Substance has the characteristic of mass that is directly related to its substantiality. In fact, the substantiality may be expressed in terms of the density of mass. The mass density may be plotted on a continuous scale meaning that it is continuous in value. It is not an integer multiple of some ultimate amount that is indivisible.

**Mass is the measure of substantiality of
substance.**

The intrinsic motion of substance is expressed as its energy. The agitation of gas molecules is an example of intrinsic motion. Another example is the Brownian motion. The intrinsic motion appears naturally between two substances of different mass densities. An example is the extremely rapid motion of electrons around the nucleus of an atom. Another example is the speed of light relative to earth. These motions are intrinsic. There are no external force generating such motion.

**Energy is the measure of intrinsic
motion of substance.**

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## Substantiality and Motion

Matter is highly substantial, and it endures at any location for a long time. Its intrinsic motion, therefore, is very small. Light, on the other hand has little substantiality, and it barely endures at any location. Its intrinsic motion, therefore, is very high. This sums up to the following observation.

**The higher is the substantiality (mass density) of substance, the lower is its intrinsic motion (energy).**

In other words, higher is its intrinsic motion, lower is the substantiality of substance. This means that near infinite speed of light must be accompanied by infinitesimal substantiality. Therefore, light must have insignificant but finite mass density.

Einstein’s assertion that light has no mass may be interpreted as follows:

- The mass density of light is insignificant and negligible compared to the mass density of matter, or
- Light does not have the property of “center of mass”.

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## Mass and energy

Per the discussion above, intrinsic
motion is inverse of mass density. In other words, energy is inversely related
to mass density. This seems to contradict Einstein’s famous equation, **E = mc ^{2}**,
which seems to relate energy directly to mass.

This contradiction, however,
resolves when we look at this equation in terms of unit conversion. Using this
equation, we can express mass in energy units, and energy in mass units. It
shows that infinitesimal amount of mass density is equivalent to a significant
amount of energy because of the large multiplier **c ^{2}**.

**Therefore, when mass expands
into energy, the amount of that energy is significantly large, and when energy
condenses into mass, the amount of that mass is insignificantly small.**

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## Time

The discussion above also provides us with a new understanding of time as the “duration of substance.” The higher is the mass density of substance, the longer is the time for which it endures at any location, and the lower is its intrinsic motion. Therefore,

**Time is directly proportional to mass
density, and inversely proportional to intrinsic motion.**

Time has the intrinsic nature of duration that depends on how dense substance is at any location. The fourth dimension at any location, therefore, is mass density.

**Each location has three dimensions of space and a fourth dimension of mass density (duration).**

Locations of infinite mass density shall have infinite duration and they shall act as points of absolute rest. Thus, points of infinite density shall act as reference points for surrounding motion. That is why we find black holes at the center of galaxies.

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