Difference between either and ether [closed]

Even though they mean completely different things why do either and ether sound so similar to eachother?

Modern English has two phonemes that developed out of one in Middle English and never quite split completely. Therefore it comes as a surprise to most literate Modern English speakers when they find out that there are two different phonemes, pronounced differently in English, but both spelled TH.

One is voiced in Modern English, like the, this, other, either, mother, father, and fathom, and represented in the IPA by the letter "edh" (ð), which occurred in Old and Middle English texts, along with another character "thorn" (þ) that was used instead. These letters were in free variation, like different versions of handwritten letters today are. (In Old and Middle English, of course, all letters were handwritten.) Both of these letters still occur in Modern Icelandic, which is very archaic.

This voiced phoneme appears in all the Germanic words that occur in the grammar, and is therefore very common, because words like the, this, that, these, those, them, their, there, etc. occur so very frequently in English speech. Most of the words with /ð/ either don't have a meaning (like the), or have a meaning that's very difficult to state without giving examples, like other or either. Looking up these words in a dictionary is unsatisfying; they're part of the grammar, not the lexicon.

The other phoneme spelled TH in Modern English is voiceless, as in theater, thistle, ether, ethylene, moth, myth, sheath, and thicken. It's represented in IPA by the letter "theta" (θ), from the Greek alphabet, where it's pronounced the same way. Words with this sound tend to be borrowed from other languages (often Greek), and they have meanings and can be looked up in dictionaries with success.

This pair of English phonemes /ð/ and /θ/ developed when Modern English began to distinguish voiced fricatives (/v ð z ʒ/) from voiceless (/f θ s ʃ h/), maybe around 1300 in some dialects, later in others. There was a common rule of voicing that we still find in some words with irregular morphology, like

• leaf (sg) /lif/ versus leaves (pl) /livz/ [plural gets voiced final stem]
• sheath (n) /ʃiθ/ versus sheathe (v) /ʃið/ [verbalization voices final stem]
• breath (n) /brɛθ/ versus breathe (v) /brið/ [verbalization lengthens stem vowel and voices final]
• house (n) /haws/ versus house (v) /hawz/ [verbalization voices final stem]

But these have faded into minor irregularities as the voicing of fricatives became widespread in Modern English. And because the dental fricatives are so split into word categories, there are very very few minimal pairs for /ð/ and /θ/ and they are easy to ignore if you're not told anything about pronunciation. And native speakers educated in Anglophone school systems are not taught anything about English pronunciation, only about its spelling. So it's easy to miss something like this.

Linguists say that the contrast between these two phonemes has a low functional yield. That means you don't need to distinguish them very often. The only minimal pairs I know, none of them very good, are

• This'll and thistle , which is comparing a noun to a contracted phrase
• either and ether, which refers to only one pronunciation of either
• thy and thigh, which compares a modern word to an archaic one

That's why you didn't notice before, and that's why we can still get away with spelling them both the same.