Quantum physics and Newtonian physics are two distinct theories that describe the behavior of the physical world at different scales. Newtonian physics, also known as classical physics, provides accurate predictions for macroscopic objects, such as everyday objects and the motion of planets. On the other hand, quantum physics is necessary to describe the behavior of particles at the microscopic scale, such as atoms and subatomic particles.
The transition between the macroscopic and microscopic scales is not a clear boundary where one theory abruptly ends and the other begins. Rather, it is a gradual transition where the laws of classical physics break down as we delve into the quantum realm. This transition occurs when the characteristic scales of the system approach the scale of Planck's constant (h), which is approximately 6.626 x 10^-34 joule-seconds.
At scales much larger than the Planck scale, classical physics accurately describes the behavior of objects, and quantum effects become negligible. However, as we approach the Planck scale, quantum phenomena become increasingly significant, and classical descriptions become inadequate. This is where a unified theory that combines quantum mechanics and gravity, such as a theory of quantum gravity, is expected to be relevant.
Currently, there is no widely accepted theory that fully unifies quantum mechanics and gravity. The most well-known attempt at a theory of quantum gravity is string theory, which postulates that elementary particles are not point-like objects but rather tiny vibrating strings. However, string theory is still a work in progress, and its experimental verification is challenging.
Studying the transition from quantum physics to classical physics at the Planck scale is difficult because it requires experimental capabilities beyond our current technological reach. The extreme energy and length scales involved make it difficult to probe directly. Moreover, the effects of quantum gravity are expected to be negligible in everyday observations, so finding experimental evidence for a unified theory is a formidable task.
Nevertheless, researchers continue to explore various approaches to quantum gravity, such as loop quantum gravity, causal dynamical triangulation, and emergent gravity models. These theories aim to bridge the gap between quantum and classical physics and provide a unified description of the fundamental forces of nature. However, more experimental and observational progress is needed to validate and refine these theories.