I think something even as mundane as a battery operated flashlight, especially a waterproof flashlight, would have people absolutely agog, all the more progressively so as you went back from as recent as the 18th century to say the time of ancient Rome. It’d be interesting to see the reaction people might have.
What would be the impact of bringing a bag full of Norman Borlaug’s dwarf wheat seeds back in time?
I think the potato would have just as much transformative impact. Introduce the Romans to blight-resistant potatoes and stand back.
I don’t know quite enough about agriculture–if we took things like Borlaug’s dwarf wheat or blight resistant potato varieties back in time, would farming practices back then work with those seeds? Or did they only work in combination with modern farming techniques?
Both would work, but you might want to introduce the idea of four field rotation at the same time, just to be safe.
Look, we’re traveling in time, so it’s well past time for worrying about “asking for trouble”. 
This sort of thing would be exactly the sort of knowledge you’d want to bring back. Things that took us a long, looong time to figure out, but which are pretty easy to teach. How much human suffering could have been avoided by such basic practices?
This is the same with the basic sanitation rules, and antiseptic procedures for treating wounds, or surgery. Just boiling your instruments and bandages, and washing your hands with soap and hot water, before treating the patient would cut down on all sorts of infections.
It’s always easier to learn something than to invent it, so send back a bunch of books with these kinds of simple lessons.
So here’s the real question: In your industry, whatever that may be, what one simple idea took a long time to figure out, but revolutionized the industry almost over night once people figured it out? We’ll write them all down in the Big Book of Duh and send that to the past.
I work in Intellectual Property, so I suppose it’s just that: the idea of giving some degree of “ownership” to new ideas, so as to propagate them, spurring new inventions on the basis of the old. IP used to be jealously guarded as secrets by guilds and the like, which seriously curtailed the spread of knowledge that could have improved the lives of a lot of people.
I’d be interested in seeing some of these, since my studying of invention says that most things are generally invented almost immediately after they CAN be invented. Or, they are invented soon after they are needed. There are very few useful inventions that could have been invented much earlier but just weren’t because no one thought of it, or because there was no need earlier.
Take the steam engine. Herotodus invented one thousands of years ago but it was essentially a toy because no one could come up with a use for one, slaves were cheaper labor, and the metallurgy didn’t exist to make a large steam vessel that could actually do useful work. The Baghdad battery is an ancient chemical cell, but it was a dead end because there was really not much use for such a thing except for perhaps electroplating something if they knew how.
Or take crop rotation. We certainly could have learned that earlier, But was there a need? Would rotating crops in the 18th century have been much of an improvement compared to the other advances we were making in agriculture? And could people of the time afford it? Was there enough crop variation in agricultural areas to even allow for rotation without leaving fields fallow for a season?
I’d say several of Newton’s most important discoveries: derivative / integral calculus, his three laws of classical mechanics and his law of universal gravitation, could have been developed many years prior. Newton inherited an intellectual tradition that paved his way, but there was no “technical” barrier, like the need for other complementary inventions you’d need to have made viable steam engines in the early 100s AD.
Well, my favorite example is the Minie Ball.
We figured out that rifled firearms were superior in both range and accuracy to smooth bored firearms pretty quickly after we invented the first firearms. But it took us several centuries after that to figure out how to load rifles as quickly as smooth bores.
The basic design of the Minie ball is pretty straightforward. Anyone with the ability to make a firearm could have also made these balls, and yet, it took centuries to figure out. And we were really trying to figure it out. Take a look at some of the bizzaro designs for rifles and bullets we tried over the years, and you’ll know how much people wanted a faster loading rifle.
But it took centuries.
Get a few basic medical textbooks. Have them translated into ancient Greek. Give them to Hippocrates or Galen.
I wonder if any Egyptologists are fluent enough in math and science to translate a textbook into hieroglyphs?
Well, when Viscount Townshend introduced the practice to Britain in the early 18th century it directly kicked off the British Agricultural Revolution. The key addition to the rotation was the addition of turnips to the equation.
“Verily, this person is practicing black magic. Burn him at the stake and his devilish contraption with him.”
Rifling was invented centuries earlier, but didn’t come into widespread use until the 1800’s because of the difficulty of mass producing rifled barrels. The first Minie balls were designed in the 1830’s, very soon after rifled muskets were widely available.
Maybe not a ‘technical’ barrier, but Newton definitely needed precursor math which was fairly recent, such as infinitetismals, tangent theory, maxima and minima calculations, the concept of derivatives and integrals, etc.
The fact that both Newton and Liebnitz both landed on Calculus at nearly the same time is often used as evidence for inventions happening as soon as possible after precursors are available and there is need, as is the simultaneous discovery of evolution. Darwin rushed his “On the Theory of the Origin of Species” to publishing because he knew someone else was writing the same thing.
Newton’s laws were also developed from discoveries and observations that were quite recent at the time.
Okay… But crop rotation as a concept goes back to 6000 BC. It was discovered almost immediately after people settled down to become farmers. What you are talking about is ‘four crop rotation’ using turnips and clover in England. But since that was a significant improvement over previous rotation methods, it certainly could count unless there is some reason I don’t know about why it couldn’t be done earlier or invented in other places.
I think if you give Archimedes classical translations of modern textbooks on analytic geometry, trigonometry, logarithms, and calculus, he would have had no problem working out all the details. The hardest part might have been the entire idea of a function and of a variable. If you could add one on differential equations, that would also be useful. A treatise on paper making would also help.
I feel like the wheel could have been invented way sooner than it actually was. Maybe save Easter Island from becoming a baron wasteland (relatively speaking).
I have often thought about this. What if I took back plans for, say, the jet engine and the plans for the F-86 Sabre to the US in the mid-'30s? Maybe along with plans for the M26 Pershing, and possibly the AK-46? You could even take back the mass production setup, even for things like the Sherman or P-51 plus designs which would have given the US a huge step up, even if they just filed it (under crazy person) until the war happened. One of the big issues was the US was really unprepared for war at all levels, and had to learn both what to build and how to build it and how to set up mass production to do so…taking those things back in time would give the US a big boost in getting the ball rolling.
And Charles Lindbergh gives those plans to the Germans and Hitler rolls over Britain and the USSR before the US even enters the war. Simpler to just go back to 1888 and introduce modern birth control to Hitler’s mon.
You never saw Star Trek Arena? Tsk.
I bet I could speed up the Civil War by a few years if I handed out some good quality walkie talkies to the Union army.