Movie 3 (:12)

Script: In a world where the speed of light is only five meters per second, a flickering streetlamp does not instantly illuminate its surroundings; rather it casts expanding shells of light into space around it.

 

Comments: Because the speed of light is so fast in our world, and the distances that are important to us ( walking across the room, digging around in the back of the refrigerator ) are so small, our intuition tells us that a flash of light is instantaneous, and illuminates all of a room at the same moment in time.

This is not at all true.

Imagine a race at the Olympics. Sprinters are crouched in their starting positions in 8 lanes, waiting for the sound of the starter's pistol to explode down the track....

But, who will hear the sound of the pistol first ?

Of course, the runner who is CLOSEST ! The sound doesn't have to travel as far.

That, of course, isn't very fair, especially when the margins of victory in the shorter races are measured in hundredths of a second. In fact, speakers installed directly behind each runner's starting position ensures each runner hears the sound at EXACTLY the same time, because each runner is EXACTLY the same distance from the source of the sound.

Extending the sound analogy to light, which is fast, but NOT instantaneous, suggests that very idea of "simultaneous" is suspect. If the starter's pistol was a light flash, or an electronic signal sent from ONE place to MANY speakers, it could not arrive at each speaker at the same instant. (In fact, in the race setup described above, the speaker signal STILL doesn't come out of each speaker at EXACTLTY the same time, and it never could. It's just that the delay induced by that means is so much smaller than the delay of a sound wave through air, that it's a big improvement)