Is there a case in which "instantaneously" can be used in which "instantly" cannot? If not, why does the former exist? If so, what are the circumstances dictating that usage?
Learn English – Difference between “instantly” and “instantaneously”
adverbsdifferences
Best Answer
According to this post on the Grammarist, there is a difference between the two. They write:
Instantly is synonymous with immediately, but instantaneously is closer to with little delay. They give the following examples of each:
So, yes: there are cases when one should use instantly instead of instantaneously and vice versa. They represent two different ideas about when something happens. However, despite the fact that one could distinguish between them, one does not necessarily need to in practice: