First, she explains what we understand a Theory of Everything to be:
Now, three of the fundamental interactions, the electromagnetic and the strong and weak nuclear force, are of the same type. They are collected in what is known as the standard model of particle physics. The three forces in the standard model are described by quantum field theories which means, in a nutshell, that all particles obey the principles of quantum mechanics, like the uncertainty principle, and they can be entangled and so on. Gravity, however, is described by Einstein’s theory of General Relativity and does not know anything about quantum mechanics, so it stands apart from the other three forces. That’s a problem because we know that all the quantum particles in the standard model have a gravitational pull. But we do not know how this works. We just do not have a theory to describe how elementary particles gravitate. For this, we would need a theory for the quantum behavior of gravity, a theory of “quantum gravity,” as it’s called.
We need a theory of quantum gravity because general relativity and the standard model are mathematically incompatible.Sabine Hossenfelder, “Do we need a Theory of Everything?” at BackRe(Action)
But she goes on to say something surprising:
So this whole idea of a theory of everything is based on an unscientific premise. Some people would like the laws of nature to be pretty in a very specific way. They want it to be simple, they want it to be symmetric, they want it to be natural, and here I have to warn you that “natural” is a technical term. So they have an idea of what they want to be true. Then they stumble over some piece of mathematics that strikes them as particularly pretty and they become convinced that certainly it must play a role for the laws of nature. In brief, they invent a theory for what they think the universe *should be like.
This is simply not a good strategy to develop scientific theories, and no, it is most certainly not standard methodology.Sabine Hossenfelder, “Do we need a Theory of Everything?” at BackRe(Action)
It gets more interesting from there.