Relativistic quantum mechanics, also known as quantum mechanics combined with special relativity, was an important step in unifying quantum mechanics with Einstein's theory of special relativity. It provides a framework for describing the behavior of quantum systems in the presence of high velocities and energies.
However, as physicists explored deeper into the realm of particle physics, it became clear that relativistic quantum mechanics had limitations when it came to describing certain phenomena and interactions. These limitations led to the development of quantum field theory (QFT), which is a more comprehensive framework that incorporates both quantum mechanics and special relativity.
There are a few reasons why quantum field theory is preferred over relativistic quantum mechanics in many areas of modern physics:
Treatment of Particle Interactions: Relativistic quantum mechanics treats particles as point-like objects and describes their interactions through scattering amplitudes. However, in reality, particles are better understood as excitations of underlying fields. Quantum field theory allows for a more natural treatment of particle interactions by describing the dynamics of these fields and their excitations, which are particles.
Creation and Annihilation of Particles: In relativistic quantum mechanics, the number of particles is fixed and does not change. Quantum field theory, on the other hand, allows for the creation and annihilation of particles through field interactions. This is essential for describing processes such as particle decay or particle creation in high-energy collisions.
Vacuum Fluctuations: Quantum field theory incorporates the concept of vacuum fluctuations, which are inherent fluctuations in the quantum fields even in the absence of particles. These vacuum fluctuations give rise to observable phenomena, such as the Lamb shift and the Casimir effect, which cannot be explained within relativistic quantum mechanics.
Renormalization: Quantum field theory provides a systematic framework for dealing with divergences that arise in calculations of certain physical quantities. This process, known as renormalization, allows physicists to obtain meaningful and finite results, particularly in the context of quantum electrodynamics (QED) and the Standard Model of particle physics.
It's important to note that relativistic quantum mechanics is still a valuable and widely used tool in many areas of physics, particularly for systems where particle interactions and high energies are not the primary focus. However, for a comprehensive understanding of the fundamental forces and particles, as well as the behavior of matter at small scales, quantum field theory has proven to be a more powerful and effective framework.