Concerning the importance of his second postulate Einstein was quite
clear:

"If the speed of light is the least bit affected by the speed of the
light source, then my whole theory of relativity and theory of gravity
is false."

If particles are four dimensional, and they exist within a much larger
four dimensional Space-Time, then much can be understood.

For instance, if these four dimensional particles have spin properties,
then the axis does not have to be limited to being just a Spatial axis.
The axis can also extend across Time.

If one changes ones direction of travel in Space-Time, one also changes
the direction of which the axis points across that Space-Time. The end
result is that the faster one moves across Space ( and therefore the
less one travels across the Time dimension ) the greater the percentage
of the spin axis extends across Time, and the lesser percentage of the
spin axis now extends across Space.

In short, this Holistic rotation of the particles axis, will cause the
Spatial properties of the particle to contract in line with the
direction of travel, while it extends further across the Time dimension
instead. The length of the particle will contract.

Now if you have a meson that is at rest in space, when it decays it
will produce two Photons that fly apart with each traveling at the
speed of light (c).

If, on the other hand, the meson is in motion across Space, the
Space-Time axis of the meson has rotated such that the Spatial spin
angular momentum has reduced in the forward direction and has increased
in the reverse direction relative to the direction of travel. When
released, the end result is that the velocity of one Photon that
travels in the reverse direction is c + v, and the other that travels
in the forward direction is c -v relative to the mesons original velocity.
The end result is that the original velocity of the meson is canceled out, and
each Photon travels at (c) velocity relative to Space-Time itself, or relative to
the universal background.

Thus the velocity of the object that emits light, does not effect the
speed of light, since the Space-Time axis of the particle that releases
the Photon is at a new angle in Space-Time, which therefore determines
a new spatial angular momentum, while at the same time has not altered
the original Holistic Space-Time angular momentum.

This also means that if a four dimensional particle receives a Photon,
then both its Holistic angular momentum and its axis angle in
Space-Time will be altered. This may happen several times over and
each time it will shift the axis angle and increase the Holistic
momentum one step further. Due to the shift in the axis angle occurring
while also there being an increase the Holistic momentum, the Spatial
angular axis always remains the same unless as discussed above, the
particles velocity is changed.

In this case, several Photons may be received, and several Photons may
then be released. The charging of the particle is actually an increase
in the particles Holistic angular momentum, along with the shift in the
particles axis angle in Space-Time.

In short, the release speed of a photon depends on the Spatial
properties of a particle, and so the rotation of the particle in
Space-Time will change the Spatial properties of the particle in
proportion with the change of velocity.

otion through time. Therefore, when it decays and splits into two
Photons, these Photons not only travel across Space at (c) velocity,
but they also retain the original velocity across the dimension of Time
which is also at the equivalent of (c) velocity.

Again, if the meson was in motion across Space, then the shift in its
axis angle will also maintain the Spatial travel of the Photon across
space to the (c) velocity, but it will also maintain the Photons travel
across Time to be the same equivalent of (c) velocity.

The end result is that the Photons have the momentum across Space-Time
that is the equivalent of 424,264 km per second if the momentum was
across Space only rather than being equally distributed across
Space-Time.