Tuesday, October 4, 2022
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Rocks that roll | Quantum Frontiers


In Terry Pratchett’s fantasy novel Soul Music, rock ’n roll arrives in Ankh-Morpork. Ankh-Morpork resembles the London of yesteryear—teeming with heroes and cutthroats, palaces and squalor—but in addition homes vampires, golems, wizards, and a sentient suitcase. Towards this backdrop, a younger harpist stumbles upon a mysterious guitar. He kinds a band with a dwarf and with a troll who performs tuned rocks, after which the trio calls its model “Music with Rocks In.” The remainder of the story consists of satire, drums, and rocks that roll. 

The subject of rolling rocks sounds prefer it ought to elicit extra yawns than an Elvis live performance elicited screams. However rocks’ rolling helped current College of Maryland physics PhD scholar Zackery Benson win a Nationwide Analysis Council Fellowship. He and his advisor, Wolfgang Losert, transformed me right into a fan of granular stream.

What I’ve been learning lately. Form of.

Grains make up supplies all through the galaxy, such because the substance of avalanches. Many granular supplies bear repeated forcing by their environments. As an illustration, the grains that type an asteroid endure bombardment from particles flying by means of outer area. The gravel beneath prepare tracks is compressed at any time when a prepare passes. 

Usually, a sample characterizes the forces in a granular system’s setting. As an illustration, trains in a selected weight class could traverse some patch of gravel, and the trains could arrive with a selected frequency. Some granular programs come to encode details about these patterns of their microscopic configurations and large-scale properties. So granular stream—little rocks that roll—can impression supplies science, engineering, geophysics, and thermodynamics.

Granular stream sounds so elementary, you would possibly anticipate us to have identified every little thing about it since lengthy earlier than the Beatles’ time. However we didn’t even know till lately learn how to measure rolling in granular flows. 

Envision a grain as a tiny sphere, like a globe of the Earth. Scientists targeted totally on how far grains are translated by means of area in a stream, analogouslly to how far a globe travels throughout a desktop if flicked. Lately, scientists measured how far a grain rotates about one axis, like a globe fastened in a body. Sans body, although, a globe can spin about multiple axis—about three unbiased axes. Zack carried out the primary measurement of all of the rotations and translations of all of the particles in a granular stream.

Every grain was an acrylic bead about as extensive as my pinky nail. Two holes have been drilled into every bead, forming an X, for causes I’ll clarify. 

Picture credit score: Benson et al., Phys. Rev. Lett. 129, 048001 (2022).

Zack dumped over 10,000 beads into an oblong container. Then, he poured in a fluid that stuffed the areas between the grains. Putting a weight atop the grains, he exerted a relentless stress on them. Zack would push one of many container’s partitions inward, compressing the grains equally to how a prepare compresses gravel. Then, he’d decompress the beads. He repeated this compression cycle many occasions.

Picture credit score: Benson et al., Phys. Rev. E 103, 062906 (2021).

Every cycle consisted of many steps: Zack would compress the beads a tiny quantity, pause, snap footage, after which compress a tiny quantity extra. Throughout every pause, the digital camera activated a fluorescent dye within the fluid, which appeared clear within the pictures. Missing the fluorescent dye, the beads confirmed up as darkish patches. Clear X’s minimize by means of the darkish patches, as dye stuffed the cavities drilled into the beads. From the X’s, Zack inferred each grain’s orientation. He inferred how each grain rotated by evaluating the orientation in a single snapshot with the orientation within the subsequent snapshot. 

Picture credit score: Benson et al., Phys. Rev. Lett. 129, 048001 (2022).

Wolfgang’s lab had been making an attempt for fifteen years to measure all of the motions in a granular stream. The feat required experimental and computational talent. I appreciated the prospect to play a minor position, in analyzing the information. Bodily Overview Letters revealed our paper final month.

From Zack’s measurements, we realized in regards to the distinctive roles performed by rotations in granular stream. As an illustration, rotations dominate the movement in a granular system’s bulk, removed from the container’s partitions. Importantly, the majority dissipates probably the most vitality. Additionally, whereas translations are reversible—nonetheless far grains shift whereas compressed, they have a tendency to shift oppositely whereas decompressed—rotations are usually not. Such irreversibility can contribute to supplies’ getting old.

In Soul Music, the spirit of rock ’n roll—conceived of as a pressure in its personal proper—provides the guitarist the chance to by no means age. He can reside quick, die younger, and luxuriate in immortality as a legend, for his guitar comes from a dusty little store not totally of Ankh-Morpork’s world. Such retailers deal in destiny and fortune, the creator maintains. Doing so, he takes a dig on the River Ankh, which flows by means of town of Ankh-Morpork. The Ankh’s waters maintain a lot rubbish, excrement, midnight victims, and different muck that they scarcely rely as waters:

And there was even an Ankh-Morpork legend, wasn’t there, about some outdated drum [ . . . ] that was imagined to bang itself if an enemy fleet was seen crusing up the Ankh? The legend had died out in current centuries, partly as a result of this was the Age of Purpose and in addition as a result of no enemy fleet may sail up the Ankh with no gang of males with shovels getting in entrance.

Such a drum would qualify as magic simply, however don’t underestimate the sludge. As a granular-flow system, it’s extra unbelievable than you would possibly anticipate.



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