As shown in Chapter 3: Matter and Substantiality, both matter and light are substantial. Matter, however, is considered to have mass, but light is not. Matter has solidity and density, but light does not.
The fundamental difference between matter and light is that matter consists of atoms, and light does not. Atoms were at first depicted as solid globules, but now they are determined to be 99.99% space, with a speck of a solid nucleus at the center. The atomic space, however, is filled with electrons whose mass density is 1/1840th of the mass density of the nucleus.
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The Nucleus
The nucleus is like a material particle that is solid and rigid, and has a center of mass. It is surrounded by electrons that do not have a center of mass. Electrons are depicted as having a probabilistic distribution in a very large volume compared to the volume of the nucleus.
This is mathematical, of course, but it tells us that electrons are thin in terms of mass, and are spread out in a large volume. They lack a rigid structure like the nucleus. We may conclude:
The substance (mass) is not rigid, solid, and concentrated except in the nucleus. It is ephemeral, flexible and sparse in electrons and elsewhere.
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The Electron
A proton or a neutron is like a mass contained in an point-like volume. If that volume is blown up 1840 times, the mass density in that volume shall also thin out 1840 times. This shall roughly be the volume and mass density of an electron.
Electrons have neither solidity nor structure, and they move rapidly making its thin mass slosh like a fluid. That is why they do not have a center of mass. This gives electron a wave-like characteristic. The electrons are discrete in terms of energy interactions because they have mass density, but in space, they seem to form a fluid-like continuum. This explains the results from the double-slit phenomenon.
Electrons are not particles that behave as waves in some medium. They are a dense fluid-like substance with a wave-like sloshing motion.
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Light
Light is an electromagnetic (EM) radiation. It has much larger spread in space compared even to the electrons. Therefore, it has much smaller mass density. The sloshing speed of photons of light is much greater. Without mass there shall neither be wave-like motion, nor discrete energy interactions. This logical view is different from what is generally assumed in science.
This is the real explanation of why the quantum of electron (and, more so, the quantum of light) cannot be pin pointed in space. Only particles with center of mass may be pinpointed. This explains the reality underlying Heisenberg’s uncertainty principle.
Light is very low density and extremely fast moving fluid-like sloshing substance. Its momentum and energy comes from its mass density and velocity.
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Summary
Matter is an atomic substance. The density of matter depends on how densely the atoms are packed. But even then, this density is averaged over the atom in which the heavy nucleus exists only as a tiny speck. The highest density of substance exists in the nucleus of an atom. The rigid structure of matter gives it a center of mass
Electrons and the electromagnetic radiation (light) are non-atomic substances. There is a very sharply declining density gradient from nucleus to the surrounding electrons. The density decreases farther away from the nucleus in the electronic region. Photons surrounding the electronic region of atoms have still lesser mass density and greater volume. One may say that mass is becoming increasingly thinner from neutron/proton to electron to photon. The mass density of photon may be estimated from its wavelength.
The mass becomes more fluid-like as it thins and spreads out in greater volume. At the same time, it increases in its dynamic characteristic and gains wave-like properties.
Matter and light are substances with mass density. The mass density gives them particle-like properties.
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