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I think the closest synonym for "toss-up" is going to be "even chance".
You could also say something like "complementary events of equal chance". This phrase gets a little closer to the point that it must be one event or the other, and that the probability of each is the same, but it may be a bit too pedantic compared to a simple term like "toss-up".
I'm not aware of a similar term that might be specific to quantum physics, but I'll keep looking and will post back here if I find anything.
Hi Ryan, maybe more context would help. It's for a novel and a quantum physicist character is referring to a fork in the road where either direction might lead to his destination. I'm looking for a mathematical term he might use to describe that, so it has to be conceptually accurate but also general enough that a layperson would be able to infer its meaning.
I was thinking about something like "an indeterminate" or "a free variable" but I don't know enough about mathematics to determine what would be an apt usage.