So I was recently watching a science documentary (Brian Greene's The Fabric of the Cosmos) and they discussed what Casimir predicted (the Casimir Effect, see link).

So, the two plates get "pushed together" by the quantum fluctuation on the outside of the plates, as the amount of such fluctuation on the inside of the plates is much lower. That's the explanation they give.

But what they didn't discuss, is how the two plates being pushed together can't be explained by the gravity of the two plates pulling on each other.

To me, that seems just as likely an explanation for what happens.

What am I missing. How do we know it wasn't gravity but the quantum fluctuation?

Please help!