If there was a large scale nuclear war, would it be possible for humanity to literally be blown back into the Stone Age. Like if all pre war knowlege and technology was destroyed could people revert back to how people lived in the early days of human history?
You are not going to go back to the Stone Age. If nothing else, only a few specialists know how to knapp a decent stone tool these days.
What you might get instead is something you might call the Salvage Age. People may not have the industrial base to manufacture many things, but they can re-use and adapt what is laying around. If you don't have the wherewithal to make brass cases & primers for modern guns, you can convert to black powder (lots of amateurs use it), or make arrows for the compound bows down at the local sporting goods store. Likewise, you don't need to make stone clubs when there's plenty of good steel that can be worked into swords.
Fundamentally, you are not going to lose all knowledge. Contrary to what Tim B suggests in his answer, there are a good number of well-educated people who've chosen to live in rural areas, and so would have a good chance of surviving a nuclear war. There are also such things as books: my own personal library (I live in a rural area) covers subjects from home repair and gardening up to general relativity. You might not be building a silicon foundry any time soon, but there'd be the knowledge of what could be done.
There's also a great deal of practical knowledge held in the minds of farmers and such. The electric grid fails? You can cobble together a system from car alternators and run it on water or wind. Haven't got petrol fuel? Not that hard to distill alcohol or make biodiesel. You might not make enough for an urban commuter culture, but there'd be enough to run farm equipment. And there'd be the accumulated knowledge of generations to tell you how to be a better farmer than Stone Age people, or even 19th century ones.
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This line of thought is not mine, but I like it a lot. The point is, to bomb a humanity into Stone Age you would need to destroy not only infrastructure, but knowledge. As in: all people, bearing that knowledge.
As long as the community has engineers, or at least people with modern technical education, it is impossible for it to descend below Renaissance. For Leonardo da Vinci level of tech, all you need is wood. Scrapping on the remaining of our current industrial civilisation would provide much more. Just surviving in the woods with modern-educated people would provide you this.
The only way to descend a human civilisation below Renaissance is either strip them of all resources (they die), or kill all adults and let the kids derive their own savage civilisation, "Lord of the flies"-style. This is not what you typically reach by carpet bombing, provided there are still survivors.
The Sentinelese are believed to be the last pre-neolithic tribe in the world. They literally are stone-age people. If they were the only people to remain, the world would be pushed back to the stone age.
There are (perhaps) a few other uncontacted peoples in the world who, if they survived, would become stone-age throwbacks. However, most are gone and none of those remaining are untainted. Nevertheless, any that still survive might also be the basis for a story (but only the Sentinelese could guarantee it).
However, if anyone else survives, you'll get a hybrid of the 1500s and the 1900s depending on what mix of skill sets and technological resources survived.
To be honest, it's realitively easy to retain basic metallurgy and manufacturing. Unless you magically killed all the adults, you'd be back to basic automobiles, functioning steam trains, and locally-generated electricity fairly quickly.
Alex makes an interesting point in his comment, but I don't fully agree with it. He's right insofar as there would be pockets of humanity that would be capable of living with at least early 20th century technology, but in the Northern Hemisphere in particular, it's possible that people could be bombed, if not back to the stone age, at least to a pre-industrial revolution society. This is not because technology would disappear; it's because the skills would go.
Just like in a pandemic scenario (killer flu, zombie apocalypse, etc.), people stop producing. They do this, not because they are afraid of disease or attack, but because they're dead. In this case, we can consider cities lost because they're prime targets for nuclear strike. Even those that aren't need massive supplies of food (and shipping) that might not be available.
The problem here is that we now specialise in our society. Some of us know how to grow crops, but we don't know how to smelt metal. Some of us know how to mine, but in practice the skills required to do this today are based around operating heavy machinery. Heavy machinery that is now useless unless we have a regular supply of fuel.
Where are all our architects? Most of our engineers? Our logistics specialists? the manufacturing plants for things as simple as nails?
They're all in cities.
Even those that aren't are now subject to Maslow's Hierarchy of needs like the rest of us. They have to learn how to grow crops and protect their family and property from raiders and the like. They're unlikely to pass on the ability to design buildings, operate heavy equipment et al to their children in lieu of how to survive in a world that only has patchy access to do these things.
The caveat here is that if you have a strong functioning government capable of mass communications with their constituents (remember all the communications and digital data is located in the cities too), then you might have a chance of getting someone to grow and supply food to an architect so he is free to design buildings. In the short to medium term however, that's not going to happen and feeding himself and his family has to be a priority over designing a new building, and he's already at a disadvantage compared to (say) a farmer in that regard.
What's he going to do? Look up farming on the internet? (Oh, wait. Those data centres and critical infrastructure was all based in cities...)
So; stone age? Probably not. But, the society and culture that strings up in the wake of such a strike would have a lot in common with a neolithic society insofar as it would likely devolve into a clan based system where those with the ability to repair things would likely be revered far more than people who can design something new. Eventually, the focus would have to change because what's left finally wears out, but by then the skills have likely died out and even if they haven't, where do you get the resources to build what you need?
Our society functions the way it does because our farming techniques mean that we only need a small percentage of our population generating food. The rest of that productive capacity can be spent on other things (including designing the machines of war). In a post-atomic scenario, that changes and all of a sudden, we need a much larger percentage of the population generating food. That means that many of the interlinked support industries that allow us to manufacture technical products are no longer possible.
Nuclear Winter is well established. the smoke from burning cities will be so thick that the atmosphere will become Turbid.
Once the sunlight is blocked all our crops will fail, the mass starvation and total environmental collapse will result in the total and complete collapse of all civilizations.
Once crops fail, other plant life will also have failed. The dearth of oxygen, due to the death of the vast majority of plant life, will result in humanity basically becoming extinct except for in a few remote locations underground where humans survive off of the heat from local volcanic activity, and the oxygen produced by bacteria, and eating troglobites to survive.
There are probably pockets of plants left near the equator and near jetstreams to help with Oxygen production. Plants clearly survived the Dinosaur killer asteroid after all.
As snowfall accumulates, but doesn't melt, glaciers will erode the ruins of our cities.
By the time the next Ice Age is over, you'll be back in the Stone Age.