions, from The Time Machine to The Flash, is essentially always hogwash. You just have to accept “glittering metallic framework” or “speed force” or “time ship” and “temporal zone” as sufficient explanations. And anyway, in an evolving intellectual landscape marked by emergent ideas of space-time, perhaps locality is the spooky action and nonlocality is the norm and things don’t actually have to have positions.
But plot continuity is still important, if for no other reason than audience engagement. To some extent a viewer looks for consistent cause and effect and, with the Arrowverse in particular, doesn’t always get it. The writers’ handling of time travel tends to walk a fine line between making some sense and making absolutely no sense. A visualization tool is helpful in this respect. Mathematician Hermann Minkowski developed a visual model of the four-dimensional universe that plots an object or an event’s “world-line” in space-time in 1908. It’s called a spacetime diagram. A simple version of the Earth’s world line in relation to the Sun’s world line looks like this (Gott 9):
The idea of a person’s world line made it to fiction early on. Heinlein’s first short story “Life-Line” (1939) describes a character as a “spacetime event having duration four ways”. His height, width and thickness change with his age, his world line “a long pink worm, continuous through the years, one end in his mother’s womb, and the other at the grave…”. Likewise Vonnegut’s Tralfamadorians, an alien race that sees humans in all four spacetime dimensions simultaneously, including post-death.
In “Pilot”, we are (re)introduced to Barry Allen. He’s transformed into the Flash by a particle accelerator accident somewhere between 2013 and 2015. Discounting later information and plot reveals, Allen’s world line would be unusual after the accident:
His transformation to superhero would introduce more crazy squiggles, bifurcations and loops than shown here as his future adventures presumably would take him all over the planet, off-world, to other dimensions and through time. The particular world line above is innacurate for another in-show reason. The pilot episode actually marks one of the last steps in a 15-year-long long con by the show’s villain Eobard Thawne. In a convoluted series of time traveling throw downs with the Flash, we learn that Thawne manipulated Allen’s world line to turn Allen into a speedster five years sooner (revealed in a flashback in the 17th episode “Tricksters”, two episodes after Barry himself accidentally created another new world line), so Allen’s world line in relation to Thawne’s really looks something like this during “Pilot”:
Thawne’s trip from the future to 2024 and then to 2000 causes Allen’s world line to split into two, “T1” (light gray) and “T2” (black). What does this mean in terms of real science? Was one reality destroyed and another created to take its place? Are there two realities? How does that work? These are questions intrinsic to the real world study of time travel. Following comic book conventions, the Arrowverse writers typically refer to these as “timelines”.
By the end of Season 3, where Team Flash finally avoids an interminable threat against Iris by Barry Allen’s nemesis Savitar, the show has introduced at least eight timelines. Reddit users have tried to make sense of it more than a few times. Depending on whatever underlying theory of time best applies to the Arrowverse (block, many worlds, many dimensions, tributaries of hypertime, etc., all discussed below), Allen’s world line, along with the world lines of all his colleagues, as well as everyone in Central City and beyond (poor Sara Diggle!), could now look like a bowl of spaghetti.
Timelines are weird. Has Barry’s T1 line been resolved? What did T1 Barry Allen do after that night saving his 10-year old self? Where did he go? The date of the future newspaper article authored by either Iris West-Allen or Julie Greer — 2024 — hasn’t changed, so T1 is still being referenced within the show’s current continuity (meaning it hasn’t been erased, suggesting some rules to the Arrowverse). So, what happened to T1 Barry Allen? That’s a person. And they just disposed of him. Like he was nothing. As we’ll see, the show writers aren’t too concerned wih tying up loose ends.
On the Arrow of Time
In the second episode of Season 3, Barry Allen has created and supposedly uncreated a timeline or reality nicknamed “Flashpoint”. In the undoing of Flashpoint, our hero discovers unnerving and even tragic discrepencies to the realities of his colleagues. So, of course, he relies on his uneducated gumption again and runs back in time to try to fix the timeline again. Wherein Jay Garrick, the Flash of Earth-3 (remember “many worlds” for later), somehow knows where he is and stops him. (Unexplained coincidences tend to pile up in the Arrowverse.) Subsequently, at a diner, Garrick uses a broken coffee cup to explain how impossible history is to fix. There are always cracks left behind.
“This coffee cup right here, think of it as the space time continuum. Whenever you go back in time, it breaks. You can reset the timeline, you can try to refix it, but no matter how hard you try, it’s never going to be exactly how it was.”
Whether coincidence or direct reference, this coffee cup analogy has precedent in an explanation of “Time’s Arrow” (Clegg 48). Consider two states of a coffee cup. The first, you’re holding a coffee cup full of coffee. Second, you’re looking at the shards of a broken coffee cup and splashes of coffee scattered all over the floor. If we film the action of the cup from hand to floor and play the breakage back and forth, isolating just those stages in the life of our coffee cup, which direction is “easier” to create? It would be much easier to break the cup. It would be much harder to reconstruct the cup. Much harder to recollect the fluid and get it back into that cup. How many arrangements of shards and splatters could you come up with? Probably an infinite number. How many arrangements of a working coffee cup containing that volume of coffee could you come up? A lot less. Arguably one. If the whole cup is order, and the broken cup is disorder, disorder is easier.
“It’s easier to go from order to disorder, to increase entropy, than it is to go the other way.” (Clegg 56)
There’s a great animation of this in Brian Greene’s PBS doc “The Illusion of Time”, an episode of The Fabric of the Cosmos, using a wine glass (and an ice cube, falling pages from a book, an egg, etc.) instead of a coffee cup.
The CW writers elegantly simplified the concept of entropy and subtly related it to the nature of time.
Entropy is fundamental to understanding the limitations on time travel, because all mechanical devices, including future time machines, require the generation and output of energy. Don’t get “entropy” and “energy” mixed up, though. They are kind of opposites. As Forensic Scientist Bartholomew Henry Allen learned in school, entropy is a description of change in an energy state and an aspect of the laws of thermodynamics, a set of rules devised during the 19th Century that guided the development of steam engines. All your Steampunk tech runs on the movement of heat from one chamber to another, on worked-up atoms losing their excitability. As Mechanical Engineer Cisco Ramon learned in school, a machine’s motive power can be produced from the fall of a body from a hot state to a cold one, and vice versa.
A lot of basic physics experiments in which one thing effects another look the same in either direction. The common example is two billiard balls. Billiard balls seem to be popular because you can imagine them as really big particles. And all this real world time travel theory is based on what microscopic particles do. So… two billiard balls, both in motion, both moving at roughly the same speed, colliding off each other. Whether a boomerang-style animated gif on Instagram of this collision is running forwards or in reverse, you really can’t tell which is which. Basic physics doesn’t distinguish between where the past stops and the future begins. The flow of time in both directions is theoretically symmetrical.
By contrast, the more complex observed world usually favors one direction, whole coffee cup to broken coffee cup, order to disorder, because it would take more energy to fix the coffee cup than to break it. While the universe doesn’t show a preference for how the first three dimensions operate, it does seem to care about which direction the 4th runs in. The known laws of the universe, i.e. thermodynamics, push things to go in the direction requiring less energy and more entropy. An “irreversibility principle”. There seems to be a “preferential direction”, dubbed the “arrow of time” (which suggests a good Cisco Ramon pun about the Arrowverse). “Time’s arrow, then, seems to be based on an inherent physical property of the universe. And it has a powerful influence on the way we experience the world.” (Clegg 60)
Consider another example: memory. Memory requires energy and an ordering of data, so entropy is going down. Forgetting requires a lot less energy and involves disorder, so entropy is going up. Over time you’re more likely to forget than to remember, unfortunately. Your natural state has a preference towards increasing entropy. And an additional, kind of weird thought… you remember events from your past but not your future. The basic operating system of your memory is fixed to time’s arrow.
The arrow of time suggests that it’s impossible to travel to the past, and even that the past doesn’t exist. That is, if it wasn’t for quantum mechanics and potentially surprising structures of the overall universe. Thankfully, some scientists and all Arrowverse writers are undeterred by these common observations of time’s nature. And Jay Garrick is probably wrong.
On paradox, “Girl, let it go”.
The Flash’s greatest nemesis is Eobard Thawne, the Reverse Flash, played with scene-chewing aplomb by two very convincing actors, Tom Cavanagh and Matt Letscher. A scientist from the year 2180, Thawne developed a personal fandom for the Flash. He becomes a speedster by reverse engineering the Flash’s origin. (That 160+ year difference? That’s like being inspired by David Livingstone, Sojourner Truth, Harriet Tubman or Charles Darwin today. Totally reasonable.) Thawne’s shift from hero to villain has not yet been revealed, but what is clear is that at some point Thawne’s inspiration turned to hatred. And as revealed in “Fast Enough”, the 23rd episode of the first season, Thawne traveled to 2000 to kill a ten year old Barry Allen.
What if he had succeeded? Doesn’t that mean no Flash, no motivation, no Reverse Flash, no time travel? If he had succeeded, Thawne would have eliminated some part of his established history. Paradox!
Paradox fuels the wackiest conflicts in time travel stories. Marty McFly begins disappearing, physically dematerializing, when his mother’s romantic feelings towards his father lessen, implying that he is never born. Typically called the Grandfather or Grandmother Paradox, changing your past or your ancestor’s past erases your future. The literal example involves traveling to the past to kill your own grandfather, effectively a form of suicide. Or going back and killing Baby Hitler. If Hitler never becomes Germany’s leader then the reason to time travel in the first place vanishes. So did you time travel or didn’t you?
A world line for McFly or Thawne would include a loop for the time travel bit and then an end point. And then non-existence. The line would disappear. But… if McFly erases his existence then he can’t go back in time. He can’t change his parents getting together. History goes about it’s normal course. McFly and Thawne are born and experience the same inspirations and events as before. Paradox!
What came first, the chicken or the egg? Aristotle freaked out about this 2,400 years ago. “For there could not have been a first egg to give a beginning to birds, or there should have been a first bird which gave a beginning to eggs; for a bird comes from an egg.”
This is a much more impressive type of paradox, called a causal loop. Also an ontological paradox or a bootstrap paradox. In Star Trek, a predestination paradox.
In the 20th episode of the first season, “The Trap”, Cisco responds favorably to the Flash’s suit design shown in the holographic newspaper article from 2024. “Wait a second. Suppose we now change the color on your suit. Will it be because we got the idea from this picture? That would mean we’re living in a causal nexus. This – wow – this is so trippy, like, Marty and the Polaroid trippy.”
Causal loop becomes causal “nexus”, perhaps an example of the writers reinventing terminology to avoid real science. And then in the Season 2 opener, Cisco goes ahead and changes the color. He’s already been inspired by the future, it’s already in his brain, so why not? The world line of Cisco’s design inspiration, this thought process, loops like a hula hoop with no apparent beginning or end.
A “closed timelike curve”, a CTC, is the established term in mathematical physics for a material version of this phenomenon, where the wordline of an object or event travels in time in such a way that its origination point is obscured. Why are professional mathematicians theorizing about time travel at all? Blame Einstein.
Einstein’s 1905 “On the Electrodynamics of Moving Bodies” paper introduced special relativity. Time travel to the future became theoretically possible, along with time-bending concepts like black holes, wormholes and string theory, and all open up the probability of paradoxes. Steely-eyed astrophysicist Igor Novikov’s 1992 paper titled “The Jinn of the time machine: nontrivial self-consistent solutions” essentially refutes the problems associated with CTCs with the nifty concept of the “Jinnee”.
The term “Jinnee” comes from the jinni/djinni/genie figure popularized for Western readers in the Arabian Nights, the supernatural creatures normally concealed from our senses but, when revealed, grant wishes. The Genie offers solutions. Novikov’s Jinnee represents the existence of self-consistent solutions to apparent paradoxes resulting from time travel. The paper explores various time travel scenarios to show how, in all cases, the probablity of a CTC is zero. Basically, that time paradoxes are impossible. The paper defines two types: “The Jinnee of the first kind–matter makes a loop” and “The Jinnee of the second kind–infomation makes a loop”. So Cisco’s design idea might be a 2nd level Jinn.
Is Season 3’s Big Bad, Savitar, a Jinn? Absolutely. Although not yet pinned down on screen, it seems that the villain’s history includes a paradoxical event in which he creates himself. While on the surface a CTC is hinted at, where a doppleganger Time Remnant hung around and eventually broke bad, in all likelihood a “self-consistent” solution can be created, too, with sufficient imagination.
Novikov suggests that his Jinnee could solve any Grandfather Paradox by staying the hand of the Grandfather-killer time traveler. Basically, the Jinnee polices time travelers, like the Time Masters, but without all the adventure. They (it’s not really a “they”) exist to magically solve paradox everywhere in the spacetime continuum.
Will The Flash solve its paradoxes? “Girl, let it go”.
Time travel on The Flash is kind of easy, more or less. Barry’s gotta run really fast. And concentrate on where he wants to go. That’s the rough version. In Season 3’s “Once and Future Flash” we see him calculating by hand how to target a specific point in the future. He’s clearly using “speed of light calculations”. You have to be super smart for that stuff.
Real world time travel is just as easy! We know how it works thanks to Einstein’s Special Theory of Relativity. This type of time travel requires a lot of planning, money and energy consumption. See? Easy. But basically, you send an astronaut out into deep space, flying away from the Earth at (very very very) high speed, then have her turn around and come back. Depending on how far away she flies before making a U-turn, the total number of years that would pass for her would be a lot less than the decades, centuries or even millenia that would pass for us. It doesn’t make intuitive sense but the science bears it out.
It’s certainly not the immediate gratification of tv show time travel. The astronaut has to sit around for a really long time, in human terms, to skip an even longer amount of time on Earth. Not narratively satisfying.
Einstein came up with his theories by just sitting around and concentrating really hard, a process he named “Gedankenexperimente”. Einstein’s first famous Gedankenexperimente happened in 1895, at the age of 16. In a cool little science fiction scenario he tried to imagine what one distinct light beam would look like if he could ride alongside it at its speed. Would it take a form? Like a wave? Like the laser blast frozen in place by Kylo Ren in The Force Awakens? “I should observe such a beam of light as an electromagnetic field at rest.” The scenario revealed to him unresolved contradictions in the established science of light, as did all the other thought experiments that followed throughout his career, which led to monumental discoveries of all kinds.
During the 1905 publications of his four most famous papers, including “On the Electrodynamics of Moving Bodies”, Einstein was working in a patent office evaluating specific patent applications that described machines that used electricity to synchronize time, often across vast distances. Modern clocks, basically. But his thought experiments always expanded to consider more extreme cases, such as wondering how two people traveling at different speeds would observe these clocks. He focused, in fact, on relativistic velocities, i.e. at things in motion at a significant fraction of the speed of light, which he knew to be “the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel”, which a lot of other dudes before him (Roemer, Bradley, Maxwell) had already figured out.
Time travel was a byproduct of Einstein’s work on light, and barely anything included in his theories as they pertain to time are relevant to the Arrowverse. But if you want to understand special relativity, The Flash is a great place to start.
In his 1916 book Relativity: The Special and General Theory, in the chapter titled “The Relativity of Simultaneity”, Einstein describes a train hurtling down a track and a passenger on that train passing a platform (or “embankment”) and an observer on that platform. And, of course, LIGHTNING strikes.
Let’s replace the train AND the passenger with the Flash. Let’s skip the platform, have Barry running down the street outside Jitters. Replace the observer on the platform with Iris West just leaving Jitters. And the Weather Wizard, hovering above the street (he can hover right?), is generating lightning bolts aimed at Barry.
Barry’s going a constant velocity, his normal 1500 mph (which in the big scheme of things is pretty slow). The Weather Wizard throws two lightning bolts at the same time, but the first, let’s label it ‘A’, strikes just behind Barry while the second, ‘B’, strikes just ahead of him. They strike equidistance from Barry’s position. To the eyes of both the Weather Wizard and to Iris, the lightning bolts strike the street at the same time. “At the same time” is the problematic phrase here, though. As Einstein did not exactly write, “Are two events (e.g. the two strokes of lightning A and B) which are simultaneous with reference to Iris West and the Weather Wizard also simultaneous relatively to the Flash? We shall show directly that the answer must be in the negative.”
Iris sees lightning bolt ‘A’ and lightning bolt ‘B’ strike the pavement at the same time, but because the Flash is moving at high speed away from ‘A’ and towards ‘B’, he will actually see ‘B’ strike earlier than ‘A’. Wha?
Again, [not exactly] according to Einstein, “events which are simultaneous with reference to Iris are not simultaneous with respect to the Flash, and vice versa (relativity of simultaneity). Every reference-body (co-ordinate system) has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.”
The speed at which you’re moving seems to alter the fabric of reality. Is this fake science? No. Are either Barry or Iris mistaken or lying to themselves? No. The scenario assumes no blinking or head turning. What the example actually illustrates is that “now” is elastic, not fixed. “Now” is a relative measurement depending on the constant velocity of the observer. Barry Allen is the “special” in the Special Theory, meaning that his unusual 1500mph accelarates his experience of the movement of light from one place to another. A “time dilation” happens. Iris is less special, but only because her constant velocity is defined by the movements of the Earth, which she shares with the rest of humanity. Thus it’s difficult to fold time dilation into our standardized understanding of how the world works, what we consider the fabric of reality, when we never experience it.
If you can overcome the slightly disturbing 3D models, someone visualized this phenomena quite accurately back in 2007. The comments of disbelief are amusing, and the additional scenario of a bomb triggered by the lightning bolts helps explain the concept even better. For instance, if another famous Rogue, the Trickster, rigged a bomb hidden in the Flash’s suit to explode when two lights flashed simultaneously, he’d probably forget to account for Einstein’s theory and set the lights to flash simultaneous to his own time reference. All Barry would have to do to avoid the bomb going off is run really fast, to create a different time reference where light behaves differently, where simultaneity is measured differently.
In effect there’s a bit of time travel happening here, although we’re talking tiny fractions of a second. The Flash is experiencing the future ever so slightly sooner than Iris. And there’s no practical way to reverse the experience.
Clegg, Brian. How to Build a Time Machine: The Real Science of Time Travel. St. Martin’s Press, 2011.
Gott, J. Richard, III. Time Travel in Einstein’s Universe: The Physical Possibilities of Travel Through Time. Houghton Mifflin Company, 2001.
Kakalios, James. The Physics of Superheroes. Avery Publishing, 2005.
Many Worlds, Self-consistency, Multiverse