What do you call an event that happens without a cause?

I used to think those are random events but someone over at physics.stackexchange.com insists that randomness means something else so I am at a loss here. Can someone help me out?

What do you call an event that happens without a cause?

I would say a spontaneous event.

It seems that the concept without cause is not a true criterion. Assuming we are not talking about supernatural events, there are physical, biological, social, political things that happened that contributed to the ultimate event.

The real sense behind the phrase is no known cause or no understood cause. I would therefore propose inexplicable

not capable of explanation; unexplainable

Well, there is always the word causeless: so you could simply say a causeless event. A rough synonym is fortuitous.

You could potentially say a random event or chance event.

The commentators on the physics forum are correct: as a scientific term, that isn't the technical meaning of random. But unless you're intending to use it as a technical term, so what? -- there are plenty of words that have a different everyday meaning to their technical meaning.

From OED...

acausal - not causal; independent of or not involving the relationship of cause and effect.

Here's an NGram showing how usage has increased during the Quantum era it fits so well with.

How about unprecipitated or unprompted?