Tangibility
In computer science random numbers are generated using data that's likely to be different each time it's ran, such as the current timestamp or values from memory. Whatever value it pulls would only ever be able to generate one specific number. The only way to apply any more randomtness to said number is to apply more calculations to it based on changing data. It can get more complex or harder to reverse engineer, but at it's core it's impossible to generate a truly random number.
By aiming a camera sensor, or other sensor, at a radioactive material, it's possible to detect stray alpha particles it emits and use their X,Y data as input for generating a random number. This is introducing a natural aspect into the equation
Half-lives are measurable lengths. Each isotope has a verified range of time that it will most likely exist for until half of it decays. This is a set unchageable value. The randomness that's seen in nature is seen as random because it reflects the same randomness that we are able to produce. If I ask you to think of a random number between one and ten your answer could depend on any number of things.
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