How closely related is hebrew and arabic (or any other semitic lanuage such as aramic or amharic). Can each be understood by speakers of the other’s language?
Ben
No.
PDF file.
http://www.erudit.org/erudit/meta/v43n01/amit-kochavi/amit-kochavi.pdf
You can read it on Google in HTML cache, but I can’t get it to make a link. 
Pretty closely. Hebrew and Arabic are both, as you mentioned, Semitic languages, as is Aramaic. Semitic languages are part of the Afroasiatic family, including Semitic, Egyptian, Cushitic, Berber, Chad, and maybe Omotic. Supposedly these languages all came from an unknown ancestor language spoken around 6 BC, but the exact relationship between them isn’t agreed upon.
As to whether they can understand each other, my WAG is that they probably get a word here and there but by and large they can’t. Apache and Navajo both derive from Athabascan languages, but if you ask each one if he can understand the other they’ll both say “You can get a word here and there, but they talk too fast.”
But then again, Czech and Slovak speakers can understand each other, so…I don’t know.
Sorry, didn’t see Duck Duck Goose’s post, I guess I do know.
(Stop me before I triplepost again)
Kinda like English and Dutch. Dutch is closer to English than just about any other language (except Friesian). But I only understood about 1 word out of 15 when I was in Holland.
I have no idea on the actual OP question, but I have to say that just because two languages are in the same group does not make them understandable to speakers of other languages within the same group. FTR IANALinguist 
(starting to get a hang of these acronyms, please tell me there is a list somewhere?)
My automatic reaction is to look at the Gaelic languages. Irish, Welsh, Manx and Breton are all members of the Gaelic group of languages. They have a common root. They are in practical terms however completely different from eachother, as different (imho) as English and Japanese. As an Irish speaker I can tell you I can’t even pronounce written words of Welsh, let alone guess what they might mean. AFAIK Finnish and Koran have the same language group root as well, together with Hungarian or something…
Well, Czech and Slovak separated a hell of a lot more recently than 6 BC. Slovak only began to be considered a separate language (rather than a Czech dialect) in the mid-1800s and was not granted official recognition as such until after WW2. So it’s not really a comparable situation.
I know…I just wanted to leave myself an out if it became necessary to backpedal 
Seriously, that was a bad comparison, you’re right.
Wrong. Irish and Manx are Gaelic languages. Welsh and Breton are Brythonic languages. Gaelic and Brythonic are both branches of the Celtic family, which is where your confusion stems from.
The Gaelic languages are, indeed, not mutually intelligible with the Brythonic languages, although they do share certain features (such as VSO structure and initial mutation). However, there is some degree of mutual intelligibility among the languages within each branch - more so in the case of the Gaelic than the Brythonic languages.
IOW you as an Irish speaker can’t understand Welsh, but Scottish Gaelic or Manx should be reasonably comprehensible to you. Welsh speakers would have a harder time with Breton, but they’d recognise more words in it than they do with Irish.
Excactly the example I was going to bring. The grammar is similar - three-letter roots, tenses - but the vocabulary isn’t really. I can understand a word of the language here and there just from similarity to Hebrew, but most of the Arabic I know is from highschool and from later on.
You meant 6000 BCE, right?
The estimate I have here places the separation somewhere between 12,000 and 6,000 BCE.
As a Tigrinya speaker I can say that we share a few words with Arabic (my sig means “Hello” in Tigrinya), but this could be from modern assimilation too. We trace most of our words (and our unique writing script) from Ge’ez, a now dead language (think Latin-English is like Ge’ez-Tigrinya). Amharinya (or often called “Amharic” by non-Ethiopians) also is related to Ge’ez, but it is more distantly related (Amharic is a southern Ethiopian language).
Which means Tigrinya shares a lot of words with other Ge’ez languages opposed to something like Hebrew. Tigrinya, Amharic, and Tigre (another Eritrean language); all relate to each other as French, Spanish, and Italian would. As for indepth info about Arabic and Hebrew, I have no idea about them (sorry).
Whoops. Yes, I meant to say “6th millenium BC”. Thanks-
They are not close enough to be understood. I’ve never studied Arabic, but I do enjoy Hebrew etymology and I can tell where some Arabic and Hebrew words have the same root, but by no means could I even make or understand a sentence in Arabic.
:smack: I stand corrected, it is exactly as you say ruadh.
I have now taken a toddle out on the net and seen that at least in written terms, I could definatly follow along with Scottish Gaelic, at least in the same respect as I can (as a Swedish speaker) with Danish and Norweigen. From the little I have seen of Manx I am not entirely sure I would have been able to understand, but that could also be to do with the typically crap level of my Irish. Thanks for the heads up ruadh! 
I’ve studied both. To a comparative linguist, the close resemblances between Arabic and Hebrew are many and obvious. The conjugation of the verb, the VSO syntax (something they share with Celtic), and the hundreds of three-letter roots they share in common. Often the shared root has the same meaning, but just as often it has a different (though possibly related) meaning.
None of this adds up to mutual intelligibility, however. There are many sound changes between the two. Hebrew sh corresponds to either th or s in Arabic. The sound correspondences are very regular, but you would have to know some comparative linguistics to see how strong the resemblances are. It’s like English and Dutch, a good analogy, or English and German.
Tigrinya is another Semitic language, so while there are no doubt Arabic loanwords in Tigrinya, there are also cognates that have always been shared by both languages from their earliest origins. Ge‘ez, Amharic, and Tigrinya belong to the branch of South Semitic, not as closely related as the West Semitic Hebrew and Arabic are to each other
In fact, there is a story told about an Arab terrorist group that were holding a group of Israeli schoolchildren hostage, demanding the release of prisoners. The negotiator spoke to them in Arabic, but occasionally tossed in a Hebrew phrase that the children could understand, to comfort them. Meanwhile, the Israeli SWAT team got in position, and the negotiator, in the midst of his Arabic, added in Hebrew “Quickly get down on the floor!” The children hit the floor, the SWAT team got the terrorists. One child had a minor wound in the leg.
Of course, that was back in the balmy days when terrorists had negotiable goals.
But the point is that Hebrew and Arabic are not mutually understandable, other than picking out a word here and there.
The Finno-Ugric language family is what you’re looking for, I believe. But what is “Koran” (aside from the Holy book, that is)? I’ve never heard of it.
The OP also asked about Amharic. Assuming that that’s an alternate name for Aramaic, that language is very close to Hebrew, and is in fact the language that most of the Talmud is in. It’s almost certain that Hebrew speakers would be able to get the gist of an Aramaic statement.