I encourage people to treat this as an exercise and avoid looking at the other answers before posting your own. (Maybe use spoiler blocks?) This is more for fun than for serious. However, it may potentially serve as an analogy to help think about what the top modern-day interventions might be.
The challenge: you have one opportunity to go back in time and change something, for the good of all humankind. Use your upvotes wisely; it’s how we’ll judge success. My suggestion: upvote based on what you think would actually do good; strong upvote what you think works best. If you want to give kudos for other reasons, such as insightfulness, I suggest using a comment instead.
You don’t have magical powers to change anything you want (so for example, you can’t just “prevent the agricultural revolution”; you have to have a plan to do that). However, you do have the ability to take modern technology and resources back with you. For simplicity, though, let’s keep it to one time-travel event—your machine is single-use-only and you can’t afford to build two. (You could bring the science of time travel back with you, but I don’t know if that’s a good idea...)
For the sake of the thought experiment, pretend that you can actually change things via time travel, giving rise to a new timeline (rather than there only being one consistent timeline, like we’d expect of real time travel).
What do you do, to do the most good?
Stretch goals: solve the problem under harsher constraints, such as time travel machines with restricted ranges. Block key things most important to your strategies so far, and see what you can still do.
I think this problem is challenging for some specific reasons, but I’ll put them behind a spoiler block for those who want to just dive into it.
ETA: I also posted a similar challenge on the EA forums.
I’m specifically interested in this challenge because of X-risk. I used to think that, in order to the most good, you basically take as much scientific knowledge as you can, as far back as you can, and try to teach it to people or something. But it’s quite plausible that that just brings X-risk nearer, for a wide variety of sciences and technologies one might try to smuggle back.
Similarly, you can just try to do basic good for people—averting large historical disasters, etc. But again, this might just accelerate progress, giving humans the power to create global catastrophic risks sooner, and thus giving humanity less time overall.
This makes things feel a little hopeless—is it possible to do very much good at all?