3 space M$_{2}$ and, thus, it does not produce inertia effect. Therefore, the real inertia effects can be an empirical indicator of absolute acceleration. We calculate the iner- tial force acting on the photon in gravitating system of particles that are bound together by their mutual gravi- tational attraction. A particular attention is given to the theoretical justification for introduction of the Principle of Equivalence. gravitation theory, we derive the line element in particu- lar case of static and spherically symmetric gravitational field. Traditionally the solar system is a laboratory that offers many opportunities to improve tests of relativistic gravity. The usual Eddington-Robertson-Schiff parame- ters β and γ used to describe these tests are perhaps in some sense the most important parameters of the param- eterized post-Newtonian (PPN) formalism [37–40]. We rather show that the agreement is satisfactory between the proposed gravitation theory and available observa- tional verifications [4, 40–55]. We complete the proposed theoretical basis of distortion of spacetime by exploring, further, two major problems of inertia and rearrangement of vacuum state in gravity. We construct the relativistic field theory of inertia which similarly to gravitation theory treats the inertia effects as a distortion of local internal properties of flat spacetime continuum. We motivate this approach as fol- lows. Unlike the GR, proposed gauge field theory of grav- itation is free of fictitious forces, because the infinite- parameter group of general covariance is no longer in use. Instead, the preferred systems and group of trans- formations of the, so-called, real-curvilinear coordinates relate solely to real gravitational fields. In spite of to- tally different and independent physical sources of grav- itation and inertia, still we might expect that the iner- tial force is of the same nature as gravitational force. Namely we ascribe the effects associated with gravity and inertia to spacetime geometry itself, and that both phenomena arise due to the distortion of local internal properties of flat space. To trace this line, we involve besides the distortion gauge fields being responsible only for gravitation, also the distortion inertial fields which account for the inertia separately. Seeking a replace- ment for the unobservable Newtonian absolute spacetime , which is necessary to assign a meaning to Newtonian ab- solute acceleration , instead we explore the geometry of two-dimensional flat space M$_{2}$ . Similar reasoning leads us, further, to the conclusion that an alteration of uni- form motion of test particle under the unbalanced force is the immediate cause of the real distortion of the lo- cal internal properties of the space M$_{2}$ conducted under the distortion inertial field. This necessarily, in the first place, with equal justice could be interpreted as a def- inite criterion for the universal absolute acceleration of test particle or coordinate frame of interest, and in the second place, will give us the fundamental RLI. This we might expect to hold on the basis of an intuition founded on a past experience limited to low velocities, and which were implicit in the ideas of Galileo and Newton as to the nature of inertia. The major premise is that the cen- trifugal endeavor of particles to recede from the axis of rotation is directly proportional to the quantity of the ab- solute circular acceleration, which, for example, concave water surface in Newton’s famous rotating bucket exper- iments. In this framework, the relative acceleration (in Newton’s terminology) (both magnitude and direction), in contrary, cannot be the cause of the distortion of the For the benefit of the reader, a brief outline of the framework of GGP are given in this Section and in Ap- pendix to make the rest of the paper understandable. We have used a combined geometrical structure known as a fiber bundle, which provides a unified picture of theory This paper is organized as follows: In Sect.2 a num- ber of useful mathematical concepts of GGP are reviewed for the reader’s convenience. We will refrain from pro- viding lengthy details of the formalism of GGP and uni- tary map. For these the reader is referred to Appendix. In Sect.3 we relate the group G$_{R}$ to the Lie group G$_{D}$ by constructing its nonlinear realization. In Sect.4 we construct the relativistic field theory of inertia and give theoretical justification for introduction of the Principle of Equivalence. In Sect.5 we address the rearrangement of vacuum state in gravity. Conclusions are presented in Sect.6. The specific topics dealt with in the Appendix are further details on the GGP. We will be brief and of- ten suppress the indices without notice. Unless otherwise stated we take natural units, h = c = 1. The quantities denoted by wiggles throughout this paper refer to dis- torted (curved) space, but the quantities referring to flat space are left without wiggles. Finally, the developments on the GGP are applied to address the rearrangement of vacuum state in gravity. The objections concerning non-uniqueness of the physical vacuum can be circumvented immediately due to one of the underlying principles that in the flat space interacting quantum field theory the vacuum is well-determined and unique | in > = | out > (up to a phase factor). In realiza- tion of G$_{R}$ we implement the simplest hidden gauge sym- metry of abelian group U loc = U (1)$_{Y}$ × diag [ SU (2)] on the M$_{6}$ which entails two neutral gauge bosons. Sponta- neous symmetry breaking is achieved in standard manner by introducing the neutral complex Higgs scalar. Non- vanishing vacuum expectation value (VEV) leaves one Goldstone boson which is gauged away from the scalar sector. But it essentially reappears in the gauge sector providing the longitudinally polarized spin state of one of gauge bosons that acquires mass through its coupling to Higgs scalar. The massless component of distortion field is responsible for gravitational interactions. In the resulting theory, simultaneously with the strong gravity, the massive distortion field component may cause a sub- stantial change of properties of spacetime continuum at huge energies above the threshold value. II. THE GGP PRELIMINARIES