My eight-year-old son Ambrose recently discovered highway mirages. He spent a long hot car journey excitedly pointing them out to me. I tried to explain them to him and ended up having to look them up. A mirage is not exactly an illusion. It is an anomaly in the field of light we observe before us. It results from light bending as it hits a different density of air, just as a straw appears to bend when it enters the denser medium of water. In the eighteenth century, sailors enduring the sunless season on a ship trapped in ice near the Arctic Circle saw the sun rise two weeks ahead of its due date. They were seeing a fata morgana, not the real sun. The sun’s light had bent around the curvature of the earth because a peculiar density of cold air redirected it.
When we witness a mirage on the highway, our brains interpret an anomaly as a silvery puddle. We can even see reflections of cars on the other side of the highway pass across its “surface.” Something that has object permanence (a red Ford Escort) is reflected in something that does not have object permanence (a silver oval or stripe that evaporates as we close in upon it). Perhaps when we see ghosts our brains are processing similar anomalies. Outside the window a man is walking down the block loudly yelling Tricksy! Tricksy! He means his disappeared cat, who may have been run over last night.