The question of whether something can be considered truly "physical" if its behavior cannot be predicted due to quantum effects or entanglement is a philosophical and interpretational aspect of quantum mechanics. Different interpretations of quantum mechanics offer varying perspectives on this issue.
Quantum mechanics, the fundamental theory describing the behavior of particles at the microscopic level, introduces inherent uncertainties and probabilistic predictions. According to the Copenhagen interpretation, one of the widely accepted interpretations of quantum mechanics, the behavior of particles is described by a wave function that evolves probabilistically until a measurement is made, at which point the wave function "collapses" to a definite state. In this interpretation, the unpredictable nature of quantum mechanics is accepted as an intrinsic feature of the physical world.
However, there are alternative interpretations, such as the Many-Worlds interpretation and the Pilot-Wave theory, that propose different ways of understanding the nature of quantum reality and attempt to address the issue of unpredictability. These interpretations posit the existence of hidden variables or multiple parallel universes to explain the seemingly random outcomes of quantum measurements. In these frameworks, the behavior of particles is not truly unpredictable but is influenced by underlying factors that are not directly observable.
From a broader philosophical perspective, the question of what can be considered "physical" is open to interpretation. Some philosophers argue that a phenomenon can still be considered physical even if it exhibits unpredictable behavior at the quantum level. They argue that physicality should be defined based on the ability to interact with other objects or have measurable effects, regardless of the underlying mechanisms.
In summary, whether something can be considered truly "physical" despite unpredictable behavior due to quantum effects depends on the philosophical perspective and the interpretation of quantum mechanics one adopts. Different interpretations provide alternative explanations and viewpoints on the nature of quantum phenomena and the limits of predictability.