We have recently found cities under the Sahara that are "only" thousands of years old. 20 years ago we had no idea they were there. If we knock ourselves back to the stone age this century (nuclear war or climate change, take your pick) will anyone in 5,000 years even know what human civilization had achieved today?

One thing that could remain is radiation. There are certain radioactive isotopes that cannot occur in nature (AFAIK). That fact combined with known half-lives could tell future civilizations that we had relatively advanced physics capabilities.

On that subject, I recently ran across an article that says we (current humans) have found a site with such a radioactive signature from an ancient civilization. Sadly I have not had time to read more than the headline or search for any corroboration from scientists and/or scientific bodies.

Very interesting subject.