Reading — 2026 Jan–Apr Recall Set 19

A The oceans of Earth cover more than 70 percent of the planet’s surface, yet, until quite recently, we knew less about their depths than we did about the surface of the Moon. Distant as it is, the Moon has been far more accessible to study because astronomers long have been able to look at its surface, first with the naked eye and then with the telescope—both instruments that focus light. And, with telescopes tuned to different wavelengths of light, modern astronomers can not only analyze Earth’s atmosphere, but also determine the temperature and composition of the Sun or other stars many hundreds of light-years away. Until the twentieth century, however, no analogous instruments were available for the study of Earth’s oceans: Light, which can travel trillions of miles through the vast vacuum of space, cannot penetrate very far in seawater. B Curious investigators long have been fascinated by sound and the way it travels in water. As early as 1490, Leonardo da Vinci observed: “If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.” In 1687, the first mathematical theory of sound propagation was published by Sir Isaac Newton in his Philosophiae Naturalis Principia Mathematica. Investigators were measuring the speed of sound in air beginning in the mid seventeenth century, but it was not until 1826 that Daniel Colladon, a Swiss physicist, and Charles Sturm, a French mathematician, accurately measured its speed in water. Using a long tube to listen underwater (as da Vinci had suggested), they recorded how fast the sound of a submerged bell traveled across Lake Geneva. Their result—1,435 meters (1,569 yards) per second in water of 1.8 degrees Celsius (35 degrees Fahrenheit)—was only 3 meters per second off from the speed accepted today. What these investigators demonstrated was that water—whether fresh or salt—is an excellent medium for sound, transmitting it almost five times faster than its speed in air. C In 1877 and 1878, the British scientist John William Strutt, third Baron Rayleigh, published his two-volume seminal work, The Theory of Sound, often regarded as marking the beginning of the modern study of acoustics. The recipient of the Nobel Prize for Physics in 1904 for his successful isolation of the element argon, Lord Rayleigh made key discoveries in the fields of acoustics and optics that are critical to the theory of wave propagation in fluids. Among other things, Lord Rayleigh was the first to describe a sound wave as a mathematical equation (the basis of all theoretical work on acoustics) and the first to describe how small particles in the atmosphere scatter certain wavelengths of sunlight, a principle that also applies to the behavior of sound waves in water. D A number of factors influence how far sound travels underwater and how long it lasts. For one, particles in seawater can reflect, scatter, and absorb certain frequencies of sound—just as certain wavelengths of light may be reflected, scattered, and absorbed by specific types of particles in the atmosphere. Seawater absorbs 30 times the amount of sound absorbed by distilled water, with specific chemicals (such as magnesium sulfate and boric acid) damping out certain frequencies of sound. Researchers also learned that low frequency sounds, whose long wavelengths generally pass over tiny particles, tend to travel farther without loss through absorption or scattering. Further work on the effects of salinity, temperature, and pressure on the speed of sound has yielded fascinating insights into the structure of the ocean. Speaking generally, the ocean is divided into horizontal layers in which sound speed is influenced more greatly by temperature in the upper regions and by pressure in the lower depths. At the surface is a sun-warmed upper layer, the actual temperature and thickness of which varies with the season. At mid-latitudes, this layer tends to be isothermal, that is, the temperature tends to be uniform throughout the layer because the water is well mixed by the action of waves, winds, and convection currents; a sound signal moving down through this layer tends to travel at an almost constant speed. Next comes a transitional layer called the thermocline, in which temperature drops steadily with depth; as temperature falls, so does the speed of sound. E The U.S. Navy was quick to appreciate the usefulness of low-frequency sound and the deep sound channel in extending the range at which it could detect submarines. In great secrecy during the 1950s, the U.S. Navy launched a project that went by the code name Jezebel; it would later come to be known as the Sound Surveillance System (SOSUS). The system involved arrays of underwater microphones, called hydrophones, that were placed on the ocean bottom and connected by cables to onshore processing centers. With SOSUS deployed in both deep and shallow waters along both coasts of North America and the British West Indies, the U.S. Navy not only could detect submarines in much of the northern hemisphere, it also could distinguish how many propellers a submarine had, whether it was conventional or nuclear, and sometimes even the class of sub. F The realization that SOSUS could be used to listen to whales also was made by Christopher Clark, a biological acoustician at Cornell University, when he first visited a SOSUS station in 1992. When Clark looked at the graphic representations of sound, scrolling 24 hours a day, every day, he saw the voice patterns of blue, finback, minke, and humpback whales. He also could hear the sounds. Using a SOSUS receiver in the West Indies, he could hear whales that were 1,770 kilometers (1,100 miles) away. Whales are the biggest of Earth’s creatures. The blue whale, for example, can be 100 feet long and weigh as many tons. Yet these animals also are remarkably elusive. Scientists wish to observe blue whales firsthand must wait in their ships for the whales to surface. A few whales have been tracked briefly in the wild this way but not for very great distances, and much about them remains unknown. Using the SOSUS stations, scientists can track the whales in real time and position them on a map. Moreover, they can track not just one whale at a time, but many creatures simultaneously throughout the North Atlantic and the eastern North Pacific. They also can learn to distinguish whale calls. For example, Fox and colleagues have detected changes in the calls of finback whales during different seasons and have found that blue whales in different regions of the Pacific Ocean have different calls. G SOSUS, with its vast reach, also has proved instrumental in obtaining information crucial to our understanding of Earth’s weather and climate. Specifically, the system has enabled researchers to begin making ocean temperature measurements on a global scale—measurements that are keys to puzzling out the workings of heat transfer between the ocean and the atmosphere. The ocean plays an enormous role in determining air temperature—the heat capacity in only the upper few meters of ocean is thought to be equal to all of the heat in the entire atmosphere. For sound waves traveling horizontally in the ocean, speed is largely a function of temperature. Thus, the travel time of a wave of sound between two points is a sensitive indicator of the average temperature along its path. Transmitting sound in numerous directions through the deep sound channel can give scientists measurements spanning vast areas of the globe. Thousands of sound paths in the ocean could be pieced together into a map of global ocean temperatures and, by repeating measurements along the same paths over times, scientists could track changes in temperature over months or years. H Researchers also are using other acoustic techniques to monitor climate. Oceanographer Jeff Nystuen at the University of Washington, for example, has explored the use of sound to measure rainfall over the ocean. Monitoring changing global rainfall patterns undoubtedly will contribute to understanding major climate change as well as the weather phenomenon known as El Nino. Since 1985, Nystuen has used hydrophones to listen to rain over the ocean, acoustically measuring not only the rainfall rate but also the rainfall type, from drizzle to thunderstorms. By using the sound of rain underwater as a “natural” rain gauge, the measurement of rainfall over the oceans will become available to climatologists.

A rare textile made from the silk of more than a million wild spiders has been on display at the American Museum of Natural History in New York City. To produce this golden cloth, 70 people spent four years collecting golden-orb spiders from telephone poles in Madagascar, while another dozen workers carefully extracted about 80 feet of silk filament from each of the arachnids. The resulting 11-foot-by-4-foot textile is the only large piece of cloth made from natural spider silk in the world today. Spider silk is very elastic and strong compared with steel or Kevlar, said textile expert Simon Peers, who co-led the project. Kevlar is a lightweight synthetic fabric, chemically related to nylon, that is used in bullet-proof vests. Kevlar is resistant to wear, tear and heat and has virtually no melting point. But the tensile strength of spider silk is even greater than Kevlar’s aramid filaments and higher than that of high-grade steel. Most importantly, spider silk is extremely lightweight: a strand long enough to circle the Earth would weigh less than 500 grams (18 oz). It is also especially ductile, able to stretch up to 140 percent of its length without breaking and to retain its strength below –40 °C, giving it toughness equal to that of leading commercial fibres. Researchers have long been intrigued by the unique properties of spider silk. Unfortunately, spider silk is extremely hard to mass-produce. Unlike silkworms—easy to raise in captivity—spiders have a habit of biting off each other’s heads when housed together. According to Peers, there is intensive research worldwide aimed at replicating spider-silk tensile properties for use in medicine and industry, but no-one has yet reproduced all the qualities of natural silk. Peers conceived the idea of weaving spider silk after reading about French missionary Jacob Paul Camboué, who worked with spiders in Madagascar during the 1880s and 1890s. Camboué built a small hand-driven machine to extract silk from up to 24 spiders at once, without harming them: the spiders were briefly restrained, their silk collected, then released. Peers built a replica of this 24-spider “silking” machine, said co-leader Nicholas Godley. As a test the pair collected about 20 spiders. “When we stuck them in the machine and started turning it, lo and behold, this beautiful gold-coloured silk started coming out,” Godley recalled. To make a textile of any significant size, the scale had to increase dramatically. Fourteen thousand spiders yield about an ounce of silk, Godley said, and the finished textile weighs about 2.6 pounds. By the end, handlers had worked with more than one million female golden-orb spiders—abundant in Madagascar and famed for their golden thread. Because the spiders produce silk only in the rainy season, all were collected between October and June. An additional 12 workers used hand-powered machines to extract the silk and twist it into 96-filament yarn. After “silking”, the spiders were released; within a week they regenerate their silk, allowing the same individuals to be used again—“the gift that never stops giving,” said Godley. Spending four years to produce a single piece of cloth is hardly practical for scientists or companies hoping to exploit spider silk in biomedicine or as a Kevlar alternative. Several groups have inserted spider genes into bacteria and even goats to make silk, but results have been only partly successful. One reason is that spider silk begins as a liquid protein produced in a special gland in the abdomen. Using the spinneret, the spider applies force that rearranges the protein’s molecular structure, transforming it into solid fibre. “When we talk about a spider spinning silk, we’re talking about how it applies forces to convert liquid to solid,” explained spider-silk expert Todd Blackledge of the University of Akron, who was not involved in the project. “Every year we get closer to mass production, but we’re not there yet.” For now, we must be content with one extraordinarily beautiful cloth—courtesy of more than a million spiders.

A In 1687, the English scientist Isaac Newton published his monumental work, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), containing his theory of gravitation and the mathematics to support it. In essence, Newton's law of gravitation stated that the gravitational force between two objects, for example, two astronomical bodies, is directly proportional to their masses. Astronomers found that it accurately predicted all the observable data that science at that time was able to collect, with one exception—a very slight variation in the orbit of the planet Mercury around the sun. B It was 228 years before anyone was able to offer a refinement of Newton's law that accounted for the shape of Mercury's orbit. In 1915, Albert Einstein's general theory of relativity was published. Using the equations of general relativity, he calculated the shape of Mercury's orbit. The results predicted astronomical observations exactly and provided the first proof of his theory. Expressing it very simplistically, the general theory of relativity presumes that both matter and energy can distort space-time and cause it to curve. What we commonly call gravity is in fact the effect of that curvature. C Among other phenomena, Einstein's theory predicted the existence of black holes, although initially, he had doubts about their existence. Black holes are areas in space where the gravitational field is so strong that nothing can escape them. Because of the immense gravitational pull, they consume all the light that comes near them, and thus they are "black." In fact, neither emitting nor reflecting light, they are invisible. Due to this, they can be studied only by inference based on observations of their effect on the matter—both stars and gases—around them and by computer simulation. In particular, when gases are being pulled into a black hole, they can reach temperatures up to 1,000 times the heat of the sun and become an intensely glowing source of X rays. D Surrounding each black hole is an "event horizon," which defines the area over which the gravitational force of the black hole operates. Anything passing over the lip of the event horizon is pulled into the black hole. Because observations of event horizons are difficult due to their relatively small size, even less is known about them than about black holes themselves. Black holes exist in three sizes. Compact ones, called star-mass black holes and which have been known to exist for some time, are believed to be the result of the death of a single star. When a star has consumed itself to the point that it no longer has the energy to support its mass, the core collapses and forms a black hole. Shock waves then bounce out, causing the shell of the star to explode. In a way that is not yet understood, the black hole may then reenergize and create multiple explosions within the first few minutes of its existence. So-called supermassive black holes, also well documented, contain the mass of millions or even billions of stars. And just recently one intermediate black hole, with about 500 times the mass of the sun, has been discovered. Scientists have postulated that the intermediate black hole may provide a "missing link" in understanding the evolution of black holes. E Current scientific data suggest that black holes are fairly common and lie at the center of most galaxies. Based on indirect evidence gained using X-ray telescopes, thousands of black holes have been located in our galaxy and beyond. The black hole at the center of the Milky Way, known as Sagittarius A* (pronounced "A-star"), is a supermassive one, containing roughly four million times the mass of our sun. Astronomers suggest that orbiting around Sagittarius A*, 26,000 light-years from Earth, may be as many as tens of thousands of smaller black holes. One possible theory to explain this is that a process called "dynamical friction" is causing stellar black holes to sink toward the center of the galaxy. It is thought that the first black holes came into existence not long after the big bang. Newly created clouds of gases slowly coalesced into the first stars. As these early stars collapsed, they gave rise to the first black holes. A number of theories proposed that the first black holes were essential "seeds," which then gravitationally attracted and consumed enormous quantities of matter found in adjacent gas clouds and dust. This allowed them to grow into the supermassive black holes that now sit in the centers of galaxies. However, a new computer simulation proposes that such growth was minimal. When the simulated star collapsed and formed a black hole, there was very little matter anywhere near the black hole's event horizon. Being in essence "starved," it grew by less than 1 percent over the course of its first hundred million years. The new simulations do not definitively invalidate the seed theory, but they make it far less likely. On the other hand, it is known that black holes existed a billion times more massive than our sun did exist in the early universe. Researchers have yet to discover how these supermassive black holes were formed in such a short time, and the origin of these giants poses one of the most fundamental questions in astrophysics. It has become practically a hallmark of the research on black holes that with each new study, more is known, more theories are generated, and yet more questions are raised than answered.