Oppenheimer. The conversation you can't leave for later.

My son asked me whether nuclear weapons still exist. We were watching Oppenheimer in sections — it's three hours long and not entirely suitable for a nine-year-old, but the question of how to handle it seemed less important than the question of what to do when he's already encountered it through other means and the conversation is already happening.

I said: yes, nuclear weapons still exist. He said: how many? I said: approximately thirteen thousand, across nine countries. He was very quiet for a moment. Then: 'Why?'

Why is a harder question than it looks. The factual answer — deterrence theory, mutually assured destruction, the logic that has kept large powers from using them since 1945 — is accurate but unsatisfying. It describes the situation without explaining the morality of it. And my son was not asking about deterrence theory. He was asking about why human beings would make and keep thirteen thousand of something capable of ending human civilisation.

I said: I'm not sure I have a complete answer to that question. He said: but you're a grown-up. I said: grown-ups have been asking this question since 1945 and haven't agreed on the answer.

He sat with this. Then: 'Did Oppenheimer ask it?' Yes. He asked it for the rest of his life. He didn't like the answer he kept getting.

Three weeks ago I would not have had a way into this conversation. Then a film opened that gave us a shared entry point, and my nine-year-old is now engaged with one of the most genuinely complex ethical questions of the twentieth century. This is not nothing.

So they went anyway. I said: so they went anyway. And we were sitting with one of the most genuinely difficult ethical questions of the twentieth century, at dinner on a Tuesday evening, because a film had given us a way in.