A time traveler goes back in time (one-way trip, and he does not get to choose the exact destination) to a low-tech society, similar to medieval Europe. His objective is to kickstart an industrial revolution as quickly as possible, and the king of the place he lands in, takes him seriously, wants this to happen and lends active assistance.

He cannot bring any modern equipment back with him, only the knowledge in his head.

He can have any obscure but available knowledge that will help. The exact formula for gunpowder, or the procedure for purifying useful quantities of penicillin? Sure, he can fortunately happen to remember those.

He cannot gain advantage from knowledge of historical events. The reason in this story is that he has gone sideways as well as backward in time, so has landed in a place that resembles medieval Europe, but is not our Europe. But if you prefer, take it as a meta-condition: the question is about the use of knowledge of science and technology, rather than knowledge of historical events, so suppose the time traveler simply happens not to know of any imminent invasions, assassinations or such like.

The king is a practical man. Revelations about the stars being other suns or the nature of atoms are well and good, but what he's actually interested in are ways to improve the security and prosperity of his kingdom. (In other words, the topic is applied knowledge. Pure knowledge for its own sake would be a different discussion.) Sooner is better than later.

I can see how there is enormous advantage to be gained from later developments like rifles, steam engines and mass production. But I'm having difficulty seeing how to gain much practical advantage quickly. It seems likely to take a long time to go from gunpowder to militarily useful firearms, for example.

What is the first innovation that could be developed with future knowledge and local tools and resources, that would provide significant practical advantage?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Jan 17 '19 at 8:17
  • $\begingroup$ not to be a wet blanket, but how are you dealing with the fact that you just brought back all sorts of futuristic germs to a society that has no way of dealing with them? $\endgroup$ – Nullman Jan 17 '19 at 12:32
  • $\begingroup$ You mention that historical information isn't allowed, but what about astronomical events? Knowing the exact date of an eclipse or comet sighting or some other observable event that could in no way be altered by changes in Earth history might be useful. $\endgroup$ – Darrel Hoffman Jan 17 '19 at 14:48
  • $\begingroup$ @DarrelHoffman Right, for the purposes of this discussion I'm not interested in knowledge of the timing of individual events, but in how to apply knowledge of science and technology. Now, if there is a modern algorithm for predicting eclipses, more effective than any algorithm known in medieval times, that could be interesting. $\endgroup$ – rwallace Jan 17 '19 at 15:56
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    $\begingroup$ @rwallace "He can have any obscure but available knowledge that will help." - I think this is where it gets way too broad, basically allowing a character with 43 engineering degrees. If you'd restrict that more, e.g. "his knowledge comes from his job as a nurse + nowaday's common knowledge", we can work with that better. But a furnace technician knowing how to implement the bessemer process, who's also pharma technician knowing how to manufacture pest medicine, who's also a chemist knowing how to create black powder... That's a Mary Sue. $\endgroup$ – R. Schmitz Jan 17 '19 at 18:00

10 Answers 10


My money is on Germ Theory. This is a good candidate for a few reasons.

1) Everyone in the modern world has at least some idea about it. Your time-traveler does not need an advanced education to get the idea across and implement it. Keep surgical equipment clean, quarantine sick people, doctors need to wash up between seeing patients, and you're done.

2) It requires no pre-existing technology or equipment. It works at any time period and on any sized society, from a stone-age tribe to Colonial England.

3) It will dramatically increase populations. More people means higher GDP, which tends to mean faster technological growth.

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    $\begingroup$ Might want to spend some time learning how to create decent quality lenses using simple tools that can be assembled to make an early microscope capable of magnifying enough to see bacteria though. Lets face it without some way to demonstrate your claims of invisible tiny creatures making people sick you would kinda come across as a madman. Worse you might find it doesn't go down well with those that like their monopoly on scaring the public with stories of invisible entities (the church) etc. $\endgroup$ – MttJocy Jan 17 '19 at 2:14
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    $\begingroup$ Also such a microscope could be useful in identifying and subsequently culturing Streptomyces griseus from soil samples. This species produces streptomycin kinda useful since while Penecillin is somewhat easier to find the source of and isolate it has the distinct disadvantage of not being effective against the bacteria that is about to kill half the population Yersinia pestis is resistant to it but Streptomyicin is effective. $\endgroup$ – MttJocy Jan 17 '19 at 2:18
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    $\begingroup$ Your patients surviving much more frequently will be pretty good proof. OP specified that the king has already been convinced you're from the future, so he should be able to get people to go along with it long enough for evidence to build. $\endgroup$ – Ryan_L Jan 17 '19 at 2:45
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    $\begingroup$ Agreed, move the lavatory away from the well. Focus on clean water and get bathing frequency up. $\endgroup$ – Jodrell Jan 17 '19 at 7:06
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    $\begingroup$ @MttJocy there is no need to prove microscopic creatures. The simple concept of 'contagion' will suffice. Cleanliness was understood to some extent and was especially important in household cheese production and brewing, which was ubiquitous. There is no need to convey the precise mechanisms of viral infection to convey the necessary information, just as it is not necessary that all 10 year old children understand the chemical structure of a viral capsid for most to have a sufficient understanding of disease transmission. $\endgroup$ – pluckedkiwi Jan 17 '19 at 14:04

Accurate World Map

Being able to draw a highly accurate map of the world would be a huge boon to any seafaring or trading nation. Moreover, knowledge of other civilizations of the time period and their valuable trade goods could kick-start a trading empire.

This doesn't precisely answer the prompt, as it doesn't really improve technology much, but it gives the time traveler the resources to actually make their technological visions a reality.

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    $\begingroup$ On the contrary I think it improves technology quite a bit. Consider how important accurate maps are for trade and for administering a government. Anything that improves these two things will speed up the development of other technologies. $\endgroup$ – Ryan_L Jan 17 '19 at 5:13
  • $\begingroup$ Better yet, a world map with the sources of natural resources marked out on it. $\endgroup$ – Arcanist Lupus Jan 17 '19 at 7:19
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    $\begingroup$ The second of OP's conditions disqualifies this answer, as the world is different from our own. So we can assume even world map is different. $\endgroup$ – Euphoric Jan 17 '19 at 7:40
  • $\begingroup$ @Euphoric Fair point, though I figured that it had more to do with people/borders rather than geography, $\endgroup$ – Bert Haddad Jan 17 '19 at 21:31

I think there is a very simple answer to this question that avoids the question of broadness, but in so doing I'm inferring the real question to be 'What knowledge should a time traveller take back to jump start technology?'

The answer is the Maxwell Equations of 1861, and the knowledge that magnetism and electricity are in effect different manifestations of the same thing.

Ultimately, the Maxwell Equations were the theoretical integrations of these two fundamental forces, but from an engineering perspective you don't actually need these equations to figure out how to use the two together. School children do it by hand cranking a wire-wrapped axle in between two magnets, and this is ultimately why the knowledge of electromagnetism is so important; it means you can create energy by turning an axle, and turn an axle by applying electricity. This is a ubiquitous element of our modern technologies by virtue of the fact that we can essentially transmit energy in a portable and useable form to wherever it's needed. That versatility revolutionised all the advancements we gained even through the Industrial Revolution, and in a medieval society, would even give them access to plentiful energy WITHOUT using coal or oil to the extent they ended up doing.

Ironically, this would result in a reverse-steampunk scenario, where fantastical ideas would emerge from electrically driven devices out of mindsets that haven't industrialised yet.

What an industrial revolution WOULD add that isn't currently in place is the ability to scale this technology up. In the first instance, I can see plenty of homes having their own windmills, watermills, etc. to generate enough power for their personal needs. You would end up with literally a cottage industry of electrical generation techniques, all powering personally designed products at different amperages, wattages and the like. So, the reverse-steampunk effect would at least be visually intriguing because of the variety of tools that would all employ electricity in different ways to solve the same problems.


Im going to go in a different direction from most of the current answers, The king would like any methods to improve his kingdom and you want to start an industrial revolution. Don't start with the natural sciences start with the people and policies:

  1. A production line drastically improves the rate of production of crafted goods
  2. Tax policies - support industries and research
  3. Foreign treaties - make it so that you can attract foreign talent
  4. Make some level education compulsory for all people
  5. Support collaboration over competition for research and have it available to all citizens
  6. incentivise innovation

Now you have a baseline to start from, within a few years the revolution should kick off on its own rather than just being based on you and your ideas. Bringing in some useful formulas would be a helpful boost but there is a limit and once you hit that limit you will need to rely on others to keep the innovation going.

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    $\begingroup$ These are mostly too far up the development scale already. Production is only useful if there is a market for the product, compulsory universal education is both extremely expensive and largely unnecessary for farm hands (the vast bulk of the population), and research competition isn't a meaningful thing. This sounds good policies for 17th century (basically reject mercantilism), but isn't likely to have a positive return for a 5th to 14th century society (wow that is a range - medieval is really too broad). $\endgroup$ – pluckedkiwi Jan 17 '19 at 14:19
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    $\begingroup$ @pluckedkiwi Agreed - these changes would most likely result in a lot of well educated people starving to death because there isn't enough food production going on to feed them. Intellectual pursuits and technological innovation was only possible because we developed efficient enough agriculture to give people the free time to do things other than food production. $\endgroup$ – Darrel Hoffman Jan 17 '19 at 15:08

There isn't really any one invention you could drop in as "the first innovation that could be developed with future knowledge and local tools and resources, that would provide significant practical advantage" that wouldn't really be just a matter of opinion.

from comments.

The printing press with movable type was invented sometime after 1400 so that would be a good one, raising literacy & availability of printed manuals led to a fairly big boost in technical advances.

Crop rotation (four field crop rotation) is another simple idea that produced fairly significant increases in crop yields, that was invented sometime in the 1600's

The concept of microbes & bacteria, boiling drinking water, sterilizing medical instruments etc is another recent one with significant far reaching effects for an early society.

Are all good in there way but (if I'm reading your question aright) you want something that's going to spark further innovation & technological advances in & of itself without much further intervention?

  • The steam engine is an easy one with the help of a blacksmith & could have all sorts of implications from agricultural threshing machines to transport, it's the bedrock of our own industrial revolution & once you've any one type of steam engine in relatively common use innovation around this particular bit of tech will rocket.
  • The microscope & telescope were invented in the 1500's & 1600's, a lot of science couldn't get started until we had those & it lets you point at those tiny little things in water & say "see this is what we're killing when we boil it so that you don't get cholera"
  • The copper zinc battery
  • The copper wire electric generator & motor

For those you just need glass-making, blacksmiths, carpenters, pottery & a few cattle (for the batteries, any idea how strong the acid in a cows stomach is?) which are all available in 1200.

Electricity seems like a really important one (alongside microscopes & telescopes) if you want to kick start science really early (which is why I threw in the battery & electric generator / motor), but for them to be anything other than curiosity or novelty items you need something to use them with & I'm at a bit of a loss as to what invention might make use of it that a medieval society would find an immediate & compelling use for so that it would be widely adopted.

Radio might be plausible as a use for electricity with foxhole crystal radios for reception & electricity needed for the transmitting sets, any medieval king would love something like that for his armies, I'm just not sure if the required materials could be sourced in a medieval society.

Watermills & windmills already existed, the Dutch certainly had windmills back then, not so sure about the English.

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    $\begingroup$ 'Spark further innovation without much further intervention' is, mind you, an interesting distinction. As I understand it, the number one invention in that regard is the printing press; it seems to be the meta-invention that makes knowledge transmission efficient enough to allow everything that comes after. $\endgroup$ – rwallace Jan 17 '19 at 0:58
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    $\begingroup$ "Though, taking 1200 as the starting point, don't you need some incremental improvements in metalworking before you can make a steam engine?" Nah! just give em the basic principles & let em have at it, so you lose a few blacksmiths along the way until they start getting it right :) you can make a steam engine with copper if you want, you only need steel rather than just iron if your dealing with excesssive heat & pressure, for less efficient / powerful steam engines no problem. $\endgroup$ – Pelinore Jan 17 '19 at 1:13
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    $\begingroup$ They did of course have steel at this point steel artifacts date back over a millenium what you would need specifically is the Bessimer Process or any of the more recent improvements on that (Though later methods bring other prerequisites like being able to produce streams of pure Oxygen at high pressure etc so industrial oxygen manufacture and industrial compressors needed etc) which allowed the mass production of steel in batches on the order of 10's to 100's of metric tons per batch rather than 10's to 100's of kilos and much more cheaply too. $\endgroup$ – MttJocy Jan 17 '19 at 2:10
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    $\begingroup$ @MttJocy : the Chinese yes, Europe, not so much, depends where he's setting his medieval kingdom. $\endgroup$ – Pelinore Jan 17 '19 at 2:16
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    $\begingroup$ @Pelinore It was still usually called carbonised iron the name steel hadn't come into usage yet but yeah the weapons and armour in 1200 were already carbonised (steel) not simply iron. Especially not in areas in or near where the Roman empire existed as they were having the stuff produced all over to supply their vast legions. Course a lot of substances had very different names at the time than they do now even though they were the same thing. $\endgroup$ – MttJocy Jan 17 '19 at 2:23

Health information: first aid, personal sanitation, community sanitation, germ theory. That gives you an immediate leg up when it comes to dealing with things like sickness, disease, and public health. A lot of it is simple behaviour and habit and doesn't require a ton of infrastructure.

After that, a lot depends on what level of tech you'd find was already present, but things like the Bessemer process for cheaply producing mass quantities of steel would be good almost any time and weren't technologically too advanced to be created. And once you have cheap steel in mass quantities, all of a sudden you're looking at a whole new world.

  • $\begingroup$ community sanitation in the form of landfills and the introduction of soap will greatly increase community health on it's own, stabilizing the population. $\endgroup$ – C Teegarden Jan 17 '19 at 17:58

All other answers have the precondition that innovation stays inside the borders of the country.

If this is basically our Earth, then the single biggest question for medieval Europe is who gets to America first - and then who can keep it. Spain became the richest nation in Europe through their gold and silver mines in Mexico. Britain then displaced Spain through state-sponsored piracy and having better ships (and captains).

If your hypothetical country has a coast, then the lesson for medieval and Renaissance Europe is that the people with the best navy win. With better ships, and by essentially controlling the seas, you free up your explorers, traders and settlers/miners to make you money.


A horse-drawn seed drill - this will allow your farmers to plant their crops much faster, much easier, and in a more regular pattern (to they do not choke each other as they grow) - of course, even a modern steel plough instead of an old-fashioned Wooden or Iron plough would make agriculture easier! (Stronger, sharper, and damp soil is less likely to stick to It and slow it down)

Over a longer term, combine this with modern crop rotation (e.g. the Norfolk four-course system), and you can ramp up food production even further

Add in chemical fertilizers, or a set of Harvesting machines, and you should find you produce more food with less farmers - allowing you to increase general prosperity. (Before attempting a Combine Harvester, create separate Reaping, Threshing and Winnowing machines. The first can be horse-drawn, the second two can be static units powered by a water wheel.)

Now that you have a surplus of both food and workers, you can start to tackle other areas of industry - remember, a Revolution requires Revolutionaries!


Already some really good answers in the comments, I'd just like to suggest an alternative approach:


While the concept of the Heliograph is simple, it wasn't invented and used in an organised, systemic way until roughly the 1820's. There may have been some isolated cases of using mirrors and shields to pass message, but evidence is spotty at best.

Your time traveller could help the King set up a series of lamp-and-mirror stations through his Kingdom, to rapidly pass messages along. This would increase response time to an invasion by a orders of magnitude, and allow information and intelligence to be more rapidly spread.

Communication would be via Morse Code, and he could also teach them a thing or two about modern cryptography and compression, to further increase their security and efficiency. Some of this could be applied to the King's normal messages as well.

Depending on our time travellers electrical knowledge, he could also attempt to build a bare-bones radio transmitter and receiver. Called a Spark-Grap Transmitter, it basically emits a large pulse of electrical energy that could be detected with a simple crystal radio set up some kilometres away - more if you manage to refine the design. Electrical power could be provided by a simple generator, perhaps turned by a windmill, waterwheel, or handy peasant.


I see four points with varying costs and high benefits:

  • Penicillin (as you noted), hygiene, simple medical advances

more population, more health, more workers

  • transportation -> trains

simple but very effective, enables the transfer of knowledge, goods and troops, freeing a lot of resources and boosting research

  • communication -> telegraph

knowledge transfer, faster reactions, etc.

  • food production, simple farming machines, some theory of farming, generally a focus on providing more food and storing food

the industrial revolution was kickstarted by agricultural advances. If 90% of people produce food all day, that's really a problem.

Really all boils down to having more people with more time able to dedicate time to learning, research and inventions. The rest is history.


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