Time travel works...just invented, by you. The tests prove it!

After many conversations and significant planning, your epidemiologist significant other has approved your plans for time travel, forward or backwards. You are both satisfied that if you do bring back some dread disease, the only person who will die is you; not everyone else.

However, as a time traveler, you're a strong student of history and are aware that ideas can be just as infectious as biological pathogens. For example, sending fully formed Marxist ideas back to the end of feudalism in the 1500's would have a profound effect on the history of Europe. Likewise, bringing an old but forgotten idea forward can have similarly profound effects.

As much as possible, you'd like to be able to rejoin society after your travels but worry about bringing some contagious idea with you when you come back. How does this time traveler handle the risks of being infected with a contagious idea then spreading it to his fellow humans on return to his own time?

The same disclaimer applies: you don't have to worry about the Grandfather Paradox or causality or any of the other nasty time travel problems. Physical security of the time machine itself is also taken care of, so no worries of theft or breaking & entering. The time machine also includes a 99% accurate universal translator that permits bidirectional communication between you and whoever you're talking to (if you don't have this translator then you don't stand much chance of getting infected, eh?).

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    $\begingroup$ @AngelPray Ideas die all the time. I've probably had a few thousand ideas today. Most of them were uninteresting, ill formed, and not worth my time. Since I cannot recollect them and haven't subjected anyone else to them they are effectively dead. $\endgroup$
    – sphennings
    Nov 28, 2017 at 18:26
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    $\begingroup$ Unless you have no ideas already in you when you time travel, you shouldn't do it at all if you are worried about the damages of the ideas you can expose to the times. Anyways memetic ideas becoming overwhelming doesn't matter if you gloss over the grandfather paradox as both depends on the consequences of you in another time. If you are worried about bring ideas to your original time, it really doesn't matter because ideas are born and will be born eventually, it literally doesn't matter if you bring some back with you. $\endgroup$ Nov 28, 2017 at 18:32
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    $\begingroup$ "Fully formed Marxist ideas" would have no traction whatsoever in the pre-Modern world. Among other problems, Marxist ideas presuppose that free market capitalism is the dominant mode of production; that goods production is done mostly in mechanized factories; that the world works essentially under the rule of law; that the normal organization of a country is a representative democracy; that most people think rationally and not magically; that religion is a spent power; etc. In other words, Marxist ideology is just as historically determined as any other ideology. $\endgroup$
    – AlexP
    Nov 28, 2017 at 21:44
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    $\begingroup$ If you don't have to worry about the Grandfather Paradox or other causality problems, then you don't have to worry about ideas spreading, since whatever is preventing said paradoxes/problems must, as a requirement, resolve the issue of potentially problematic ideas being brought back in time. $\endgroup$
    – Aify
    Nov 28, 2017 at 23:09
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    $\begingroup$ I just wanted to say that I find this a VERY great question and I enjoyed it much. Wish I could upvote it multiple times! $\endgroup$
    – Maciej
    Nov 29, 2017 at 10:17

10 Answers 10


Memes don't exist in a vacuum. Like a bacteria or a virus they need the right host to settle in. For instance when I play Rick Astley's 1987 hit song Never Gonna Give You Up to my 90 year old grandmother, she remarks "What a nice song." without any of the groans and grumbles associated with being rickrolled.

Since as a time traveler, lacking cultural context, you're probably not the ideal host for most memes. Now it is possible that an ideal translator would translate the speech acts in such a way that the context is perfectly translated as well. If your translator is 99% accurate it's probably not preserving context well enough.*

* The importance of cultural context in communication is quite well demonstrated in Darmok, the 102nd episode of Star Trek the Next Generation

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    $\begingroup$ Well, you think she liked it. If you get a giant, hot pink knitted sweater for Christmas, you'll know she was just biding her time... $\endgroup$
    – Machavity
    Nov 28, 2017 at 19:18
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    $\begingroup$ @Machavity I'd be elated if my grandmother had enough energy to knit me a sweater at 90. $\endgroup$
    – sphennings
    Nov 28, 2017 at 19:22
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    $\begingroup$ @Xan That's only 5 years. We're rather resistant to 5 year old memes. Note how in the last panel other people exposed to the memes are not spreading the memes further. $\endgroup$
    – sphennings
    Nov 28, 2017 at 23:34
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    $\begingroup$ Quite right. Memes can only be contagious if the host is receptive. This means the cultural, social, politic & economic conditions must be right for its transmission. An ideal translator will be OK for ordinary speech, but confronted with an unusual idea it will squark "concept incomprehensible!" in the time traveller's ear. The OP's example of Marxism in the medieval causing revolution is improbable. Medieval folk wouldn't understand Marx's economic theories let alone why capitalism, which they didn't have though its seeds were present, was evil. Plus one. $\endgroup$
    – a4android
    Nov 29, 2017 at 3:34
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    $\begingroup$ The importance of cultural context in communication is even better demonstrated by your footnote. You may think you've communicated an idea, but the (probably quite large) number of people who either have never watched that program or haven't memorized every episode are just going "What?" $\endgroup$
    – jamesqf
    Nov 29, 2017 at 4:12

A "contagious idea" seems like a strange way of saying "a good idea".

  1. You go to some far-away place. (Another planet, another time, another culture).

  2. You hear about some idea you think is awesome.

  3. You come home and tell everyone about it. They think it's awesome too.

  4. It's widely adopted.

  5. Hooray!

Normally, learning new ideas and philosophies is encouraged, unless you explicitly want people to stick to some fixed belief system... then you would keep them away from knowledge. But in your case, I think the "contagious idea" is one that only seems good, but will probably have some dire consequences in the future?

You can always check up on that with your time machine. "Woah guys, virology is tricky." "Turns out that the aliens aren't friendly, humans are just delicious." "The kool-aid was poison! Go figure!" etc.

If that interferes with the "no paradox" rule, then you could have a council you need to report to after each trip. Psychologists who specialize in helping people who were trapped in cults, scientists who can evaluate the danger of new discoveries, historians specializing in different forms of government, and so forth. A filter of experts.

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    $\begingroup$ "Jews are the cause of our woes!" doesn't sound like a good idea, unless you're a proud Aryan looking for a scapegoat. $\endgroup$ Nov 28, 2017 at 23:44
  • $\begingroup$ @CeesTimmerman Coverd by "one that only seems good, but will probably have some dire consequences in the future". I take it that we all know what those consequences were... :-( $\endgroup$
    – wizzwizz4
    Nov 29, 2017 at 19:27
  • $\begingroup$ @CeesTimmerman Actually this idea was a common knowledge in medieval times. A time traveler may rather cause troublesome time paradox by explaining that epidemics spread because of microbes and not because of Jews poisoning wells. $\endgroup$
    – Shadow1024
    Nov 30, 2017 at 7:52

This really is just another version of the grandfather paradox; if you can't go back in the past and kill your own grandfather, then you can't (by extension) go into the future and bring back an idea that changes the future.

New and dangerous ideas are introduced to society all the time; the only difference is that the time that they're introduced is called the 'present', and we see the present change and unfold before us as we travel through time in a forward direction at 1x 'speed'. So, we know that we can change the future with our ideas, but we're doing it with a single stake in the ground; we know everything from the past up to this point, but we know nothing about the future.

If killing your grandfather can't happen, then you can't introduce Marxism to peasants in the Middle Ages; that's changing the past even more than killing your grandfather, so simply won't work (it might even BE killing your grandfather if you obliterate the conditions that led to his birth in the first place).

BUT - if you go to the future and pick up an idea, you can't bring it back and introduce it for the very same reason. If you do, then it's always happened. This is because all the people in the future you've visited all have grandfathers, and introducing this idea could introduce changes to the future you've just visited, including potentially causing those people's non-existence for the same reason that you can't change the past.

Another way of putting this; if you can visit the future as well as the past, and paradoxes can't exist, then that means that the future is already written. That means that whatever you do, you can't change it just like you can't change the past.

  • $\begingroup$ Time is simply a measurement of space, and TV allows us to safely see in relatively real time how, say, a form of government works out. $\endgroup$ Nov 28, 2017 at 23:49
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    $\begingroup$ Just because the universe is deterministic doesn't necessarily mean that consciousness is all an il[MESSAGE REMOVED FOR STARTING A FLAME WAR ACROSS THE INTERNET] $\endgroup$
    – wizzwizz4
    Nov 29, 2017 at 19:29

Brainwash- ahem Re-Education

You are a genius (obviously - how else would you have constructed this perfect time-travel scenario?) so it should be easy for you to encapsulate your current self, namely in the form of your regular thought processes and the amount of ideas, in a special automated training program.

You need a machine.

Humans might get infected. But if you build a machine that is able to interact with you (think ELIZA, but better) you can safely test yourself for contagious ideas.

To make this fool-proof, you should allow your machine to hypnotize you and then ask you about what you learned on your adventures. If hypnotizing does not work, drugs might be helpful.

The program should then proceed to help you bury the dangerous new ideas that you encountered under a layer of other thoughts - pain springs to mind. Electroshock-therapy in combination with hypnosis should prevent you from accidentally spreading your ideas.

Remember to bury the ideas veeeeeeery deep!

What's a little torture if you can be sure that you won't harm humanity through contagious ideas?


For 99.99% of the cases, meme containment will be automatic. As long as you're traveling to a time period where easy access to information isn't readily available, the majority of the people aren't going to have the means to propagate your meme - thus it dies on the vine, as it were.

In other cases, the context of the meme won't make sense to someone without the context - they will infer you're just an outsider speaking about things they don't understand or don't care about. Who cares if you 'Rick-rolled' the King. Is that even a bad thing? What is this 'Rick-Roll' you speak of? Is it edible?

So, for the very small subset of cases where a change would actually have some sort of effect, you have three categories of how that change would affect the timeline.

In the first, we make the assumption that time is either somewhat self-repairing as in most changes are 'smoothed' out to fix mistakes... OR....

Changes in time propagate up the timeline, making the future-past you remember change as you make them; meaning you'll remember your past as if you'd already made the change you just made.

OR... you're creating another universe based on the changes you're making now, meaning the new universe was always the way you just changed it and your old universe never had you changing the past to begin with - because it already happened and never had you visiting there.

Of those three choices, I would believe either A or B to be the most likely. And I personally prefer the A scenario; meaning the past will somewhat correct itself of any anomalies you introduce anyway.

Thus, to answer your question, I don't think the meme containment is something you need to really be worried about. Most things will be handled on their own. The best way to ensure this is to make sure of the following: Bring back nothing that's out of period, minimize contact with the time-period natives, don't have random philosophical conversations with the few people of the period that might understand the deeper implications of your remarks.

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    $\begingroup$ And avoid any butterflies. ;) $\endgroup$ Nov 28, 2017 at 23:41
  • $\begingroup$ It could become a story about how "spreading memes" isn't as easy as it's made out to be, and as the protagonist thought it was. $\endgroup$
    – A. B.
    Mar 11, 2021 at 3:19

You might want to watch Dr. Who as he time travels all the time.

If you get an idea, first jump 50 or so years into the future and use research materials at libraries or internet or etc to see how the idea turns out.

  1. If it has a negative effect, then you won't spread it.
  2. If it has a neutral effect or unknown effect jump back in the TARDIS and try again in 50 more years.
  3. If there is no mention of it, the idea was probably replaced with a better idea.
  4. If it has a positive effect, then spreading it might be ok.

The key is to be like a wild life observer who documents but does not interfere. Don't talk about religious, political, or etc topics that might get you in trouble. Try the weather and generic things.

  • $\begingroup$ "If there is no mention of it, the idea was probably replaced with a better idea" or you didn't spread it. This may be because it had a negative effect... Oh dear. $\endgroup$
    – wizzwizz4
    Nov 29, 2017 at 19:33

Dogmatic bullheaded zealotry.

I am interested in time travel only to spread the true faith, which possesses my every thought. I see the world through the lens of this faith which distorts all things to match my preconceptions. I am supremely confident in the correctness of my ideas and the irrelevance of any other which does not match.

These ideological blinders ensure I am safe from new ideas or different ideas, which I interpret as actually being the same as my own ideas if I like them or some sort of pathetic aberration if I do not.

In fact, the whole reason for my travels is to proselytize and spread my own faith through history. I am confident in my ability to infect many. No; infect ALL. I must just get to them before (and after) my enemies do.


The premise is flawed.

This is not a real 'infection'. It's about the time traveler getting a new idea. As such, the solution is not to spread bad ideas. Given that you are both the one concerned about this and the one doing the time travel, you can fix this your self either by not telling anyone of your time about that (not ideal, though), or ensuring before communicating anything that it is indeed a good idea. Even if your old-you would think badly about it, the new one is expected to have, increased, that same knowledge, and thus be able to a better decision. Traveling opens your mind :)

If these were different people, I see how it could be a problem, though. Our gracious government may not be keen on a time-traveler discovering that a few centuries ago democracy actually worked acceptably, and spreading such disturbing idea amongst our loyal citizens…


TD;DR -- Don't worry 'bout it

Remember, another phrase for memetic infection is "liking the idea".

As others have commented, your own upbringing will inoculate you from many ideas which are too outre for your current worldview to assimilate. You'll experience an "ick factor" when someone suggests something your birth culture finds repellent. Imagine you go check out the Aztec Empire ... they are unlikely to convince you that people must make human sacrifices lest the Sun fail to rise.

You are in more danger if you go back to the past of your own civilization (I'm assuming you are not Aztec descended here, mind). Their ideas will resonate harder, because those assumptions are lurking in your own meme-pool already.

Could be you are impressed with the fervent faith of your ancestors in their villages, and outraged that pilgrims to various shrines in the Middle East are being mistreated.

So you come back, screaming "Deus Vult" every ten minutes. You rally support, assemble a huge army, sail across the ocean and disembark in the Holy Land, ready to kick some Fatimid Caliphate heinie. Imagine your chagrin when an irritated JDF colonel grates out that the Fatimids haven't bothered anyone since, oh I dunno, 1171 AD, and can you please show your tourist visas?

Which is to say, your native time culture is also inoculated against ideas which are too far away from what they're used to. Only the ideas that mesh with what's already there will catch on. And if they do, that's not a bad thing.


Hypnosis could be used to reset the time traveler's mind to its previous state, especially if the hypnotic suggestion was inserted before the start of the journey.

The hypnotist speaks, "You will forget everything which happens between now and the moment when you hear this little brass bell ring.

Then you get in the time machine and wiz away. Moments later you reappear with a wild gleam in your eye and powerful words about to spill out from your fevered mind.


That is how a hypnotist saved the world!

Extending This Idea Into A Multi-Layered Air-Lock For Ideas...

Before each time travel journey, the traveler, a medical doctor and 3 interrogators (a psychologist, a physicist and a historian) are all hypnotized as described above. The time traveler then goes on the journey. Upon his return, he is locked in a room with the doctor who conducts a detailed medical exam then monitors the traveler during a multi-week quarantine. During the quarantine, each of the interrogators will (from their individual quarantine cells) take turns debriefing the traveler using a hardwired and isolated telephone connection. At the end of the quarantine, the hypnotist and a team of additional psychiatrists will evaluate each of the interrogators, the doctor and the traveler. If any aberrant becomes evident, the bell will be rung and the interrogators notes will be burned. If not, they all get to keep their memories and anything which was learned during the journey.

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    $\begingroup$ Hopefully no one in the past/future rings a little brass bell in your vicinity, or you will become a very confused individual (and if you manage to return, will be infected...). $\endgroup$ Nov 28, 2017 at 20:47
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    $\begingroup$ It may not work, as even if the traveller cannot consciously remember the idea, it would still be in the unconscious. Even worse, being repressed, it may emerge in warped forms and/or have adverse effects on the traveller's mental health down the line. $\endgroup$
    – Eth
    Nov 29, 2017 at 15:49

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