In the darkness of the deep sea, where sunlight can barely reach, a single photon(分子) of light can blow an animal’s cover.

Scientists have now discovered at least 16 species of ultra-black fish, each equipped with specialized skin that allows then to evade detection while hunting or hiding in the dark. Absorbing 99.95 percent of all photons, this blacker-than-black surface creates a cloak of invisibility against the ocean’s dim backdrop.

Incidentally, this is what initially aroused Karen Osborn’s interest. As a research zoologist at the Smithsonian’s National Museum of Natural History, she grew upset trying to photograph a striking black fish that had been pulled from the deep sea. “It didn’t matter how you set up the camera or lighting — they just sucked up all the light,” says Osborn.

Taking their curiosity to the lab, Osborn and her colleagues carefully analyzed the skin from 18 species of black fish, collected during trawls(拖网) in the Gulf of Mexico and in Monterey Bay, California. All of the creatures had an ultra-black surface that reflected less than 0.6 percent of available light, and 16 species reflected less than 0.5 percent. What’s more, this ultra-black skin was found across the body, and in the near-complete darkness of the deep sea, this cloak of darkness probably evolved to absorb bioluminescent(生物性发光的) light, emitted from prey or predators.

“This low reflectance puts deep-sea fish at the same level with the blackest known animals,” the authors write, “surpassing(超过) the darkness of ultra-black butterflies(0.06 percent—0.5 percent reflectance) and equaling the blackest birds of paradise(0.05 percent—0.31 percent reflectance).” In fact, the black skin of these fish is nearly at the same level with Vantablack, which absorbs 99.96 percent of light—once a record for the blackest material.

Analyzing the structure of this unbelievable fish skin, researchers known to science found pigment cells(色素细胞) were closely packed together, with very few, if any, gaps, almost like a gumball machine(糖果机).

In one species, this ultra-black skin was actually found around the gut(内脏), and this might be used to hide the light of a recent bioluminescent snack. “Effectively what they’ve done is to make a super-efficient and super-thin light trap,” Osborn explains. “Light doesn’t bounce back; light doesn’t go through. It just goes into this layer, and it’s gone.”

Osborn says this is the first time we’ve found an animal that uses just the pigment itself to control unabsorbed light. And it actually seems quite common among 16 distantly-related fish, researchers found that light scattering is caused by the melanosomes(黑色素体) themselves without the need for other structure.

The most detailed study yet of the physics of a finger snap (响指) has shown that in terms of acceleration (加速度), no other body movement comes close. Using high speed cameras, the researchers found that it reached a speed about 20 times faster than the blink of an eye. This “rotational (旋转的) acceleration” was nearly three times greater than the previous record for humans, set by a professional baseball player's arm.

The research, published in the Journal of the Royal Society Interface, divides the finger snap into three stages. First, force is first built up by pressing the thumb and middle fingers together. This appears to be stored in springlike muscle in the fingers and forearm. Friction (摩擦力) between the two fingers acts like a latch (门闩), preventing the energy from being released. Eventually, an “unlatching” process begins, with the thumb moving sideways and the middle finger quickly sliding past the thumb, starting the snap motion. The middle finger then slips into the palm, generating shockwaves that result in a “pop” sound.

The study was inspired by the Hollywood movie Avengers: Infinity War and the character Thanos, who destroys half the universe by snapping his fingers. He does this while wearing a metal glove. When they saw this scene, Dr Saad Bhamla, the leading researcher, and his colleagues asked themselves if this was possible - could you snap your fingers while wearing a metal glove?

They explored the role of friction by covering fingers with different materials, including metallic thimbles (金属套管), which reduced friction. The researchers concluded that a “Goldilocks zone” of friction was necessary. Too little friction and not enough energy was stored to power the snap. Too much friction led to displacement of energy as the fingers took longer to slide past each other, “wasting” the stored energy as heat. “Our results suggest that Thanos could not have snapped because of his metal armoured fingers. So, it's probably the Hollywood special effects, rather than actual physics, at play,” said Bhamla.

If you visit the new Mummies(木乃伊) show at the American Museum of Natural History(AMNH), in New York City, don’t miss the Gilded Lady. Scientists say she probably died of lung disease when she was in her 40s---about 2,000 years ago---in Egypt. They even have a model of her skull(头骨). Based on that, an artist was sure how she may have looked when she was alive. And yet, the mummy’s coffin has never been opened. Instead, scientists used a machine called a CT scanner(扫描仪) to look inside.

CT scanners were developed to help doctors examine patients. But the machines turned out to be perfect for studying mummies, too.

“A hundred years ago, scientists would usually open mummies’ coffins. This did a great deal of harm to those mummies,” say AMNH president Ellen V. Futter. “We just don’t do that anymore. We can do so much better. By using CT scanners, scientists can know about an ancient person’s diet from some of a mummy’s hair. Pieces of bone can show who was related to whom.”

Some mummies in Peru were buried with a skull hanging from their neck. Scientists thought these mummies wore the skull of an enemy. But DNA evidence showed that the mummies were buried with the skull of an ancestor(祖先).

The 19 mummies in the show are from Egypt and Peru. They had completely different reasons for mummifying the dead. The ancient Egyptians believed that mummification allowed an ancestor to live on in the next world. The people who prepared the Gilded Lady thought she would continue to see, hear, and smell. In ancient Peru, people practiced mummification to stay connected with their ancestors. Some families kept mummies in their home.

Futter calls the mummies in the show “messengers from another time.” With the help of technology, scientists will continue to uncover mummies’ secrets so we can understand it.