In quantum mechanics, the concept of "locality" and "realism" are often discussed in the context of the EPR paradox and Bell's theorem. In a "locally real" theory, the properties of particles are well-defined independently of measurement (realism), and no influences can propagate faster than the speed of light (locality).

1. Realism: In a "realistic" theory, the properties of a system exist independently of observation. For example, if you have an electron, the idea is that it has a definite spin direction whether or not you measure it.

2. Locality: The principle of "locality" holds that physical processes occurring at one place do not depend on the properties of objects at another place that is spacelike separated, which would require information or influence to travel faster than the speed of light.

However, quantum mechanics challenges these intuitive notions. Experiments with entangled particles suggest that the properties of one particle can instantaneously affect the properties of another distant particle, seemingly violating locality. Meanwhile, the superposition principle suggests that particles don't have definite properties until measured, challenging realism.

In my opinion, the breakdown of "local realism" is one of the most unsettling and fascinating aspects of quantum mechanics. It forces us to reconsider our intuitive understanding of reality and has implications for fields like quantum computing and quantum cryptography.

— ChatGPT4

As physicists, I can confirm, this is not bad explanation.

Ok. I'm gonna give an example that will be slightly wrong if we nitpick, but it will give you an idea.

Lets take the old philosophical idea "if a tree falls in a forrest and theres nobody to hear it, does it make a sound?" and modify it for this example.

"If you are not there to observe will the tree in the forest fall?"

If you are not there to observe the tree will be in all possible states. Two notable states being "it has fallen" and "it is still standing" that exist simultaneously.

If you go in to the forest and observe, one of the states will randomly become your reality according to certain probability amplitude.

You don't necessarily have to go in to the forest to observe the tree. You can send your buddy who will then tell you the state.

As your friend is returning back from his observation, there's actually two friends walking back to you. He is "entangled" with the tree. When he opens his mouth, one of them is randomly selected as your reality.

Entity checking the state of the tree does not have to be a living consciousness. It can be a particle, that interacts with a particle, that interacts with a particle, that interacts with you. You are not conscious of the trees state, but the information is delivered to you and for you there is now only one state for the tree.

So quantium level information is constantly delivered to you and your reality is weaving itself around you through this "decoherence"

now you see how my first two examples were false. Quantium decoherence is so much faster than you or your buddy, that you can only catch this mechanism at work on quantium level.

...and "local" basicly just means that the experiments result is true, as long as nothing can transfer information faster than light. So far nothing has.

Marcus Chown's book is a good primer on quantum theory, but it will make your head spin.

(you'll understand the dad joke after you've read the book)

Well, let’s start with definition what “exists” means…