In his presentation Making Humans a Multiplanetary Species, Elon Musk said "There are many examples in history where civilizations have reached a certain technology level and then have fallen well below that and then recovered only millennia later." https://www.youtube.com/watch?v=H7Uyfqi_TE8&t=48m15s
Is the statement accurate, and if so, what are some of these examples?
There are of course many examples in history where a major civilization "falls" and fades from importance for a long time, but often this seems to be more a matter of the rest of the world catching up rather than a decline in absolute technology. Besides, "millennia" is a very long time. Ancient Roman engineering (aqueducts, etc.) is the obvious example where significant technology really was lost, though recent historiography seems to give a more nuanced view by emphasizing continuity and all the new technology that was developed during the so-called Dark Ages. Even if we take the ~1000 years from the fall of Western Rome to the Modern period in Europe as an example supporting Musk's point, was this exceptional or something that really has happened many times in history?
This comment from /u/random-dent started with concrete but talks about this in depth. Concrete, steam engines, calculators.
This sort of question brings up the importance of remembering how it is that we learn about human history, particularly the ancient past: by examining what they left behind. This means that our understanding of the past is inevitably skewed towards the great empires that produced astounding monuments and works that could last centuries or even millennia. However, these empires were not necessarily what drove humanity forward. In fact, many of these empires stagnated significantly technologically and socially.
For example, Rome made significant advances in civil architecture and legal doctrine, but in terms of technologies and practices that saved labour or made life easier, there was significant stagnation (and regression with regards to the growth of latifundia). Neil Faulkner makes the observation that advances in technologies such as water wheels and farming tools were not developed by Rome itself, but rather from smaller groups living on its fringes.
Aside from Rome, there are also numerous examples of this during (and around) the postulated Late Bronze Age Collapse that struck several societies causing the transition from palace economies to something like the isolated villages of the Greek Dark Ages. While the cause of this collapse is contentious, I believe that Faulkner and Chris Harman (and to a lesser degree, Joseph Tainter) make a convincing argument in looking at the dynamics of these societies. While climate changes and other factors may catalyse or exacerbate the issue, it is the underlying social mechanics of the great empires that led to the collapse. It is this that points to an answer to your question.
As empires develop, the social layer at the top begins to invest less and less resources into developing new technologies and instead begins to consume more surplus as luxuries. Look at the enormous inequality of ancient Rome or Egypt. Both societies had abject slavery contrasted with consumption by the rich on truly staggering levels. Eventually, this over-consumption tends towards vulnerability to social crisis, uprising, vulnerability to environmental change, proclivity for increased warfare and destruction, etc. These stagnating societies were then followed by long periods of decline where isolated communities did not have the surplus production needed to free members of the society from production so that they could pursue endeavours such as learning and inventing. In this situation it was possible for generations of learning to be lost and formerly routine practices to be completely forgotten.
Research into this cyclical rise and collapse is still ongoing, but archaeology at the sites of early cities in Asia Minor is fascinating, with numerous sites showing signs of continuous construction then collapse, destruction and rebuilding. These periodic collapses were usually followed by extended periods of a return to traditional hunter-gatherer lifestyles before urbanisation began anew. The works of Faulkner, Harman and St de la Croix also examine this tendency in some depth.
With regards to Musk's quote though, it is not possible to conclude that modern societies continue this historical trend. It is certainly true that our world is currently plagued by similar issues to those that ancient societies faced, and in fact on a larger scale. War, environmental crises, funding issues and social conflict all remain key issues threatening society and even technological development. However, modern capitalist societies do not seem to involve the same stagnation that underpinned the vulnerability of these more ancient societies. This difference was noted as early as Marx who in the Communist Manifesto remarked "The bourgeoisie cannot exist without constantly revolutionising the instruments of production, and thereby the relations of production, and with them the whole relations of society. Conservation of the old modes of production in unaltered form, was, on the contrary, the first condition of existence for all earlier industrial classes. Constant revolutionising of production, uninterrupted disturbance of all social conditions, everlasting uncertainty and agitation distinguish the bourgeois epoch from all earlier ones. All fixed, fast-frozen relations, with their train of ancient and venerable prejudices and opinions, are swept away, all new-formed ones become antiquated before they can ossify. All that is solid melts into air, all that is holy is profaned, and man is at last compelled to face with sober senses his real conditions of life, and his relations with his kind."
Not really an answer, sorry, but I did come across a quote yesterday that expands on Musk's line of thinking and offers a bit more background information than that single line he spoke during the IAC speech:
The spaceships from Earth could stop coming for other reasons—it could be WWIII, it could be that Earth becomes a religious state, it could be some gradual decline where Earth civilization just sinks under its own weight. At one point the Egyptians were able to build pyramids, and then they forgot how to do that. And then they forgot how to read hieroglyphics, until the Rosetta Stone. Rome as well—they had indoor plumbing, they had advanced aqueducts, and then that fell apart. China at one point had the world’s biggest fleet of sailing ships and they were sailing as far as Africa, then some crazy emperor came along and decided that was bad and had them all burnt. So you just don’t know what’s gonna happen. The key threshold to pass is the number of people and tons of cargo required to make things self-sustaining. And that’s probably something like a million people and probably something like 10-100 million tons of cargo.
Source http://waitbutwhy.com/2016/09/spacexs-big-fking-rocket-the-full-story.html
If there are millennia separating the fall and renewal of the "civilization" in question, then I can't really consider them the same civilization. Over millennia, culture and language change. Heck, even at the genetic level, the people who constitute the renewed society may not be of the same "people" who constituted the old, due to migration and interbreeding with outsiders. I don't think you could call the society of Renaissance Italy, even in the region around Rome, to be ancient Rome risen again, just because of geography - and there only centuries separating Renaissance Italy from the fall of ancient Rome. Here you are talking about millennia. The oldest human civilization, Sumer, collapsed just over 4,000 years ago. Agriculture was developed some 10,000 years ago. Perhaps Mr Musk was misquoted.