The issue you raise regarding the description of macroscopic objects, such as a tennis ball, with a wave function is a topic of ongoing debate and research in the field of quantum mechanics. While it is true that macroscopic objects consist of an enormous number of particles and degrees of freedom, the practical limitations of dealing with such a large amount of information often make it impractical or infeasible to describe them using the full quantum wave function.
In the standard interpretation of quantum mechanics, the wave function describes the state of a quantum system and contains the information about the probabilities of various outcomes when measurements are made. According to this interpretation, in principle, the wave function can be applied to any physical system, regardless of its size.
However, the challenge lies in the practical aspects of dealing with complex wave functions that describe macroscopic objects. The computational resources required to manipulate and solve the equations for large systems quickly become unmanageable. Additionally, the measurement process itself, which collapses the wave function to a definite state, becomes extremely difficult to implement at such large scales.
As a result, physicists often rely on approximations and simplified models to describe macroscopic objects. One such approach is called decoherence theory, which explains how the quantum behavior of a system can appear to "disappear" at macroscopic scales due to interactions with the environment. Decoherence effectively leads to the emergence of classical behavior, where objects exhibit well-defined properties and appear to obey classical physics rather than quantum mechanics.
From a practical standpoint, when dealing with macroscopic objects, physicists often use statistical methods and classical theories, such as classical mechanics, thermodynamics, and electromagnetism, to describe their behavior accurately. These classical theories have been highly successful in describing and predicting the behavior of macroscopic objects in most everyday situations.
It is important to note that the relationship between quantum mechanics and the macroscopic world is still an active area of research. Scientists are actively exploring ways to bridge the gap between the quantum and classical realms, seeking a better understanding of how macroscopic classical behavior emerges from the underlying quantum nature of the universe.