Originally Posted by anonymouse
Article 1, Section 9, Clause 3 of the U.S. Constitution:
So, it's debatable, at least, whether they can, through legislation, invalidate patents already granted.
Note, the repeal of Prohibition didn't make anything that happened during Prohibition a crime retroactively.
Nuremberg Trials: http://www.globalpolitician.com/print.asp?id=620
Retroactivity is a complicated matter, because it can be successfully argued (as this extreme case show) that some things, even if legal, can not be tolerated by society.
You could do something legal and still so wrong that you should be punished for it, because you knew you were wrong when doing it, and you only were using the slowness of Law to abuse society.
Such examples could be people destroying natural habitats, pillaging natural resources, causing deaths. Maybe in 5 years, the remaining responsible persons for the Bhopal "catastrophe" (is it still an accident when you knew such a thing would end up happening?), or the invasion of Irak, will be tried for their crime, even if it was "legal" at the time. Maybe this will never happen under another interpretation of the limits of retroactivity.
Law is not an immutable thing, it depends on the morals of society, which themselves change as society evolves. Nobody can say if such evolution is good, or bad, because living in the midst of society makes you convinced that it is.
If you were born George Washington, slavery would be a natural thing to you. If you were born Saudi Arabian, male superiority over women would be perfectly normal to you. Morals are dependent on society.
Deciding on the limits of retroactivity is dependent on morals. Who can say if in a few years, infringing on the freedom to innovate by patenting obvious ideas won't be considered worse than killing millions of people (hey, it's far fetched, but not _impossible_ )? Not that I'd want to live in such a world, but just to say that retroactivity, really, is a complicated matter.