Particles do not have rest mass in 5-dimensional space-time. Different kinds of particles in 4-dimensional space-time can be unified into a single particle field in 5-dimensional space-time. Particles have the property of a quantized intrinsic length. This intrinsic length can be identified using the Planck length. This phenomenon can be called length quantization. This intrinsic length of a particle, which originates from quantization, is invariant in all frames of reference, much like physical quantities such as charge, spin, and zero-point energy. Based on the propagator and intrinsic length of particle fields, a pure quantum gravity theory can be derived using the methods of quantum field theory. Gravitational force is only an additional effect of the particle field, and there is no independent gravitational field. The commutators that transmit the gravitational force are the particles themselves, and there is no independent graviton. Gravitons do not exist. In other words, gravitons are the particles themselves. There is an unknown mapping between 5-dimensional space-time and 4-dimensional space-time. Through this mapping, the intrinsic length of particles in 5-dimensional space-time is divided into two physical quantities in 4-dimensional space-time, one is the Compton wavelength of the particle and the other is the gravitational radius of the particle. The particle's Compton wavelength and gravitational radius form a dual relationship, and the product of the two is equal to Planck length squared. This unknown mapping brings two results in 4-dimensional space-time. The first result is the rest mass of the particle in 4-dimensional space-time, and the second is the gravitational effect in 4-dimensional space-time. The gravitational force in 4-dimensional space-time is the mapping result of the gravitational force in 5-dimensional space-time. The gravitational effect of the particle field limits the maximum energy of the virtual particle, preventing the divergence of the integrals of propagators. Gravitational properties are an indispensable part of quantum field theory, and only quantum field theory that includes gravitational effects can be formed into a complete theoretical framework. The gravitational potential and the Coulomb potential have similar inverse distance properties, and the phenomenon has a common origin, both of which are the result of quantum field theory propagators. In 4-dimensional space-time, gravity has a special property of spatial background field, which can be treated as a space-time structure. Gravity imposes a limitation on coordinate transformations, which do not eliminate the intrinsic length of particles.