The photoelectric effect and the discovery of the particle nature of light were indeed significant in our understanding of quantum mechanics. However, it's important to note that the photoelectric effect doesn't directly prove that light is a particle, but rather it demonstrated that light behaves in a manner consistent with particle-like behavior. This phenomenon led to the development of the concept of photons as discrete packets of energy associated with light.
Regarding your question about the graviton particle, it's important to understand that the situation is different. The photoelectric effect dealt with the behavior of electromagnetic radiation (light), while the gravitational wave detection involves gravitational waves, which are disturbances in the fabric of spacetime caused by accelerated masses.
Gravitons are hypothetical particles associated with the gravitational force in the framework of quantum field theory. However, to date, gravitons have not been directly detected or observed. The direct detection of gravitons, similar to the photoelectric effect, would require experimental verification through well-designed experiments. However, due to the extremely weak interaction of gravity compared to electromagnetism, detecting individual gravitons is an enormous technical challenge.
The current state of research focuses on indirectly detecting gravitational waves through advanced instruments like interferometers (such as LIGO and Virgo) that can measure the tiny distortions in spacetime caused by gravitational waves. These experiments provide valuable data and insights into the behavior of gravitational waves, but they do not directly confirm the existence of gravitons.
Exploring the quantum nature of gravity and attempting to directly detect gravitons are active areas of research in theoretical physics. Scientists are working on developing theoretical frameworks that unify quantum mechanics and general relativity, which would help provide a deeper understanding of gravitons and their potential experimental implications. However, at present, direct experimental verification of gravitons remains a significant challenge.