Calculation of the Gravitational Force
In an object, the non-uniform distribution of electric charges may be very complicated, making the analysis and calculation of the produced electric field difficult. To simplify this, two net “charge centers,” similar to the “mass center” used in mechanics, are defined.
Regardless of the complexity of the net positive and net negative charge distributions, all electric forces generated by all net positive charges can be synthesized into a single positive electric force. The point from which this synthetic force is exerted on other object is the net positive “charge center.” Similarly, the net negative “charge center” is defined.
With this simplification, any object with non-uniform charge distribution can be regarded as having a net “positive charge center” and a net “negative charge center.” All net positive charges are considered concentrated at the “positive charge center,” and all net negative charges are considered concentrated at the “negative charge center.” As emphasized above, because these two charge centers are almost never at the same point for almost any object, and the two kind of the electric charges have equal amount, nearly any object with a non-uniform charge distribution can be regarded as an electric dipole.
The theoretical electric dipole is a pair of two point electric charges with equal magnitude |
| and opposite signs, where |
| is the absolute value of electric charge
. A small distance
is between the two point charges. The electric dipole moment
is the product of the distance
and the charge
.
where
and
are vectors. The direction of the dipole moment
is from the negative point charge to the positive point charge. The electrical field strength
of the electric dipole at the distance of
R is [
12]
In Eq. (2),
is unit distance vector along the
R direction.
is the electric constant. Note that, in deduction of Eq. (2), the terms in
and higher order terms are neglected. This approximation is justified if
keeping the product
finite [
13], where
,
R and
P are absolute values of the vectors
, and
. This approximation is used for to calculate the gravitational force easily and quickly.
Sometimes, the distance between the net “positive charge center” and the net “negative charge center” in an object may not be small. However, if the size of the object is much smaller than the scale related to the problem being considered, such as considering the attraction between a planet and a star in the universe, the size and shape of the object are less important because . Thus, the distance between the net positive and net negative “charge centers” in the object may be treated as enough small, and such objects with non-uniform charge distributions may be reasonably regarded as ideal electric dipoles.
From Eq. (2), we know that when the direction of the vector
changes, the electric field strength
changes too. When the direction of
is the same as or opposite to the direction of
, the electric field strength
becomes
or
The
in Eq. (3) is positive, which expresses a repulsive force away from the dipole to a positive point charge. The
in Eq. (4) is negative, which expresses an attractive force toward the dipole to a positive point charge. When the direction of the dipole moment is perpendicular to the direction of
, the electric field strength
becomes
The in Eq. (5) is negative. Please note that is a deflective force.
Then, a problem arises: if the electric field strength of an object with non-uniform charge distribution is not isotropic, why we have not observed that the gravitational force between two objects varies with their relative direction? For instance, why are many planetary orbits around stars nearly circular? In other words, why is the gravitational force always attractive?
The reason is that while the electric field of an electric dipole may exert repulsive, attractive, or deflective forces on another electric dipole, in most cases, the repulsive and deflective forces will quickly change to attractive force. When the interaction between two electric dipoles is repulsive, both dipoles are in states of highest electric potential energy, which is unstable. Any change in the charge distribution of one object will produce a deflective force between the two dipoles. This deflective force will rotate the dipoles, eventually making the force between them attractive.
The process by which repulsive and deflective forces become attractive forces, that is, the rotations of electric dipole moments, can occur automatically and continuously. This is a continuous self-calibrating process due to the mutual influence of the electric fields of two electric dipoles. As mentioned above, the change in direction of the electric dipole only requires the redistribution of the non-uniform electric charge within the object. Since the movements of free electric charges and the shifts of charge distribution centers occur at very high speeds and are not visible, this process can happen dynamically and be completed almost instantly. Consequently, the interaction force between two objects may always appear to be attractive, whether they rotate around each other or not, resulting in a seemingly isotropic gravitational force between them.
Of course, in some cases, the gravitational force between the two objects is still affected by their relative direction, which is one of the reasons why some planetary orbits around stars are elliptical.
Below, the interaction force between two electric dipoles is calculated. First, considering the case when the directions of two dipole moments are the same. In
Figure 3, the first dipole moment
consists of positive pinot charge
and negative point charge
. The second dipole moment
consists of positive pinot charge
and negative point charge
. The length of the first dipole moment
is
. The length of the second dipole moment
is
. The distance between the centers of two dipoles is
R.
From Eq. (2), the first electric dipole will produce the electrical field
at the distance
R from its center
In the electrical field
, the positive point charge
of the second dipole will feel a force
as
The direction of force
is along the direction of
. The negative point charge
of the second dipole will feel a force
as
The direction of force
is also along the direction of
.
Because the absolute values of
and
are equal, and the absolute values of
and
are also equal, when
and
, the total force felt by the second dipole in the electric field
is
approximately
In Eq. (9), if
is replaced by
,
is replaced by
,and
is replaced by a constant
, then Eq. (9) becomes
We can see that Eq. (10) is very similar to Newton’s gravitational law
Eqs. (10) and (11) show that the physical essence of the mysterious mass is the amount of electric charge. More strictly speaking, the mass is another expression of the amount of the electric charge multiplied by the length of the related electric dipole and divided by the distance between the two related dipoles.
Compare Newton's law of the gravitational force
with Coulomb’s law of the electric force
We can see that their expressions are very similar. Eqs. (10) and (11) express the attractive gravitational forces generated by two point-like objects with masses of
or
and or
and
. Eq. (12) expresses the attractive electric force generated by two point electric charges of
and
.
G is gravitational constant. K is Coulomb's constant. Their numerical values depend on the system of the units used. If two charges in Eq. (12) are replaced by two masses in Eq. (10), then Eq. (12) becomes Eq. (10), and both
and
K have the same numerical value and dimension. Thus, we profoundly see the physical rationality of the understanding that gravitational force is originated from the electric force.
The striking similarity between Eq. (10) and Eq. (12) strongly hints that the gravitational force is the electric force, because if the natures of two physical forces are different, their expressions should be significantly different. However, where else can we find such similarity between the expressions of two different physical forces?
When the directions of two dipole moments are opposite or perpendicular, the interaction forces between two dipoles become repulsive or deflective because both of the dipoles are pushed by repulsive electric fields, expressed by Eq. (3), or twisted by deflective electric fields, expressed by Eq. (5).
However, as explained above, in most cases, two interacting electric dipoles cannot push or twist each other for long. As soon as an electric dipole is affected by a deflective force, even if small, its dipole moment direction will rotate and eventually align with the direction of that force. The rotation of the electric dipole doesn’t require the rotation of the real physical body of the object. It only needs a change in the distribution of net electric charges, including free and induced net charges in the object. Such changes are easy, and very fast and cannot be seen visibly. This explains why the gravitational force is always attractive. Thus, when considering the interaction force between two objects, because the directions of two electric dipoles are almost always the same, the interaction force, that is, the gravitational force between the two objects, can be expressed by Eq. (9) or (10) only.
When three objects interact with each other, the free and induced net charges in each object are redistributed in response to the electric fields generated by other two objects. If each object is simplified as an electric dipole, it will interact with the electric dipoles of the other two objects. Thus, the redistribution of charges in each object may be regarded as forming two electric dipoles. Each dipole in one object responds to an external dipole in one of the two other objects, leading to the analysis of interactions among six electric dipoles. When more objects are involved, the gravitational force analysis becomes more complex. However, the redistribution of net charges in each object may still be regarded as forming multiple electric dipoles, each responding to a dipole in each of the multiple other objects.
The dynamically self-calibrating process may apply to multiple electric dipoles, or even multiple groups of electric dipoles, causing them to mutually and continuously attract each other, whether they rotate around each other or not. The attractions between multiple electric dipoles, or even multiple groups of electric dipoles, may be understood from a different perspective too. This is because all of the charge distributions of the multiple electric dipoles or multiple groups of electric dipoles can form a larger combined charge distribution. As long as this larger combined charge distribution is non-uniform, it will still act as a synthetic electric dipole. Thus, any other electric dipole will be attracted to this synthetic electric dipole. The other electric dipole may be any individual electric dipole, including anyone within or outside the mentioned multiple electric dipoles or multiple groups of electric dipoles. Therefore, based on the introduced self-forming and dynamically self-calibrating electric dipole model, multiple electric dipoles, or even multiple groups of electric dipoles, can continuously attract each other.
Eq. (9) or (10) expresses the interactional electric force between two electric dipoles when the dipoles have the same direction. This electric force is the gravitational force between two objects. According to Eq. (9) or (10), the gravitational force has the following properties:
First, since the gravitational force is produced by net electric charges and , the gravitational field is actually the electric field.
Second, the strength of the gravitational force depends on the direction of the electric dipole, meaning that the gravitational force is essentially anisotropic. However, due to the aforementioned dynamically self-calibrating process, a constant and seemingly isotropic attraction can be maintained between two objects, whether they are rotate around each other or not. In other words, although at any given moment, the gravitational force is anisotropic, however, after successive and numerous micro self-calibrating with very fast speed, the overall performance of the gravitational force will appear as if it has an isotropic attraction capacity.
Third, replacing with and replacing with in Eqs. (9) and (10), because in most cases, and , so the magnitudes of and are much smaller than the magnitudes of and . This is why, although the gravitational force is the electric force, the strength of the gravitational force is much smaller than the strength of the electric force.
Fourth, the values of , , and in Eq. (9) are not fixed because they are determined by net charge distributions in two objects. Since variations of charge distributions in two objects can change the values of , , and , the gravitational force between two objects is not fixed even their so-called masses don’t change.
Fifth, although the strength of the electric field of an electric dipole is inversely proportional to the cube of the distance from the dipole, the strength of the gravitational force is exactly inversely proportional to the square of the distance from the object. This is highly consistent with the results of countless precise measurements of the gravitational force. The inverse-square form of Eq. (9) or (10) is the result of strict theoretical derivation based on Coulomb's law.
Eq. (9) states that the gravitational force between two objects depends on their net electric charge and electric dipole moment lengths. Thus, the gravitational force between two objects appears to be an easily variable quantity. However, according to our usual understanding, the gravitational force is considered an invariant quantity (excluding changes with the distance between the two objects), because their masses are assumed to be constant and independent of other bodies.
This understanding has limitations. The reason for this is that we typically observe the gravitational force between objects, or the weight of objects, just on the Earth's surface or in near-Earth space. In this context, the net charge distribution within the object is mainly determined by the physical structure and states of all atoms and molecules within the object, as well as the influence of the Earth's electric field on it. Since the change of overall strength of the Earth's electric field acting on an object on the Earth's surface is minimal usually, and the physical structure and states of all atoms and molecules within an object on the Earth's surface also undergo no large changes, the so-called mass of an object on the Earth's surface, or the weight of an object on the Earth's surface, can be considered approximately constant.
These properties make the gravitational force exhibit strange and mysterious behaviors, causing confusing phenomena observed in the universe. Some of these phenomena have puzzled humans for a long time. However, by using new understandings of the gravitational force introduced above, these confusing phenomena can be explained simply and effectively. On the other hand, these phenomena may be regarded as indirect evidences of the correctness of this introduced new understanding.