Some physicists, even today, may think that it is impossible to connect relativistic quantum mechanics and general relativity [1][2][3]. This impression, however, is false, as demonstrated, among many other recent works, in an article published not long ago in this journal [4]. In order to understand the subtlety, let us remember the meaning of first [5,6], second [7,8], and third quantization (see Refs. [9][10][11]). In first quantization, what was previously a well-defined particle trajectory now becomes smeared and defines a probability density of the quantum mechanical particle. In second quantization, what was previously a well-defined functional value of a physical field at a given space-time point now becomes a field operator, which is subject to quantum fluctuations. E.g., quantum fluctuations about the flat space-time metric can be expressed in terms of the graviton field operator, for which an explicit expression can be found in Eq. (5) of Ref. [8].In third quantization, what was previously a well-defined space-time point now becomes an operator, defining a conceivably noncommutative entity, which describes space-time quantization. E.g., the emergence of the noncommutative Moyal product in quantized string theory is discussed in a particularly clear exposition in Ref. [11], in the derivation leading to Eq. (32) We thus observe that space-time quantization is not necessary, a priori, in order to combine general relativity and quantum mechanics. Indeed, before we could ever conceive to observe effects due to space-time quantization, we should first consider the leading-order coupling of the Dirac particle to a curved spacetime. Space-time curvature is visible on the classical level, and can be treated on the classical level [3]. Deviations from perfect Lorentz symmetry caused by effects other than space-time curvature may result in conceivable anisotropies of spacetime; they have been investigated by Kostelecky et al. in a series of papers (see Refs. [13][14][15] and references therein) and would be visible in tiny deviations of the dispersion relations of the relativistic particles from the predictions of Dirac theory. Measurements on neutrinos [15] can be used in order to constrain the Lorentz-violating parameters. Other recent studies concern the modification of gravitational effects under slight global violations of Lorentz symmetry [16]. Brill and Wheeler [17] were among the first to study the gravitational interactions of Dirac particles, and they put a special emphasis on neutrinos. The motivation can easily be guessed: Neutrinos, at the time, were thought to be massless and transform according to the fundamental ( ) representations of the Lorentz group. Yet, their interactions have a profound impact on cosmology [18]. Being (almost) massless, their gravitational interactions can, in principle, only be formulated on a fully relativistic footing. This situation illustrates a pertinent dilemma: namely, the gravitational potential, unlike the the Coulomb potential, cannot simply be inserted into th...