A.K.A. How do we actually move through time?

Chronotopology is tells us how the timeline reacts to time travel, but what happens to the time traveler themself? How is time travel achieved? These topics comprise chronoportation, the physical movement through time (a portmanteau, à la teleportation).

So as best I can figure, there are 4 main components to chronoportation:

Departure. What happens when time travel is initiated and the traveler leaves their timeline/era.
Transit. What happens while the traveler is traveling.
Arrival. What happens when the traveler finishes their traversal, landing in their destination era.
Conveyance. How time travel is achieved (e.g. time machine, superpowers, etc.).

For each of these components, there are a few options to choose from, and looking at them is the subject of this post.

Conveyance

Time Machine

Ever since H.G. Wells’ The Time Machine, the use of a device to travel through time has become the standard for time travel. The reason is straightforward: If time travel is a scientific phenomenon, then it can be achieved by technological means. We already use machines to travel through space, so time is a natural extension.

Sometimes the time machine is an artifact of unknown origin, sometimes it’s an accident of engineering, sometimes it’s the result of years of research, sometimes it’s an enchanted hourglass. What makes a thing a time machine is that it physically exists, is not itself a living thing (that’s a separate category), and that it can send things through time. The time machine itself may or may not travel with its passenger.

Traditionally, a time machine is seen as a vehicle: a vehicular time machine is one that moves with the traveler; it is itself a time traveler too. The alternative is that your time machine can’t come with you. The time traveler is sent out by the conveyor, but the conveyor stays behind. The time traveler is effectively stranded. Such a time machine would be a catapult time machine. It is also common for catapulting time machines to have some form of recall functionality–the travel can be reversed and the traveler brought back to the original departure point.

This is a valuable narrative device, as it vastly decreases the power that a chrononaut wields. If they mess up, they can’t just go back and try again. They only get one chance, unless they can last until the time machine is reinvented.

Examples: The Time Machine, Back to the Future

Chronokinesis

Chronokinesis is the ability to manipulate time without a time machine. Essentially it is time control as a superpower. If a living thing is used as a time machine, they have chronokinesis. There aren’t any inherent limits on the concept of chronokinesis; it is a blanket term for any temporal abilities.

Examples: Umbrella Academy, Life is Strange, X-Men: Days of Future Past

Portal/Wormhole

A portal is a gateway in space. We often imagine them as being ovals, but they could be door-shaped or spherical or any other 2- or 3-dimensional shape. The portal could even be mobile. Any location that can be traveled by normal means (e.g. walking) through to arrive in a different time is a temporal portal. Theoretically, this is how wormholes work, linking two separate spacetime locations. In this context, we will use the term portal for any such spatiotemporal connection between two points in spacetime.

Departure

Ejection

Ejection is departure in which the traveler physically leaves the era, leaving nothing behind. Ejection is in line with our natural ideas of travel–any trip you take from one location to another requires you to physically leave before anything else happens.

If we treat it like traveling 3-dimensionally, then traveling through time most likely does not involve fading in and out. It will be more like walking through a door; the traveler will disappear piecewise like a wipe transition and the air around them will rush in to take their place.

Examples: Bill & Ted, Timecop, Back to the Future

Projection

The traveler doesn’t leave the timeline, but they are projected or duplicated from it to the next one. This could include forms of astral projection. Depending on the specifics, a connection may be maintained between the original and the projection. If not, it is like catapulting, in that the traveler is cut off from their home era.

Physical Chronesthesia: Mental Time Travel

A person to sends their consciousness into their younger self. Maybe it’s a curse or maybe they have a way to send their brain waves back in time, but somehow they are inhabiting the body of their younger self. Everything that happened to them before is happening again, but now they know what’s going to happen. And they probably want to change it.

The term “chronesthesia” refers to a person’s ability to recall past events or imagine future events and mentally reconstruct the experience. In psychology, it is often referred to as mental time travel. So it makes sense that when it actually physically happens, it could be classified as a physical chronesthesia. It is sometimes also called “time leap”.

At first glance, mental time travel might seem inherently inferior to physical time travel–you can’t bring anything with you, you’re restricted to your world line (i.e., times and places you’ve visited in your lifetime), which prevents you from going very far back in time, and you can’t benefit from any body-training. But these all stem from the key advantages of mental time travel: the traveler keeps their memories, experience, and knowledge of the future without any physical deterioration. Mental time travel is the only way to use time travel to circumvent the effects of aging; sending their consciousness back in time at regular intervals, the traveler could live through several lives worth of time. With ejection, if you go back in time and spend 5 years in the past before returning, you will be 5 years older when you return than when you left; it will appear that you have aged 5 years instantly to everyone in your home era.

There are a fair number of ways a person may initiate mental time travel:

Automatic. The archetype for this type of time travel, a character is sent back regardless of their circumstances, once some specific criteria are fulfilled. Generally the destination always has the same universal time regardless of the circumstances, whether it be back one day or all the way to their birth every single time. The two most common criteria are that a certain date has passed or that the traveler dies (or loses consciousness).
Examples: Groundhog Day, Palm Springs, Re:Zero, All You Need is Kill/Edge of Tomorrow, Happy Death Day, Muv-Luv, The First Fifteen Lives of Harry August

At will: A person can send their consciousness back in time at will. Often a form of chronokinesis, it’s simply a power a character has.
Examples: Madoka Magica, Life is Strange, X-Men: Days of Future Past

Engram Implantation: When science is used to allow for mental time travel, it’s probably a matter of a character physically implanting their current memories into their younger self. The matter of if and how engrams could be implanted is beyond the scope of this book, as it is mostly a matter of biology and biotechnology. We will take as a given that memories can be stored as data and reimplanted. As mentioned in the previous section, this limits what can be transferred in order for the personality to remain intact. It also means that the memory transfer mechanism is entirely separate from the time travel mechanism, as the information must be sent through time, then implanted.
Examples: Steins;Gate, X-Factor comics

Chronesthesia and the Body

Generally, we look at chronesthesia as the person’s consciousness being sent through time, either splitting in two or leaving their body entirely. If consciousness leaves the body without splitting, the body will just slump over dead randomly in any era it abandons, because the soul has left it to go elsewhen. If it splits, then one copy of the traveler goes through time while the other remains and keeps going forward. The traveler will never know which one of the two they will be. Of course, the issue of psyche-splitting is sidestepped if chronesthesia is activated by the traveler’s death.

There are major pitfalls involved in sending one’s consciousness into their future self. If the consciousness is duplicated and one version stays in the body while another is sent forward, the version that remains in the body is left to wonder what went wrong and why they didn’t go forward. They might realize their consciousness has split, and that the traveling version will overwrite them at some point in the future, which would be a terrifying existential experience.

Examples: About Time, Hot Tub Time Machine

Transit

There is distance between two points in spacetime. Sometimes that distance is made up of space–-if you are calling someone on the phone, you both occupy the same point in time but not in space. Sometimes those two points are separated by time–-if you watch a show on your favorite streaming service, you don’t move in space, but the time changes. Sometimes the two points are separated by space and time–any time you drive anywhere, you must travel over space and it takes time. In a multiverse, there is an interuniversal distance as well that must be traversed, the distance from one universe to the next.

Intrauniversal Transit

Intrauniversal transit means that the traveler never leaves the spacetime of the universe they depart from. In modern scientific models of time travel, the traveler must go through every point in spacetime between their departure and arrival. For example, in relativity, an object stays in its own timeline all throughout traversal. It simply experiences time differently from everything around it. In theory, someone could be observing it the whole time. Similarly, hypersleep could be considered intrauniversal travel to the future.

Intrauniversal travel backward through time is a bit more intricate. If you want to leave New York on December 7 and arrive in LA on December 5, you’ll need to be in Oklahoma or Missouri on December 6.¹ So if someone else left LA on December 5, they could follow you as you moonwalked to New York on December 7. In turn, you’d see them moonwalking their way to LA.

Some models would liken this to the behavior of antimatter. That’s its own whole discussion, but in short, if travelling backward through time transformed the traveler into antimatter, they would need to not touch any normal matter lest they explode in a giant matter-antimatter annihilation reaction.

Teleportation

When we think about time travel, usually we don’t think of someone moving backward through time to get to where they want. They depart spacetime completely and then arrive at their destination rather suddenly.

Teleportation is the simplest form of transit–no time passes between departure and arrival. Chronotraversal is instantaneous. Narratively, this is a convenience because we don’t want to bog down a story by talking about the specifics of the trip itself. Scientifically, teleportation lets us focus on the relevant effects on the traveler and the chronoverse, even if it’s not realistic. Teleportation as a concept has its own associated technical issues, so for now we will have to assume that the issues of teleportation have been readily solved for us.

Even if teleportation is not possible, we will often approximate time travel to be teleportational in our discussions so that we can focus on the chronodynamic effects rather than the issues of travel across spacetime.

Finite Velocity

It is perhaps more intuitive to believe that time travel takes time, that the traveler must follow a continuous path from source to destination. After all, like we said when driving from New York to LA, you must travel through all the points in between, no matter how fast your vehicle.

However, we generally don’t see travelers going backwards through time within the universe itself. This raises a question, though: If a traveler exits a timeline to travel to another point in time, where exactly are they in between departure and arrival?

The Timestream

The extrauniversal space in between timelines is the Timestream. Unless otherwise specified, I will be treating time travel as movement through the Timestream. A time traveler leaves their timeline, enters the Timestream, travels through the Timestream to the point they want to arrive, exits the Timestream, and emerges onto the destination timeline at that point. Of course, the Timestream does not come into play of a traveler is able to move backwards through time intrauniversally.

The nature of a Timestream location is subject to all manner of effects, based on the properties of the chronoverse. While the Timestream could take up all the empty volume in the chronoverse, it could also happen that timelines take up most of the chronoverse, and the Timestream makes up small paths between points. The Timestream may appear as a hyperspace tunnel between points in time. If the multiverse forms a continuous spectrum, the traveler might never actually enter the Timestream, as they move continuously from universe to universe. In a way, this allows them to exist outside of the timelines’ conceptions of time.

Arrival

Arrival is generally determined by the method of departure: whatever happened to the traveler on departure should happen in reverse on arrival.

Displacement

Displacement occurs when the traveler appears at the point of arrival as an addition to the universe. This is perhaps the most obvious form of arrival, as it matches our common conceptions of movement, just like ejection. And in fact, displacement generally matches with ejection–-departure that involves taking the traveler out of the source era entirely leads to the traveler entering their destination.

When you arrive you push the air out of the way, as though walking through a door, because that’s essentially what’s happening–walking through a multidimensional door. Consequently, if you’re going to arrive in a solid building, it’ll be like trying to walk into a wall. Whatever method you use to track your location will have to account for this.

With displacement, there is also generally a possibility of a separate version of the traveler being on the timeline, regardless of the topology.

Replacement

Replacement occurs when the traveler arrives at a point where they already existed on the timeline. The native version ceases to be and is supplanted by the traveler. Physical chronesthesia is a common form of replacement. A more conventional replacement would have an entirely separate version of the traveler physically overtake themself; the native version would disappear when the traveler arrives on the timeline via displacement. This can be a way to work around issues of thermodynamics, by redirecting the energy from the time traveler themself. The replacing traveler generally takes on the physical state of the native version, often with transplanted memories. For example, if someone goes back 10 years via replacement, they will be 10 years younger, wearing what they were wearing 10 years ago, but retaining memories from 10 years in the future.

A traveler doesn’t necessarily need to occupy the same space as the native version to replace themself, which results in something odd. If you walk 10 feet and then jump back in time 10 seconds and arrive with replacement, the old version of you will disappear when the new version arrives. It will look like you teleported 10 feet.

Example: Life is Strange, Groundhog Day, any Physical Chronesthesia

Complete systems

Special Relativity

Special relativity provides a full framework for time travel. While we have only sent people fractions of a second into the future so far, it is of course theoretically possible to go much further. It simply hasn’t been done yet, for a number of reasons. Building a rocket that can accelerate someone on a round trip into space with enough time and acceleration to make it relativistically worth it would not be financially worth it. But it can be done with currently existing technology plus enough money and willpower. Just because something isn’t feasible doesn’t mean it’s not possible.

The common conjecture with relativity is that if time slows down and stops as we approach lightspeed, then it will reverse when we surpass it. There are two issues that crop up in the math though. First, it takes an infinite amount of energy for any object with any mass to reach c. No amount of energy lets us overstep c. Second, if we just skip the practical issue and just put superluminal velocity into our special relativity equations, time doesn’t go negative, it becomes imaginary. For example, if we want to go double lightspeed, our time factor becomes 0.577i. There are some sources² that suggests a mathematical way for time to go negative, but the solution is not conclusive.

Prophecy

There are many terms for knowledge that comes from beyond one’s perception, but I will use prophecy as the general term to cover them. Prophecy is a form of time travel in which information is the traveler. It can include such phenomena as precognition,³ clairvoyance, psychometry, or even a general ability to sense danger. The person who draws or receives this information is a prophet. Prophesying is the act of making a prophecy (yes, the only difference between the noun and the verb is the letter s). “Prophecy” usually isn’t referring to extrapolation or prediction; it indicates the existence of some future that the information must come from. Being an innate ability, it is often a form of chronokinesis. The information is projected out of its era, teleporting to the destination where it is superimposed on the prophet.

We can classify prophecy based when the information comes from: precognition is knowledge from the future and retrocognition is knowledge from the past.⁴ We can also subdivide prophecy by how it is achieved.

Prophecies are the oldest form of time travel, being involved in some form in almost every myth, legend, and religion.

Laplace’s Demon: The Hyper-Extrapolator

An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.
— Pierre Simon Laplace, A Philosophical Essay on Probabilities

We already know some things about the future. The Earth will complete one revolution around the sun in the next 365 days (approximately). Halley’s Comet is projected to return to Earth in 2061. Our sun will die in about 5 billion years. We can be pretty certain about a fair number of future astronomical phenomena.

Humanity currently has a limited ability to predict the future using physics and simulations. As you might have guessed from the preceding paragraph, our ability to predict is not extremely precise. The larger the scale of the phenomena, the less it can be altered by humans and chaos. In order to make more precise predictions, more precise information is required. For example, if someone is diagnosed with a terminal illness or if a hitman is hired to assassinate them, their life expectancy can be more accurately calculated. If a machine is built and operated at specifications, its performance can be predicted.

Of course, chaos theory tells us there is always room for error in such predictions. There are countless variables that we cannot account for.

But, what if we could? What if we had a supercomputer that could account for variables in such a way far beyond the abilities of us pathetic humans?

Laplace’s Demon is the logical conclusion to this line of reasoning. It is a theoretical being that knows the location and state of everything in the universe and all the equations that govern their behavior. The unpredictability of chaos would not come into play, because the Demon is precise enough to account for it. In theory, Laplace’s Demon could then perfectly compute and predict the future. Such precise knowledge of the future is effectively indistinguishable from time travel. In its traditional form, Laplace’s Demon is like prophecy.

Example: Overclockwise (Futurama), The Day I Became God

Footnotes

Of course, our current models require you to travel through an extreme gravitational field to do this, so you couldn’t really do it on Earth. Also, I know no person could physically walk fast enough to go from NY to LA or LA to NY in 2 days. It’s a thought experiment, people!
Hill, James & Cox, Barry, Einstein’s special relativity beyond the speed of light.
In general, precognition and prognostication have the same meaning, though prognostication can imply uncertainty, as if a guess, while precognition is more certain.
Retrocognition only covers events that weren’t learned about my mundane means. You can learn about the past by doing research, but that would not be retrocognition.

The Basics of Time Travel series:

Basics of Time Travel: Chronoportation