I would not classify sunshine and air as "free". Sunshine requires the Sun to undergo continuous fusion. It is invaluable as opposed to "free". Air is invaluable asset too. Without both we would be dead. The price of both is literally price of my being alive every second of my existence.

Public domain on the other hand is legally only possible if/when copyright has expired. That means the owner has enjoyed proceeds from copyright protection for more than his own lifetime. That is fair. It is still not comparable.

EDIT: since you tacked on more like "wind, gravity, radioactivity" etc, I would still not classify them as "free". They are invaluable to very existence of life.

"Knowledge passed on" is also after someone (in ancestry) has paid for it through blood, sweat and tears. It isn't "free". "Public domain" is legally recognized form of "knowledge passed on".