the sqrt -1 is not "unreal" - it is "imaginary" by naming convention but it is as real as a rock

Yes, it belatedlty occurred to me that there probably had been some usefulness found in j, Bryan, so actually I was talking through my hat a bit there.

Many of these so-called "theoretical" particles are not "unreal" but they may be "virtual".
But does their existence always get proven eventually? And if not, do they get "uninvented"? I'd imagine there must be cases where what was once considered to be a (theoretical) single entity is actually identified as a composite that overlaps with neighbouring particles/wavicles or whatever.

I suppose we're taking issue a bit with what looks like indulgent and perhaps excessive "advance terminology". But I'm happy to admit talking from a position of almost complete ignorance.