In theory you can, but in practise it won't happen, because this would prevent central banks from inflating the money supply (no matter which country), which they need to inflate to cover the costs of running the country and stimulate economic growth, that's in their own words. We won't discuss now how exactly they succeed to stimulate economic growth, suffice it to say that they need to inflate the money supply, which they can't do if they peg the currency to something of finite quantity. And they won't do it. Hence, there is no backing, except faith and law.