Normally, the existence of prior art containing matters for which a United States utility patent makes claims to patent rights prevents a patent from being issued for those claims, and if a patent somehow issues anyway, it provides a defense to a defendant in a patent infringement action.
But suppose that there exists prior art that is not know to anyone alive at the time that the patent is applied for and issued, but ten years later when the patent holder brings a timely patent infringement lawsuit, the prior art which had been unknown to anyone alive at the time is rediscovered. Does that invalidate the patent?
For example, suppose that Sharp Co. develops a process of making steel for knives that is superior in some way (e.g., it makes it much cheaper and quicker to manufacture the knives than an alternative status quo process that produces exactly the same end result), by incorporating flax oil into the process of forging its knives. It is impossible to discern that the flax process was used by physically inspecting a knife made with it after the fact.
Assume that this patent easily qualifies at valid by every relevant legal test evaluated as of 2012-2014 and under all changes to patent law since then that do not pertain to prior art. This process was something that no published work available to living people was aware of at the time that Sharp Co. applied for its patent in 2012, and the patent was issued in 2014.
In time, Sharp Co. discovers that Dull Co., a company next door to it, is using the same flax oil process to forge its knives and brings suit in 2022 for patent infringement, having copied the Sharp Co. process from its patent application, and Dull Co. gains 50% of the market share of knives made with this process costing Sharp Co. US$50 million a year in lost profits.
But after filing an answer in the patent infringement lawsuit in U.S. District Court, the patent infringement defense lawyer for Dull Co. who reads archaeology and linguistics texts in his spare time happens to learn that in 2016, archaeologists discovered an inscription in Old Persian from 1100 BCE in a newly unearthed tomb and translated it and published their findings in the journal Nature, describing precisely the same lost art of using flax oil to forge knives that Sharp Co. reinvented independently in 2012 and for which it secured its patent. No one affiliated of Sharp Co. learned about the newly discovered Old Persian texts until after they brought their infringement lawsuit and the lawyer for Dull Co. brought it to their attention. Dull Co.'s lawyer had raised the defense of invalidity based upon prior art in the answer that Dull Co. filed even though he didn't know about this particular prior art at the time.
Evidence from the ancient Old Persian text and some related authentic texts written by Old Persian priests from a few centuries later indicates that there were hundreds of copies of the description of this process which were included in every prayer book of the predominant religion of the region now called Iran in 1100 BCE that was freely available in each of hundreds of temples in the region to anyone interested, but it was not preserved by later scribes who decided that it was considered an obvious piece of knowledge among metal workers that wasn't worth recopying at some point in the early Iron Age, even though they were wrong and the process was later entirely forgotten.
Does the previously lost prior art, that is rediscovered before the patent of the lost prior art process expires, provide Dull Co. with a meritorious prior art invalidity defense to the validity of Sharp Co.'s patent infringement lawsuit against Dull Co.?
I can imagine this question functionally hinging on when prior art has to exist, and I could also imagine it hinging on the definition of what has to happen for something to constitute prior art. But, I'm not really sure exactly what the proper analysis would be.
Differences from prior similar questions
This differs from the similar question "What impact does prior art discovered after (US) patent awarding have on enforceability and even validity of that patent?", because in this question, by design, there was nothing that would have operationally counted as prior art available at the time the patent was applied for and at the time that it was issued, while that question implicitly assumes that the only reason that prior art wasn't discovered was due to an inadequate prior art search when the patent was issued, and that a sufficiently thorough search for prior art at the time the patent was applied for and issued could have possibly found it.
This also differs from the fact pattern where someone already uses the patented process in a secret manner protected as a trade secret that is not disclosed until after the patent is issued, because, as I understand it, to be prior art, something must be knowable to people in the general public at some prior time (usually in a publication that is not confidential).
The example is loosely inspired by Rahil Alipour, Thilo Rehren, Marcos Martinón-Torres. "Chromium crucible steel was first made in Persia." Journal of Archaeological Science (2020); 105224 DOI: 10.1016/j.jas.2020.105224 (recounting their discovery that there was widespread manufacturing of chromium steel in Southern Persia around 1000 CE, a metallurgy technique that was subsequently lost and then only rediscovered about nine centuries later).