Here is an archetypal vignette for Newtonian mechanics. Two compound atoms called and have an interaction with each other by swapping another particle which is called the *exchange* particle. The interaction is caused when emits at event which is called the initial event of the interaction. This is written as

Particle then has an effect on by being absorbed at event which is called the final event of the interaction. We express this by writing

For EthnoPhysics, the interaction is described using three repetitive chains of historically ordered events written as

Since and are composed from atoms, we assume that they can be described by space-time events with a position and time of occurrence . We do not assume that is an atom, rather we often take it to be a photon or a graviton. So we cannot always describe using a trajectory. And the position of is well-defined only for the initial and final events where it is included as part of an atom. Overall, the interaction is characterized by the following quantities.

## Momentum Change by Emission

The interaction is caused when emits at an event which is called the initial event of the interaction

Momentum is conserved so a total over all momenta are the same before and after the interaction

And the change in ‘s momentum due to the emission of is given by

so that

## Momentum Change by Absorption

Particle has an effect on by being absorbed at event which is called the final event of the interaction.

Momentum is conserved so a total over all momenta are the same before and after the interaction

And the change in ‘s momentum due to the emission of is given by

so that

## Elapsed Time between Events

The difference in the time of occurrence between initial and final events is

If the frame of reference is inertial and is isolated (apart from the interaction under consideration) then the period of does not vary between events. So the elapsed time is given by

or in terms of the frequency

Then using Planck’s postulate gives the elapsed time in terms of the mechanical energy as

## Distance between Events

The distance between initial and final events is given by

If the frame of reference is inertial and is isolated (apart from the interaction under consideration) then the wavelength does not vary so that

Then using de Broglie’s expression for the wavelength in terms of the momentum gives