One of the prints in El Castillo Cave’s Panel of Hands
was created more than 37,300 years ago.
A new study has revealed that Spain’s El Castillo Cave contains the oldest known cave paintings in Europe, with a handprint dating back 37,300 years and a red circle that was daubed onto the wall at least 40,600 years ago.
Instead of testing the paint’s age, a team of British and Spanish researchers measured the age of the stone that had formed around the drawings. In a cave, mineral-rich water drips over the walls, eventually depositing stalactites, stalagmites, and the sheet-like formations called flowstone. Some prehistoric artists had painted over flowstone made out of the mineral calcite, and then water flowed over the paint and deposited even more calcite, leaving the drawings sandwiched between mineral layers. The researchers used uranium-thorium dating to accurately determine the age of the mineral layers and therefore the window when the art itself was created; unlike the similar, more conventional carbon-14 method, uranium-thorium dating gives accurate results without damaging the subject.
Forget about fancy metamaterials that can make microscopic objects invisible–researchers at two different universities have independently shown that larger objects can be rendered invisible using a mineral that’s both naturally occurring and common: calcite.
This latest step in physicists’ ongoing quest to create an invisibility cloak come from an MIT lab, with a paper published in Physical Review Letters, and a University of Birmingham lab, whose paper just came out in Nature Communications. Both teams explained that they used calcite to make objects that are large enough to be seen with the naked eye invisible.
“By using natural crystals for the first time, rather than artificial metamaterials, we have been able to scale up the size of the cloak and can hide larger objects, thousands of times bigger than the wavelength of the light,” said Shuang Zhang, the University of Birmingham physicist who led the research…. “This is a huge step forward as, for the first time, the cloaking area is rendered at a size that is big enough for the observer to ‘see’ the invisible object with the naked eye.” [BBC]
The researchers constructed their cloaks from two glued-together calcite crystals, which have a convenient optical property called birefringence–that means they can bend a ray of light in two different directions. Then they placed the objects to be concealed in a notch beneath the crystals.