Darkness is Not a "Thing" That Travels: This is the crucial point. Darkness is not an independent entity that moves or arrives. It's simply the absence of light. Darkness is what you perceive when there are no (or very few) photons reaching your eyes. The Explanation for Why Darkness Seems to "Arrive" Instantly: 1. When the Light Is On: When you turn on a light, it emits photons that travel to your eyes, allowing you to see. 2. When You Switch the Light Off: When you turn off the light, you stop the emission of photons. There are no more photons being generated from that source to reach your eyes. 3. The Apparent "Arrival" of Darkness: The darkness isn't "arriving;" it's the immediate cessation of photons reaching your eyes. Because light travels so incredibly fast, the transition from photons reaching your eyes to no photons reaching your eyes happens almost instantaneously.