>>14326147P =E/c
The energy momentum equation due to special relativity.
E = hf
Planck's constant. Energy is discrete.
Therefore P =hf/c
Momentum is Planck's constant times frequency of emission per how fast something can possibly go.
But f/c = Lambda = L.
Light goes at one speed, as fast as possible. Wavelength times frequency equals speed. Frequency over speed is wavelength.
P = h/L
Momentum is Planck's constant per wavelength which is the definition of a photon.
What's happening:
Things use their internal configuration as a medium of interaction.
There is only so much stuff in the universe.
To exist everything is interacting with all that stuff in some causal chain.
To exist you need enough not just to take it but overcome it and take interactions.
There's a delay to that interaction just like there's a delay between the light of the moons of Jupiter and Galileo's telescope.