# Two excellent physics explanations.

I have just come across to these two incredibly lucid accounts of two key features of modern physics. Worth sharing

Semiconductors –

Silicon is a poor conductor of electricity because all of its four outer electrons are bound up in the chemical bonds holding the crystal together. However, by adding a tiny amount of phosphorous, which has five outer electrons, you effectively add a free electron to the crystal and make it conduct moderately well. Similarly, you can add boron, which has only three outer electrons, and effectively do the same thing, only now the conducting charge is called an electron hole.

The magic comes when you put a phosphorous silicon layer next to a boron silicon layer: the holes and the electrons cancel each other out at the junction but create an electric field that means that electrons only like to flow in one direction across the junction. This is called a diode.

There are many flavours of diodes, each having a different junction architecture. Light-emitting diodes (LEDs) emit light when electrons flow across the junction but the opposite effect also works: light hitting the diode creates an electric current, and this is how a solar cell works.

The perovskite lightbulb moment for solar power (The Guardian)

### Field Theory in Physics – Adam B. Barrett

Contemporary physics postulates that “fields” are the fundamental physical ingredients of the universe, with the more familiar quantum particles arising as the result of microscopic fluctuations propagating across fields, see e.g., Oerter (2006) for a lay person’s account, or Coughlan et al. (2006) for an introduction for scientists. In theoretical terms, a field is an abstract mathematical entity, which assigns a mathematical object (e.g., scalar, vector) to every point in space and time. (Formally a field is a mapping F from the set S of points in spacetime to a scalar or vector field X, F: S ? X.) So, in the simplest case, the field has a number associated with it at all points in space. At a very microscopic scale, ripples, i.e., small perturbations, move through this field of numbers, and obey the laws of quantum mechanics. These ripples correspond to the particles that we are composed of, and there is precisely one fundamental field for each species of fundamental particle. At the more macroscopic level, gradients in field values across space give rise to forces acting on particles. The Earth’s gravitational field, or the electromagnetic field around a statically charged object, are examples of this, and the classical (as opposed to quantum) description is a good approximation at this spatial scale. However, both levels of description can be considered equally fundamental if the field is fundamental, i.e., not some combination of other simpler fields. Note that the electromagnetic and gravitational fields are both examples of fundamental fields, with the corresponding fundamental particles being the photon and the graviton. Particles are divided up into matter particles and force-carrying particles, but all types of particle have associated fields; all the forces of nature can be described by field theories which model interactions, i.e., exchanges of energy, between fields.

An integration of integrated information theory with fundamental physics – (Frontiers in Consciousness research) 