The phenomenon of entanglement in quantum mechanics is indeed puzzling and can seem counterintuitive. When two particles are entangled, their quantum states become correlated in such a way that the measurement of one particle instantaneously affects the state of the other particle, regardless of the distance between them. This non-local correlation is often referred to as "spooky action at a distance."
The issue of whether the measurement of an entangled particle causes the collapse of the other particle's wave function or whether it was deterministic from the beginning is a topic of debate and interpretation in quantum mechanics. Different interpretations offer different perspectives on this matter.
In the Copenhagen interpretation, the act of measurement causes the wave function of the entangled particles to collapse, and the outcome becomes definite at the moment of measurement. This collapse is non-deterministic, and the measurement outcome appears random.
However, there are alternative interpretations, such as the Many-Worlds interpretation or pilot-wave theories, that suggest a deterministic underlying reality. According to these interpretations, the entangled particles' states were determined from the beginning, and the measurement merely reveals the pre-existing properties.
The "spooky" aspect of entanglement lies in the non-local correlation between the particles. Regardless of the interpretation, entanglement allows for instantaneous correlations that cannot be explained by classical physics. This property has been experimentally confirmed through various tests and is one of the distinguishing features of quantum mechanics.
It's important to note that quantum mechanics has been extensively tested and verified through numerous experiments. While the interpretations of quantum mechanics are still a matter of debate, the predictions and results of quantum mechanics have been highly successful in describing the behavior of particles and systems at the quantum level.