I am also quibbling with your claim “it's impossible for law to work backwards.” (IANAL)

1) It is unconstitutional to prosecute an individual for a crime that was legal at time of perpetration.

2) However, the opposite — decriminalizing previously illegal behavior — can be retroactive.

3) Changing future interpretation of copyright, etc., isn’t the same as case #1. If I’m not mistaken, Congress has passed e.g. the Mickey Mouse copyright law and the DMCA, which both extended “protection” & duration of copyright on previously created works.

Maybe.

http://www.scotusblog.com/case-files/cases/golan-v-holder/

You are mistaken on retroactive copyright; publishers and artists who failed to secure copyrights the first time around did not get retroactive postmortem copyright granted to them. The classic example is the library of H. P. Lovecraft, whose stories are considered public domain as there is nobody who has a reasonable claim of copyright to them.

Retroactive laws are common place: http://en.wikipedia.org/wiki/Ex_post_facto_law

I didn't know that. I was referring to http://en.wikipedia.org/wiki/Nullum_crimen,_nulla_poena_sine... . Obviously, history shows that governments can do whatever they want if opportunity arises.

> Remember, it's impossible for law to work backwards.

Really? I think it's just politically less convenient than making laws that apply to future research. There's nothing that would keep the government from removing copyright all together if they wanted to.