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?

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.

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.

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.

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.

sliced bread from what I hear was the biggest thing when it came out