Let’s say we left one single very smart guy (not necessarily with the knowledge: they may be able to understand hard stuff when taught it, but not know it already) alone on a copy of the earth. That person is also immortal. Could that person, by themselves, gain back all knowledge, maybe also experimental, or even surpass that is already available to us right now, before the planet gets inevitably engulfed by a sun turning red giant?
Our brains are like hard drives. We know a fair amount but over time, we forget some things to make room for newer things. Some things, we can’t access because some part of it was overwritten. Some other things, we can’t simply recount at all.
So it’d be an eternity spending time figuring it out, forgetting, misplacing the knowledge, coming to a realization, forgetting again .etc .etc