(Pronunciations taken from Cambridge Dictionaries Online)
I guess the easiest explanation is that the vowels that are represented by the English e are different in the Greek words.
beta => βήτα
meta => μετα
Pronunciation of (originally) Greek words and their individual phonemes is, alas, not always straightforward, consistent or logical.Different words were borrowed at different times, and from different sources (straight from Greek, or through Latin, Italian, French, etc.).
In general, we should also make a distinction between words and prefixes from Greek, and the names of Greek letters. Especially in the case of the names of Greek letters, there are many differences in pronunciation, as you can see in the link that Josh61 mentioned in his comment. Why do we pronounce those names in so many ways? Well, it seems that people have been using Greek letters in science for a long time, and the first thing we learn when we learn Greek is reciting the alphabet. However, modern Greek pronounces those letters quite differently from how the ancient Greeks used to do it, and our understanding of how they were pronounced has evolved over the years.
At the same time, the names of the letters are used in everyday speech and they are used by people that do not study classical Greek. Those speakers may well pronounce the names of the letters based on the English spelling.
With words that were borrowed, pronunciation often has little to do with spelling (rather, spelling follows pronunciation!) and the (perceived) pronunciation of the borrowed word is often largely retained.
So when we look at the η, we notice that it usually gets rendered as an
i: in English (similar to modern Greek), for example:
Ἀκαδημία -> academia
ᾍδης -> Hades
When we look at the ε, it usually gets rendered as
μέλας -> (melas) -> melancholy
μέθοδος -> method
There are, however, exceptions in both cases, like ἐγώ -> ego.
So, in short, beta gets pronounced the way it does because it is not an exception to the general rule that an η becomes an
i: and meta gets pronounced because it also is no exception to the general rule that an ε gets pronounced as an
However, there are plenty of exception to those "rules", especially when it comes to the pronunciation of the names of Greek letters.
There is probably no real reason, but I think the following interesting extract can help understand:
- Both have come in to American English, or gotten POPULAR or WELL KNOWN in American English, at very different times. ... Remember that 500 and 600 years ago, education (European, but also British -- 'American' hadn't been invented yet :-) and scholarship was passed on by those who learned Latin, Hebrew, and ... Greek. (Not English.) I'm betting all those late-medieval 'scholars' had to be taught their 'alpha beta' before they even began with the language study, proper, of real Greek. A 'bey-ta' pronunciation may have stuck with us from the 'alpha beta' of those years? .. Well, maybe.
- 'Meta' (mu is the Greek alphabet letter for 'm', right?, not 'meta'...) got better known and more widespread in English words much later -- whether in metaphor or metabolism or other words -- compared to the 'alpha-beta' rote learning/chanting that was common 300-500 years before it. Different 'entry-times,' different pronunciations for what looks identical.... For whatever reason, meta is 'meh-ta' and not 'mey-ta'. (No, I don't know why the difference; but I suspect that's what happened, and in rough outline, how it might have happened...)
- second letter of the Greek alphabet, c. 1300, from Greek, from Hebrew/Phoenician beth (see alphabet); used to designate the second of many things. Beta radiation is from 1899 (Rutherford). Beta particle is attested from 1904.
Metaphor: ( probably one of the first meta- words that entered the English language.
- late 15c., from Middle French metaphore (Old French metafore, 13c.), and directly from Latin metaphora, from Greek metaphora "a transfer," especially of the sense of one word to a different word, literally "a carrying over," from metapherein "transfer, carry over; change, alter; to use a word in a strange sense," from meta- "over, across" (see meta-) + pherein "to carry, bear" (see infer).
I think the difference is basically how the language pronounces the "e".
(I pronounce those identically)
On wiktionary this is mentionned :
(US) enPR: bāʹtə, IPA(key):