Let particle P be characterized by its enthalpy and the work required to bring together its component quarks. Then the **rest mass** of P is defined by

Mass | Definition |
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heavy | |

material | |

ethereal | |

imaginary |

where is a constant. This definition distinguishes several types of particles by their mass. If is positive then P is a *material* particle; an ordinary particle of matter, like a coin or a bullet. There is an important special case for material particles when the work required to make them is negligible compared to their enthalpy, then we say they are *heavy* particles. If the mass is zero then P is *ethereal*. Finally if then P has an *imaginary* mass.^{1}The term *imaginary* is used here with its mathematical meaning . Particles with an imaginary mass are no more fictitious than any other sort of nuclear particle. They carry momentum and transmit forces like other particles. The main thing about having an imaginary mass is that it puts a particle in a logical category that is different from Newtonian particles. So they are not necessarily required to follow Newtonian laws of motion. Roughly speaking, the rest mass describes how much internal energy is leftover after the work of assembling a particle has been completed. We may use the mass to describe the hardness or *density* of a particle. Recall that is the norm of the radius vector of P. Then the **density** of P is defined as

Particles and anti-particles have the same mass as each other. We have already seen how and when conjugate symmetry is assumed. But the mass depends on these quantities squared. So

Photons are ethereal because they are mostly phase anti-symmetric. The radius vector of a photon is null, and so no work is required to assemble the quarks in a photon, . Phase anti-symmetry also means that the net number of quarks is nil. Substituting this condition into the definition of enthalpy shows that as well. Then the definition of mass given above implies that

## Sensory Interpretation

Enthalpy characterizes the magnitude of all classes of sensation, whereas the work represents just somatic and visual sensations. The mass is established by their difference, which is mostly due to thermal sensation. So for heavy particles, thermal perceptions are more important than visual sensations. And for particles with an imaginary mass, audio-visual sensations dominate awareness. Next we consider the lifetime of a nuclear particle.

1 | The term imaginary is used here with its mathematical meaning . Particles with an imaginary mass are no more fictitious than any other sort of nuclear particle. They carry momentum and transmit forces like other particles. The main thing about having an imaginary mass is that it puts a particle in a logical category that is different from Newtonian particles. So they are not necessarily required to follow Newtonian laws of motion. |
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