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Movie special effects allow characters to pass through objects unharmed. Some types of sand dunes can do this in the real world. According to a report published today in the journal Nature, so-called barchan sand dunes, which are crescent-shaped and highly mobile, can pass through one another while still maintaining their shape.
In dry, sandy regions, barchan dunes can disrupt infrastructure such as roads and pipelines because of their transient nature. In one year, a dune can move tens of meters, with smaller dunes moving more quickly than bigger ones do. Veit Schwammle and Hans J. Herrmann of the University of Stuttgart in Germany developed a computer model to "describe what happens when a small barchan dune bumps into a larger one." The researchers found that the two dunes initially form a hybrid state in which they become fused in a complex pattern. What happens next depends on the size of the two dunes, particularly their heights. At an intermediate tallness with a specific difference between the height of the two dunes, they cross through one another and exactly maintain their sizes and volumes.
At other heights, small changes to the crossing dunes will occur. For example, the emerging dune may become either slightly larger or smaller than it was when it encountered the second sandpile. Whether other types of dunes can behave in a similar manner remains unclear. One hindrance to real-world studies of entire dune fields is the amount of time required to acquire sufficient data: it often takes several decades to compile thorough measurements.