What is randomness?

I want a good physical understanding of randomness. Is there something really random?

Classically when we say the event is random. We usually do so because we don't know the exact information. Are we known that there was a reason that this particular outcome occurred. But is there such analogue in Quantum Mechanics.

The philosophical dogma that I face is that symmetry demands that any outcome that occurred must have a reason that why this particular outcome occurred, while other was equally likely. If both Heads and Tails equally likely then there must be a reason why tail came in a particular outcome, while heads was equally likely. It seems to me break down of symmetry so the concept of parallel universe must exist.

This is the strangest conclusion from Quantum Mechanics in my opinion: causality fails. Causality, just as a reminder, is the principle which states that every effect has one and only one cause, definable by the effect.

In Quantum Mechanics we have two types of particle evolution: a dynamical and a instantaneous. The dynamical one tells us that as time goes by, the particle evolves obeying the Schrödinger’s equation, that is, in a causal and continuous fashion.

But whenever a particle interacts with some other particle it's properties change instantly, in a random manner. This is the moment where we loose causality. So Quantum Mechanics isn't totally random. There's some prediction you can make, but after that it is impossible to say anything about the particle.

The result that which sides come up can be calculated in principle in classical mechanics. It’s only our inadequate knowledge of the system that we use probability to explain the outcome.

I think it is the causality that I have taken for granted. I so far believe that there must be a reason that this happened this way.

Randomness is like asking to define time. This is what physics is trying to do but we may never have a final answer because the subject is infinitely rich and Physics, and science in general, actually spends more time defining what a thing is not than what a thing is.

Classically when we say the event is random we usually do so because we don't know the exact information. Be we know that there was a reason that this particular outcome occurred.