Can ghosts affect the physical world?

Question

In the movie production and book Harry Potter and the Sorcerers Stone, we see ghosts floating through walls, without affecting them (i.e. smashing through them and leaving rubble behind).

In the movie production Harry Potter and the Chamber of Secrets, we see Moaning Myrtle surprise Ron Weasley as he asks why they're brewing Polyjuice Potion in the girls lavatory. After revealing herself, Myrtle does a demented loop-de-loop around the lavatory and then dive into the toilet.

Yet later, both in the book version and in the movie version of The Goblet of Fire, we see Myrtle soundlessly enter the prefects bathtub without displacing water.

Therefore, can ghosts (not poltegeists, who as we know can) interact with the physical world, and have an effect upon their surroundings?

Answer 1

The answer is a very definitive yes, they can indeed interact with physical matter but, as the Harry Potter Wikia notes, they can "pass through solid objects without damaging themselves or the material, but create disturbances in water, fire and air".

This description seems pretty consistent with both the books and the films.

Myrtle gave a tragic sob, rose up in the air, turned over and dived head first into the toilet, splashing water all over them and vanishing from sight; from the direction of her muffled sobs, she had come to rest somewhere in the U-bend.

_{^{Harry Potter and the Chamber of Secrets}}

Answer 2

Ghosts affect liquids and gasses, but not solids.

This is best shown by the Rowling's Pottermore writing on Ghosts.

Ghosts can pass through solid objects without causing damage to themselves or the material, but create disturbances in water, fire and air. The temperature drops in the immediate vicinity of a ghost, an effect intensified if many congregate in the same place. Their appearance can also turn flames blue. Should part or all of a ghost pass through a living creature, the latter will experience a freezing sensation as though they have been plunged into ice-cold water.

_{Pottermore - Ghosts}

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This is also collaborated by Rowling's previous writings:

Ghosts cannot affect solids

In the second book we see that Nearly Headless Nick is unable to move a cabinet by himself, and has to persuade Peeves to do it, showing that ghosts cannot move solid objects.

Nearly Headless Nick came gliding out of a classroom. Behind him, Harry could see the wreckage of a large black-and-gold cabinet that appeared to have been dropped from a great height.

“I persuaded Peeves to crash it right over Filch’s office,” said Nick eagerly. “Thought it might distract him —”

_{Harry Potter and the Chamber of Secrets - Chapter Eight}

Ghosts can affect liquids

Myrtle gave a tragic sob, rose up in the air, turned over, and dived headfirst into the toilet, splashing water all over them and vanishing from sight, although from the direction of her muffled sobs, she had come to rest somewhere in the U-bend.

_{Harry Potter and the Chamber of Secrets - Chapter Nine}

Ghosts can affect gasses

The second of Rowling's W.O.M.B.A.T. tests included a question asking to identify a false statement from a list of five statements.

Which of the following statements is FALSE?
☐ Ghosts can cause movement of both liquid and gas
☐ Freshwater merpeople are less warlike than salt [correct answer]
☐ The werewolf's snout is shorter than that of the true wolf
☐ There are no male Veela
☐ Hags have four toes on each foot

W.O.M.B.A.T. Grade Two - Part One: Magical Beings - Question 2

According to Roonwit's analysis of the scoring, full credit was given for the merpeople choice, and no credit for the ghosts choice. Thus we can conclude that the statement about ghosts was not a false statement (i.e. it's a true statement), and that "ghosts can cause movement of both liquid and gas".

Answer 3

I answered this here:

Sir Patrick: "So, Harry me lad, care for a little headless bowling?"
Harry: "I'm not sure. What is it?"
Sir Patrick: "Just take your head off... not yours, we'll find one for you to borrow... roll it down the alley, and knock over the pins."

Clearly, if Harry can bowl something to knock over pins, they aren't immaterial pins. And since ghosts can bowl them down, they are interacting with them.