Even if we were driven back to the 1820s, the existence of any evidence of what we were able to achieve since then, along with some basic knowledge of the right mathematics, theoretical physics, biology, etc. on which it was built, should massively accelerate the return to present technological levels.

@R..GitHubSTOPHELPINGICE You are not wrong! As I said, we've been asked this many times before and few OPs stop to think about recovery time. (Those that do usually want a 500 year recovery period... 99.9% of human innovation happened in the last 150 years but they want Mad Max for 500 years.) The simple truth is that unless humanity was driven from a population of 9B to about 90IK worldwide, even a devastating event would recover in 3-4 decades. Removing core dependencies without destroying the entire population is, IMO, impossible.

@Hobbamok : "it was just seeing that something is possible to make people actually try" - not really. It had to be economically viable. There were many inventions possible long before they became commonplace. One of the best examples is repeating rifles. They only really became a thing in the 1860s during the US civil war (while still most of the guns were muzzleloaders) but the first repeaters were built in the 1500s! It's just that without industrialization they were too expensive to make and too expensive to maintain (they had to be custom-built and parts weren't interchangeable).

If those people will have to prioritize basic survival skills over keeping own culture alive it will be possible. Memories about old world will survive while people that have them are alive. Next generations will only have distorted retold legends. Books would be useless, as soon the last person who can read them dies. Check Greek Dark Age for example of literacy lost.

If we exploit every easily reached resource of fossil fuel before the apocalypse, recovery from 1820 tech level might be impossible because there would be no energy source available to fuel an industrial revolution again. And nuclear or solar is not possible with 1820 tech.

@Hobbamok: That's true for modern high-end CPUs and GPUs, not a hard requirement for semiconductors. And most of the increase in compute power over the past 30+ years is just completely wasted on inefficient software. Basic semiconductor fab can be done in a garage with some decent lab equipment (see: Sam Zeloof); with actual organized human effort by a decent sized team, you could have computers sufficient to aid in re-developing the next generation of computers within a few years.

The "stuff" of our lives is all over the place. The "stuff" of the Roman Empire existed all over Western Europe in the year 700, and everyone had forgotten how to build it. Even the monks out in the far corners of Ireland who knew how to read couldn't build a Roman road or aqueduct.

@GeorgPatscheider Fossil fuels are cheap and convenient, but not irreplaceble. Engines can run on alcohol or wood gas for example. Solar is possible with mirror collectors and steam engines. Wind and water power was used even long before the industrial revoution and only fell partly out of favor.

The Sentinelese people have arrows. That means they have metal, and are therefore not in the stone age but in the metal age. Having said that, if almost all people die, the survivors would probably be thrown back to a medieval technology at best, and would have to relearn the old trades and do things we take for granted.

@gerrit: Why do you consider metal to be a prerequisite for arrows? At least the Wikipedia article "History of archery" suggests that bows and arrows were already used in the Paleolithic Age as weapons.

@gerrit Schmuddi's right, metal isn't a prerequisite for arrows. But more to the point, it would take an incredibly small group of survivors to significantly move back further than the early 1800s. Regardless the amount of devastation, there would be goods and materials (in the industrialized areas) all over the place. Diesel generators that can run on vegetable oil, light bulbs... We all suspend our disbelief for a good Mad Max movie and readers of the OP's story would (hopefuly) suspend their disbelief... but if the OP is looking for justification - it ain't there.

@GeorgPatscheider If we exploit? Are you imposing a condition on the OP's question that obsoletes my answer? Eight 2-mile-long trains carrying coal roll through my town every day. Our world is nowhere near low on coal or oil. If we exploit all those resources suggests the apocalypse occurred sometime after the year 2500 - by which time I would hope solar panels useful for powering motors and lights could be picked up off the ground after the disaster. Heck, I'd hope millennial AA batteries could be picked up off the ground.

@Schmuddi Metal-working was evident

@JBH You can scavenge what's there, but something as simple as a screw or a wire is not at all easy to produce if you don't have a community of people studying crafts. I don't know "Mad Max", but humans are supremely dependent on each other even for quite simple things, even at a 1750s level of technology.

@gerrit: Thanks. With this additional information that the Sentinelese use metal arrowheads, your previous implication is of course clear (without that information, not so much).

@gerrit The world is full of tools. Step down to your local devastated hardware store and pick a tap-and-die set off the shelf - or from the shelf of your own (or your neighbor's) garage. Use any spool of the millions of miles of wire hanging on hooks and sitting on shelves. People have been making wire since 2,000 BC. Then there's the factories and industrial centers... You're not accounting for the incomprehensible volume of available material and tools. People aren't starting from scratch. Besides, my answer says impossible unless we contrive a solution, then it's 1820 (not 1750s).

You're not thinking big enough. The stone age lasted 3.4 MILLION years, that's plenty of time for books to disappear, technology to disintegrate, and civilization to be consumed back into the natural landscape.

@GeorgPatscheider One really big problem with that hypothesis: Humans (and virtually all animals) also run on hydrocarbon fuels. If there is food for humans and animals, there are fuel sources that can power machines and generators. You cannot eliminate hydrocarbon fuels without eliminating humans. Oil and coal just happen to be convenient ones for running machines. They are very, very far from being the only usable hydrocarbon fuels. Crops and trees will burn just fine.

@JeffUK The point, though is that it would not last that long with books and technology still lying around. Realistically, it wouldn't start at all.

@JeffUK An answer must be given in the context of the OP's question: which identifies an apocalypse or event - not an era or age. If the sun's output increased 5% then, yes, over millions of years there wouldn't be much left - but that's not at all what the OP asked about. Can you think of an event that over the course of weeks or months (heck, I'll give you up to two years) would result in an irreversible stone age requiring humanity to start over, but NOT kill all the humans? That would probably be very useful to the OP.