Placement of the monolithic stones at Stonehenge could be related to an acoustic phenomenon created when two pipers play the same note, according to a new theory from archaeoacoustician Steven Waller.
A person walking in a circle around the pipers hears the note's volume decrease at certain points where the two sound waves collide and cancel one another out. At these spots, it sounds as though a giant pillar is blocking the sound, Waller says.
According to New Scientist:
Perhaps, Waller proposes, the ancients thought these silent points were invisible walls from the spirit world. They may have then arranged Stonehenge and the stone circles like it as very physical, 40-tonne incarnations of these walls.
To test whether the illusion holds true in real life, Waller took blindfolded volunteers out into an empty field and led them in a circle around two recorders that were playing the same note. Then he asked the volunteers to draw the space they thought they had walked through.
All six of the volunteers drew a circle of pillars or archways between themselves and the recorders.
"It's a very mystical phenomenon," Waller says, and would have been totally inexplicable to ancient people who didn't understand the physics of sound waves.