Galilei transformations are formulated in a 5-dimensional form and on such basis the non-relativistic quantum mechanics is reconstructed. Our approach has the following advantages over others'. (1) The Schrödinger wavefunctions are taken to be vector (not projective) representations of the Galilei group, so that (2) the problem here is considerably simplified. (3) The formulation proceeds in a way quite parallel to that of relativistic quantum mechanics, and what is most important, (4) Galilei covariance is manifest throughout.
In nonrelativistic quantum mechanics and in relativistic quantum field theory, the time coordinate
t is a parameter and thus the time-reversal operator
T does not actually reverse the sign of
t. In contrast, in the five-dimensional approach to relativistic quantum mechanics introduced by Feynman, time
t is a quantum-mechanical operator. In this paper it is shown how one can use this five-dimensional approach to extend
T and
PT symmetry from nonrelativistic to relativistic quantum mechanics and implement time-reversal as an operation that effects $\mathcal{T}\text{ }t\mathcal{T}=\ensuremath{-}t$ just as
P effects $\mathcal{P}x\mathcal{P}=\ensuremath{-}x$, with
PT thus effecting $\mathcal{P}\mathcal{T}\text{ }{x}_{\ensuremath{\mu}}\mathcal{P}\mathcal{T}=\ensuremath{-}{x}_{\ensuremath{\mu}}$. Some illustrative relativistic quantum-mechanical models are constructed whose associated Hamiltonians are non-Hermitian but
PT symmetric, and it is shown that for each such Hamiltonian the energy eigenvalues are all real.
We argue that special relativity, instead of quantum theory, should be radically reformulated to resolve inconsistencies between those two theories. A new relativistic transformation recently-proposed renders physical laws form-invariant via transformation of physical quantities, instead of space-time coordinates. This new perspective on relativistic transformation provides an insight into the very meaning of the principle of relativity. The principle of relativity means that the same physical laws hold in all inertial frames, rather than their mathematical formulas are Lorentz-covariant under the Lorentz transformation of space-time coordinates. The space-time concept underlying this new relativistic transformation is Newtonian absolute space and absolute time. With this new perspective, quantum theory becomes compatible with the principle of relativity. A new theory of relativistic quantum mechanics is formulated. The new relativistic quantum mechanics thus obtained maintains the essentials of non-relativistic quantum mechanics, for instance, the statistical single-particle interpretation. Studies on standard problems in quantum mechanics, square step and barrier potential, show that the new relativistic quantum mechanics is free from such anomalies as the negative probability density, Zitterbewegung and the Klein paradox bedeviling the current relativistic quantum mechanics. Furthermore, a remarkable result is found as applying the new relativistic quantum mechanics to study the problems of square potential well --- a particle can not be confined within an infinite square well of width less than half of the Compton wavelength of the particle. As implications of this finding in nuclear physics, there is a lower bound on the size of atomic nucleus. Neither an electron, nor a positron, can be confined inside the nucleus.