Three different quantum field theories deal with three of the four fundamental forces by which matter interacts:

• Electromagnetism: It explains how atoms hold together.
• The strong nuclear force: It describes the stability of the nucleus at the centre of the atom.
• The weak nuclear force: It explains why some atoms undergo radioactive decay.

The three theories are known as the "standard model" of particle physics.

Understanding how electrons move or not through a solid material and so make a material a metal, an insulator or a semiconductor, for example, requires the development of "effective field theories" that don't go into all the details.

But it is difficult to construct such a theory, and the reason why many important questions in solid-state physics remain unresolved.

Quantum particles can act as particles located in a single place, or they can act as waves that are distributed all over space or in several places at once.

How they appear seems to depend on how we choose to measure them. Before we measure, they seem to have no apparent properties at all. This fuzziness leads to paradoxes such as Schrödinger’s cat (a cat is left dead and alive at the same time). Quantum particles also seem to affect one another at the same time even if they are far away from each other, known as entanglement.

• A fourth fundamental force of nature that quantum theory cannot explain, namely gravity, doesn't even involve particles. None of the decades of effort can bring gravity under the quantum umbrella to explain all of fundamental physics within one "theory of everything."
• Then it appears that over 95 per cent of the universe consists of dark matter and dark energy. There is currently no explanation within the standard model.