In the context of quantum mechanics, the term "real" is often used in contrast to "virtual." In this context, a real particle is one that exists as a detectable entity, while a virtual particle is a mathematical construct that arises in certain calculations but cannot be directly measured or detected. The distinction between real and virtual particles is not dependent on the mass of the particle.
Massless particles, such as photons, are considered real entities in quantum mechanics. They are described by quantum fields and can be detected and measured in experiments. The wave-particle duality of quantum mechanics means that photons can exhibit both particle-like and wave-like properties depending on the experimental setup. In the case of the double-slit experiment, photons are emitted from a source and then detected on a screen after passing through two slits.
When researchers say they have "released a photon" in the context of a double-slit experiment, they mean that they have prepared the experimental setup and initiated the process that will allow a photon to interact with the slits and produce an interference pattern on the screen. The photon itself is considered real throughout the experiment, even before it is detected or measured.
It's important to note that quantum mechanics deals with probabilities and uncertainties. Before a measurement is made, the photon's state is described by a probability wave, or wavefunction, which encodes the likelihood of finding the photon in different states or locations. The act of measurement "collapses" the wavefunction, and the photon is observed to have a particular position or property. Until that measurement occurs, the photon's behavior is described by the wavefunction, which can exhibit interference effects in the case of the double-slit experiment.
The concept of local realism, often referred to as the violation of Bell's inequalities, addresses the question of whether the outcomes of measurements on entangled particles can be explained by local causes and pre-existing properties. Experimental observations, such as those involving entangled photons, have shown violations of Bell's inequalities, suggesting that local realism is incompatible with the predictions of quantum mechanics. This means that the behavior of entangled particles cannot be explained solely by pre-existing properties and hidden variables, as would be the case in a local realistic theory.
In summary, massless particles, like photons, are considered real entities in quantum mechanics, and their behavior is described by quantum fields. The concept of reality in quantum mechanics is not dependent on the mass of a particle but rather on its detectability and measurability. The notion of "releasing a photon" in experiments refers to initiating the process of allowing the photon to interact and produce observable effects. The quantum nature of particles, including massless ones, can exhibit both particle-like and wave-like behaviors, as described by the wave-particle duality.