tabula rasa: We have no idea about Ancient Egypt

What if there had been almost no surviving records about Ancient Egypt left, the language completley unknown (Champollion does not decode it) , the monouments somehow shattered and eroded , the valley of the king´s plundered and devasted . Im thinking of the Egyptian time like the 400 year dark age in Greece where no records were left. We know nothing about Ramenses II., Osiris and Horus etc.

My name is Ozymandias, king of kings, ye mighty and despair !
Of that colossal wreck, boundless and bare, the lone and level sands strech far away.
Percy Shelly
 
What if there had been almost no surviving records about Ancient Egypt left
I'm not too sure it would be doable, safe a systematical campaign of destruction continuously and consistently made since the end of pharaonic era. Ancient Egyptians simply written down a lot of things during more than 5,000 years of existence, far too much to not let a lot of remains.

This pretty much ammounts to asking about Greek or Romans civilisation being entierly unknown because all of what they left somehow systematically disappeared.
Even civilisation largely unknown before their discoveries, as Hittits, simply let too many traces.

the language completley unknown (Champollion does not decode it)
While Champollion can be credited without problem having genial intuition on this, he was hardly the only one : every linguist with two cents of ambition was on the thing. Sooner or later, someone would have figured it out.

Im thinking of the Egyptian time like the 400 year dark age in Greece where no records were left
But Greek "Dark Ages" didn't destroyed the previous writings, and Mycenean Greece clues can still be used.
As for the Egyptian case, it was simply far more develloped culturally than Mycenean, too important and influent to simply know a "dark age" :

roughly speaking, the disappearance of Linear B in Greece is largely tied with the fall of palatial organisation of Myceneans, when invasions in Egypt generally "fit in the shoes" of pharaonic organisation.

Assuming, tough, hieroglyphic script in spite of being largely used in Egyptian society, simply disappear. We'll end with another script on a same language, a bit like Champollion used coptic and demotic to decode it : you'd just add one layer on the whole thing.

- - -

That said, we could have a more parcellar knowledge of Ancient Egypt, but that would require, IMO, important changes in its history. Roughly, no Middle or New Empire with important acculturation (a wild exemple that I don't consider being viable : Nubianisation on the South, Berberisation on the West, Indo-Europeanisation in the North, etc.).
A good part of Antiquity's Egypt language/culture would be distinct enough (altough the sheer force of acculturation of Ancient Egypt was huge and more than prooved, so I don't think that's even a plausible outcome) that using native languages to decode hieroglyphic script would be a pain.

But at worst, save systematical destruction from omniscient immortal vandals, we'll be more on an Etruscean-like situation : writings are there, we can maybe decipher it, but we can't really say what the hell it's about.
 
A big part of why Egypt is so well documented relative to most other civilizations of comparable age is that Egyptian had materials available and cultural choices that favored preservation of their stuff, particularly written stuff, in the environment they lived in. Papyrus preserves fairly well, and preserves a lot better in dry climates, like the one Egypt has. They also liked to write stuff on monumental walls (much more so than most Mesopotamian societies, for example, it seems), and used stone for most said walls (as opposed for instance to brick, or wood, etc.) .
They invested considerable resources into funerary practices that at times included very visible landmarks and involved a lot of art, writing, and storing of objects in general, and they often buried their prominent dead in chambers cut into stone that resist reasonably well to time (AFAIK, we don't have even remotely the same level of funerary evidence from Sumer, I'd guess as a combination of Sumerians having less conspicuous funerary practices and their living in an alluvial plain where burials tend to get flooded, eroded away or silted).
Therefore, Ancient Egypt is optimally placed to leave useable traces.
Also note that that these environmental constraints contribute to skew our record in favor of Upper and Middle Egypt, where the majority of intact archaeological findings is found, by a huge margin. The Delta a lot more poorly documented.

If suddenly Egypt becomes wetter, and the Nile somehow floods the thin arable strip so that most remains are eroded, the record might thin out considerably.
Mind you, not even that would get you tabula rasa.
It might take a lot longer to decipher the script, but we'd figure out eventually, at least with some approximation. There are enough bilinguals outside Egypt proper. We'll have lots of references pointing to a very important culture and state there, so someone is going to start digging around. And the sheer amount of material that Pharaonic civilization left indicates that stuff would be found (like in Mesopotamia etc.).
Even going with massive climate change, I think we'd have a decent amount of evidence.
 
Top