Dear Amanda,
Wow! Your essay deserves the highest possible note -- and you got a 10 from me -- because it brilliantly explores (In the right direction!) which of our basic physical assumptions are wrong. Many thanks to Ben Dribus, who told me yesterday evening about your excellent essay.
So I would not argue with you when you "... argue that the basic assumption of a single universe shared by multiple observers is wrong." On the contrary, I fully agree with your perspective, and in my essay From Minkowski's Diagram to the Multispace Model of the Universe I make the first step in the same direction by explaining "why" and most important "how" it happens that this universe is a multispace.
Yes, as you say, "... each observer lives in their own unique universe", but I realized that telling this fact like that was quite scary for some people, and that they had a tendency to disconnect themselves from my subject. Didn't you notice the same thing?
So in my essay, I preferred to use 'spacetime reference frames' instead of 'universes' and 'multispace' instead of 'multiverse,' nicely reminding the audience that this confirms Minkowski's 1908 declaration that the world is composed of an infinite number of spaces, as well as his multispace relativity.
You say "that we must never speak of more than one observer or universe at a time."
On one side, I completely agree. Each observer exists in his own universe. It is not for nothing that we instinctively feel we live in a 'bubble of perception.' And it's also true that we perceive the surrounding reality through our sense organs, filters that project this reality into our 'bubble universe,' our conscious self. So yes, each of us is 'one observer in his own universe.'
Yet, on the other side, it's not so simple. When do we start to talk about this 'own unique universe?' Is it the space surrounding our body? If you and me are ten feet away are we still in the same universe? What if we hug each other? If I look at my hand, is it inside or outside my universe?
Even more, we agree that in a multispace/multiverse world, "... each observer lives in their own unique universe." But each observer -- in fact each of us -- is made out of various organs, cells, and so on until these nagging pieces of matter called elementary particles. Do we have the right to say that quantum spaces are also little universes? After all, besides the size, the only difference between me looking at you through our universes, and seeing you, and looking at an elementary particle through its universe -- its quantum space -- is that I don't see this elementary particle. But there is a simple and very physical explanation for that, which I also advance in my essay From Minkowski's Diagram to the Multispace Model of the Universe.
In fact, what I want to say is that this essay is just the tip of the iceberg. I spent several years to prove the Multispace Model beyond any possible doubt by finding convincing supportive evidence not just in physics, but also in cosmology and in Earth sciences.
I hope you have a moment to check it out and share your thoughts with me.
Best of luck in the contest.
Eugeniu Alexandrescu