Inspired by this question:
In my answer, I had stated that Patronuses appear to not be physical beings because they have no effect on anything other than Dementors.
- They don't cause leaves to stir in the movies.
- They passed straight through the tent.
- They don't appear to be bound by normal laws pf physics and can travel great distances almost immediately (Shacklebolt was presumably in London when his Patronus issued the warning).
- They can apparently walk across water without sinking (as they did in The Prisoner of Azkaban).
- They can jump, walk, and float in mid-air (as seen in the Room of Requirement in the movie version of The Order of the Phoenix).
- They make no footprints in lightly powdered snow (The Deathly Hallows Chapter 19) and make no sound of crunching snow as Harry does (further down the same page in Chapter 19).
I stated incorrectly that the only place in canon where there is an exception is with Dementors, where the Patronus physically fends them off with its horns. A Dementor is clearly a physical being, if it prised Dudley's mouth open and (in the movie anyway) physically slid open the train door.)
Based on comments and further thought, I can think of at least two other examples when a Patronus interacts physically with a physical being.
- As pointed out by Slytherincess, a Patronus can fend off a Lethifold (Living Shroud).
- When training, the Patonus took form against a Boggart, which had taken the form of a Dementor.
All three of these things have, AFAIK, only three things in common (but I may be off).
- They are evil
- They are magical
- They are, indeed, physical beings.
So the question is, why can a Patronus interact with all three of these, but seems to have no physical effect on everyday things?