Classical mechanics is a branch of physics that describes the motion of objects on macroscopic scales, while quantum mechanics deals with the behavior of particles at the atomic and subatomic levels. As we move from the macroscopic to the microscopic realm, classical mechanics becomes less accurate, and quantum mechanics provides a more comprehensive description of nature.
In classical mechanics, objects are treated as point masses or extended bodies with well-defined positions and velocities. However, at the quantum level, particles exhibit wave-particle duality, uncertainty in position and momentum, and other quantum phenomena that are not accounted for in classical mechanics.
There is no strict cutoff point where classical mechanics suddenly becomes invalid and quantum mechanics takes over. Instead, the transition occurs gradually as we move to smaller and smaller scales. As a general rule, classical mechanics provides accurate predictions for macroscopic objects such as everyday items, planets, and galaxies, where quantum effects are negligible and their behavior can be well approximated using classical principles.
The exact size at which classical mechanics fails to accurately describe an object and quantum mechanics becomes necessary depends on the specific system and the desired level of precision. However, as a rough estimate, quantum effects become significant at the atomic and molecular scale, typically around the nanometer (10^(-9) meters) range.
Therefore, the largest objects that can be accurately described solely by classical mechanics and not by quantum mechanics are macroscopic objects that are much larger than atoms and molecules. This includes everyday objects such as tables, chairs, buildings, and even larger structures like planets, stars, and galaxies.