I'm late getting back to this interesting thread with so many things I wish to respond to, but I'll jump in here and backtrack to some other points later from earlier:
Let me just clarify what I mean here: consider this feeling of Oneness that people achieve when they meditate. It seems to be pretty universal--lots of people get the same feeling. This might be a worthwhile practice, in fact it might expand your mind, give you peace, make you healthy, help you "think outside the box" etc. But the fact is that lots of people get the same feeling when they take LSD--they feel like they can fly. Does this tell us something about the entire universe, or does it tell us something about the human brain? Lots of photographs show similar artifacts--again does this have cosmic significance, or is it something interesting and peculiar about cameras? (These are rhetorical questions, please bear with me until the end.)
Yes. What can be said is this. The experience of the Absolute (I'm not going to call it Oneness here, which I'll explain later) is the highest awareness of human experience. It has nothing to do with physics. It has to do with Awareness through human experience, in this body, with this brain. It is the highest state of awareness known in our experience.
That said however, ALL experience, even scientific inquiry is experienced through the brain and its mechanisms. So the point is, get this, how you experience the world through using the tools of science is limited because you are not really actually opening up the mind itself to its highest level of awareness. That highest level of awareness, is the mystical experience.
Through that type of awareness comes an ability to surpass reason. And as such it can illuminate reason. It does not REPLACE reason, but rather offers it greater perception than what reason alone can penetrate into.
Now to the rest....
Suppose just hypothetically that all is not "One", whatever that means. Suppose that actually, we discover that the Milky Way galaxy is just completely, 100% different and separate from Andromeda. In fact Andromeda isn't even energy, it's something totally different. If the idea that "all is One" is meaningful, we must be able to entertain, at least, this hypothetical situation where "all is NOT One".
All of this is dealing with material existence, physics, as if that defines the world, or the universe. In reality (pun intended), the Universe is more than just matter and physics. It is also the world lived in our heads. It is conceptual reality, it is lived experience, it is knowledge through relationships. It is the world of sensed being. It is mind. It is also spirit.
Now one could try to argue as a reductionist that mind is just brain, and who cares. The reality of it is, it is a world of its own that you navigate and live within, just the same way you do the physical world. It cannot be reduced to brain, as much as you cannot tell your wife you love her by displaying your EEG to her! That is as much apart of the Universe as matter is. The Greeks in speaking of the Cosmos, meant just that, not what you see through the Hubble telescope. And this is just the tip of the iceberg!
So, when someone says everything is One, it means we see the connecting layers of all that is arising as One, and we experience the "Oneness" of this. But it is truly, really, best described instead as Unity. Unity. Diversity and Unity.
"Oneness" as you are describing it rather is a type of Monism. Monism is a scientific way of saying everything is of the same substance. That is not the mystical realization of Oneness. In the highest experiences of consciousness, the universe is seen as Nondual. Nondual does not mean Monism. Nonduality is, "Not one, not two". This is impossible to grasp rationally as it is paradoxical because all language is dualistic in nature, with subject/object separation. Monism, is actually a type of duality, believe it or not. It is still looking at the world as "That" and describing it. Nonduality rather includes monism and dualism. It is an experienced reality. (You can read more about the difference here:
http://www.yogicbuddhism.org/content/not-duality-not-non-duality ).
And that experience of the Universe, the nondual, is experienced as the highest awareness of "what is". It includes everything. There, is your "theory of everything", not just simply, lowest level
prana, or the material realm. All, means All. Our Being. Not just the bits that make up matter.
Is Reality beyond what even the Nondual experience of mystics sees? Oh hell yes!!
I'll leave it at that for now.