digraph G {
    // https://dreampuf.github.io/GraphvizOnline
    rankdir=LR;
    
  subgraph cluster_1 {
    "t: time";
    "E: energy";
    "p: momentum";
    "F: force";
    "m: mass";
    "\\gamma: lorentz factor";
    "v: velocity";
    "B: magnetic field";
    "E: electric field";
    "h: Planck constant";
    "a: acceleration";
    "i: sqrt of -1";
    "f: frequency";
    "S: entropy";
    "k_B: Boltzman constant";
    "\\Omega: configurations";
    
    label = "variables and constants";
    color=blue
  }
    
  subgraph cluster_2 {
    "F = m (dv/dt) = m a";
    "p = \\gamma m v"
    "\\nabla \\cross E = -(\\del B)/(del t)";
    "i (h/(2 \\pi)) (d/dt)\\Psi = H \\Psi";
    "E = h f";
    "\\lambda_{deBroglie} = h/p";
    "S = k_B ln \\Omega"
    
    label = "expressions";
    color=red
  }
    
    
  "domains of Physics" -> "classical mechanics (slow, big)";
  "domains of Physics" -> "non-relativistic quantum mechanics";
  "domains of Physics" -> "relativity";
  "domains of Physics" -> "quantum field theory";
  
  "classical mechanics (slow, big)" -> "Newton's laws of motion";
  "Newton's laws of motion" -> "F = m (dv/dt) = m a";
  "F = m (dv/dt) = m a" -> "m: mass";
  "F = m (dv/dt) = m a" -> "p: momentum";
  "F = m (dv/dt) = m a" -> "t: time";
  "F = m (dv/dt) = m a" -> "F: force";
  "F = m (dv/dt) = m a" -> "a: acceleration";
  "F = m (dv/dt) = m a" -> "v: velocity";
  
  "relativity" -> "special relativity";
  "relativity" -> "general relativity";

  "special relativity" -> "p = \\gamma m v";
  "p = \\gamma m v" -> "p: momentum";
  "p = \\gamma m v" -> "v: velocity";
  "p = \\gamma m v" -> "\\gamma: lorentz factor";
  "p = \\gamma m v" -> "m: mass";
  
  "classical mechanics (slow, big)" -> "fluid dynamics";
  "fluid dynamics" -> "Navier-Stokes equation";
  
  "classical mechanics (slow, big)" -> "electrodynamics";
  "electrodynamics" -> "Maxwell's equations";
  "Maxwell's equations" -> "\\nabla \\cross E = -(\\del B)/(del t)";
  "\\nabla \\cross E = -(\\del B)/(del t)" -> "t: time";
  "\\nabla \\cross E = -(\\del B)/(del t)" -> "E: electric field";
  "\\nabla \\cross E = -(\\del B)/(del t)" -> "B: magnetic field";
  
  
  "classical mechanics (slow, big)" -> "kinematics"
  "kinematics" -> "non-relativistic particles"
  
  "non-relativistic particles" -> "plasma physics"
  "electrodynamics" -> "plasma physics"
  "plasma physics" -> "magnetohydrodynamics"
  "fluid dynamics" -> "plasma physics";
  
  "statistical mechanics" -> "S = k_B ln \\Omega";
  "S = k_B ln \\Omega" -> "S: entropy";
  "S = k_B ln \\Omega" -> "k_B: Boltzman constant";
  "S = k_B ln \\Omega" -> "\\Omega: configurations";

  
  "non-relativistic particles" -> "statistical mechanics"
  "non-relativistic particles" -> "thermodynamics"
  "thermodynamics" -> "entropy"
  "non-relativistic particles" -> "non-relativistic quantum mechanics"
  "non-relativistic quantum mechanics" -> "deBroglie wavelength"
  "deBroglie wavelength" -> "\\lambda_{deBroglie} = h/p"
  "\\lambda_{deBroglie} = h/p" -> "h: Planck constant"
  "\\lambda_{deBroglie} = h/p" -> "p: momentum"
  
  "non-relativistic quantum mechanics" -> "Schrödinger equation"
  "Schrödinger equation" -> "i (h/(2 \\pi)) (d/dt)\\Psi = H \\Psi"
  "i (h/(2 \\pi)) (d/dt)\\Psi = H \\Psi" -> "i: sqrt of -1"
  "i (h/(2 \\pi)) (d/dt)\\Psi = H \\Psi" -> "h: Planck constant"
  "i (h/(2 \\pi)) (d/dt)\\Psi = H \\Psi" -> "t: time"
  
  "Planck–Einstein relation" -> "E = h f"
  "E = h f" -> "E: energy"
  "E = h f" -> "h: Planck constant"
  "E = h f" -> "f: frequency"
}