So,
You have a cat in a box, with a 50% chance that said cat is alive or dead.

In classical physics (and, indeed, common sense), the cat is either alive or dead.

In quantum physics, because you cannot know whether the cat is alive or dead, the cat exists in a "superposition of states" - that is to say, it is simultaneously alive and dead.

Where quantum and classical physics agree is in saying that you cannot know the state of the cat until you look inside the box. Where quantum differs is that it recognises the effect of the observer on the system, i.e., the effect of you opening the box to look at the cat. In doing so, you are interfering with the system. But until you interfere with the system, its state is not determined.

So, this leads us back to your question about alternative dimensions. What happens at the instant you open the box? The two most popular / widely accepted suggestions are:
(i) aka 'The Copenhagen interpretation' - the cat exists as a fuzzy cloud of quantum uncertainty, in which it is simultaneously alive and dead, until the box is opened, and these two states 'collapse' into one state, either a live or dead cat.

(ii) aka the 'Many Worlds interpretation' - the cat again exists in its superposition of live and dead states, but when the box is opened, the universe diverges into two separate paths: a universe in which the cat is alive, and a universe in which it is dead.

Does that help or, indeed, make any sense?

I think it was Niels Bohr who aptly stated that "Anyone who is not shocked by quantum mechanics has not understood it."
(LongJohnBaldry, Mon 18 Aug 2008, 11:03, Reply)