We know that our readers simply cannot live without up-to-the-minute news about capillary water bridges and their impact on the shear modulus of sand. But, before we get into that, here's a quick history lesson.
The ancient necropolis at Giza is by far the most famous example of the 138 pyramids sprinkled across the Egyptian landscape. Like many of the country's age-old monoliths, the Great Pyramids there are constructed entirely of stone, and while archeologists have long agreed that they were dragged and lifted there solely by human power — likely in the form of skilled workers, not slaves — they haven't determined exactly how they were brought there in the first place.
What does that have to do with sand? Well, there's a hell of a lot of it surrounding the pyramids, and that can either hurt or help you when you're dragging a 2.3-metric-ton limestone block across the desert. Recently, physicists at the University of Amsterdam employed some simple beach science and discovered that wetting the sand makes pulling something across is far easier.
Anyone who's built a sand castle knows that, in certain amounts, water can glue sand together and keeps your towers from immediately crumbling. As the Dutch researchers wrote in a paper published this week in Physical Review Letters, water fills the gaps between the grains — forming something called "capillary bridges" — and acts as adhesive. But, it also reduces the sliding friction of objects moving across the sand, and prevents the sand from forming mounds or "berms" that would block a moving object's path.
Could ancient Egyptians have known this? It's quite possible, the researchers note. They point to a wall painting on the tomb of a feudal ruler named Djehutihotep, which depicts a man pouring water in front of a colossal statue as it is dragged by 172 laborers.
"Egyptologists had been interpreting the water as part of a purification ritual and had never sought a scientific explanation," said Daniel Bonn, one of the physicists, to the Washington Post. "And friction is a terribly complicated problem; even if you realize that wet sand is harder — as in a sand castle, you cannot build on dry sand — the consequences of that for friction are hard to predict."
While there's no evidence that the practice was necessarily widespread or long-established — Djehutihotep was buried in Middle Egypt over 600 years after the Giza's biggest pyramid, the Pyramid of Khufu, was completed — Bonn is convinced that this is the answer to the question that these wonders of the worlds have long-posed to scientists.