> “the universe is the extension of the self” is precisely the idea we are talking about.

Idealists can take several possible approaches to the issue of how many people/minds exist:

1) Solipsism: only I exist, and everyone else is a figment of my imagination

2) Many minds: only minds ultimately exist, but many distinct minds exist (George Berkeley, John McTaggart)

3) Open individualism: I exist and everyone else exists too, but we are all ultimately the same person, and the idea that we are different people is an illusion (not necessarily an idealist view, but one open to an idealist to adopt; most famous notable proponent is Daniel Kolak; but Kolak in the introduction of his book I Am You extensively quotes the physicists Freeman Dyson and Erwin Schrödinger as expressing the same view)

4) Pan(en)theism: only one mind/person ultimately exists, but we are somehow "sub-minds"/"sub-persons" of that ultimate person. One might call that single ultimate person "God", albeit it is defining the term "God" in a very different way than classical Western theism does. Or, maybe we could call it the "Universe", or borrow Plato's term "the World Soul". (Maybe there is not much difference between (3) and (4), but (4) would view the distinction between different "sub-minds"/"sub-persons" as more "real" than (3) does.)

5) Panpsychism: everything in the universe (even individual atoms) is conscious, and hence has a distinct mind. This in a sense is a variant of (2), but proposes far more minds than Berkeley or McTaggart would ever have admitted. Not all panpsychists are idealists, but you can certainly be an idealist panpsychist

Critics of idealism tend to focus on (1), but in practice (1) has never had any serious proponents. All serious idealists have espoused (2)-(5) (or maybe some other variation I've missed)

See also the philosopher David Chalmers' paper in which he proposes his own taxonomy of idealisms, different from mine: https://philpapers.org/archive/CHAIAT-11.pdf

As an idealist, my starting position is (2), although I have some sympathy for (4).

> And yes you can derive new “physics” with this idea alone.

I don't know exactly what you mean, but I'm not a fan of that kind of talk.

We have to distinguish between physics the natural science, and the philosophical discipline of the philosophy of physics, which is a sub-discipline of the philosophy of science

The idealism debate fundamentally belongs to metaphysics (although contemporary presentations often focus on it through the lens of philosophy of mind instead), but it has obvious consequences for other philosophical disciplines, including epistemology, philosophy of mind, philosophy of science, and indeed philosophy of physics

But while adopting idealism must lead us to a different philosophy of physics, the actual content of physics the natural science is unchanged. Physics the natural science is ultimately just a bunch of mathematical tools for predicting future observations. Those tools, and how you use them, are exactly the same whether you are a materialist, an idealist, a dualist, or none of the above. The only difference is your answer to the philosophical debates about what those tools ultimately are, or what they ultimately mean.