Is there an idiom for when two things don't interact with each other? For example, two bacteria next to each other don't interact with each other and behaves like they are not near each other. It doesn't have to be a person or anything and I mean it in the most general way possible. I couldn't think of any idiom that refers to this situation or fact.

  • I had thought the idiom "like ships that pass in the night" means "take no notice of each other" but the Cambridge Dictionary disagrees, saying it means "rarely". Dictionary.com has yet another meaning: like "people who meet for a brief but intense moment". – Weather Vane Mar 31 '20 at 18:48
  • Give us an example sentence in which you'd use this idiom, and how you'd phrase the sentence yourself (without the idiom). – user3395 Apr 4 '20 at 14:48
  • like oil and water
    (idiomatic) Two things which are incapable of mixing or coexisting harmoniously with each other.

I don't know an idiom to describe that situation, but there is a word. You could say the two things are "oblivious of" or "oblivious to" each other.

Merriam-Webster "oblivious"
1 : lacking remembrance, memory, or mindful attention
2 : lacking active conscious knowledge or awareness —usually used with of or to

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