What are the rules governing how a Patronus can interact with the physical world?

Question

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?

Answer 1

I would pose that the mechanism of action of Patronuses is mental/psychic more than physical.

• Dementors

Since Patronus is basically a manifestation of the positive feelings/emotions of a caster, and Dementors are the magical equivalent of Depression, it repels the Dementors (for lack of a better expression, tastes/smells bad).

• As pointed out by Slytherincess, a Patronus can fend off a Lethifold (Living Shroud).

This one is harder to explain, mainly since we don't know much about Lethifold. But it's not implausible that they are also not repelled psychically.

Please note that it seems a bit contradictory to assume that Patronus is what repelled Lethifold in the first place - there was only ONE report, and it explicitly states that in all other cases there were fatalities since the victim was asleep. So it could very well be argued that Lethifold was repelled by the fact that a wizard was awake and casting ANY magic (and thus went after safer pray) as opposed to the specific Patronus charm.

Another - IMHO less likely - possibility is that the Living Shroud has the same distate for positive emotions as Dementor.

• When training, the Patonus took form against a Boggart, which had taken the form of a Dementor.

Same here. Boggart reacts to caster's positive emotions (laughter), which Patronus would help rise from a person who's afraid of Dementors. No physical ineeraction involved.

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All three of these things have, AFAIK, only three things in common.
- They are evil
- They are magical
- They are, indeed, physical beings.

Not quite.

• Dementor is - while a physical being - a lot more of a mental force (again, JKR meant them to be an equivalent of depression). And Bogart doesn't ACT as a physical being, in either shape shifting towards the fearsom image, OR shape shifting away from it after Riddiculus.

• Also, Lethifold isn't strictly speaking "evil" - no more than you when you eat a cow or a chicken. They are predators.