In the double-slit experiment, photons are typically emitted from a source and then allowed to pass through two closely spaced slits before reaching a screen where they create an interference pattern. This pattern arises due to the wave-like nature of photons.
In the traditional setup of the experiment, the photons are not physically returned to their source. Instead, the interference pattern is observed on a screen placed some distance away from the slits. The photons that pass through the slits spread out and interfere with each other, creating regions of constructive and destructive interference on the screen.
It's important to note that individual photons behave probabilistically and can only be detected at specific points on the screen. Each photon behaves as if it passes through both slits simultaneously and interferes with itself. However, over many repetitions of the experiment, the pattern of individual photon detections builds up to reveal the interference pattern.
If you are referring to a modified version of the experiment where photons are somehow retroactively "sent back" to the source, it would imply a reversal of their trajectory and violate the principles of causality in physics. In the standard interpretation of the double-slit experiment, the photons travel from the source to the screen without being redirected back to the source.