The principles of quantum mechanics, as currently understood, apply to all objects, regardless of their size. However, when it comes to macroscopic objects, the effects of quantum mechanics are typically negligible and overshadowed by classical physics.
Quantum mechanics describes the behavior of particles at the quantum level, where phenomena such as superposition (particles existing in multiple states simultaneously) and entanglement (correlations between particles) are observed. These effects are more pronounced for microscopic particles like atoms and subatomic particles.
For macroscopic objects, such as everyday items we encounter, their large number of constituent particles and interactions with the environment cause quantum effects to "decohere." Decoherence refers to the loss of quantum coherence, where the object behaves classically and its behavior can be described using classical physics.
However, there are ongoing research efforts to explore and observe quantum effects in macroscopic systems. These investigations aim to push the boundaries of quantum coherence and study phenomena like macroscopic superposition or quantum entanglement on larger scales. This field of research is known as quantum macroscopicity or macroscopic quantum phenomena.
While we do not currently need to modify the principles of quantum mechanics to apply them to macroscopic objects, further research may uncover new aspects and refine our understanding of quantum behavior in larger systems.