The change of frequency of a photon in a gravitational field has been demonstrated by the Pound-Rebka experiment. When a photon falls in a gravitational field, it acquires energy equal to E=Dmc2 which separates into three parts; one part behaves like a positive electric field and another part behaves like a negative electric field. These neutralize each other in the structure of the photon (a photon itself is neutral) and the third part behaves like a magnetic field.

In quantum mechanical theory, every field is quantized. In addition, force is described as energy per distance shown by:

If we consider this equation from the aspect of quantum mechanics, a graviton enters into the structure of a photon, carrying gravitational force. As a result, a graviton disappears and the energy of the photon increases.

Similarly, Red Shift has the opposite effect. As a photon escapes from a gravitational field, its frequency shifts to red and its energy converts to gravitons. How can we describe this interaction between photons and gravitons on a sub-quantum scale such as in the structure of a photon?

Another consideration is virtual particles, implied by the uncertainty principle in the form:

In the vacuum, one or more particles with energy ?E above the vacuum may be created for a short time ?t. So, any analysis of the ZPE should be able to explain this particle production and even Hawking radiation in a strong gravitational field such as that of a black hole. To do this, the best way is to use the Dirac equation, which originally demonstrated the possibility of a particle pair, that is, a particle and anti-particle.

The relationship between energy and momentum for a massless particles is given by;

Now it is possible to change the definition of the rest mass of a particle. As we know, some particles such as photons are never seen at rest in any reference frame. According to relativity however, they do have mass that derives from their energy. For example, a photon has a mass given by: