The concept you're referring to is often associated with the idea of wave-particle duality in quantum mechanics. According to quantum theory, particles such as electrons or photons can exhibit both wave-like and particle-like behavior. The behavior observed in an experiment depends on how the particle is measured or observed.
In quantum mechanics, the act of measurement or observation is often described as a process that "collapses" the particle's wavefunction, which represents its probabilistic behavior. When a measurement is made, the wavefunction is said to collapse into a particular state corresponding to the measurement outcome.
However, it's important to note that the interpretation and understanding of this process are still subjects of debate and philosophical inquiry in the field of quantum mechanics. Different interpretations offer various perspectives on what is happening during the measurement process and how the collapse of the wavefunction should be understood.
One commonly accepted interpretation is the Copenhagen interpretation, which states that the act of measurement causes an abrupt change in the quantum system, bringing about the observed outcome. According to this view, the measurement itself determines whether the particle is observed as a particle or a wave.
Other interpretations, such as the many-worlds interpretation or the de Broglie-Bohm theory, propose alternative explanations for the measurement process. These interpretations suggest that the wavefunction doesn't collapse but rather evolves deterministically, and the observed outcome is determined by interactions between the particle and its environment.
It's important to emphasize that these interpretations are still matters of ongoing research and philosophical discussion. While quantum mechanics provides a highly successful mathematical framework for describing the behavior of particles, the fundamental nature of measurement and the process by which observation determines the existence of a particle are still open questions.