A Linear's Guide To Time Travel

What's that? You're time traveling, but you're scared of messing something up and don't know what type of time travel you're dealing with? Did you skip Time Travel 101 in school or something?

Well, don't worry, you've come to the right place.

In this article I, as a human located on Earth's surface while progressing through time at roughly 1 second per second relative to other humans, will do my utmost to assist you in your time travel efforts.

Just to clarify, I can't provide much scientific proof for everything that follows, since time travel doesn't actually exist. Probably. Also, I don't really know the technical names for these things, or if they even exist, so I'll just use my own.

Types of time travel

There are, primarily, two types of time travel.


Retconnable time travel is kind of difficult to illustrate with a short example, since it all comes down to the idea that you can go back, change something, then return to the future and things will be different. Now, whether or not you keep your memories (and your body and everything else) from the original version of the universe is essentially impossible to say. The movie Back To The Future (the first one) illustrates retconnable time travel decently (though with some artistic license).

It is worth noting that retconnable time travel causes some immediate concerns. Primarily paradoxes. You probably would have heard of things like the grandfather paradox if you hadn't skipped that Time Travel 101 class. The general idea is that you go back in time and kill you own grandfather before your mother, thus preventing her from birthing you, which means that you can't go back in time to kill your own grandfather. This is a paradox in the traditional time travel sense. If only it were that simple.

I can't exactly say how this would work, since, again, time travel doesn't exist, but, scientifically speaking, your very appearance and existence in another time would start to cause issues very quickly. Don't forget that your body is made out of atoms and that, every second, you shed dead hair and skin cells, which are also made of atoms. Almost more importantly is the fact that an observer influences the state of whatever it's observing, as you may have learned in one of your science classes. Unless you skipped those, too. You should really stop skipping classes, you know.

Anyway, your atoms, your observations and the radiation you emit (heat, for example) will inevitably result in some changes by the very nature of their existence and influence on the other matter in the universe. Okay, it's getting a bit technical and off-topic here, but it all comes down to this:

Don't expect the future to be the same, even if you don't think you changed anything. (If you're dealing with retconnable time travel, of course.)


Non-retconnable time travel, or stable time travel, is essentially the idea that everything that ever will happen has already happened. In other words, you cannot technically change anything.

An example you might think of is: You see someone die and go back to save them, but you will always be prevented from doing so, since you've seen them die.

I would argue that this example is incorrect, however. A more accurate example would be as follows: You see someone die and go back to save them, but you will always think that you see them die. While you going into the past could save their life, it won't change the fact that you thought you saw them die.

Make sense?

In short, the universe is static across 4 dimensions. Past, present, and future already exists, always has, and always will. As I understand it, this follows Albert Einstein's (hopefully you've heard of him) theory of general relativity, so this is what I would consider to be most likely. Fun is limited, but at least you don't have to worry about blowing up the planet.

Test Procedure

If you wish to test whether you're dealing with retconnable or non-retconnable time travel, I recommend the following procedure:

  • Find a good spot. Preferably right beneath your feet, so it's a constant.
  • If there is a circle there, go back in time (to when nothing is there) and paint a circle there.
  • If there is a cross there, go back in time (to when nothing is there) and paint a cross there.
  • If there is nothing there, go back in time and paint a circle.

If there is nothing or a circle, it's retconnable time travel. If there is a cross, it's non-retconnable time travel.

This procedure functions by simply conveying information across loops. If there is a nothing, you're the first in the line of retconnable time travel. You will make a circle, which indicates to the next you that somebody (not specifically you, but just somebody) along the line experienced a point where there was nothing painted. That version of you will see the circle and repaint it, which indicates the same to the next. If there is a cross, you painted a cross because there was never a time when there was nothing there. There is no start or end.

Note that the above procedure is subject to interruption or outside influence, so it may be wise to adapt it to your circumstances or perform it several times in different locations to be sure.

Please also note that this particular logic only works if you're not dealing with some pretty specific multiversal conditions. Unfortunately, I haven't come up with a test for that. Sorry.


In addition to the types of time travel, the structure of the metaverse (No, not Metaverse. It's just a term to cover the outer bounds of existence.). The metaverse may end with the universe, in which case our universe is the only one in existence. However, there may be more layers around our universe, like a multiverse, and more universes beside it.

Single universe

This is the pretty intuitive and follows our current scientific understanding. There is just one universe. You can expect non-retconnable and retconnable time travel to function the way I've explained before.

Now, there are also a few types of multiverses. Knowing about them probably won't be too useful for you, but let me at least describe them for the fun of it.

Multiverse (finite)

There exists a multiverse with several finite universes within it. This multiverse could be expanding or static, but since this idea arises from applying the rules of our own expanding universe to it, I would say that, if this is the case, an expanding multiverse is most likely. Regardless, there is a limited amount of universes within the multiverse. Take care of them.

Multiverse (infinite)

There exists a multiverse. It contains an infinite amount of universes. Your possibility for fun is boundless. Please still take care of them, though. If this is the case, you should probably also be careful when you arrive somewhere new. There's no telling what those infinite universes hold.

Multiverse (branching)

There exists a multiverse with some amount of universes within, but whenever time travel occurs, you are sent to a new universe, which branched off the one you left. The most likely way that this would work is that when you time travel, a new universe is created. No matter what. You can never go back to a previous universe under any circumstances. If this is the case, hope that the multiverse is expanding, and that it is fast enough to account for all the new universes you're making. It could be argued that this is a "type" of time travel, as well, but it's kind of hard to categorize, so you'll have to excuse me placing it here.

As an aside, note that, if a multiverse does exist, it is also hypothetically possible for there to exist some outer layer around the multiverse(s) in a similar way that the multiverse surrounds our universes. If that is the case, I would expect them to be functionally similar to the multiverses within them. I won't cover those ideas in too much detail here, since they're probably impossible. More probably impossible than just time travel, anyway.


When travelling, safety is important. Not just for other people, but for you as well.

Be prepared for the possibility that humanity doesn't exist in a lot of the universes or times you travel to, and that the planet is either a burnt rock floating in space or space dust. If it were me, I would try to limit the time machine to times and universes with a planet roughly in the same location as ours, and with radiation similar to ours, since it should help indicate the existence of intelligent life through our technology. Also maybe wear a space suit. Alternatively, make your time machine a capsule with a built-in gyroscope designed to hold yourself upright regardless of any rough landings.

Speaking of landings, I recommend implementing several features into your time machine, if possible. For one, I would recommend checking whether or not something already exists in a radius around where you're aiming. You don't want to embed yourself in anything. Second, I would try to lock time travel to the rotation and location of the planet Earth to avoid accidentally flying off into nothingness. Of course, this also requires space travel; Not just time travel. Spacetime travel then. Hopefully you can apply some of those theoretical wormholes to your advantage here, so you won't be restricted to the speed of light.

I would like to provide you with advice on keeping other people safe, but I'm afraid I can't really do that. If you're dealing with retconnable time travel, the most I can do is urge you not to destroy the planet or blip me into non-existence. If you're dealing with non-retconnable time travel... well, what can I do? It's already happened. Maybe don't get involved in anything too critical, though. Just in case.


Thank you, dear time traveler, for reading. Good luck out and take care out there. Don't forget to say hello to me on the way.