What word means "explicitly forbidden by the most fundamental laws"?

I'd suggest fundamentally impossible when writing for an audience which is not familiar with the field, while obviously impossible would work for an audience which is familiar with it.

Since you're talking about physics, why not "physically impossible"?

I would suggest intrinsically unworkable where the important part is intrinsic, expressing the fact that the problems with the experiment are internal, not related at all to any changeable conditions.

I realize this recasts your question a bit, but I think it better conveys the underlying concept that a given proposal can't ever work. Intrinsically flawed could also do the job.