Difference between "instantly" and "instantaneously"

Question

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?

Answer 1

According to this post on the Grammarist, there is a difference between the two. They write:

Instantly means at once or immediately. Instantaneously means happening so soon (in relation to something else) that no delay is perceptible. The difference between these adverbs is subtle, and there is plenty of gray area between them, but careful writers keep them separate.

Instantly is synonymous with immediately, but instantaneously is closer to with little delay. They give the following examples of each:

I don’t use Twitter often, but I like how my Tweets appear instantly on my Facebook page. Buzz delays them for hours. [Boston Globe]

Mr. Lingamfelter argues that advances in technology mean that gun sellers can now verify that buyers are not felons instantaneously through the National Instant Check System . . . [Washington Post]

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:

But while drawing this distinction between instantly and instantaneously is nice, in practical usage the words are usually used interchangeably.

Answer 2

I suspect that 'instantly' is the newcomer...

Instantaneously means taking zero time (or almost zero time). Instantly means occurring now.

You can use 'instantaneously' in contexts such as 'when the ball hits the wall, it instantaneously reverses direction', and I don't think you could possibly use 'instantly' instead (not in British English, anyway). Clearly, depending on the level of detail you're looking at, a ball bouncing might or might not be instantaneous (say, human reaction time vs atomic vibration time).

You can use 'instantly' in contexts such as 'the line for the movie will be forming over there instantly' and the process will take a long time - a lot longer than 'instantaneously'.

Answer 3

"Instantly" originally carried the added connotation of "persistently", in addition to "urgently." It only came to mean "immediately" some eighty or a hundred years after it was first introduced. Then, shortly after that, "instantaneously" was introduced, along with the words "simultaneously" and "spontaneously".

Basically, they mean the same thing, but if there is a difference it probably resides in the ancient connotation of "persistence" that "instantly" once held. "He was instantly transformed into a frog", or "his face instantly shifted to a furious expression"; contrast that to "it was an instantaneous event, past in a moment, never to be recaptured", or -- as above -- "instantaneously, the ball bounced from the wall and into the waiting arms of the goalkeeper."

Answer 4

They actually both mean "happening immediately". Perhaps the difference here might be just the tone. Instantaneously sounds a lot more formal than instantly.