Os fantasmas podem afetar o mundo físico?

A resposta é um sim muito definitivo, eles podem de fato interagir com a matéria física, mas, como a Harry Potter Wikia observa eles podem "passar por objetos sólidos sem danificar a si mesmos ou o material, mas criar distúrbios na água, fogo e ar".

Esta descrição parece bastante consistente com os livros e os filmes.

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}}

Fantasmas afetam líquidos e gases, mas não sólidos.

Isso é melhor mostrado pelo Pottermore de Rowling escrevendo sobre 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}

Isso também é colaborado pelos escritos anteriores de Rowling:

Os fantasmas não podem afetar os sólidos

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}

Fantasmas podem afetar líquidos

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}

Os fantasmas podem afetar os gases

O segundo do W.O.M.B.A.T. de Rowling Os testes incluíram uma pergunta pedindo para identificar uma declaração falsa de uma lista de cinco declarações.

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

De acordo com a análise de Roonwit da pontuação , todo o crédito foi dado pela escolha do seriado, e nenhum crédito pela escolha dos fantasmas. Assim, podemos concluir que a afirmação sobre fantasmas não foi uma afirmação falsa (ou seja, é uma afirmação verdadeira), e que "fantasmas podem causar movimento de líquido e gás".

Eu respondi aqui :

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."