Achilles’ Tortoise

“Gather ’round, everyone”, said the tortoise to Achilles, who happened to be the only person present. “I’m going to talk about something very profound – something known as Einstein-Minkowski Spacetime. Draw nigh!”

“In this case, I offer to you this wonderful color illustration:

“I love ice cream” mumbled Achilles, as the Tortoise appeared not to hear and continued his pontification:

“This illustrates the principle of the light cone. Notice that there are in fact two cones, and the point where they touch is here and now. Since anything we do is ultimately slower than light, it makes sense that we ignore humans, gods and reptiles and simply talk about the past and future of light. The upper cone represents the possible paths of light from the here and now. Since time moves forward (in the positive Y axis [up]), and since light has a limit to how fast it can travel, it stands to reason that light cannot ‘move’ outside of the cones. It can travel to any point contained within the cone, but cannot (without breaking the fundamental theory that the speed of light is a constant) travel to points outside of the cone.”

Achilles began to glaze over and stare at clouds…

“The lower cone represents the past, so any light that arrives at the here and now had to come from within the lower cone (and not any point outside of it). The blue lines represent rays of light that come from the past, travel through the here and now and continue into the future.”

Achilles heaved a great sigh and said “Yes, but what is your point?”, clearly growing weary of the theoretical discussion. “Feh!” exclaimed the tortoise, “Pearls before swine!”, losing interest… “If you care at all about this, just read this article about it… But mark my words.. This subject will come up again!”