In this simple circuit, the Zener diode regulates a constant voltage across the load. I don't understand how the Zener diode is affecting the voltage drop across the resistor, making it so that the voltage is maintained? What are the internal workings behind this phenomena. I want to intuitively...

I have been working with the concept of matter (or any 3D field) power spectrum for a while, but still struggle to understand it precisely. The power spectrum $P(k)$ is defined by the equation $$ \left\langle \delta (\mathbf k)\delta^* (\mathbf k') \right\rangle =(2\pi )^3 \delta_\text D(\mathbf ...

After reading that the Schwarzschild solution is static, i.e., it is independent of time, I am wondering in what frame. Changing frames means mixing space and time through coordinate transformations. So in which frame the Schwarzschild solution is static?