The causality paradox of the double-slit experiment, often referred to as the delayed-choice experiment, is a thought experiment that raises questions about the nature of causality and the behavior of particles, such as photons, at the quantum level.
In the double-slit experiment, when particles such as photons or electrons are sent through two slits and allowed to interfere with each other, they exhibit wave-like behavior, creating an interference pattern on a screen behind the slits. However, when a detection apparatus is placed to determine which path the particles take (which slit they pass through), the interference pattern disappears, and the particles behave like particles rather than waves.
The delayed-choice version of the experiment introduces the possibility of making the decision of whether or not to place the detection apparatus after the particles have already passed through the slits, even after they should have made their "choice" of behaving as particles or waves. It seems as if the post-observation determines the behavior of the particles in the past, creating a causal paradox.
It is important to note that the behavior of particles, including photons, in the double-slit experiment is well-described by the mathematical framework of quantum mechanics, which incorporates principles such as wave-particle duality, superposition, and the collapse of the wave function upon measurement. These principles apply to all particles, including massless ones like photons.
The idea that massless particles like photons do not "experience" space or time in the same way as massive particles is related to their behavior according to special relativity, where the speed of light is constant and acts as an absolute limit on the velocity of information transfer. However, this aspect of special relativity does not directly explain the causality paradox in the double-slit experiment.
The delayed-choice experiment challenges our intuitions about causality and raises profound questions about the nature of reality at the quantum level. Various interpretations of quantum mechanics, such as the Copenhagen interpretation or the many-worlds interpretation, have been proposed to address these questions, but the debate is ongoing, and there is no consensus on a definitive explanation for the paradox.