I first noticed in this answer that there is something sneaky going on with the word photon: its ‹t› is the stressed allophone of /t/, a fully aspirated [tʰ]. It does not reduce to [t] or [ɾ] the way it does in words like voting. Other words with the same issue include proton and lepton.
The only way I can explain this would be if the second syllable in such words bears secondary stress, so [ˈfoʊˌtʰɑn].
Even so, the question remains: why does this happen? Is it because these are all “new” words? Or does Greek somehow enter into it?
I notice other new scientific terms have the related issue of an unreduced vowel in the syllable without primary stress, such as in hadron, quasar, protein, baryon, genome.
So the ‹t› in proton and photon works more like it does in Motown or cow town, but those are at morphemic boundaries. The only “old” word I could think of where it might work the same way (remain aspirated) might be in canton. But there the vowel in the unstressed syllable does seem a bit reduced, just not all the way.
What is really going on here?
Edit: The mid-word aspirated ‹t› also occurs in futon and wonton (as in wonton soup), but not in Briton, Milton, tartan, titan, wanton (as in a wanton woman).
Best Answer
As Bruce Hayes (UCLA) puts it,
Thus,
proton [ˈpʰɹouˌtʰɑn] in AmE.
There's no secondary stress in "voting" in AmE.
See a note on aspiration in the Longman Pronunciation Dictionary (3rd ed., Wells 2008: 45) - those rules are rather comprehensive; at least, that should be enough for someone who's not a professional phonetician.
The most important thing to remember is that aspiration is not binary, aspirated or unaspirated, but rather gradual. See fig. 6.6 in Ladefoged 2006, A Course in English Phonetics.