The correct negative form (past participle)

Question

I've been searching the answer to my question wherever it is possible, but I haven't managed to get the strict rule (or guideline) for it.

As we know the negative form of the past participle is created by the addition of not (Heard - not heard; caught - not caught), but also in the English language there is one other way of creating the negation of such Participles, by dint of attaching prefixes to them, mainly it is Un- (Unheard, uncaught, unseen). The latter, with Un-, work as adjectives.

Earlier I was quite sure that Not + participle is used in long clauses having additional objects, whereas Prefix + participle by itself.

For you to have better understanding, there are some examples.

1- We complained but as usual our voices went unheard.(Adjective)

2- The robber, yet not caught by the police, is said to be armed and particularly dangerous. (Participle clause).

However, there are a couple of cases I'm completely in doubt with. Here they are, below.

3 Our Milky Way galaxy is strewn with billions of planets, alien worlds still unseen by human eyes - at least for now.

(This one I found on YouGlish, the phrase is said by a native speaker, but Why unseen, I think it should be not seen - still not seen by human eyes)

4 I managed to sneak into the hall unnoticed by the security / not noticed by the security

(I think the second one is correct, but on the analogy of the first one I'm not sure.)

5 In the world there are a lot of wonderful places still untouched and unseen by humans / still not touched or seen by humans.

(Personally, I'd write the second, but, as I said earlier, I have no guideline to answer for certain.)

Hope you'll be able to help me, If there is needed to be any clarification from my side, just let me know.

P.S: Before posting the question I'd read all possible sources however connected to relustative adjectives (they are related to the post in some way). I was baffled mainly because in my mother tongue there is a clear rule for that particular case, and if I wrote an adjective instead of a participle in such sentences, it would be considered an outrage mistake. As we all know, languages are different and it is stupid to apply the same rules to every one of them. Nonetheless, I am not alone, there is at least one more persone having trouble here; take a look if you will link. To tell the truth, I'm quite sure that I'll fail any thorough test on this very theme as the vagueness is none the less. It is very strange that there's no strict rule for this part of grammar.

Answer 1

Both ways are grammatical. In some cases, one form will be more idiomatic than another form, but there is no general rule as to which one you should use.

All the following sentences are correct:

Our Milky Way galaxy is strewn with billions of planets, alien worlds still unseen by human eyes - at least for now.
Our Milky Way galaxy is strewn with billions of planets, alien worlds not yet seen by human eyes - at least for now.
In the world there are a lot of wonderful places still untouched and unseen by humans.
In the world there are a lot of wonderful places not yet touched or seen by humans.

The reason that you can do this is that in these sentences, seen, unseen, noticed, unnoticed, touched, untouched are adjectives. If you have a past participle in a verb, not noticed, you cannot replace it by unnoticed. For example,

*The security guard had unnoticed them,

is truly wrong; you need to use had not noticed in that sentence.

For cases where the adjective is an predicative adjunct, the un- form is much more idiomatic.

I managed to sneak into the hall unnoticed by security.

and not

*I managed to sneak into the hall not noticed by security.

and

In the attic, he could work undisturbed for hours,

and not

*In the attic, he could work not disturbed for hours.

Answer 2

Where there is the negated adjective, it is usually the idiomatic choice:

• 1 We complained but as usual our voices went unheard.
• 1' *We complained but as usual our voices went not heard.

(went unheard / unnoticed / unseen ... are fixed phrases, and by far the more natural-sounding choice. As stated, these are adjectives rather than past participles, which are not used after 'go' / 'went' (*went hit, *went noticed, *go seen ...).

• 2' The robber, as yet unapprehended by the police, is said to be armed and particularly dangerous.

• 2 ?The robber, not yet caught by the police, is said to be armed and particularly dangerous.

• 2'' ?The robber, who has not yet been caught by the police, is said to be armed and particularly dangerous. (sounds more natural than the deleted form)

• 3 Our Milky Way galaxy is strewn with billions of planets, alien worlds still unseen by human eyes.

• 3' Our Milky Way galaxy is strewn with billions of planets, alien worlds still not seen by human eyes. (sounds less idiomatic than (3) or 'which have still not been seen ...')

• 4 ... '[still] unnoticed' or 'still not having been noticed' are more natural

• 5 This has me stumped. I agree, the not-version sounds at least as natural (to my ears). I think it's because 'still unseen by humans' sounds vaguely less natural than 'still not seen by humans'.