Subject: Special relativity, conceptual question on E-p

Let k = E-p (assuming use of natural units superfluizing the "c"s, such that 
for instance E^2 = m^2 + p^2).

Is k to be thought of as reflecting the system's internal structure & internal 
motions in an observer's frame? I.e., does k represent momenta "tied up" in the 
system due to the system's traveling slower than light?

This idea makes sense to me if I consider a slower-than-light system consisting 
of two photons moving perpendicularly to each other, but when massive particle 
are involved, I get in over my untrained head, since I have to start thinking 
about attributing "potential" momenta to massive particles in virtue of their 
convertibility to kinetic energy by collisions with suitable antiparticles at 
which point they would have p=e=E.

Anyway, it would be nice because then "E = p+k" would be a physically 
significant statement about E as a kind of sum of different kinds of momentum, 
just as "E = m+e" is a physically significant statement about E as a sum of 
energy & rest mass. It would be nice, but is it true?

Is there a technical term for k?

m+e = p+k = E = sqrt(p^2 + m^2)

p _______ e
____ E ____
m _______ k

I like this, it reminds me of a logical Square of Opposition.

Thank you for your attention.

Truly yours,
Ben Udell

Re: Special relativity, conceptual question on E-p