The observer-dependent nature of quantum physics refers to the role of measurement and observation in determining the properties of quantum systems. In classical physics, observation is considered separate from the system being observed, and the act of measurement does not significantly affect the properties of the system. However, in quantum mechanics, the act of measurement can profoundly influence the state of the system being observed.
According to the principles of quantum mechanics, before a measurement is made, a quantum system can exist in a superposition of multiple possible states. These states are described by a mathematical object called a wavefunction, which contains information about the probabilities of different outcomes when measured. The wavefunction evolves over time according to Schrödinger's equation, which is a fundamental equation of quantum mechanics.
When a measurement is performed on a quantum system, it "collapses" the wavefunction to a specific state corresponding to the measurement outcome. This collapse is probabilistic, and the probabilities of different outcomes are given by the squared magnitudes of the coefficients in the wavefunction. This process is often referred to as wavefunction collapse or the measurement postulate.
The observer-dependent nature arises because the act of measurement selects a specific outcome from the range of possible outcomes, and the system is no longer in a superposition of states. Different measurement outcomes can be obtained depending on the observable being measured and the basis chosen for the measurement.
Furthermore, quantum mechanics introduces the concept of entanglement, where two or more particles become correlated in such a way that the state of one particle is entangled with the state of the other particles. When entangled particles are measured, the measurement outcomes for one particle can become correlated with the outcomes of the other particles, even if they are physically separated. This phenomenon has been experimentally confirmed and is often referred to as "spooky action at a distance."
The observer-dependent nature of quantum physics challenges our classical intuition and raises philosophical questions about the nature of reality and the role of observation in shaping it. Various interpretations of quantum mechanics have been proposed to explain this observer dependence, such as the Copenhagen interpretation, the many-worlds interpretation, and the pilot-wave theory, among others. These interpretations offer different perspectives on how to understand and interpret the quantum phenomena, but the debate on their implications and philosophical consequences is ongoing.