Incan numerical recordkeeping system may have been widely used

The Inca Empire hung by a thread—literally.

Inca bureaucrats recorded all the goings-on in their bustling empire using knotted cords called khipu, where the position and order of the knots represented numbers. They relied on the khipu system to track people, taxes, produce, livestock, and products like woven cloth and beer.

Because khipu were so vital to the Inca government, and because the khipu itself is such a sophisticated way of recording numbers, colonial writers decided that these tools must be the exclusive knowledge of a very specialized, elite class of bureaucrats. But a recent study, analyzing hair from a khipu made around 1498 CE, suggests that even common folk had a good grasp of this intricate way of recording numbers.

A lasting signature on a gorgeous piece of work

University of St. Andrews archaeologist Sabine Hyland and her colleagues recently analyzed a stand of hair from a 500-year-old khipu—one they expected to be the handiwork of an especially high-ranking member of the Inca empire, based on how beautifully it was crafted.

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Ice discs slingshot across a metal surface all on their own

VA Tech experiment was inspired by Death Valley’s mysterious “sailing stones” at Racetrack Playa.

Scientists have figured out how to make frozen discs of ice self-propel across a patterned metal surface, according to a new paper published in the journal ACS Applied Materials and Interfaces. It's the latest breakthrough to come out of the Virginia Tech lab of mechanical engineer Jonathan Boreyko.

A few years ago, Boreyko's lab experimentally demonstrated a three-phase Leidenfrost effect in water vapor, liquid water, and ice. The Leidenfrost effect is what happens when you dash a few drops of water onto a very hot, sizzling skillet. The drops levitate, sliding around the pan with wild abandon. If the surface is at least 400° Fahrenheit (well above the boiling point of water), cushions of water vapor, or steam, form underneath them, keeping them levitated. The effect also works with other liquids, including oils and alcohol, but the temperature at which it manifests will be different.

Boreyko's lab discovered that this effect can also be achieved in ice simply by placing a thin, flat disc of ice on a heated aluminum surface. When the plate was heated above 150° C (302° F), the ice did not levitate on a vapor the way liquid water does. Instead, there was a significantly higher threshold of 550° Celsius (1,022° F) for levitation of the ice to occur. Unless that critical threshold is reached, the meltwater below the ice just keeps boiling in direct contact with the surface. Cross that critical point and you will get a three-phase Leidenfrost effect.

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Misunderstood “photophoresis” effect could loft metal sheets to exosphere

Photophoresis can generate a tiny bit of lift without any moving parts.

Most people would recognize the device in the image above, although they probably wouldn't know it by its formal name: the Crookes radiometer. As its name implies, placing the radiometer in light produces a measurable change: the blades start spinning.

Unfortunately, many people misunderstand the physics of its operation (which we'll return to shortly). The actual forces that drive the blades to spin, called photophoresis, can act on a variety of structures as long as they're placed in a sufficiently low-density atmosphere. Now, a team of researchers has figured out that it may be possible to use the photophoretic effect to loft thin sheets of metal into the upper atmosphere of Earth and other planets. While their idea is to use it to send probes to the portion of the atmosphere that's too high for balloons and too low for satellites, they have tested some working prototypes a bit closer to the Earth's surface.

Photophoresis

It's quite common—and quite wrong—to see explanations of the Crookes radiometer that involve radiation pressure. Supposedly, the dark sides of the blades absorb more photons, each of which carries a tiny bit of momentum, giving the dark side of the blades a consistent push. The problem with this explanation is that photons are bouncing off the silvery side, which imparts even more momentum. If the device were spinning due to radiation pressure, it would be turning in the opposite direction than it actually does.

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Apple Watch gets reformulated, non-patent-infringing blood oxygen monitoring

Feature had been removed from US Apple Watches after Apple lost a patent fight.

In early 2024, Apple removed a blood oxygen monitoring feature from its then-current Apple Watch Series 9 and Ultra 2 watches, following a ruling that the feature violated light-based pulse oximetry patents from a California-based company called Masimo. Removing the feature allowed Apple to circumvent an import ban and continue selling the Series 9 and Ultra 2 (and, later, the Series 10) in the US.

Today, Apple announced that it's restoring a version of the blood oxygen monitoring feature to affected Series 9, Series 10, and Ultra 2 watches. Apple says these updates were "enabled by a recent U.S. Customs ruling."

The feature has been redesigned, presumably to get around the Masimo patents that prompted the import ban in the first place. Apple says that sensor data from your watch will be sent to your iPhone for processing, and you can find the data in the Respiratory section of the Health app on your phone. The old version of the feature allowed results to be viewed directly on the Apple Watch, as shown in Apple's documentation.

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Thermoelektrik: Leistungsschub für Solarzellen durch breiteres Spektrum

Eine 15-fache Erhöhung der Effizienz macht die Thermoelektrik zu einer nutzbaren Energiequelle. Nicht nur die Kombination mit Photovoltaik ist denkbar. (Energiewende, Solarenergie)

Eine 15-fache Erhöhung der Effizienz macht die Thermoelektrik zu einer nutzbaren Energiequelle. Nicht nur die Kombination mit Photovoltaik ist denkbar. (Energiewende, Solarenergie)