radiation - Why were the fathers of quantum mechanics so sure radioactive decay was indeterministic?

The classic example of an indeterministic system is a radioactive isotope, e.g. the one that kills Schrödinger's cat.

I get there are arguments against hidden variables in quantum mechanics, but how could they be so sure, back in the twenties, that the strong nuclear forces involved in radioactivity were not governed by hidden variables rather than true randomness?

Einstein was very unhappy about the indeterminism of quantum mechanics regarding even well understood effects like Young's slit experiments, but it seems kind of ideological and brash on behalf of Heisenberg & Co to extend the indeterminism over to phenomena they hadn't even begun to understand, like alpha decay.

Is there a reason for this early self-assuredness in postulating indeterminsm?

Answer

Schrödinger came up with the cat in 1935, which was relatively late in the development of quantum mechanics.

Back in the 1920's there had been a lot more uncertainty. The Copenhagen school had wanted to quantize the atom while leaving the electromagnetic field classical, as formalized in the Bohr-Kramers-Slater (BKS) theory. De Broglie's 1924 thesis included a hypothesis that there were hidden variables involved in the electron. In the 20's virtually nothing was known about the nucleus; the neutron had been theorized but not experimentally confirmed.

But we're talking about 1935. This was after the uncertainty principle, after Bothe-Geiger, after the discovery of the neutron, and after the EPR paper. (Schrödinger proposed the cat in a letter discussing the interpretation of EPR.) By this time it had long ago been appreciated that if you tried to quantize one field but not another (as in BKS), you had to pay a high price (conservation of energy-momentum only on a statistical basis), and experiments had falsified such a mixed picture for electrons interacting with light. It would have been very unnatural to quantize electrons and light, but not neutrons and protons. Neutrons and protons were material particles and therefore in the same conceptual category as electrons -- which had been the first particles to be quantized. Ivanenko had already proposed a nuclear shell model in 1932.