2.2 The Granular World

summary

Quantum mechanics established the discrete nature of light.

- Quantum mechanics began after observations of atomic emission spectra showed that atoms do not emit light at any arbitrary frequencies but rather at specific well-defined frequencies.
- This eventually led to the notion that light, in addition to being a wave, is also a particle, and that particles of light can only have energies that are discrete multiples of a fundamental value.
- In the case of electromagnetism, the continuous electromagnetic field is replaced with the photon.

Heisenberg's interpretation of quantum theory

- According to Heisenberg, we cannot assume that particles exist continuously between observations. Reality itself is discrete, not continuous.
- Just as there is a minimum fundamental value for the energy of photons, there too exists a minimum fundamental distance of space. Particles do not move smoothly through space but rather “jump” from one segment to the next.
- Furthermore, in standard quantum theory, there is no strong deterministic law that predicts when and where particles will next appear. Quantum mechanics only provides probabilities about particle locations and behavior.
- Finally, one way to interpret quantum theory is that reality is relational — particles only become well-defined during interactions, and they are defined in relation to the system they interact with.

Almost every aspect of reality has proven to be fundamentally discrete.

- Quarks and leptons are the class of elementary particles that compose matter. Quarks can combine to create protons and neutrons, while the electron is the most commonly known lepton.
- As mentioned above, the photon is the quantum of the electromagnetic force.
- The quantum particles of the strong and weak nuclear forces have also been discovered. The strong force is mediated by gluons, while W and Z bosons mediate the weak force.
- The only fundamental force for which no quantum particle has yet been discovered is gravitation. The best current description of gravitation, general relativity, is a purely classical field theory.