As a follow-up to Would 私は怖い mean "I'm scared" or "I'm scary"?
We know that 怖い can mean both "to be scared" and "to be scary to somebody".
What about the difference between 怖い and 恐い? Or more general 怖 and 恐? How does 恐 come to mean such different things as:
恐らく （おそらく） – regrettably
恐竜 （きょうりゅう） – dinosaur
For the general difference between these two kanji it may be better to avoid a direct comparison between the versions of こわい and look at their other uses. In particular I associate the kanji 恐 with the word 恐ろしい, which you would usually not alternately write as 怖ろしい, which is very similar to 怖い but carries more of an objective sense of "scary" while 怖い tends to be more of a subjective thing.
I think this example sentence taken from here shows the difference pretty succinctly:
「草原で恐ろしい毒蛇にあい、怖かった」
So you see a snake, which we can all probably agree was pretty frightening (in appearance, etc.), and the speaker felt afraid on a personal level as well. That's why we write 恐竜, essentially a scary dragon. This answer also backs up this idea. If you want to use 恐い I'm sure the nuance can carry over along with the kanji.
As for the origins of おそらく, if the explanation on this site is to be trusted, then the kanji 恐 in Chinese includes a meaning of possibility, and it was absorbed into Japanese hundreds of years ago through scholars and their 漢文.
怖い is for more in-your-face "terror", while 恐い is a more abstract notation of "fear", or "dread". 怖い is used more for something seen or experienced first-hand, while 恐い is used for things one may never actually encounter first-hand. Compare the following:
- 地震は恐い → "Earthquakes are scary"; a general concept - you don't have to have actually experienced one to believe this
- 地震は怖かった → "The earthquake was scary"; you were in this earthquake - it gave you personal, scary/terrifying feelings
As for 恐らく, it means "regrettable" in the sense of "I'm afraid that..."
I'm afraid I don't know the answer. = Regrettably, I don't know the answer.
In this day and age, "not knowing the answer" to something does not actually make you scared or fearful. Although I can see how this modern usage might have carried on from older times, when a servant had to tell regrettable news to a master/king/etc., and might actually have feared receiving some punishment for delivering bad news.
恐らく is really more translated as "perhaps" or "probably", but you can see the overlap in the following sentence:
恐らくそれは見つからないだろう → I am afraid it will not be found. / It is not likely to be found.
