Here's why:
* Authentic: Genuine, real, original, true to its nature.
* Mechanical: Relating to machines, automatic, lacking spontaneity or emotion.
Instead of a direct opposite, think of "mechanical" as a contrasting concept:
* Authentic emphasizes genuineness and originality.
* Mechanical implies artificiality or a lack of genuineness.
Some words that could be considered opposites of "authentic" in different contexts are:
* Fake
* Counterfeit
* Artificial
* Inauthentic
* Forged
* Copied
The best word to use depends on the specific context and what you want to emphasize.