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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.

Question

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?

Assumptions

  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 - Reinstate Monica Jul 18 '20 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 Jul 18 '20 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 Jul 18 '20 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 Jul 18 '20 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 Jul 18 '20 at 14:05

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In Europe, the stirrup. They were in use in China by the fifth century, but were unknown in Europe, and give a very significant advantage to cavalry as well as facilitating other uses for ridden horses. The Wikipedia article says:

Some argue that the stirrup was one of the basic tools used to create and spread modern civilization, possibly as important as the wheel or printing press.

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    $\begingroup$ I have to upvote this for being one of the only things that a laymen could actually get made. he won't get global domination out of it, but he will make some money until people just start copying his stirrup. damn that is another consideration, anything John is capable of telling a craftsman how to make, are simple enough for other people to copy without giving John a dime. $\endgroup$ – John Jul 18 '20 at 13:56
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    $\begingroup$ Great idea. There's even built-in advertising. John rides around on his horse and passers-by ask. Pardon me sir, but what are those metal things on your feet? John leaps out of his saddle and produces a shiny new pair from his bag. These sir are the latest thing. All the way from Nonsylvania. Can I interest you in buying a pair? Of course, there was probably no patent law but brands are important. Look, Mary, I have a genuine pair of John stirrups. Aren't they fine? Also of course, John will manufacture ahead of demand. He will 'invent' mass production and beat everyone else to the market. $\endgroup$ – chasly - supports Monica Jul 18 '20 at 22:36
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    $\begingroup$ @chasly-reinstateMonica mass production requires a bit more than just being "invented". $\endgroup$ – njzk2 Jul 19 '20 at 11:07
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    $\begingroup$ @chasly-reinstateMonica many people making the same thing at the same time requires a few things: enough skilled workers in close enough proximity, which requires the supply chain to provide them with food and resources. 5th century is not densely populated, so that's not easy. It also requires a market that can accept lots of goods, which again is not obvious given the population and the organisation at the time. (During Rome peak, it would have made more sense. The army's uniform, weapons, armors, were mass-produced, after all) $\endgroup$ – njzk2 Jul 19 '20 at 18:12
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    $\begingroup$ There's a good chance that John's stirrups cause him to roll sideways off of the horse, to much general amusement (and no one buying stirrups). Functional stirrups require a really good saddle tree (and should be attached to the tree), and I don't think Europeans had commonly available saddles of that quality at that time. Making a saddle tree is hard, and John (who's never even heard of a saddle tree) is not going to get it right. $\endgroup$ – user3067860 Jul 19 '20 at 22:06
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Hygiene: A powerful battlefield weapon

If you really want to get the attention of powerful people, teach them how to keep their soldiers alive when the enemy is dying. Through much of history, diseases have ravaged armies around the world. Even with the improvements of technology made in the next 1,400 years, two-thirds of fatalities in the US Civil War came from disease. Start teaching a few things like not collecting drinking water downstream of your latrines. Then move on to advanced lessons, like hand washing and properly storing and cooking food. Charge heavily for each lesson. Then train employees to teach these skills and collect fees to have them go to every military unit in the country and explain hygiene.

Pros: You don't have to bring anything from the future and you can prevent a lot of human suffering.

Cons: You might upset the military balance of power with unknown consequences.

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    $\begingroup$ I think this is a very good candidate. Though I worry that the time traveler may suffer the same fate as Ignaz Semmelweis who basically invented medical hygiene, and was ignored, discredited, and died in an insane assylum. $\endgroup$ – cowlinator Jul 17 '20 at 21:35
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    $\begingroup$ I doubt the acceptance of this too. People today laugh in your face today even when you tell them to wash their hands. And unlike ancient people, these are people who know microbes exist, have greater accessibility to wash facilities, and are used to higher levels of hygiene than those of the past. $\endgroup$ – DKNguyen Jul 17 '20 at 21:37
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    $\begingroup$ hygiene was not as uncommon as most people think, the problem was achieving modern levels of hygiene which require modern technology. $\endgroup$ – John Jul 17 '20 at 22:10
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    $\begingroup$ @John Ehh, are you sure? Sure, if you're doing endoscopic surgery, yeah, but the basics of sterilization and hygiene (boiling water, soap, high-proof alcohol) aren't terribly technologically advanced yet extremely effective. The real problem historically was the lack of germ theory and improper chain of custody in sterile or cleaned materials and tools. $\endgroup$ – Dragongeek Jul 18 '20 at 5:29
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    $\begingroup$ @Dragongeek and ancient people used them, people washed with soap and boiled drinking water, made alcohol for hygiene, what they lacked were things like sewage systems, large scale water treatment, bathing that is not labor intensive, and effective ways to treat disease. actual knowledge about anatomy and germs would be a big improvement but John doesn't have those. $\endgroup$ – John Jul 18 '20 at 13:17
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Since this question has the reality check tag, I'm going to have to point out that people in ancient times weren't stupid. They just lacked the means and the knowledge base. They were far more in tune with is possible with manual labour and handcrafting.

So for John, as uneducated as he is and with no research, to try and bring back a concept which they were not already aware of, yet have the means to achieve (both technically and economically) seems almost insurmountable to be. A lot of stuff we take for granted now, if it was doable back then was crazy expensive and so simply wasn't done.

If it's so basic that John is aware of it and they can do it, but they haven't done it, then they either don't need it or they need it but it is too expensive. Even if we put the technical hurdles specific to the product aside, the more challenging hurdle is to make it less expensive (aka feasible). That probably means enabling technologies which are the most challenging of all. You aren't inventing enabling technologies without knowing what you're doing, which John doesn't. The value of the enabling technology would also dwarf the value of any particular product.

You're on their turf, doing what they do, with their means, trying to outwit them with no preparation. I'm almost inclined to say it's modern hubris, which might just be up John's alley.

So I'm going to present this frame challenge: Are you wanting something that John would actually be believably successful commercializing? Or just something he thinks he would be successful at?

A laymen bringing back the concept of a steel hammer or lenses might as well be bring back the concept of a computer if he has no idea how to make steel or glass and forge them or grind them in the first place. Ideas are a dime a dozen.

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    $\begingroup$ "Are you wanting something that John would actually be believably successful commercializing?" Yes, see my comment following @Mike Scott's answer. John is good at staying ahead of the game. He can organise people, he has the gift of the gab. He's a great salesman. He can easily convince and harness a team to the the dirty work. Who knows he may set off a premature industrial revolution. Don't underestimate John. President Trump knows zero about Covid 19 (as proved by his daft statements). He could not have designed or constructed Trump Towers without architects, engineers and labourers. $\endgroup$ – chasly - supports Monica Jul 18 '20 at 22:52
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    $\begingroup$ @chasly-reinstateMonica I still don't know why you must continue to insist that he has done no research. That's completely unbelievable to me unless he was an incompetent entrepreneur and being just a businessman and not a scientist does not excuse this just like how it doesn't excuse a scientist trying to commercialize a product without learning something about business. $\endgroup$ – DKNguyen Jul 19 '20 at 3:39
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    $\begingroup$ Because the OP has made that one of the assumptions of the question: "John has done no research before leaving." Personally, I think this is silly. An hour spent on the internet would give him a decent background for an invention. $\endgroup$ – NomadMaker Jul 19 '20 at 5:41
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    $\begingroup$ @NomadMaker in fact, John could just place an innocent looking question on the Worldbuilding site and get a load of options in minutes. $\endgroup$ – John Dvorak Jul 19 '20 at 8:35
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    $\begingroup$ @chasly-reinstateMonica the problem is there were people like John alive back then, and they have a large advantage over john they have connections and knowledge of daily life. John is nothing special, the only advantage he could have, better knowledge, you are specifically denying him. I have a hard time John would even survive in the setting since the cultural practices are so drastically different and he is dropped in the middle of major military upheaval. $\endgroup$ – John Jul 19 '20 at 13:50
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I'd honestly go for the printing press.

John doesn't really have to know how to make one, he just has to describe the basic pieces to a craftsman and demonstrate it to the right people.

It's utility would be obvious within a highly bureaucratic government structure.

It's utility would be obvious to the Church. Plenty of Catholic scholars in that century. Probably some Pagan ones, too.

The press doesn't require fancy knowledge or technology to make and can easily be operated by slaves, paid workers, or monks, depending. And eventually, people much more knowledgeable than John will work out the bugs and improve its design.

As far as world changing --- that's obvious! More books means higher likelihood of ancient works surviving to the present day. Will eventually mean a surge to the nascent scientific and scholarly explosion of the medieval period. Perhaps, in the hands of the right people, that revolution could be brought forward. We might be seeing proper universities 500 years before they got going in our timeline. We could see the invention of the scientific method much earlier.


Address to comments:

  • what about paper? They have papyrus and they have vellum and they also have cloth. All of these can be printed on. They'll figure out rag paper soon enough! Or perhaps bark paper. Even regular leather can be printed on.
  • Ink & metalwork? Do you think they didn't have ink and bronze and even iron in the 5th century AD? Anyway, all John needs to do is demonstrate how letters carved onto blocks of wood can be rearranged to print different words. His audience will take over from there!
  • all John has to do here is plant the seed. This is why I specified him showing this to the Church: it is within the next three centuries that the Church will be rescuing the Empire from its own decay and bequeathing all the ancient knowledge on to the empires of the future
  • trust me: if they lack rag paper in John's time, they will figure something out soon enough! Who knows? Maybe the libraries & universities of the 7th century will be founded on books of linen or amate!
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    $\begingroup$ I like the idea of going back as far as you can and introducing the idea to the really early writing cultures. I can just see a Mesopotamian noble handing out the clay business cards.. $\endgroup$ – Joe Bloggs Jul 21 '20 at 8:03
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"Lest darkness fall" protagonist made his first money in 6th century Europe by producing brandy. His revolutionary tool was "pot still" (though I don't know if it qualifies).

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    $\begingroup$ Nice find. I should have guessed that a very similar scenario would have been proposed in 1939 by a science-fiction author! Is there anything left for the rest of us to do!? $\endgroup$ – chasly - supports Monica Jul 17 '20 at 22:19
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    $\begingroup$ IIRC the first thing he did to get money was to introduce Hindu-Arabic numerals & double entry book keeping. Then he used the money from that to introduce distilling. Those sound like better first 'inventions' than any 'tool' in the sense of the original question. BTW should I put those an an answer to the original question rather than a mere comment? $\endgroup$ – Jim Baerg Jul 18 '20 at 4:02
  • $\begingroup$ @Jim Baerg I would call that rather "secured a loan" (as opposed to made money). But sure, a revolution in math can change the world. $\endgroup$ – Alexander Jul 18 '20 at 4:59
  • $\begingroup$ Technically the still didn't change the world in the book. The still let the protagonist make enough money for him to keep producing "inventions" that would change the world eventually. It was his most popular invention though. $\endgroup$ – DJClayworth Jul 18 '20 at 20:27
  • $\begingroup$ +1 for most profitable option. Not as world-changing as a stirrup, but easier to prevent copycats (you're selling brandy, not stills), and very lucrative. $\endgroup$ – user3294068 Jul 20 '20 at 16:01
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Basic maths using the Arabic (decimal) number system, basic economic theory and double entry book keeping.

This includes introducing the concept of zero. Basic statistics, compound interest formula's, forecasting, financial analysis and algebra and calculus. As a businessman he should hopefully be numerate and have and a high school education or better.

He would soon find himself in a senior administrative or teaching post. Better still his knowledge is portable and can't be stolen only learnt.

Amendment: Forgot to include concepts and formulas used in logistics, project management etc plus currency and commodities hedging**.

Another amendment; All the other inventions mentioned in this thread are worthy of development. The problem is the character by default knows about them but cannot (unless otherwise specified) re-invent them himself. So if he were just show up and to try and explain gunpowder or germ theory etc chances are he would be seen as either mad or a witch. BUT- after establishing himself as a mathematical prodigy and gaining peoples trust? Then he can start talking about how other scholars in his home land had studied these things and describe them in general. Result? before you know it experiments and new ideas start being conducted/tried across the land. And all because he's proved his worth first.

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    $\begingroup$ He's going to have to teach a lot of modern mathematical notation, too. Pretty much every symbol or numeric convention that says "mathematics" to him was adopted in the Rennaisance or later. Even the Plus Sign is fifteenth-century. Medieval mathematical texts tended to describe equations in lengthy worded descriptions and geometrical diagrams. $\endgroup$ – notovny Jul 18 '20 at 11:49
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    $\begingroup$ What makes you believe that they didn't know the formula for summing a geometric series? (That's what "compound interest" is. Hint: they did.) And what makes you believe that they did not have a practical method of writing numbers in mathematical or technical works? (Hint: they did. "Roman" numerals were only used for writing numbers in ordinary text.) Nobody calculated in writing because writing materials were expensive. If you want to introduce calculating in writing then a much more practical proposal is chalk and slate chalkboards. $\endgroup$ – AlexP Jul 18 '20 at 12:19
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    $\begingroup$ @Nosajimiki I'd check your sources on that. In Europe, multiplication of large numbers was considered the mark of an elite mathematician before Fibonacci. Before the renaissance, mathematics as a general framework was restricted to what you could prove geometrically with a compass and a square. Long division didn't exist until 1200 AD, and exponents didn't have a notation, so it was difficult to become adept at using them. Just having the system of notation we use today in math would turn a decent mathematician into a world-class scholar. $\endgroup$ – Riet Jul 20 '20 at 15:53
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    $\begingroup$ It might also be useful to note that although many of these mathematical concepts might have been proven in antiquity, it wasn't until the renaissance that people translated the books they were recorded in and made them available to the public. $\endgroup$ – Riet Jul 20 '20 at 15:55
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    $\begingroup$ @Riet They did not use long division as we know it because they did not do written math. 5th century Europeans primarily used counting boards (a form of abacus) that could be used to add, subtract, multiply, and divide. Sure you might throw a few new ideas out there, but unless you also introduce the printing press, it (like Ancient Greek Mathematics) will be very slow to proliferate and will likely just remain some obscure piece of arcana that may or may not be rediscovered hundreds of years latter. $\endgroup$ – Nosajimiki Jul 20 '20 at 17:01
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Lenses

John does not knows how to precisely make lenses nor how to shape the glass into the correct form nor the mathematical optical formulas. However, he have some crude ideas of how they work. Also, he knows that it is (a) possible, (b) reachable with the 5th century technology and (c) revolutionary.

So, John just need to recruit some loyal people with good skills in glass-making and/or maths. In a few months, with some trial and error, he and his workers would be able to produce glasses for people with no good eyes, spyglasses for soldiers, and also telescopes and microscopes. And this would happen more than a thousand years before than when they were actually invented.

With a telescope, astronomers/astrologers would quickly start to use them to observe the skies and soon heliocentrism and gravitation would be figured out, much before than they actually did.

With microscopes, the microorganisms would be discovered and would help to make advances in medicine happen much faster. Also, it would also be used in other areas like metallurgy, alchemy and secure communications which would also enable further development in other areas.

In fact, lenses already existed in that age, but they were crude and not widely used. What he needs to do is to lead his workers to meticulously produce them in mass with the best quality achievable. And then, he just sells them, a skill that surely any good enterpreneur have.

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    $\begingroup$ This might work, but would require resources which John does not have so he would have to be politically savvy and charismatic and position himself to have a wealthy patron. Technical knowledge not needed. Just extreme charisma and knowledge of what is possible. On the other hand, he is essentially trying to tackle an enabling technology here which is probably the greatest of tasks. $\endgroup$ – DKNguyen Jul 17 '20 at 23:45
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    $\begingroup$ Lenses already existed, the Romans actually had them, but the technology was largely lost in Europe. He might be able to hire enough people reinvent the techniques and make make telescopes (spyglasses) possible. But he will need to invest a LOT of money. Easily enough to build a small kingdom. enough money that he could live just as comfortably by not trying to sell anything and just buying a small estate. $\endgroup$ – John Jul 17 '20 at 23:49
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    $\begingroup$ @John Convincing a king to finance it in exchange of having spyglasses exclusive for his soldiers and not for other kingdoms (at least before other people might be able to copy them) is a way for achieving that. $\endgroup$ – Victor Stafusa Jul 17 '20 at 23:56
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    $\begingroup$ @VictorStafusa except he will need a working spyglass to convince a king to invest in it, which he doesn't have. he does not have the political clout to gain access long enough to even try to convince him without a working example, ideas are a dime a dozen and scam artists are older than coinage. $\endgroup$ – John Jul 18 '20 at 0:02
  • $\begingroup$ @John Well, anyway, that is doable. Might not be easy to convince enough people for that, but it is surely doable. $\endgroup$ – Victor Stafusa Jul 18 '20 at 0:05
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Cowlinator mentions it but it seems it didn't get much attention. Pasteurization is no small thing to invent!

For some strange reason, John can maintain milk for longer than other residents, his milk just won't spoil in a couple of days. There is no need to make perfect, standardized Pasteurization. John remembers from school that pasteurization entails heating. Amateurish low-heat warming can definitely get John a couple more days worth of milk. Assuming John has absolutely no idea, I can imagine him practicing by warming his milk for half an hour every day, practicing with the heat levels to avoid boiling it, maybe even wasting much milk in the process.

However, as noted in other answers and comments, what is no more important than how, of course. By setting up some apparatus, mainly for show, to attract the attention, John can definitely start offering to preserve the milk of neighbors. See, everybody observes that John buys more milk than others, while he is rarely seen throwing it away.

Little by little, John can perfect the "machinery" (which is simply used to slowly warm milk, of course) et voila! John is the alchemist du-jour, as milk really lasts somewhat longer after he has it flow through his machine. So, John has "invented" a tool that can be used to preserve milk.

EDIT

As Nosajimiki notes in a comment, obviously, even pasteurized milk only lasts a couple of hours at room temperature, unless it's quite cold... Unfortunately, John needs to remember that this is most effective in winter times. But as John's profits go up the first winter, he thinks there must be a way to make money during summers too. With time, he concocts a sort of a dripping machine, letting milk drip slowly through a hot furnace-like space. Droplets pass through the heat, drying up, with only the residue reaching the bottom surface (which is not warmed). Week by week, he perfects the layout to ensure adequate travel time for perfect drying. John has invented an arrangement to produce dehydrated (powdered) milk! Stored properly, powdered milk does actually last unimaginably long compared to the contemporary expectations.

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    $\begingroup$ they kept it in their cow. $\endgroup$ – Jasen Jul 19 '20 at 1:55
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    $\begingroup$ or turned into cheese or butter. $\endgroup$ – John Jul 19 '20 at 3:42
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    $\begingroup$ @DKNguyen Don't take it the wrong way, but I find it lovely when people get so much out of touch with a culture that the utterly obvious solution just doesn't appear :D Milk wasn't something you had to preserve; you had a fresh supply every day. It's not like you went to the supermarket to buy it. In fact, it was probably one of the safer fresh drinks to drink. Milk and alcohol tolerance are a massive giveaway of European ancestry - the Europeans who didn't have it died out. Of course, this also kills Vector's idea; why ruin milk for no benefit? $\endgroup$ – Luaan Jul 19 '20 at 14:24
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    $\begingroup$ Preserving milk even for 1 day more makes it possible to distribute it in distances farther apart. Wouldn't people of the 5th century be able to capitalize on an idea like that? $\endgroup$ – Vector Zita Jul 20 '20 at 6:57
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    $\begingroup$ +1. Milking your own cow every day for fresh milk is more convenient than buying some at the market every day, but perhaps not as convenient as buying milk from the market once or twice per week. There's still a market for milk that keeps longer. Plus, even in the 5th century, there were some large cities. Not everyone had room to have their own cow, and milk that keeps longer could be sold at a premium. $\endgroup$ – Alexandre Aubrey Jul 20 '20 at 15:15
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How about a compass? Even without detailed scientific knowledge it should be possible to make one with a bit of experimentation. Lode stones were known in ancient times as were various forms of iron. A bit of experimentation with small fragments / splinters from flattened nails and a lode stone should be able to produce a magnetised pointer. A small marked wooden box would be easy to make and even a small glass cover might be possible.

It would be of great use to mariners travelling across the Mediterranean.

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  • $\begingroup$ Do you need to actually bring back a compass for that though? You can just bring back the idea in your head. $\endgroup$ – DKNguyen Jul 17 '20 at 21:41
  • $\begingroup$ @DKNguyen - where does "bringing back" come in? I've said he can't take any artefacts back in time. $\endgroup$ – chasly - supports Monica Jul 17 '20 at 21:49
  • $\begingroup$ @chasly-reinstateMonica Oh I misread the question. I thought you said he wasn't allowed to but he was going to smuggle something. That really limits options if he's as uninformed as you say he is and with no research. People in ancient times weren't stupid. They mostly just lacked the means and knowledge base. They probably know far better than John what can be done with their hands so him trying to bring low-hanging fruit that they don't already would be pretty tough. $\endgroup$ – DKNguyen Jul 17 '20 at 21:50
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Even on the wikipedia list you provide, there are quite a few inventions that rely on principles so simple that even a layman would probably understand well enough to invent.

These include:

  • The brace (and auger)
  • The thermometer
  • The coil spring (assuming ironworking is sufficiently advanced to make wire)
  • Buttons (and buttonholes)
  • Eyeglasses/Telescope/Microscope (it might take a layman a long time to get the specifics right, but I think they'd get it eventually)
  • Barbed wire (assuming ironworking is sufficiently advanced to make wire)
  • The zipper
  • The hook-and-loop fastener (a.k.a. Velcro)
  • Movable type and the printing press
  • The hot air balloon (and thus flight!)
  • Reinforced concrete
  • The Windmill
  • Banknotes (though it requires people to buy into the idea to be useful)
  • Medical hygiene
  • Variolation (The most rudamentary form of vaccine. You probably know the principle, though you probably don't know it by this name.)
  • The pasteurization process

As well as the general idea for several other basic scientific principles. (E.g. the atomic nature of matter, and the parts of an atom).

Which one is the best? I would have to go with the hot air balloon. Inventing flight one and a half millennia early just seems so cool... and it would revolutionize exploration, travel, and warfare.

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    $\begingroup$ Movable type was invented in around 1000 AD in china, books were made common by inventing matrix molds for movable type. coil spring require advanced metallurgy, velcro requires plastic, zippers require high precision manufacturing, hot air ballons require chemistry. things like thermometers, buttons, and zippers will have very little impact on a medieval society. Also buttons predate iron, they were just not very useful. $\endgroup$ – John Jul 17 '20 at 22:22
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    $\begingroup$ movable type was invented in 1040 AD in China, buttons were invented in mohenjo-daro, they became common in 13th century Germany after bespoke tailoring became common. Thermometers won't be big sellers because doctors at the time will not see them as useful, no germ theory, nor is John going to be able to invent them. A coil spring requires a wire made of spring steel not just any iron wire. $\endgroup$ – John Jul 17 '20 at 22:45
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    $\begingroup$ @John Don't you need to be able to evacuate the glass tube as well? Which, as far as I know, would involve like mercury and some intricate glass work. $\endgroup$ – DKNguyen Jul 17 '20 at 23:01
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    $\begingroup$ @DKNguyen you also need to be able to pull a vacuum and hermetically seal the glass,and you need to produce them with high consistency, otherwise they will not measure accurately. $\endgroup$ – John Jul 17 '20 at 23:07
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    $\begingroup$ "live your life without buttons and zippers and tell me if it has no impact on your life" - Shirts don't need buttons. Pants don't need buttons either if you keep them up with a belt or even just a cord. Really, I'm struggling to think of how my life would be any more than slightly inconvenienced with no buttons or zippers. $\endgroup$ – Matthew Jul 18 '20 at 3:13
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The invention that would have The most impact if John could get it accepted would be 3 field crop rotation. A 50% increase in farm output would be huge. Persuading farmers to change their ways would be a tough sell though.

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    $\begingroup$ Nice plan, but take a survey in the street today and 90% of the people you ask will have no idea what you are talking about. I know crop rotation is supposed to be a good idea but I can't precisely remember how - let alone which crops to use in rotation. John is a town boy, he has probably never visited a farm in his life. Good suggestion though, thanks. $\endgroup$ – chasly - supports Monica Jul 19 '20 at 13:15
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    $\begingroup$ Wheat, beans and fallow. And no I didn't Google that. $\endgroup$ – DJClayworth Jul 19 '20 at 19:52
  • $\begingroup$ @chasly-reinstateMonica You could probably figure out what crops would work from local farmer knowledge about their crops and how fields of different crops behave from year to year. The farmers must notice something. But at the same time, certain crops may not have reached a region yet. Check out the food section in Guns, Germs, and Steel for the history of when and where crops arrived in a region $\endgroup$ – DKNguyen Jul 19 '20 at 20:06
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    $\begingroup$ Ancient Romans used the “food, feed and fallow” 3 crop rotation system; so, it is likely that this was still common in 500AD throughout much of thier former territories. The issues of single crop farming throughout history typically stems from cases where farms become corporate assets and businessmen wanting to maximize profits starting telling the farmers how to do thier jobs. $\endgroup$ – Nosajimiki Jul 20 '20 at 17:25
  • $\begingroup$ @Nosajimiki Brittanica and other sources say 3-field rotation was not introduced in Europe until the Middle Ages, so after 500AD. None of the sources mention the Romans. Even if the Romans had 3-field rotation and it had been forgotten, introducing it would still have a huge impact on European agriculture. $\endgroup$ – DJClayworth Jul 20 '20 at 18:17
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The wheelbarrow.

This is simple enough that John can understand it and build one using available tools without much enhancement.

It seems like wheelbarrows should always have existed, because they're so obvious - but the ancient Romans didn't have them. (They were invented in China in ~ 200 AD but didn't make it to Europe until the 13th century.) But once you see one and use one, their utility is obvious. They have a dramatic impact on small project agricultural and construction productivity, relative to the capital investment required to create one.

Raising these two types of productivity at such an early date will dramatically change history as a whole.

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  • $\begingroup$ This is pretty good if it is indeed true. I think. If there were sufficient flattened earth in the fields. Probably would not use metal though $\endgroup$ – DKNguyen Jul 20 '20 at 17:52
  • $\begingroup$ Also they had hand carts, wheel barrows are not that big of an improvement. $\endgroup$ – John Dec 15 '20 at 3:46
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since others bring invention from other region or just a view century later already, i may as well just jump to the wagon then.

heres mine

KITE

its easy to make and quite known for common folk and kid in this era, though i dont know is this true to modern western or specifically to john, you can use leather, cloth, or even use broad leaf as kite not necessary to be made from paper. it can help measure distance, testing wind, lifting men, and signal, or if you near shore you can help fishermen using kite fishing, also help proto benjamin franklin or even your john to proof regarding lightning and electricity theory thing and early air plane invention.

AIR BALOON

already known in china using sky lantern and even made from egg shell, can be use as alternative for kite stuff.

FORK OR OTHER FOOD UTENSILS

most europe people in that times eating using bare hands, this can help hygienic alot especially made of silver, this will be prestigious enough to captivate monarch at least. or john can try chopstick, it can be made from simple wood, precious metal/stone to ivory so it can also captivate monarch, and its quite simple enough and less material require, though need to teach them how to use it to make it a booming, which depend on do john know how to use chopstick or not, this can change european culture as a whole, or at least can make kebab booming earlier, since kebab exist since prehistoric, its meat in stick after all, but not that well used during this period.

PRINTING

like woodblock printing or xylograph and movable clay block press printing no machine required, it use by china and its simple enough, just add paint to the block and press it to the object, can be applied to even stone, and single man is enough to do single printing.

PAPER

ask the carpenter permissions to get the sawdust to get the fiber, asking farmer for their grass, put it into water and mix it, after cleaning it, boil it in water, beat or find something to flattened it until the fiber is thin, dry it, make paper, thats at least the crude way i know, the result is probably not as high grade paper today though, also i may make some mistake there but through trial and error it can be perfected.

  • and if John come there with his modern clothing, it can also revolutionize clothing or fashion, such as sewing pattern/technique or clothing forms, heck even john way of wrapping clothes to look exotic without any sewing can boom a fashion and change european culture if it captivate the noble or merchant fashionista.

  • also concrete or cement, it made of water, sand, and gravel. modern people may know this, even though they dont know the correct mixture same as me, though this depend on john knowledge.

  • oh yeah also if John know how to make wax or clay, he can try make 3d map, akin to 3d printing stuff but more primitive (already a thing in ancient china btw, so basically most modern non electric invention is already an invention or figure out in other regions in the past, and most of it is far before 5th century even, including robot or automaton).

  • since others says scientific theory is ok, maybe metric system or unified measurements (i dont know the correct english, i hope this is understandable enough regarding what i mean), since europe dont have base or unified measurement in that time or quite inconsistent as far as i know, i believe modern western know it well even without advance educations, though my math suck so i cant say much regarding it.

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    $\begingroup$ Forks already existed, they were just too expensive for most people to bother with. Printing requires paper, and paper making requires actually knowledge which John does not have, what you described will make a mess but not even the crudest of paper. Concrete has one other ingredient the one that actually makes it concrete and John does not know how to make it. hot air balloons that can do something useful, that is something more than a side show act, requires liquid fuels, metal burners, and a lot of engineering. $\endgroup$ – John Jul 18 '20 at 13:45
  • $\begingroup$ I have to agree with @John; all of these ideas are things that either (a) existed but were absurdly expensive or (b) didn't exist, but require tools-to-make-tools which the protagonist doesn't have. $\endgroup$ – The Daleks Jul 18 '20 at 22:50
  • $\begingroup$ @TheDaleks well yeah, i did say this thing already exist somewhere or exist a view century later most answer so far like that, regarding tools that require other tools invention at least mine is not, as far as i notice, since my printing idea doesnt necessary need to be use with paper to print. $\endgroup$ – Li Jun Jul 19 '20 at 1:39
  • $\begingroup$ @John i dont know that, i though they only know spoon and use knife during that era, well maybe i can try chopstick, that can change europe culture at least though i dont know do john know how to use chopstick or not, well thats how traditional paper making process i know though, can you tell me what the concrete ingredient is? thats maybe true if your aim is to carry human but outside of that, it still can be use like how kite is use that i mention there. $\endgroup$ – Li Jun Jul 19 '20 at 1:55
  • $\begingroup$ @TheDaleks outside of the concrete ingredients i believe. $\endgroup$ – Li Jun Jul 19 '20 at 2:10
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Eyeglasses. Glassmaking was millennia old by that point, some experimentation with lens shapes and a bit of uncomplicated metalwork later, and you've started improving vision. The aristocracy and the rich would pay through the nose for better sight as they get older.

Similarly, the telescope.

The printing press. The screw press was in use by the Romans by the 1st century, so no need to reinvent the wheel there, and the printing press is a relatively simple modification.

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    $\begingroup$ Sorry, you're too late. The first lenses were made way before the 5th century! en.wikipedia.org/wiki/…. $\endgroup$ – chasly - supports Monica Jul 17 '20 at 22:41
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    $\begingroup$ John is not going to know how to make a printing press $\endgroup$ – John Jul 17 '20 at 23:29
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    $\begingroup$ @Matthew The idea for something often occurs long before it is feasible, especially if it doesn't require any unknown principles which is the case for a printing press. You can bet people were sick of scribing books by hand for ages and trying to figure a way out of it. Something like a propeller, on the other hand, might not be so obvious because it uses a fundamentally different principle than a paddle wheel. How obvious the application of lenses would be is a bit more up in the air if glass is rare commodity thereby reducing people's experience with its incidental magnification and setup. $\endgroup$ – DKNguyen Jul 18 '20 at 3:59
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    $\begingroup$ @Matthew, the printing press is not a simple piece of machinery, and the process itself is even more complex. Worse he lacks the quality paper needed to make it work. You are thinking of an engraving/etching press, where the entire page of text is carved on a single piece of wood, which he could make (well pay someone to make) but again he doesn't have paper to use it with. paper making did not make it to Europe until the 11th century. $\endgroup$ – John Jul 18 '20 at 13:30
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    $\begingroup$ @chasly-reinstateMonica The short answer is you don't and make do with spherical lenses instead. The non-spherical lenses you are referring to are called aspherical lenses and have only become mass produced very VERY recently (first mass produced in 1956). Not only have lensgrinders known for a thousand years that spherical was not the optimal shape, they knew how to calculate the optimal shape but there was no feasible way to manufacture them. It took 700 years to figure out how to make aspherical lenses of a quality that outperformed spherical lenses and another 300 to mass produce them. $\endgroup$ – DKNguyen Jul 19 '20 at 3:26
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The horse collar.

From Wikipedia:

A horse collar is a part of a horse harness that is used to distribute the load around a horse's neck and shoulders when pulling a wagon or plough. The collar often supports and pads a pair of curved metal or wooden pieces, called hames, to which the traces of the harness are attached. The collar allows the horse to use its full strength when pulling, essentially enabling the animal to push forward with its hindquarters into the collar. If wearing a yoke or a breastcollar, the horse had to pull with its less-powerful shoulders. The collar had another advantage over the yoke as it reduced pressure on the horse's windpipe.

From the time of the invention of the horse collar, horses became more valuable for plowing and pulling. When the horse was harnessed in the collar, the horse could apply 50% more power to a task in a given time period than could an ox, due to the horse's greater speed. Additionally, horses generally have greater endurance than oxen, and thus can work more hours each day. The importance and value of horses as a resource for improving agricultural production increased accordingly.

The horse collar was very important to the development of many areas of the world. Wherever oxen were used and could be replaced with horses, the use of horses boosted economies, and reduced reliance on subsistence farming. This allowed people more free time to take on specialized activities, and consequently to the development of early industry, education, and the arts in the rise of market-based towns.

The horse collar was developed in China in the 4th, possibly 5th century. It was not introduced into Europe until the 12th century.

To invent the horse collar in 5th century Europe, John will have to (1) remember roughly what it looked like, (2) realize what problems it solved, (3) obtain a horse, and (4) experiment. He should also interview anyone he can find who has been anywhere near China.

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  • $\begingroup$ this needs research. $\endgroup$ – carlo Jul 18 '20 at 14:41
  • $\begingroup$ it also requires widespread horses, horses were rich mens animals. $\endgroup$ – John Jul 19 '20 at 13:52
  • $\begingroup$ That's not enough to make horses practical, though. There were huge amounts of advances in tools and land management before horses became cost-effective, not just the horse collar. $\endgroup$ – Luaan Jul 19 '20 at 14:29
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    $\begingroup$ Clarissa Dickson Wright (A History of English Food ch. 9) writes: "Good-sized horses had been bred in England since the Middle Ages... But when it came to pulling ploughs... oxen were what were used." Circa 1750 Robert Bakewell develops the Shire horse. "When horses are of the right size and quality, they are much easier to plough with than oxen... By the end of the Georgian period virtually no ploughing was done with oxen." Tentative conclusion: "obtaining a [suitable] horse" may be the devilishly hard part of this plan. $\endgroup$ – Quuxplusone Jul 19 '20 at 19:03
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The Instant Legolas!

Joerg Sprave did a lot to try to prove how simple a magazine system (and even a draw assist) can be. It's a bit of complex wood/metal work, but not outside the ability of a hand craft tools. Check out his videos he made as he developed the design.

One of the reasons people moved to crossbows and guns is because they're easier to make and easier to train soldiers to use - but they were often poor replacements to a truly well trained bowman. Joerg's invention let's a bowman hold their draw almost effortlessly, he even later strapped a second bow to the front of it as a draw assist desvice to make pulling a heavy bow easier without effecting the strength of the bow.

There's another video of someone who made one with hand tools and then let someone who actually shoots heavy bows try it. There was a learning curve, but apparently they thought with some extra time to practice with it it could have made shooting very easy.

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    $\begingroup$ Here's the version of the one made with medieval technology: youtu.be/TcDP9jN_FFQ The problem is that it takes a lot of awareness of woodworking, and it's coming from a medieval timeframe. I don't think it reasonable for "John" to make it in the 500s. $\endgroup$ – Binyomin Jul 19 '20 at 6:23
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    $\begingroup$ Fascinating video, thanks. The trouble is, John never spent much of his time watching YouTube, he was (in the future) too busy making money. I'm looking for something that the average punter in the street would know about and have some idea how to make. $\endgroup$ – chasly - supports Monica Jul 19 '20 at 13:20
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    $\begingroup$ I agree with the OP the instant legolas is not easy to make, and also may not functions as well as an archer in most circumstances becasue it drastically reduces the power of the bow. The same video (I love Tod's shop) showed that even a trained archer was quickly exhausted trying to use it with a realistically powered bow, and that the power was drastically lower due to friction under high load. $\endgroup$ – John Jul 19 '20 at 13:40
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    $\begingroup$ Bugger, you stole my answer! $\endgroup$ – Mikesplace Jul 19 '20 at 20:31
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    $\begingroup$ This is another of those things where it may be common knowledge among the world building community, but if you walk down a the street and ask 10 normal people what an Instant Legolas is, they will not know what you are talking about. $\endgroup$ – Nosajimiki Jul 20 '20 at 17:34
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Semaphore and/or Telegraph

Whatever time you are in, military technology and faster communication are always good.

When the Spanish Armada attacked England in 1588, the best system the English had to warn of the attack was to light a series of preconstructed bonfires - faster than a horserider, but a pretty crude (yes/no) message. Land semaphore was invented in 1792 in France and enabled more complex messages to be sent for military and other purposes. Signalling between ships with hand flag semaphore was common up to world war 2.

Needle telegraph is also not beyond what could be achieved with 5th century materials. A battery could be constructed from any two dissimilar metals (iron, and copper or lead) and any available mineral or organic acid (vinegar, rancid wine) with sufficient power to deflect a magnetic needle. The issue would be finding enough metal to make wires with. Here's a video on how to make a telegraph (with modern materials) in 6 minutes https://www.youtube.com/watch?v=1r2eOpkBTOo

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    $\begingroup$ they are however beyond someone with zero technical knowledge or research. $\endgroup$ – John Jul 19 '20 at 3:36
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    $\begingroup$ @John Semaphore is not. It simply requires the idea of a semaphore code, which he can reinvent himself, it doesn't have to be the same as the original. Braille invented his code for the blind when he was 15. The rest is within the reach of local craftsmen: erect a pole with wooden arms (and possibly some pulleys) on a hilltop. Needle telegraph would require that he be generally aware of its function, but he may at least have made a battery from a lemon at school. $\endgroup$ – Level River St Jul 19 '20 at 9:22
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    $\begingroup$ maybe semaphore but making a telegraph is way beyond him, you don't just need a battery you need a strong batter and an electromagnet. he may have made a battery from a lemon, but lemons will not make it to Europe for another thousand years and zinc had not been discovered yet and he doesn't know how to make an electromagnet. $\endgroup$ – John Jul 19 '20 at 13:22
  • $\begingroup$ @John You don't need zinc to make a battery. ANY two dissimilar metals will work. I mentioned iron as the more reactive metal, and copper / lead as the less reactive metal. They appear to be using ungalvanized iron here youtube.com/watch?v=sOmKdIKz9EU Nor do you need a lemon. In this example they use salt water. The current required to deflect a telegraph needle is tiny Though they used a D cell in the video I added to my answer, Something like this (0.15A) should be enough to power a proof of concept telegraph to impress the local nobles youtube.com/watch?v=S5sDvXmcV4w $\endgroup$ – Level River St Jul 19 '20 at 13:56
  • $\begingroup$ My point was John doesn't understand the principles of how a battery operates, and he cannot just copy what he did in school because the materials are not available. And again you still need an electromagnet which John will not know how to make. $\endgroup$ – John Jul 19 '20 at 15:05
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John is a businessman after all. He might have no scientific skills, so inventing things which also require deep knowledge about chemical/physical processes would not fit his role. Unfortunately it also requires a lot of time and money. There is a lot of trial and error involved. Since neither John nor the blacksmith know how to make new alloys or invent things so that they work, it is not worth the try. In addition, who says that those new invention will sell like crazy and bring him a fortune? Not every inventor got rich within in his lifetime. Often inventions need time to get accepted - and this is even more true the further you go back in time. And more time is needed to 'sell' them to people. Further, you need either a rich patron to pay for the investments or already have some money.

So it is too risky and I propose another business method. John should not invent tools, produce them or try to sell 'new' technology. Instead, he should 'invent' what he knows best, e.g.:

Stock market

John knows about stocks and how a stock market works in principle. He is also affine with numbers. It is easy for him to evaluate the value of a business (e.g. the local blacksmith) and to calculate the share value. With some money from the priests, leaders or a wealthy trader, he can build up the first stock exchange.

Arabic numerals

The [insert whatever numerals are used in your location] are very inconvenient for book keeping and the stock market idea. So John invents the arabic numerals, which he uses every day. This will revolutionize not only mathematics, but will speed up accounting.

Banking - deposition and withdrawal system

The first prototype banks were invented around 2000 BC in Assyria, India and Sumeria. So he could also invent new things related to banking, which are not known at this time. Likely it is the deposition and withdrawal system - IMHO its early system was invented by the Templars in the 12th century. With this idea he could increase the safety of merchants and trading routes.

Marketing

John could earn some money making extensive advertisements for local businesses. He could design wall posters or invent logos printed on coins. Let him invent the basics of trademarks. So only local businesses are allowed to sell 'special' papyri or bricks, all marked with their logo.

Job market

You need skilled people for a certain job? John can help. He can establish a sort of recruiting service in every bigger city. With the 'invention' of CVs and resumes John can always find the best person.

Insurances

Everybody needs them. Together with a good marketing strategy John could establish a big insurance company. However, it needs a lot of risk management/analysis and I'm not sure if John knows how to handle that.

Homeopathy?

I'm not a fan of it, but John might be. Let him 'invent' a potion which heals every known disease. Paired with his business skills he knows how to market it. He can 'print' advertisements or bribe people to spread the word that this potion can heal everything.

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John Is Not A Very Smart Man.

John needs to utilize something that already exists!

Penicillium, the mold that was discovered to contain a powerful antibiotic, would be an absolute historical game changer.

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    $\begingroup$ How would John discover penicillin and purify it? I personally wouldn't know how to do it. I have a vague historical idea about leaving something to go mouldy and then scraping the mould off. However most moulds are poisonous AFAIK. Note that John is not stupid - he just didn't have much of an education. He first learned his business skills in his local town market. $\endgroup$ – chasly - supports Monica Jul 17 '20 at 21:00
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    $\begingroup$ An answer on how difficult it is to turn wild penicilium into a workable drug from scratch under mdedieval conditions can be found here. worldbuilding.stackexchange.com/a/23908 $\endgroup$ – notovny Jul 17 '20 at 21:38
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    $\begingroup$ @notovny never before has such a brilliant essay on why I'm wrong been preemptively written, presumably by time travelers. $\endgroup$ – Aww_Geez Jul 17 '20 at 21:39
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    $\begingroup$ @Aww_Geez It's the last attempt to dissuade you. If you continue down this path someone will pop in and assassinate you yesterday. $\endgroup$ – DKNguyen Jul 17 '20 at 21:47
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    $\begingroup$ I'm now going to assume every time I have an idea for something that has already been invented, I was simply thwarted by time travelers. $\endgroup$ – Aww_Geez Jul 17 '20 at 21:48
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I'm going give you a frame challenge here. The fact that John has what I would consider sub-par levels of knowledge of modern technology means he's not really likely to be able to 'invent' anything with earth-shattering ramifications that didn't already exist and just wasn't seen for how useful it was at the time (see steam engines and hot air balloons as canonical examples of this, both were invented in antiquity but neither was recognized for it's utility until early modern history).

Let's put this in a slightly different context. How likely is it that a high-school dropout could explain to you how to make something like a mould-board plough? That's about as simple and useful as you can get for tools that hadn't been invented yet (and, were I setting out on a mission like this myself, is actually what I would pick, though I would likely also make the effort to get people to realize that the aeolipile was actually useful rather than just a curiosity) but it's not something almost anybody in modern times who hasn't actually used or made one is likely to be able to tell you how to make. Most other truly 'simple' tools that hadn't been invented yet hadn't been invented because there was no use for them. There's no need for a screwdriver if you don't have screws for example.

Essentially, your issue here is that he doesn't have enough knowledge to 'invent' physical tools. In fact, even less tangible stuff like modern business management techniques wouldn't have worked back then (because there just wasn't the population to support it effectively). Based on this, I contend that there is nothing he can do other than point out utility of inventions that have already been made but are not seen for how useful they are.

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    $\begingroup$ Thanks. I can truthfully say that I have no idea how to make a mould-board plough and I consider myself quite well-educated. People on this site have all sorts of arcane knowledge that most people don't (for example black-powder - that is only talked about in re-enactment circles and the history of weaponry). I think the best counter-example to your frame challenge is probably the stirrup that @Mike Scott suggested in his answer. Easy to mould in bronze or wrought iron, no moving parts, simple design, huge implications. $\endgroup$ – chasly - supports Monica Jul 18 '20 at 22:30
  • $\begingroup$ @chasly-reinstateMonica While the stirrup is kind of a counter-argument, it's not exactly common knowledge that they weren't used throughout Europe until relatively (historically speaking) recently, and many people would not put together how much of an advantage it confers when riding a horse. Also, given your question, you might be interested in howtoinventeverything.com, it fits this type of thing your asking about here perfectly. $\endgroup$ – Austin Hemmelgarn Jul 18 '20 at 22:55
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    $\begingroup$ I thought I mentioned somewhere that John's daughter had riding lessons - Maybe that was in the draft version of my question. Anyway, John would certainly notice people riding around without stirrups and ask a local why. They reply, "What are stirrups? Never heard of them. What do they do?", "Hmm...", thinks John, "something to investigate here. I'll make some enquiries further afield. Meanwhile I'll get my team onto it to produce a few prototypes and test them out. $\endgroup$ – chasly - supports Monica Jul 18 '20 at 23:02
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First, may I just say that, in your story, John not being the type of person to prepare at ALL for his trip back in time makes absolutely no sense. If John is at least smart enough to have been able to make enough money to afford time travel, he should be smart enough to figure out what it is he is going to "invent" before going back if his entire reason for going back is to "invent" something. Someone who's fully-formed skills are "making money, influencing people, spotting gaps in the market, organising teams to do the things he doesn't have the expertise for, etc." is NOT going to go back in time with the sole purpose of "inventing" something that is already around in his own time and not HAVE THAT THING IN MIND. If John were truly that STUPID, he would probably already be dead.
That being said, with the limitations of John not being allowed to prepare at all and considering his skill set, my answer to you is:

John should "invent" the idea of fractional reserve banking, where one only keeps a fraction of the deposits available in order to loan the rest out and make money on the interest charged on those loans. With his skills of organizing people to do what he cannot coupled with his sole knowledge/expertise being economics learned "from the street", this should suit him to a tea.

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  • $\begingroup$ I can't tell all of the story and all of the reasons. I would have to write a novel right here. I've already allowed myself to get into this sort of discussion in a conversation that took place in the comments of the original question. Let's just say there is a reason for John's behaviour. Now the idea of banking is interesting. However banking goes back a long way (2,000 B.C. according to Wikipedia). Are you sure this form of banking is new (in the 5th century)? en.wikipedia.org/wiki/History_of_banking $\endgroup$ – chasly - supports Monica Jul 19 '20 at 23:21
  • $\begingroup$ Fair enough:) I am pretty certain that fractional reserve banking was not practiced until fairly recently, but to tell you the truth I am not able to find any confirmation. I will say that if anybody did it, it was probably the Romans but that likely ended when Christianity came in an put restrictions on it, since charging interest was considered a sin. This could be interesting in your story; John becoming a powerful enough individual around that time via frac.res. banking could change how Christianity came about, etc. $\endgroup$ – Ron Kyle Aug 6 '20 at 17:46
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The map of the world

The discovery of the world was very much incomplete from a European perspective in the 5th century. However any ordinary modern person has a much better grasp of how the world map is laid out.

John could sell the idea of exploring new (and rich) lands. If he could fund an expedition, the rest is alternate history.

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Globalism... well kind of

Your best bet would probably not be any singular tool or idea but the knowlede of where to find more knowledge.

John remebers that different countries had different ideas at any given time, even though he doesn't exactly know who had what. But his greatest strenght would be that he has a LOT of hindsight.

Some examples:

  • Where others see "a black powder that burns quick and stinks" he sees guns, dynamite and so much more.

  • Where others see just a horse collar, he thinks of quick carriages, drawn by eight horses (eg. post coaches).

  • He knows what lenses, maths, paper and so much more really are capable of beyond their obvious applications

He knows what is possible and the amount of ideas and concepts he has, that are already proven to work, is unrivaled. Most of these may not be doable in that era, but he can already make huge steps in the right direction (or make other people make huge steps) just by spreading this knowledge and nudging people in the right direction.
Plus he knows how important it is to properly document each step and make frequent backups of that documentation (especially for the black powder reasearch groups).

And even if none of these could possibly be expected to work during his lifetime, he DOES know how countries would benfit from the already existing inventions of other countries.

If he is rather attentive, he can travel one country to get a hold of their common knowledge and then travel further to countries like China or Arabia to get a good look at their current level. This might come with huge hurdles, as he would be considered a foreigner in any country and might not get easy access to their knowledge.

If he plays it right though, he might end up with some good connections to any country he visits, maybe even with the opportunity to send some bright minds over to learn anything he deems useful with his knowledge of modern times.
He would essentially establish the first global corporation ever.

How though?

He might be able to identify himself as some sort of travelling scholar.
If he is a foreigner anyway, he might as well be a reputable foreigner.
This might actually grant him some boons like becoming the guest of some wealthy people and getting their attention for a little while, which enables him to make connections and find investors/trading partners early on (or at least have a safe place to sleep and eat for a night or three).
Plus a scholar would of course seek knowledge. Nothing suspicious about that.

This way he can get access to the most promising ideas and inventions from all over the world (or their prototypes anyway) and start spreading them everywhere to make a huge profit off of them.

This way you pretty much get all the inventions mentioned here at once.
And all of this is possible with "just" entrepreneurship and soft skills. No previous research required.

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  • $\begingroup$ Nice idea. Thought provoking. $\endgroup$ – chasly - supports Monica Jul 22 '20 at 10:51
  • $\begingroup$ @chasly - reinstate Monica The thing is, most people forget (or never realize) how much accumulated knowledge we are being spoonfed today (or more like shovel fed). Coming up with all the possible implementations of something new is pretty much impossible. But if you already KNOW, you have unbelievable advantage (say paper: there are books, newspaper, flyers, napkins, toilet paper, carbon paper, templated paper like lines, squares, tax forms... I could go on). One person alone won't get far, but have groups of people, feed them ideas and filter bad ones out and you can achieve so much. $\endgroup$ – SRMM Jul 22 '20 at 15:20
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The talent of John is to make people do things for him.

That's great! Instead of making a part which is then quickly copied, he should introduce some process. Processes normally need to be controlled, that's John's part, and they are less easy to copy from mere observation. Finally, you can cut a process in single task slices so that even the involved never learn all of it.

That can be mass production of a part (arrows?), modern organization of a restaurant, shoe making, what you want.

The key is, as a business man, he can then kick the concurrence off the market by being cheaper and higher quality at the same time.

Being a leader and business man he will certainly be fully capable of dealing with the mafia-like reactions and consequences that were normal at the times.

You're setting John into a quite violent time there!

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  • $\begingroup$ Yes, I did consider making John a Mafia-type boss before sending him back in time. I still don't rule that out! I wonder if you could flesh this out in terms of a single product and how he would go about it. Note: I'm not saying that I would accept this as the answer, but I would be interested to know more. $\endgroup$ – chasly - supports Monica Jul 20 '20 at 11:40
  • $\begingroup$ yeah, ford type stuff $\endgroup$ – MolbOrg Dec 15 '20 at 10:17
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The Rocket Stove

Just a few slight modifcations to a traditional brick/steel/clay stove (not needing any additional materials, just a modified shape) drastically increases the burning temperature and fuel efficiency of a stove.

One of the things that held back development is the difficulty of smelting and metal working, the effective cost and difficulty of cooking, the expense of fuel, and much more.

A Rocket Stove, invented in the 1900's, is pretty much an amusing sidenote of technology today since we have more high-tech solutions of improving our fires such as electric blowers, bellows, space heaters, electric coils, blast furnaces, etc.

The rocket stove, had it been invented anytime between 4000 B.C. and 1700 A.D. (the whole range of which had the prerequisite technology), would have drastically revolutionized technological advancement.

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    $\begingroup$ This would work if a rocket stove were conventional knowledge which I do not believe it is. This is the first time I've heard of one so I doubt John has heard of it either having done no research. $\endgroup$ – DKNguyen Jul 21 '20 at 17:22
  • $\begingroup$ I guess "common" knowledge changes depending on the culture group you're part of. Rocket stoves are pretty well known to homesteaders, lots of non-glamping campers, and a bunch of rural people. $\endgroup$ – lilHar Jul 28 '20 at 18:40
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    $\begingroup$ Agreed. For example, I have no idea what glamping means. $\endgroup$ – DKNguyen Jul 28 '20 at 19:04
  • $\begingroup$ Glamping means "Glam camping." Originally the term was coined by a resort that advertised "A camping experience without the hassle" (you came, they already had a tent set up with cots, nice grills, wood, everything you need, etc.) However, since then, the term glamping has expanded to mean "Any lazy camping done by groups who don't actually do the work part of camping." which seems to include those who use RVs, rent cabins, the original glamping services, etc. and is occasionally expanded to include those who camp with enough gear to basically be at an "at home or near at home" comfort level. $\endgroup$ – lilHar Jul 30 '20 at 15:18
  • $\begingroup$ The "rocket stove" uses a stove design that has existed since the roman times, the fact it needs specifically sized fuel makes it mostly useless except for specialized use. roman pottery and glass ovens used a similar set up. $\endgroup$ – John Dec 15 '20 at 3:51
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Aeolipile / Simple steam engine

The Aeolipile was made as a curiosity in the first century AD, its essentially a sphere filled with boiling water that has two exhaust pipes that allow the sphere to eject steam and spin the sphere.

I think an average person like John would know how to make a really simple steam engine like this and that using more water, more heat and bigger boiler could provide more power which makes it more useful.

Once you have a basic steam engine John knows enough history to figure out what comes next, lathe is an obvious first step, make an Aeolipile attach a rod on axis of rotation, skewer something on the rod and now you have a lathe. Power source for an Archimedes screw All of a sudden you now have a powerful siege weapon, a self running pump to drain your enemies water supply UP a hill as well as fast water distribution for permanent installs in cities. Grinders for metal work, windless windmills (just called "mills" if you will) for grain, etc...

All John needs to know is a steam engine is essentially a kettle with an exhaust pipe and that engines of any kind are really useful.

He will simply take seriously and push development what was seen as a mere novelty or curiosity.

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    $\begingroup$ John is explicitly stated to have done zero research, he possess no technical knowledge, he is not making a steam engine. $\endgroup$ – John Jul 18 '20 at 13:52
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    $\begingroup$ come on, a steam engine is very simple, the most difficult thing here is to craft it. he could pay a blacksimth for that $\endgroup$ – carlo Jul 18 '20 at 14:24
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    $\begingroup$ An aeropile is simple but refining the tech to achieve rotational energy greater than what animal power can do is the real challenge. $\endgroup$ – SurpriseDog Jul 18 '20 at 14:46
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    $\begingroup$ It's a nice idea but an aeolipile can be stopped by a single finger (apart from getting a nasty burn). The gearing you would need to power anything big would have so much friction that nothing would happen. That is why (as far as I know) it was only ever a toy in ancient Greece. Otherwise the industrial revolution would have started back then. Actually, on second thoughts, maybe you could use it to drive a clock. Hmm ... now we're talking. They weren't invented until much later. $\endgroup$ – chasly - supports Monica Jul 18 '20 at 23:13
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    $\begingroup$ @carlo ... no. Steam engines were the result of massive improvements in metallurgy, mining and economies of scale. And what does a steam engine give you? How do you fuel it? Why would you need a steam engine to power a lathe? People used wooden lathes for millennia. An aelipile, no matter how big you scale it, is not going to power a lathe powerful enough to work steel anyway. And it would be completely uneconomical anyway - people have been using water wheels for the same purpose since ancient times, requiring far less maintenance and no fuel. $\endgroup$ – Luaan Jul 19 '20 at 14:36
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Well, here's two things I can think of off the top of my head.

Germ Theory and Sanitation

The Germ Theory was truly groundbreaking when it first came out. Before bacteriological causes, every disease was assumed to be caused by either poison or an angry deity. As a result, most "cures" were ineffective, if not harmful (Pliny recommended eating the offending dog's head as a cure fore Rabies). Furthermore, they did almost nothing in the way of sanitation. As a result, introducing the Germ Theory, along with some basic sanitation techniques, would save many lives. Indeed, the "basic sanitation techniques" part probably wouldn't even be necessary - they knew "basic sanitation techniques" in the 5th century (have you ever read Leviticus?), they just didn't use them.

Modern Mathematical Notation

The ancients were really good at math (many of the theorems and equations you learn in Algebra and PreCalc predate the 5th century). However, actually doing the math was a pain, and transferring that knowledge to others was expensive due to the verbosity of their notation. The introduction of modern mathematical notation (Arabic numerals and arithmetic operators being the most important) would make math much easier. While "John" probably couldn't use this to make much money*, it could probably get him into a monastery (which were just starting to become rich during that period).

* EDIT: On second thought, tutors were paid huge sums back then - it wasn't every day that you could find someone who was both literate and looking for a job.

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    $\begingroup$ Germ theory was also not accepted for decades until so very in depth research provided outstanding evidence. you may want to look up the broad street pump. Sanitation requires technology, and more importantly public/government support John does not have available. It also has already been used in an answer. $\endgroup$ – John Jul 19 '20 at 3:39
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None.

There is common misconception that history is linear and things get invented and change the world. It's false.

Things get invented, then they fall into obscurity and get reinvented anew. Sometimes they even get forgotten after a period of widespread success, like phalanx formation.

John needs the same thing as Beth from 2348 who traveled to today with knowledge how to beat COVID-19 in 3 simple steps: being trusted by powerful and rich people who can make it happen. The thing that an outsider, like a time traveler, can never have.

John can't change the ancient world, just like Beth can't stop what's going to happen in early 2021.

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Cutlery and plates?

Spoons, forks and plates are pretty basic and we use them every single day but nobody thinks "That spoon! I like that spoon! That spoon is great! What would I do without that spoon?"

Nobody really appreciates forks, spoons, plates and how important that simple stuff is, because it's something so simple and basic that we think that it's a totally normal everday tool. People ate back then with their hands and put their food on bread cause they didn't had plates and cutlery. Putting a fork in your meat and having clean hands after eating would change a lot. And now imagine how much money you can make with that invention. Change their lifes and throw a spoon in society!

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  • $\begingroup$ They don’t have the materials science to make decent cutlery. Every affordable metal makes the food taste funny, until aluminium and then stainless steel come along. And silver cutlery is too expensive. $\endgroup$ – Mike Scott Jul 20 '20 at 17:44
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The Toothbrush

According to Wikipedia:

The first bristle toothbrush resembling the modern one was found in China. Used during the Tang Dynasty (619–907), it consisted of hog bristles.

So, if John introduced it to Europe a couple hundred years earlier (and got people to believe it would help them live longer and actually use it), it might change things significantly.

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