'unless' and 'if ... not' have the same meaning?

[anhminh]

Could you please tell me what the meaning of the following sentence is?
Unless they get protection, they will not testify.

I think it means
If they do not get protection, they will not testify.

The two sentences have an relatively equal meaning, right?
Thank you.

[Alan]

In this case, the two sentences happen to be equivalent in meaning.

There are, however, many cases in which 'if not' does not constitute a meaningful replacement for 'unless'.