John from our future, takes the time-travel bus back in time and steps off in 5th century Europe. He heads for the secret location where travellers are kitted out with authentic clothes, food and currency to keep them alive until they can establish themselves. He has learned enough of the local language to pass for a foreigner. He tells the locals he is from "Nonsylvania" or "far to the North" or such.

By time-travel law, he was not allowed to carry any artefact back with him and he didn't. He purchased a return ticket and and can use it by going back to the hideout according to the timetable. He is not allowed to make repeat journeys for any reason.

John is an entrepreneur. He left school early to work in a local market. He doesn't have scientific or detailed historical knowledge. There is no way he could invent even electric power. He doesn't know how to make rubber. If he wants to "invent" something that he knows about from his own century, he must do it with ancient tools and technology and get the locals to make it.

However John, contrary to all the rules of time-travel, wants to change the ancient world.

He settles on one simple invention from later than the 5th century that he can make and sell to build his business. He can employ local crafts people but they can only use their ancient tools.

My first thought was scissors but it turns out they were invented around 1500 BC in ancient Egypt.


I then thought of a can-opener (invented around 1810) but then he would have to invent cans. Given that John knows little about metals or where to find them, is it feasible that he could make this "invention" and set up his 5th century worldwide business empire?

Can he make cans? If he can't make cans, what can he make?


  1. John has done no research before leaving. He is a businessman not a historian or scientist. His plan is to look around when he gets there and see if they are missing any everyday objects that he knows about. He only has to have a rough idea how to make them and maybe draw a sketch for the local blacksmith to try and emulate. He must rely on local skills, tools and knowledge to do the actual manufacturing.

  2. By tool I mean a solid object, e.g. scissors, paperweight, hammer, axe, etc. Remember that John was not allowed to take any artefacts back in time (and didn't). He has to work from memory. Hence he will probably pick something simple but very useful.

Detailed timeline of inventions from Wikipedia https://en.wikipedia.org/wiki/Timeline_of_historic_inventions

EDIT - in defence of John

NOTE - What follows, superficially looks like story-line but it is actually an explanation of the conditions I set. Without clear conditions a question like this becomes far too open-ended.

Many people have queried John's lack of knowledge and research. John does not spend all his time on Worldbuilding, so he does not know about "black powder" and other arcane subjects. He is a man-in-the-street who made good. All his expertise lies in: making money, influencing people, spotting gaps in the market, organising teams to do the things he doesn't have the expertise for, etc. He was moderately rich and successful when he lived in his own century. Thus his skills are fully formed. Now that he can afford time-travel (it's very expensive), he is setting out on the adventure of his life. Instead of being a big fish in a medium-sized pond, he wants to be a big fish in the pond that is 5th century Earth. He didn't do research before starting because he isn't that kind of guy. He enjoys landing on his feet in a new situation and thinking fast. His intention is to spot gaps in the market - at first locally and then more and more widely. He will diversify when necessary and concentrate on efficient manufacturing and distribution - out-competing others when he can, and forming partnerships with them when he can't

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Jul 18, 2020 at 3:00
  • $\begingroup$ does invention from other regions ok ? such as paper? or maybe silk making? or this must change the ancient world as a whole? no one invented yet? does john come there naked ? or he also come with his modern clothing? or do he know concrete making? $\endgroup$
    – Li Jun
    Commented Jul 18, 2020 at 3:49
  • $\begingroup$ Think of the world in the first half of the 19th century (Napoleonic wars, American War of Secession). Compare it with the world in the Antiquity. Do they look the same to you? No, they don't. In the (classical, Greco-Romano-Persian) Antiquity they didn't have -- buttons, paper, steel, cotton gins, water powered looms, lathes for machining metal, tailored clothing, uniform units of measurement, telescopes, efficient sailing ships, efficient rudders for the ships, strong alcoholic beverages, mariner's compasses, firearms and cannon, horse collars, cheap glass, bottles with stoppers, &c. &c. &c. $\endgroup$
    – AlexP
    Commented Jul 18, 2020 at 5:32
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    $\begingroup$ Mike Scotts answer, which would actually work, brings up another problem, anything John is capable of describing how to make, other people can copy without giving him a dime. If it doesn't require technical knowledge there is nothing stopping people from copying it, patent laws were extremely rate and basically only granted by royalty, so only good for a single country. $\endgroup$
    – John
    Commented Jul 18, 2020 at 14:03
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    $\begingroup$ Cans would require a degree of metallurgy that was not available in ancient times. It took until the 1800s until sheet metal was available that could be pressed into cans in a cost-effective way. $\endgroup$
    – papirtiger
    Commented Jul 18, 2020 at 14:05

37 Answers 37


I'm going to take the crazy approach and try for something new: polycarbonate and/or epoxy.

Just think for a second. Transparent fairly light shields that wont budge with a direct crossbow or war bow impact that could shatter an iron shield. Seafaring ships inmune to sea worms. Lightweight vehicles and structures that needs less horse power to move the same cargo. The sky is the limit!

  • $\begingroup$ These require petrochemicals and all the associated refining technology, which in turn requires all the associated metalworking and metallurgy and and knowledge of chemistry. $\endgroup$
    – DKNguyen
    Commented Jul 21, 2020 at 1:18

I searched through to see if I could find any that haven't been mentioned yet (there's a lot). Mostly my answers all relate to one of the most expensive things people owned at the time: clothing.

Innovations in clothing production actually had a huge impact on society because it changed farming from being an inefficient, communal, subsistence-based practice, to a profitable, merit-based system, which lowered the amount of farmers and resulted in more people doing other stuff.

The Treadle Spinning Wheel

Spinning at that time was a slow and arduous process. The great wheel was considered a huge improvement, and the treadle spinning wheel is many times faster.

The Shuttle Loom

No longer will your people have to manually move the weft threads between the warp threads. Now they press a peddle to move the warp, and a shuttle carries the weft thread across.

The Carding Machine

Instead of manually combing your wool to get out the chaff and straighten out the fibers, you can just turn a crank and do a bunch at the same time.

And most importantly:

The Enclosure Movement

Just find the best farmers and put them in charge of the land. They will get profits from using the land more efficiently and getting higher returns. Maybe they will raise more sheep. Anyway, people work better when they have a stake in the results.

The enclosure movement will result in people moving from farms to cities. Before they can find jobs, a lot of people will starve to death, but after that, they will be making pretty much everything other than food which we have in society today.



Now I know that a modern pistol or even older guns are no joke. They require a lot of work.

But many of the lessons that people learned the hard way can be skipped over as he already knows them.

Even an hour reading and seeing videos will provide enough information about the basic concepts of a gun.

He won't aim for an automatic rifle, not yet.

All he wants to make is a one shot gun of a sort or another.

He already saw a hundred revolver, he knows the trigger, the cartridge...etc. And smokeless self contained cartridges are not magic, they existed a long time ago.

So all he needs to do is experiment enough with gunpowder and machinery to create a gun. Even a single shot gun would be like magic at the time.

Starting with that he can make a fortune, and I mean a fortune, that will allow him to further improve his craft all the way to traditional revolvers.

He will have a monopoly and, if he invested sometime in it, can easily further improve metalworking and even machinery as a whole.

The YT channel forgotten weapons has amazing examples of people copying famous weapon designs with little tools and producing real guns.

Again all he want to make first is a single functional revolver.

All the other stuff, safety, standards, sights, rifling...etc come after he already make a lot of money.

Cannons are much "easier". Again just make one cannon and the local lord/jarl...etc would be so impressed that you will get enough money to make as many as you want.

Reading on those topics for a couple of hours would be enough to give theoretical knowledge.

I highly recommend he tries to do both in our world with limited tools to get a feeling of them though.

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    $\begingroup$ I only saw this answer after I posted my own... but I stand by "saltpetre" being a better answer than "guns", simply because you can do a whole lot with black power, regardless of using it as a propellant. But same answer effectively, no? $\endgroup$ Commented Jul 18, 2020 at 8:50
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    $\begingroup$ John does not know enough to make effective black powder nor enough metallurgy to make a cannon that will not explode and kill him if he did. $\endgroup$
    – John
    Commented Jul 18, 2020 at 13:50
  • $\begingroup$ @John, If he is stupid enough to not understand the most simple and basic principles of science and common sense, then he can't make anything. Start small, measure the quantities, experiment with small amounts of gunpowder, write the method, repeat...etc. Basic scientific method and common sense. Also it is simply impossible to do anything, almost, without investing time to learn it. If he can't bother reading a bit then he won't bother and read my answer. I'm also under the impression he is aware of how dangerous they are. Lastly trial and error. He knows it works. Just try it. $\endgroup$
    – Seallussus
    Commented Jul 19, 2020 at 5:23
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    $\begingroup$ making a cannon is not just common sense, making cannons strong enough not to explode is very difficult, and John is not going to have the slightest clue what he is doing wrong, assuming he lives through the failed trails. $\endgroup$
    – John
    Commented Jul 19, 2020 at 13:13
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    $\begingroup$ People were killed by cannons all the time, because their failure was unpredictable, a cannon could fire fine six times then explode on the seventh. I doubt john will know how to make a fuse but even if he did fuses were also a problem they did not burn consistently so you could light one and watch it burn down almost instantly, historical people were not idiots they understood cannons were dangerous and they were still killed by them all the time. metal fatigue and faulty chemistry are not things you can see with the naked eye. there is a reason cannons took hundreds of years to develop. $\endgroup$
    – John
    Commented Jul 20, 2020 at 13:15

Saltpetre. Potassium nitrate. Explosives and firearms. Although he equally could have com from the past (and be South Asian) to 5th century Europe as the future. But gunpowder would only reach China in the 9th century, then Turkey, the Middle East, and finally Europe in the 13th century.

You asked, if it is "feasible" he could make this "invention" and "set up his 5th century worldwide business empire". This would allow him to establish a worldwide "empire" (no qualifications like "business" necessary), a good 800 years early.

And yes, with just a local blacksmith, etc. He just would have to know which rocks to use; and how not to make it NOT explode before he wanted it to, but um, that's the foundation for empire. Business comes afterward.

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    $\begingroup$ Gunpowder does not ensure military success, especially the more primitive your firearms. Plus you would need to set up a complex logistical supply train to deal with squirreling these resources (particularly sulfur). It might be more helpful to OP if you detailed out the challenges/benefits of this scenario. $\endgroup$ Commented Jul 18, 2020 at 13:40
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    $\begingroup$ Plus John does not know how to make it, he is explicitly stated to have done zero research, so he will not know what rocks to use, in what proportions, nor how to properly refine them to make something useful. making black powder is not easy. $\endgroup$
    – John
    Commented Jul 18, 2020 at 13:47
  • $\begingroup$ @John I disagree. "John Q. Public" generally does know how to make gunpowder without explicit prior research (every heard of "science class"?), although some of the necessary chemicals were called by different names back then. $\endgroup$ Commented Jul 18, 2020 at 22:54
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    $\begingroup$ @TheDaleks I would argue most don't making gunpowder is more more than just mixing three chemicals, you have to know how to refine said chemicals and how to prepare the powder, you can get something that burns easily getting something that explodes requires knowledge. Potassium nitrate in particular is almost impossible to get if you don't know how to refine it. $\endgroup$
    – John
    Commented Jul 19, 2020 at 3:24
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    $\begingroup$ @TheDaleks That's a ridiculous notion. Even if you could get the raw materials (pretty hard without research!), you would have no idea what the proper ratio is - and that's the tricky thing about making gunpowder. The real world is not like Minecraft where you throw stuff on the crafting table and get gunpowder :) $\endgroup$
    – Luaan
    Commented Jul 19, 2020 at 14:41


There's a good chance that our hero has read about the accidental discovery of penicillin in a high school textbook. Given this, it shouldn't be hard to recreate the experiment, even with very primitive cooking utensils. You need some kind of jelly that's good for cultivating bacteria, a source of bacteria, and a little speck of mold.

You have to be lucky enough to find mold that contains the penicillium organism, and it may take a few tries before you succeed. After that, you need to learn how to cultivate penicillium and how to to purify penicillin, at least to some degree. It doesn't have to be anywhere near the standards for today's drugs.

Penicillin is a powerful weapon against many diseases caused by bacteria. Once the people have begun using penicillin against disease, they will never go back to the way they were before.

  • $\begingroup$ Thanks for your answer. Don't forget to check other people's suggestions before posting yours.. This has already been proposed by @Aww_Geez - Others have posted their objections to the idea in the comments. $\endgroup$ Commented Jul 19, 2020 at 12:46

Better agricultural practices

By introducing better plow technology like mouldboard plows and introducing techniques like crop rotation, you can start the agricultural revolution 1000 years earlier. This means a drastic increase in the yield of farmland, which allows to sustain a higher population density with less labor bound to food production. So you have a lot more labor available for all kinds of economic branches... or if you are more cynical, more disposable manpower to send into wars.

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    $\begingroup$ Well, crop rotation is way older than Antiquity. It's even mentioned in Old Testament. This only proves my point: what's necessary is not the ability to make something, but the ability to convince large amount of people to do it. $\endgroup$
    – Agent_L
    Commented Jul 21, 2020 at 15:10

John needs to invent the sulphur match. We can presume he has done some chemistry at high school and that he played with matches as a child. Maybe a small known fact is stuck in his head, and he knows enough to begin experimenting. According to Wikipedia, John's invention would anticipate the real thing by less than a hundred years, the first mass-produced match began in England around the 1840s.

A note in the text Cho Keng Lu, written in 1366, describes a sulfur match, small sticks of pinewood impregnated with sulfur, used in China by "impoverished court ladies" in AD 577 during the conquest of Northern Qi.[5] During the Five Dynasties and Ten Kingdoms (AD 907–960), a book called the Records of the Unworldly and the Strange written by Chinese author Tao Gu in about 950 stated:

If there occurs an emergency at night it may take some time to make a light to light a lamp. But an ingenious man devised the system of impregnating little sticks of pinewood with sulfur and storing them ready for use. At the slightest touch of fire, they burst into flame. One gets a little flame like an ear of corn. This marvelous thing was formerly called a "light-bringing slave", but afterward when it became an article of commerce its name was changed to 'fire inch-stick'.

Further information: https://medium.com/study-of-history/a-short-history-c52a80c69d5e


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