China’s first emperor broke the mold when he had himself buried with a terra-cotta army. Now insight into the careful crafting of those soldiers is coming from the clays used to build them. Custom clay pastes were mixed at a clay-making center and then distributed to specialized workshops that cranked out thousands of the life-size figures, new research suggests.

Roughly 700,000 craftsmen and laborers built Emperor Qin Shihuang’s palatial mausoleum in east-central China between 247 B.C. and 210 B.C. A portion of those workers gathered clay from nearby deposits and prepared it in at least three forms, researchers propose in the August Antiquity. On-site or nearby workshops used different signature clay recipes for terra-cotta warriors, parts of mostly bronze waterfowl figures and paving bricks for pits in which the soldiers originally stood.
Around 7,000 ceramic foot soldiers, generals and horses — equipped with a variety of bronze weapons — make up the army, which was accidentally discovered in 1974 by farmers digging a well. The emperor would have regarded the ceramic statues as a magic army that would protect him as he ruled in the afterlife, many researchers suspect.

Building and assembling the multitude was an enormous task. Workers poured clay mixtures into casts of torsos, limbs and other body parts, and then assembled the bodies, taking care to create different facial features for each soldier. Finished statues, now mostly gray, were covered in colored lacquers and likely fired in kilns. Most figures were placed inside one giant pit. Earthen walls formed 11 parallel corridors where statues stood in battle-ready rows.

Still, no workshops or debris firmly linked to the statue-making process have been found. As a result, the number, size, location and organization of workshops involved in producing the emperor’s ceramic troops remain uncertain.

Archaeologist Patrick Quinn of University College London and three Chinese colleagues studied the composition of clay samples from the site. The pieces were taken from 12 terra-cotta warriors, two acrobat statues found in a second pit, five clay bricks from the floor of the largest pit, clay fragments from inside three bronze waterfowl statues found in a third pit and part of an earthen wall in the acrobat pit.

Microscopic analysis of the samples revealed that the clay came from deposits near the tomb’s location, the scientists say. But the recipes for different parts varied. Paving bricks contained only a mixture of dark and light clays, while the clay used for warriors and acrobats had sand worked in. Sand and plant fragments were folded into a clay mixture that formed the core of the bronze waterfowl.
Sand may have made the clay more malleable for shaping into ornate figures and increased statues’ durability, the researchers speculate. Plant pieces may have helped reduce the weight of birds’ clay cores. A clay-processing site at or just outside the emperor’s mausoleum must have doled out the appropriate clay pastes to an array of workshops where potters made statues, bricks or other objects, the scientists propose.

What’s more, many statue and waterfowl samples show signs of having been slowly heated in kilns at maximum temperatures of no more than 750˚ Celsius. That’s lower by 150˚ C or more than some previous estimates, the investigators say. Fires set in an attack on the tomb after the emperor’s death may have refired some of the clay, accounting for the temperature discrepancy, the researchers say.

“I’m not at all surprised by the new findings,” says East Asian art historian Robin D.S. Yates of McGill University in Montreal. Legal and administrative documents previously found at two other Qin Empire sites describe workshops that specialized in various types of craft production, Yates says.

In some cases, artisans’ stamps and inscriptions on terra-cotta warriors match those on excavated roof tiles from Emperor Qin’s mausoleum. The markings suggest that some workshops made several types of ceramic objects, says East Asian art historian Lothar Ledderose of Heidelberg University in Germany. Inscriptions on statues also indicate that artisans working at off-site factories during the Qin Empire collaborated with potters at local workshops to produce the terra-cotta army, Ledderose says.

On astrophysicists’ charts of star stuff, there’s a substance that still merits the label “here be dragons.” That poorly understood material is found inside neutron stars — the collapsed remnants of once-mighty stars — and is now being mapped out, as scientists better characterize the weird matter.

The detection of two colliding neutron stars, announced in October (SN: 11/11/17, p. 6), has accelerated the pace of discovery. Since the event, which scientists spied with gravitational waves and various wavelengths of light, several studies have placed new limits on the sizes and masses possible for such stellar husks and on how squishy or stiff they are.
“The properties of neutron star matter are not very well known,” says physicist Andreas Bauswein of the Heidelberg Institute for Theoretical Studies in Germany. Part of the problem is that the matter inside a neutron star is so dense that a teaspoonful would weigh a billion tons, so the substance can’t be reproduced in any laboratory on Earth.

In the collision, the two neutron stars merged into a single behemoth. This remnant may have immediately collapsed into a black hole. Or it may have formed a bigger, spinning neutron star that, propped up by its own rapid rotation, existed for a few milliseconds — or potentially much longer — before collapsing. The speed of the object’s demise is helping scientists figure out whether neutron stars are made of material that is relatively soft, compressing when squeezed like a pillow, or whether the neutron star stuff is stiff, standing up to pressure. This property, known as the equation of state, determines the radius of a neutron star of a particular mass.

An immediate collapse seems unlikely, two teams of researchers say. Telescopes spotted a bright glow of light after the collision. That glow could only appear if there were a delay before the merged neutron star collapsed into a black hole, says physicist David Radice of Princeton University because when the remnant collapses, “all the material around falls inside of the black hole immediately.” Instead, the neutron star stuck around for at least several milliseconds, the scientists propose.

Simulations indicate that if neutron stars are soft, they will collapse more quickly because they will be smaller than stiff neutron stars of the same mass. So the inferred delay allows Radice and colleagues to rule out theories that predict neutron stars are extremely squishy, the researchers report in a paper published November 13 at arXiv.org.
Using similar logic, Bauswein and colleagues rule out some of the smallest sizes that neutron stars of a particular mass might be. For example, a neutron star 60 percent more massive than the sun can’t have a radius smaller than 10.7 kilometers, they determine. These results appear in a paper published November 29 in the Astrophysical Journal Letters.

Other researchers set a limit on the maximum mass a neutron star can have. Above a certain heft, neutron stars can no longer support their own weight and collapse into a black hole. If this maximum possible mass were particularly large, theories predict that the newly formed behemoth neutron star would have lasted hours or days before collapsing. But, in a third study, two physicists determined that the collapse came much more quickly than that, on the scale of milliseconds rather than hours. A long-lasting, spinning neutron star would dissipate its rotational energy into the material ejected from the collision, making the stream of glowing matter more energetic than what was seen, physicists Ben Margalit and Brian Metzger of Columbia University report. In a paper published November 21 in the Astrophysical Journal Letters, the pair concludes that the maximum possible mass is smaller than about 2.2 times that of the sun.

“We didn’t have many constraints on that prior to this discovery,” Metzger says. The result also rules out some of the stiffer equations of state because stiffer matter tends to support larger masses without collapsing.

Some theories predict that bizarre forms of matter are created deep inside neutron stars. Neutron stars might contain a sea of free-floating quarks — particles that are normally confined within larger particles like protons or neutrons. Other physicists suggest that neutron stars may contain hyperons, particles made with heavier quarks known as strange quarks, not found in normal matter. Such unusual matter would tend to make neutron stars softer, so pinning down the equation of state with additional neutron star crashes could eventually resolve whether these exotic beasts of physics indeed lurk in this unexplored territory.

Galileo’s most famous experiment has taken a trip to outer space. The result? Einstein was right yet again. The experiment confirms a tenet of Einstein’s theory of gravity with greater precision than ever before.

According to science lore, Galileo dropped two balls from the Leaning Tower of Pisa to show that they fell at the same rate no matter their composition. Although it seems unlikely that Galileo actually carried out this experiment, scientists have performed a similar, but much more sensitive experiment in a satellite orbiting Earth. Two hollow cylinders within the satellite fell at the same rate over 120 orbits, or about eight days’ worth of free-fall time, researchers with the MICROSCOPE experiment report December 4 in Physical Review Letters. The cylinders’ accelerations match within two-trillionths of a percent.

The result confirms a foundation of Einstein’s general theory of relativity known as the equivalence principle. That principle states that an object’s inertial mass, which sets the amount of force needed to accelerate it, is equal to its gravitational mass, which determines how the object responds to a gravitational field. As a result, items fall at the same rate — at least in a vacuum, where air resistance is eliminated — even if they have different masses or are made of different materials.

The result is “fantastic,” says physicist Stephan Schlamminger of OTH Regensburg in Germany who was not involved with the research. “It’s just great to have a more precise measurement of the equivalence principle because it’s one of the most fundamental tenets of gravity.”
In the satellite, which is still collecting additional data, a hollow cylinder, made of platinum alloy, is centered inside a hollow, titanium-alloy cylinder. According to standard physics, gravity should cause the cylinders to fall at the same rate, despite their different masses and materials. A violation of the equivalence principle, however, might make one fall slightly faster than the other.

As the two objects fall in their orbit around Earth, the satellite uses electrical forces to keep the pair aligned. If the equivalence principle didn’t hold, adjustments needed to keep the cylinders in line would vary with a regular frequency, tied to the rate at which the satellite orbits and rotates. “If we see any difference in the acceleration it would be a signature of violation” of the equivalence principle, says MICROSCOPE researcher Manuel Rodrigues of the French aerospace lab ONERA in Palaiseau. But no hint of such a signal was found.

With about 10 times the precision of previous tests, the result is “very impressive,” says physicist Jens Gundlach of the University of Washington in Seattle. But, he notes, “the results are still not as precise as what I think they can get out of a satellite measurement.”

Performing the experiment in space eliminates certain pitfalls of modern-day land-based equivalence principle tests, such as groundwater flow altering the mass of surrounding terrain. But temperature changes in the satellite limited how well the scientists could confirm the equivalence principle, as these variations can cause parts of the apparatus to expand or contract.

MICROSCOPE’s ultimate goal is to beat other measurements by a factor of 100, comparing the cylinders’ accelerations to see whether they match within a tenth of a trillionth of a percent. With additional data yet to be analyzed, the scientists may still reach that mark.

Confirmation of the equivalence principle doesn’t mean that all is hunky-dory in gravitational physics. Scientists still don’t know how to combine general relativity with quantum mechanics, the physics of the very small. “The two theories seems to be very different, and people would like to merge these two theories,” Rodrigues says. But some attempts to do that predict violations of the equivalence principle on a level that’s not yet detectable. That’s why scientists think the equivalence principle is worth testing to ever more precision — even if it means shipping their experiments off to space.