In quantum mechanics, the concept of "now" or "the present moment" is not explicitly defined. The fundamental framework of quantum mechanics does not include a concept of absolute time that applies universally to all quantum systems. Instead, time is treated as a parameter that is relative to the observer and the particular system under consideration.
In the formalism of quantum mechanics, the evolution of quantum systems is described by the Schrödinger equation or other time-dependent equations. These equations govern the time evolution of the wave function, which represents the state of a quantum system. The wave function evolves continuously over time, allowing for the calculation of probabilities and predictions of future events.
However, the measurement process in quantum mechanics introduces an element of indeterminacy. According to the Copenhagen interpretation, when a measurement is made on a quantum system, its wave function collapses into one of the possible eigenstates of the measured observable. This collapse of the wave function is probabilistic, and the outcome of a measurement is determined by the probabilities encoded in the wave function.
Given the probabilistic nature of quantum measurements, it is not possible to determine a precise "now" moment when a specific event occurs in the quantum realm. The concept of time in quantum mechanics is more about the evolution and correlations between states rather than a fixed notion of the present moment.
It's important to note that the philosophical implications and interpretations of time in quantum mechanics are still subjects of ongoing debate and research among physicists and philosophers of science. Different interpretations, such as the many-worlds interpretation or the transactional interpretation, offer alternative perspectives on the nature of time and its relationship with quantum phenomena.