system
Given the following post, respond as if you're commenting on Reddit:
human
Is quantum mechanics truly random or merely unpredictable?

because theres a huge difference

Edit: What im asking is whether we actually KNOW if the universe is not deterministic or whether it might be but we just cant tell. What brought this idea to mind was thinking about the idea of parallel universes and whether randomness is necessary for that to be possible.
gpt
The idea you're looking for is that of *hidden variables*. Einstein disliked the idea that quantum mechanics was non-deterministic so much that he proposed there exists a theory deeper than quantum mechanic that is deterministic but reproduces the seemingly random results of quantum mechanics. These theories also fixed what Einstein saw as a problem with non-locality (faster than light communication) in quantum mechanics.

A decade or two after Einstein died a John Bell showed by contradiction that a hidden variable theory could never produce the results we see in quantum mechanics experiments.  Using the following assumptions,
    1. Experimenters have free will,
    2. Logic is valid way to reason,
    3. Hidden variables exist,
    4. Hidden variables are local,
he derived a set of inequalities. He then found an experiment that violated these inequalities. This means at least one of the assumptions listed above is wrong. 

The commonly accepted one is that 3. is faulty and that quantum mechanics is actually random. The idea that hidden variables, an idea specifically introduced to ensure quantum mechanics was local, were in fact non-local would probably upset Einstein much more than the idea of reality not being deterministic.

edit: added the assumption that free will exists, as ichthyic points out as being necessary.