The relativistic character of the laws of physics began to be apparent very early
in the evolution of classical physics. Even before the time of Galileo and
Newton, Nicolaus Copernicus1 had shown that the complicated and imprecise
Aristotelian method of computing the motions of the planets, based on the assumption
that Earth was located at the center of the universe, could be made much simpler,
though no more accurate, if it were assumed that the planets move about the Sun
instead of Earth. Although Copernicus did not publish his work until very late in
life, it became widely known through correspondence with his contemporaries and
helped pave the way for acceptance a century later of the heliocentric theory of
planetary motion. While the Copernican theory led to a dramatic revolution in human
thought, the aspect that concerns us here is that it did not consider the location of
Earth to be special or favored in any way. Thus, the laws of physics discovered
on Earth could apply equally well with any point taken as the center—i.e., the
same equations would be obtained regardless of the origin of coordinates. This
invariance of the equations that express the laws of physics is what we mean by the
term relativity.
We will begin this chapter by investigating briefly the relativity of Newton’s
laws and then concentrate on the theory of relativity as developed by Albert Einstein
(1879–1955). The theory of relativity consists of two rather different theories, the
special theory and the general theory. The special theory, developed by Einstein and
others in 1905, concerns the comparison of measurements made in different frames
of reference moving with constant velocity relative to each other. Contrary to popu-
lar opinion, the special theory is not difficult to understand. Its consequences, which
can be derived with a minimum of mathematics, are applicable in a wide variety of
situations in physics and engineering. On the other hand, the general theory, also
developed by Einstein (around 1916), is concerned with accelerated reference frames
and gravity. Although a thorough understanding of the general theory requires more
sophisticated mathematics (e.g., tensor analysis), a number of its basic ideas and
important predictions can be discussed at the level of this book. The general theory
is of great importance in cosmology and in understanding events that occur in the
1-1 The Experimental
Basis of
Relativity 4
1-2 Einstein’s
Postulates 11
1-3 The Lorenz
Transformation 17
1-4 TimeDilation
and Length
Contraction 29
1-5 The Doppler
Effect 41
1-6 TheTwin
Paradox and
Other Surprises 45
3
4 Chapter 1
Relativity I
vicinity of very large masses (e.g., stars) but is rarely encountered in other areas of
physics and engineering. We will devote this chapter entirely to the special theory
(often referred to as special relativity) and discuss the general theory in the final
section of Chapter 2, following the sections concerned with special relativistic
mechanics.
Dinosaurs died out more than 65 million years ago (not counting birds, their modern-day relatives). So, it’s a bit surprising that scientists know so much about these ancient creatures. Now, a new study reveals that a certain type of duckbilled dinosaur lived in the Arctic year-round. These animals also traveled in herds that included many age groups, they find. The creatures even appear to have gone through a “teenage growth spurt.”
Just as interesting, however, is how these insights emerged. Scientists didn’t look at a single fossil bone. Instead, they analyzed a large number of preserved footprints on a mountainside located toward the southern end of central Alaska.
Anthony Fiorillo works at the Perot Museum of Nature and Science in Dallas, Texas. As a vertebrate paleontologist, he studies the fossils of creatures with backbones. In 2007, he was part of a research team exploring Denali National Park. “We rounded the corner and there they were,” he recalls. Thousands of footprints had been preserved in stone. “It was amazing.”
Thousands of tracks cover this rocky mountainside in Alaska’s Denali National Park. They provide a wealth of information about the size, age and lifestyle of certain dinosaurs.
COURTESY OF PEROT MUSEUM OF NATURE AND SCIENCE
Those tracks pepper a steep patch of exposed rock about twice as long as a football field and up to 60 meters (roughly 200 feet) wide. They sit at least 160 kilometers (100 miles) north of the Gulf of Alaska. Between 69 million and 72 million years ago, that now-rocky material was muddy sediment on a floodplain near a seacoast, Fiorillo explains. The hadrosaurs walked across the squishy mud. Later, the footprints they left turned to stone.
Previous studies suggested adult duckbills took care of their young, says Fiorillo. The new evidence that these dinosaurs truly traveled in herds with multiple age groups confirms that parents cared for their young well beyond the time they left the nest, his team concludes. The researchers published their findings June 30 in Geology.
Evidence for herds of dinos
Small meat-eating dinosaurs called theropods had left behind a few of the tracks that Fiorillo’s team found in Denali. Birds had left some others. But the vast majority came from creatures called hadrosaurs. These large plant-eating duckbilled dinosaurs had been quite common during the Cretaceous Period. That helps explain one of their nicknames: “cattle of the Cretaceous.”
For the new study, the researchers focused only on the hadrosaur tracks. More than half of the footprints were preserved so well that they had clear impressions of the skin on the dinosaurs’ feet.
A hadrosaur footprint made roughly 70 million years ago. For scale, the long blue bar at right is 10 centimeters long; each small blue or white bar measures 1 centimeter.
COURTESY OF PEROT MUSEUM OF NATURE AND SCIENCE
Most tracks had a similar level of preservation. That suggests all were probably left within a short period. Other fossils in the nearby rocks, including insect burrows, suggest these hadrosaurs had left their footprints during the summer. These are trace fossils — evidence of ancient life other than a preserved carcass or bone.
At the time these dinosaurs lived, Fiorillio says, the average temperature in the warmest months was between 10° and 12° Celsius (50° and 54° Fahrenheit). That’s about what conditions are like today along the border between Canada and the lower 48 U.S. states, he notes.
The team measured a large sample of the duckbills’ footprints. They fell into four distinct size ranges. The largest tracks, presumably made by adults, measured about 64 centimeters (25 inches) across. The smallest tracks, 8 centimeters (3 inches) wide, were likely left by young duckbills. They would have been no more than a year old. Tracks of two other size groups were probably made by juveniles and near-adults.
These data suggest the community of hadrosaurs included four different age groups.
These dinosaurs didn’t migrate
About 84 percent of the tracks sampled for the new study had been left by older hadrosaurs — adults or near-adults. Roughly 13 percent came from the youngest members of the herd. And a mere 3 percent came from herd members considered to be juveniles, says Fiorillo. The rarity of tracks by these tweens suggests that the young of this species had a rapid growth spurt. If true, they would have spent relatively little time at this vulnerable size — and therefore left very few tracks.
“What’s really neat is how many small tracks there are,” notes Anthony Martin. An ichnologist — or expert in trace fossils — he works at Emory University in Atlanta, Ga.
Other scientists had analyzed fossil bones from duckbills. These studies had hinted that the equivalent of adolescent hadrosaurs would have experienced growth spurts. But the new findings are “the best evidence that I’ve seen,” says Eric Snively. He’s a vertebrate paleontologist at the University of Wisconsin-La Crosse. “This is a great study,” he adds, “and further evidence that juvenile hadrosaurs grew up in an eye-blink.”
Also previously, researchers had proposed that Arctic dinosaurs migrated farther south for the winter. That’s because even if the region was much warmer than it is today, nights in the high Arctic would have been 24 hours long. So, with no sunshine for several months, Alaska would have had long periods of very bleak, chilly weather.
Field work is often harsh. Paleontologists studying the dinosaur footprints here on an Alaskan mountainside sometimes worked in cold and fog.
COURTESY OF PEROT MUSEUM OF NATURE AND SCIENCE
But finding juveniles in the herd strongly suggests that these dinosaurs remained in the Arctic all year. That’s because adolescents and preadolescents wouldn’t have had the strength or stamina to make those long treks, Fiorillo maintains.
The presence of very young dinosaurs might have been expected, he notes: If this were a nesting region, the babies would have hatched sometime just before summer. And remember, that’s when these tracks were left. But that wouldn’t explain the juveniles, he says.
The team’s findings “suggest that these dinosaurs were overwintering in Alaska somehow,” says Snively. At the time, the average temperature in the region remained above freezing even during the winter, he notes. But, he adds, “this study raises interesting issues about how the dinosaurs could live in the region when it was pretty dark for several months at a time.”
Power words
Cretaceous Period A geologic time period that included the end of the Age of Dinosaurs. It ran from roughly 145.5 million years ago until 65.5 million years ago.
dinosaur A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. They are distinguished by their hips. The lizard-hipped line became saurichians, such as two-footed theropods like T. rex and the lumbering four-footed apatasaurus (once known as brontosaurus). A second line of so-called bird-hipped, or ornithischian dinosaurs, led to a widely differing group of animals that included the stegosaurs and duckbilled dinosaurs.
floodplain The nearly flat land that runs along the side of a river, for some distance out from the water. When the river floods, it spills over into this plain, which is built up, over time, with the silt left as the waters recede. That silt tends to be soil that eroded off of the upstream lands during rains.
fossil Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils.
geology The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.
hadrosaur A duck-billed, plant-eating dinosaur that lived during the late Cretaceous Period.
histology The anatomical study of the microscopic structure of animal and plant tissues. The microscopic structure of tissue.
ichnologist A scientist who studies trace fossils such as footprints, burrows or chew marks on bones.
paleontologist A scientist who specializes in studying fossils, the remains of ancient organisms.
theropod A meat-eating dinosaur of a group whose members are typically bipedal (walk on two legs) and range from small and delicately built to very large.
trace fossil Evidence other than a preserved carcass or bones of ancient life. Footprints, burrows or chew marks on a bone are examples of such trace fossils. These can provide information that’s more valuable than body fossils. For instance, trace fossils can hint at a creature’s behavior. Evidence of that is usually scant.
tracks and trackways Impressions, usually footprints, left behind by an animal. The spacing and arrangement of individual footprints can provide clues about a number of things, including how large a creature had been and how fast it had been moving.
vertebrate The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes all fish, amphibians, reptiles, birds, and mammals.
On June 12, the Kansas City Royals played at home against the Detroit Tigers. When Royals centerfielder Lorenzo Cain stepped up to the plate at the bottom of the ninth, things looked grim. The Royals hadn’t scored a single run. The Tigers had two. If Cain struck out, the game would be over. No player wants to lose — especially at home.
Cain got off to a rocky start with two strikes. On the mound, Tigers pitcher Jose Valverde wound up. He let fly a special fastball: The pitch whizzed toward Cain at more than 90 miles (145 kilometers) per hour. Cain watched, swung and CRACK! The ball flew up, up, up and away. In the stands at Kauffman Stadium, 24,564 fans watched anxiously, their hopes rising with the ball as it climbed through the air.
The cheering fans weren’t the only ones watching. Radar or cameras track the path of virtually every baseball in major league stadiums. Computer programs can use those tools to generate data about the ball’s position and speed. Scientists also keep a close eye on the ball and study it with all those data.
Some do it because they love baseball. Other researchers may be more fascinated by the science behind the game. They study how all of its fast-moving parts fit together. Physics is the science of studying energy and objects in motion. And with plenty of fast-swinging bats and flying balls, baseball is a constant display of physics in action.
Scientists feed game-related data into specialized computer programs — like the one called PITCH f/x, which analyzes pitches — to determine the speed, spin and path taken by the ball during each pitch. They can compare Valverde’s special pitch to those thrown by other pitchers — or even by Valverde himself, in previous games. The experts also can analyze Cain’s swing to see what he did to make the ball sail so high and far.
“When the ball leaves the bat with a certain speed and at a certain angle, what determines how far it will travel?” asks Alan Nathan. “We’re trying to make sense of the data,” explains this physicist at the University of Illinois at Urbana-Champaign.
When Cain swung his bat that night, he connected with Valverde’s pitch. He successfully transferred energy from his body to his bat. And from the bat to the ball. Fans may have understood those connections. More importantly, they saw that Cain had given the Royals a chance to win the game.
Precision pitches
The 108 stitches on a baseball can slow it down and cause it to move in unexpected directions. Credit: Sean Winters/flickr
This photo shows how a knuckleball pitcher holds the ball. A knuckleball is a pitch that spins little, if at all. As a result, it seems to wander to home plate — and it’s hard both to hit and to catch. Credit: iStockphoto
When a bat hits the ball, it can briefly deform the ball. Some of this energy that went into squeezing the ball will also be released to the air as heat. Credit: UMass Lowell Baseball Research Center
Lorenzo Cain, No. 6 on the Kansas City Royals, saved his team from defeat when he blasted a home run on June 12 in a game against the Detroit Tigers. Credit: Kansas City Royals
Physicists study the science of a moving baseball using natural laws that have been known for hundreds of years. These laws aren’t regulations enforced by the science police. Instead, natural laws are descriptions of the way nature behaves, both invariably and predictably. In the 17th century, physics pioneer Isaac Newton first put into writing a famous law that describes an object in motion.
Newton’s First Law states that a moving object will keep moving in the same direction unless some outside force acts upon it. It also says that an object at rest won’t move without the prodding of some outside force. That means a baseball will stay put, unless a force — like a pitch — propels it. And once a baseball is moving, it will keep moving at the same speed until a force — such as friction, gravity or the swat of a bat — affects it.
Newton’s First Law gets complicated quickly when you’re talking about baseball. The force of gravity constantly pulls down on the ball. (Gravity also causes the arc traced by a ball on its way out of a ballpark.) And as soon as the pitcher releases the ball, it starts to slow due to a force called drag. This is friction caused by air pushing against the baseball in motion. Drag shows up any time an object — whether a baseball or a ship — moves through a fluid, such as air or water.
“A ball that arrives at home plate at 85 miles per hour may have left the pitcher’s hand 10 miles per hour higher,” says Nathan.
Drag slows a pitched ball. That drag depends on the shape of the ball itself. The 108 red stitches roughen a baseball’s surface. This roughness may change how much a ball will be slowed by drag.
Most pitched balls also spin. That also affects how forces act on the moving ball. In a 2008 paper published in the American Journal of Physics, for example, Nathan found that doubling the backspin on a ball caused it to stay in the air longer, fly higher and sail farther. A baseball with backspin moves forward in one direction while spinning backwards, in the opposite direction.
Nathan is currently researching the knuckleball. In this special pitch, a ball barely spins, if at all. Its effect is to make a ball seem to wander. It may fly this way and that, as if it were indecisive. The ball will trace an unpredictable trajectory. A batter who can’t figure out where the ball is going won’t know where to swing either.
“They’re hard to hit and hard to catch,” Nathan observes.
In the Royals game against the Tigers, Detroit pitcher Valverde threw a splitter, the nickname for a split-finger fastball, against Cain. The pitcher throws this by placing the index and middle fingers on different sides of the ball. This special kind of fastball sends the ball zipping quickly toward the batter, but then causes the ball to appear to drop as it nears home plate. Valverde is known for using this pitch to close down a game. This time, the baseball didn’t drop enough to fool Cain.
“It didn’t split too good and the kid hit it out of the park,” observed Jim Leyland, the Tigers manager, during a press conference after the game. The ball soared over the players on its way out of the field. Cain had hit a home run. He scored, and so did another Royals player already on base.
With the score tied, 2-2, the game headed into extra innings.
The smash
Success or failure, for a batter, comes down to something that happens in a split-second: The collision between a bat and the ball.
“A batter is trying to get the head of the bat in the right place at the right time, and with as high a bat speed as possible,” explains Nathan. “What happens to the ball is mainly determined by how fast the bat is moving at the time of collision.”
At that moment, energy becomes the name of the game.
In physics, something has energy if it can do work. Both the moving ball and the swinging bat contribute energy to the collision. These two pieces are moving in different directions when they collide. As the bat smacks into it, the ball first has to come to a complete stop and then start moving again in the opposite direction, back toward the pitcher. Nathan has researched where all that energy goes. Some gets transferred from the bat to the ball, he says, to send it back where it came from. But even more energy goes into bringing the ball to a dead stop.
“The ball ends up kind of squishing,” he says. Some of the energy that squeezes the ball becomes heat. “If your body is sensitive enough to feel it, you could actually feel the ball heat up after you hit it.”
Physicists know that the energy before the collision is the same as the energy afterward. Energy cannot be created or destroyed. Some will go into the ball. Some will slow the bat. Some will be lost to the air, as heat.
Scientists study another quantity in these collisions. Called momentum, it describes a moving object in terms of its speed, mass (the amount of stuff in it) and direction. A moving ball has momentum. So does a swinging bat. And according to another natural law, the sum of the momentum of both has to be the same before and after the collision. So a slow pitch and a slow swing combine to produce a ball that doesn’t go far.
For a batter, there’s another way to understand the conservation of momentum: The faster the pitch and the faster the swing, the farther the ball will fly. A faster pitch is harder to hit than a slower one, but a batter who can do it may score a home run.
Baseball tech
Baseball science is all about performance. And it starts before the players step onto the diamond. Many scientists study the physics of baseball to build, test and improve equipment. Washington State University, in Pullman, has a Sports Science Laboratory. Its researchers use a cannon to fire baseballs at bats in a box outfitted with devices that then measure the speed and direction of each ball. The devices also measure the motion of the bats.
The cannon “projects perfect knuckleballs against the bat,” says mechanical engineer Jeff Kensrud. He manages the laboratory. “We’re looking for perfect collisions, with the ball going straight in and going straight back.” Those perfect collisions allow researchers to compare how different bats react to the pitched balls.
Kensrud says they’re also looking for ways to make baseball a safer sport. The pitcher, in particular, occupies a dangerous place on the field. A batted ball can rocket right back toward the pitcher’s mound, traveling just as fast or faster than the pitch. Kensrud says his research team looks for ways to help the pitcher, by analyzing how long it takes for a pitcher to react to an incoming ball. The team is also studying new chest or face protectors that might lessen the blow of an incoming ball.
Beyond physics
The 10th inning of the Tigers-Royals game went unlike the previous nine. The Tigers didn’t score again, but the Royals did. They won the game 3-2.
As the happy Royals fans headed home, the stadium went dark. Though the game might have ended, information from it will continue to be analyzed by scientists — and not just physicists.
Some researchers study the hundreds of numbers, such as the tallies of hits, outs, runs or wins that every game generates.
These data, called statistics, can show patterns that otherwise would be hard to see. Baseball is full of statistics, such as data on which players are hitting better than they used to, and which aren’t. In a December 2012 paper published in the research journalPLOS ONE, researchers found that players perform better when they’re on a team with a slugger who is on a hitting streak. Other researchers may compare statistics from different years to look for longer-term patterns, such as whether baseball players overall are getting better or worse at hitting.
Biologists, too, follow the sport with keen interest. In a June 2013 paper published in Nature, biologist Neil Roach from George Washington University in Washington, D.C., reported that chimps, like pitchers, can throw a ball at high speed. (Though don’t look for the animals on the mound.)
As for Cain, the Royals centerfielder, by halfway through the season he had hit only one more home run since that June 12 game against the Tigers. Still, statistics show Cain had by then improved his overall batting average to .259, after a slump earlier in the season.
That is just one way the scientific study of baseball continues to improve the game, for both its players and its fans. Batter up!
This video of a knuckleball thrown by the Toronto Blue Jays’ R. A. Dickey shows how unpredictable these balls can be.
Power Words
biology The scientific study of living things.
drag The slowing force exerted by air or other fluid surrounding a moving object.
force An influence that tends to change the motion of a body or produce motion or stress in a stationary body.
energy A property of matter or radiation that describes the ability to do work.
friction Resistance to motion that arises when one object moves over another.
gravity The force that attracts any body with mass, or bulk, toward any other body with mass. Earth’s gravity keeps things on the planet’s surface and pulls down flying baseballs.
momentum The quantity of motion of a moving body, measured as a product of its mass and velocity.
Newton’s First Law A description of how things move in nature, namely that a body continues in a state of rest or uniform motion in a straight line unless it is acted on by an external force.
physics The scientific study of the nature and properties of matter and energy.
statistics The practice or science of collecting and analyzing numerical data in large quantities.
trajectory The path traced by a flying object.
work The transfer of energy from one system to another, by the application of a force.
Some people really don't like to be alone with their thoughts. Indeed, people taking part in one new study chose to receive an electric shock rather than spend 15 minutes alone in a room with nothing to do.
“The human mind wants to engage with the world, even, it appears, if that involves pain,” Timothy Wilson concludes. A psychologist, he works at the University of Virginia in Charlottesville.
The brains of mammals evolved, or changed slowly over generations, to watch for dangers and opportunities. Only humans evolved the ability to think about thinking, he says. Still, people can become uncomfortable controlling their own thoughts, Wilson says. They may not be able to steer those thoughts in a pleasing or comforting direction. And that may help explain the popularity of meditation. It’s a process by which people learn to control their thoughts and benefit from quiet contemplation.
For its new study, Wilson’s group recruited 146 college students. The researchers asked each student to sit alone in a room at their lab without any devices, such as cell phones. This solitude lasted for 6 to 15 minutes. Most participants said they not only had trouble concentrating but also found the experience unpleasant.
The researchers then asked 44 of these people to sit alone at home, too. Again, the students had about the same reaction as they had in the lab. They found the experience very uncomfortable. About one-third also reported they had cheated. They listened to music or read instead of just thinking in silence.
Finally, the scientists recruited 66 adults between the ages of 18 and 77 at a farmer's market or church. Each recruit took the same test of contemplation. And like the students, these people reported finding it unpleasant to sit at home alone with their thoughts.
But here’s the shocker — quite literally. The scientists asked 18 men and 24 women to sit quietly and just think about anything. Their quiet, alone time lasted a full 15 minutes. But for this experiment, the participants could give themselves a painful electric shock if they wished. That shock, though, would not make their participation in the experiment end any earlier. Still, 12 men and six women chose to receive a shock, rather than just continue to sit in peace.
Psychologist Jonathan Schooler of the University of California, Santa Barbara, did not work on the new study. He says its findings point to how many people find it unpleasant to reflect on issues by themselves. They’d rather feel pain than endure quiet time alone.
Power Words
evolve To change gradually over generations, or a long period of time. In living organisms, the evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs).
contemplation The act of thinking silently and deeply for a long while.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.
meditate To think deeply or focus one's mind for a period of time, in silence or with the aid of chanting. Sometimes it’s done for religious or spiritual purposes. It also can become a method of relaxation.
psychology The study of the human mind, especially in relation to actions and behavior. Scientists and mental-health professionals who work in this field are known as psychologists.
recruit (in research)New member of a group or human trial, or to enroll a new member into a research trial. Some may receive money or other compensation for their participation, particularly if they enter the trial healthy.
Researchers have squeezed diamonds to a record-setting pressure — 14 times as high as that inside Earth’s core. This super-compressed carbon could reveal the type of extreme conditions present deep inside supersized exoplanets, those out beyond the solar system. That’s what scientists report in the July 17 Nature.
Diamond is a crystal made from carbon. It also is the least compressible material known. But that didn’t daunt physicist Ray Smith of Lawrence Livermore National Laboratory, in California, and his team. They harnessed the world’s largest laser. At theNational Ignition Facility, in Livermore, Calif., it’s the powerful device designed for laser-fusion research.
Smith’s team focused 176 laser beams onto hair-thin layers of gold and artificial diamond (see setup in photo above). This created waves of pressure. The gold layers helped disperse heat, Smith says. This helped avoid a problem that can mean diamonds aren’t forever: As pressure mounts, diamond can liquefy, he notes. And that would have ruined the experiment. But his team found that an initial small wave of pressure helps prevent that melting before the researchers gradually and massively ramped up the material’s compression. How massively? To 5 terapascals, which is about 50 million times Earth’s atmospheric pressure at sea level.
The entire process lasted a mere 20 billionths of a second!
Smith says the properties of carbon under pressure could help researchers better simulate the insides of Neptune-like gassy exoplanets. Some, he notes, may have diamond cores. To mimic the cores of giant rocky planets, he plans to compress iron by exerting similar pressures.
Power Words
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
core In geology, Earth’s innermost layer.
density A measure of the consistency of an object, found by dividing the mass by the volume.
diamond One of the hardest known substances and rarest gems on Earth. Diamonds form deep within the planet when carbon is compressed under incredibly strong pressure.
exoplanet A planet that orbits a star outside the solar system.
Neptune The furthest planet from the sun in our solar system and its fourth largest.
nuclear fusion The process of forcing together the nuclei of atoms.
pascal A unit of pressure in the metric system. It is named for Blaise Pascal, the 17th century French scientist and mathematician. He also developed what became known as Pascal’s law of pressure. It holds that when a confined liquid is pressed, that pressure will be transmitted throughout the liquid in all directions, without any losses.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. It’s an alternative to quantum physics in explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
solar system The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
simulate To deceive in some way by imitating the form or function of something. A simulated dietary fat, for instance, may deceive the mouth that it has tasted a real fat because it has the same feel on the tongue — without having any calories. A simulated sense of touch may fool the brain into thinking a finger has touched something even though a hand may no longer exists and has been replaced by a synthetic limb. (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.
tera A prefix for units of measurement meaning trillion in the international metric system.
Everyone knows that African elephants boast versatile snouts. They can toss logs, grab food and spray water. But the towering mammals also may be the world’s best smellers. That’s what scientists conclude in a July 22 report in Genome Research.
The team looked at bush elephants (Loxodonta africana). These are the larger of Africa’s two species. They tend to live in fairly open, grassy areas (hence the term “bush” in their common name). This species hosts some 2,000 different genes for sensing odors. Scientists refer to these sensors as olfactory receptors; olfaction (Oll-FAK-shun) refers to the sense of smell. These sensors are found on the outside of scent-sensing cells. They’re in a nasal cavity, near the top of the animal’s trunk.
Renowned sniffers like rats have around 1,200 genes for scent-sensing. Bloodhounds and other dogs get by with about 800 of these different genes. Humans and other primates possess relatively poor sniffers. They also have only about 40 olfactory genes.
The researchers think that long ago, when mammals split into a broad range of new species, the original smell-sensing gene began copying itself — and morphing somewhat — over and over again. This appears to have happened the most in ancestors of today’s elephants.
The elephant’s ability to detect a broad range of odors perhaps explains why scents play a big role in its behavior. African elephants, for instance, can communicate aggression via scents. And the animals also can distinguish people from two ethnic groups living near them in East Africa — the Maasai and Kamba. That’s helpful because the Maasai herders, in Kenya, hunt elephants. Mostly farmers, the Kamba pose no threat to the pachyderms.
Power Words
behavior The way a person or other organism acts towards others, or conducts itself.
cavity A large open region surrounded by tissues (in living organisms) or some rigid structure (in geology or physics).
gene A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
genome The complete set of genes or genetic material in a cell or an organism.
nasal Having to do with the nose.
olfaction The sense of smell.
pachyderm A name for elephants and other nonruminant animals with hooves (or nails that look like them) and thick skin. A rhinoceros or hippopotamus also fits within this group, although most people use the term as a synonym for elephant.
receptor (in biology) A molecule in cells that serves as a docking station for another molecule. Thatsecond molecule can turn on some special activity by the cell.
Tibet has been called the “roof of the world.” It’s a vast region in southwest China that sits on the highest plateau on Earth. It's cold and dry. What’s more, its air has 40 percent less oxygen than does the air at sea level. That should make it difficult for people to live there. But Tibetans comfortably survive such extreme conditions. And it’s due in part to an ancient genetic gift from their ancestors. These were amazingly ancient ancestors, a new study finds. Those long-extinct folk were essentially cousins of Neandertals.
Scientists have known that Tibetans carry an unusual form of a gene called EPAS1. A gene is a snippet of DNA, found inside cells. It tells a cell how to make the molecules that will help it function. The EPAS1 gene helps the body use oxygen. The version of this gene in the native people of Tibet helps them deal with their homeland’s extremely low levels of oxygen.
Researchers had wondered when that altered gene first evolved. Its small but important genetic tweak almost certainly came from an ancient group of pre-human relatives, scientists now report. They share their analyses in the July 2 Science.
Anna Di Rienzo, who studies genes at the University of Chicago, did not work on the new study. She says the new findings provide evidence that humans picked up genetic changes from other ancient species. The gene may have been passed along when two closely related species produced offspring together, she told Science News. Such breeding across closely related species likely introduced other helpful genetic changes as well, she adds.
Rasmus Nielsen led the new study. Earlier, this geneticist at the University of California, Berkeley, had looked for the source of the Tibetan gene variant in other peoples. In one study, he showed it probably did not come from the Han Chinese. They are closely related to Tibetans. In another study, he probed the DNA of ancient Neandertals. Again, no match.
“So we just put it aside because we couldn’t make sense of the data,” Nielsen told Science News.
But in 2010, researchers discovered a finger bone from a previously unknown ancient group of pre-human folk. Scientists named these hominids Denisovans (Deh-NEE-so-vuns). In 2012, researchers finished describing the entire Denisovan genome (all of an individual’s DNA). To their surprise, when Nielsen and his team looked at this genome, they now found a match. The Denisovans had the same EPAS1 change that helps Tibetans survive in low-oxygen environments.
The discovery indicates that the Tibetans probably inherited the genetic tweak from those Denisovans.
Pontus Skoglund is not entirely convinced, however, that the genetic journey from Denisovans to modern humans is as straightforward as it might at first seem. Skoglund is a geneticist at Harvard Medical School in Boston, Mass. Denisovans interbred with that other ancient group, the Neandertals, he notes. And scientists have found evidence that ancient humansinterbred with Neandertals. Modern people, including Tibetans, still have some DNA inherited from Neandertals. So, Skoglund told Science News, “there's still a little wiggle room for [the EPAS1 variant] to have come from Neandertals.”
Power Words
archaic An adjective meaning ancient. Something from a far earlier period in time, often thousands, if not millions, of years ago.
Denisovans An ancient humanlike population whose existence is known only because of a few fossils discovered in a cave in Siberia.
DNA (short for deoxyribonucleic acid)A long, spiral-shaped molecule inside most living cells that carries genetic instructions. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
evolution A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.
gene A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics.
hominid A primate of an animal family that includes humans and their fossil ancestors.
Neandertal (sometimes spelled Neanderthal) A species (Homo neanderthalensis) that lived in Europe and parts of Asia from about 200,000 years ago to roughly 28,000 years ago.
oxygen A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.
variant (in genetics) A gene having a slight mutation that may have left its host species somewhat better adapted for its environment.
Sun-loving tourists never question much about the Bahamas beyond what the weather will be like during their visits. But scientists have had plenty of questions about this island nation in the Caribbean. Chief among them is why the islands even exist. The conditions that allowed other Caribbean islands to form aren’t present in the region of the Bahamas. But some geoscientists now think they’ve discovered what makes the Bahamas possible: Africa’s Sahara Desert.
In a new study, these scientists present data that suggest the islands built up as a result of dust carried across the Atlantic Ocean from the Sahara. This immense desert covers most of the northern expanse of Africa.
Yet the Bahamas aren't made from dust. Instead, the scientists suspect that Saharan dust fed ocean microbes called cyanobacteria. As these bacteria grow, they produce a material called calcium carbonate. This carbonate is the building block of the islands of the Bahamas. The geoscientists presented their idea June 30 in Geology.
For some 100 million years, that calcium carbonate accumulated on the seafloor. It initially formed an underwater shelf as large as Iceland. That shelf is called the Great Bahama Bank. The Bahamian islands are the uppermost peaks rising from that shelf.
“Without this dust, the Bahamas might never be there,” Peter Swart told Science News. “We’d just have a big open ocean and maybe a little underwater bump.” Swart is a geochemist at the University of Miami in Florida.
Carbonate structures such as the shelf usually form from the skeletons of creatures like coral and mollusks. But few of those ocean creatures live near the Bahamas. That set scientists looking for another explanation.
Visitors to the Bahamas have long observed whitings. These milky-white clouds in the water can stretch up to 100 kilometers (62 miles) long and wide. That’s an area about the size of New York City’s island borough of Manhattan. Whitings are made of calcium carbonate. More than 20 years ago, scientists found cyanobacteria within the whitings. Cyanobacteria also produce calcium carbonate. So scientists reasoned that the microbes could have made the shelf.
But the cyanobacteria would have needed nutrients in order to grow and produce the calcium carbonate. “The Great Bahama Bank sits in a nutrient-poor environment,” Swart says. Where the microbes were getting their nutrients was a mystery.
So Swart’s team collected samples from the seafloor along the shelf. They discovered that samples from areas with more whitings had an unusual abundance of one nutrient — iron. Cyanobacteria eat iron. And where levels of iron are high, they can make more calcium carbonate.
The scientists also found that Andros Island, the largest island in the Bahamas, had the most whitings. It also had the highest levels of iron. And it sat on the thickest part of the shelf.
Dust blown across the ocean from the Sahara Desert contains high levels of iron. Swart and his team suspect this dust gets caught on islands like Andros and runs off into the water. It also falls into the surrounding ocean. There, it fuels the cyanobacteria. The microbes then produce huge plumes of calcium carbonate that build the Bahamanian islands.
Other scientists say Swart's idea makes sense. “It needs some testing, but it ties everything together neatly,” Lisa Robbins toldScience News. Robbins is an oceanographer at the U.S. Geological Survey in St. Petersburg, Fla. She did not work on the new study. She says: “Anyone who has a biology background will say, ‘Oh yeah, this makes sense.'”
Power Words
bacterium (plural bacteria) A single-celled organism forming one of the three domains of life. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.
calcium A chemical element which is common in minerals of the Earth’s crust, and is also found in bone mineral and teeth.
calcium carbonate The main chemical compound in limestone, a rock made from the tiny shells of ancient marine organisms. Its formula is CaCO3 (meaning it contains one calcium atom, one carbon atom and three oxygen atoms).
coral Marine animals that often produce a hard and stony exoskeleton and tend to live on the exoskeletons of dead corals, called reefs
cyanobacterium A type of bacterium that can convert carbon dioxide into other molecules, including oxygen.
geochemistry A science that deals with the chemical composition of and chemical changes in the solid material of Earth or of another celestial body (such as the moon or Mars).
microbe Short for microorganism. (see microorganism)
microorganism A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
nutrients Vitamins, minerals, fats, carbohydrates and proteins needed by organisms to live, and which are extracted through the diet.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Farmers often use chemicals to kill the bugs that dine on crops. But insects aren't the poisons' only victims. A new study links widespread use of one insecticide with a drop in the number of local birds. A second study finds relatively high levels of this chemical — and related ones — taint rivers that drain off of U.S. farmlands.
Both studies focused on bug killers known as neonicotinoids (nee-oh-NIK-uh-tin-oydz). Earlier research showed these chemicals can harm honeybees and bumblebees. Both are important because they pollinate plants. That includes many that people eat. Now scientists are linking the chemicals to shrinking bird populations. Researchers don’t yet know what role the chemicals play. They may poison the birds or just deprive them of the insects they need to eat.
Whatever the reason, “the effects could be more widespread in the ecosystem than we thought,” Ruud Foppen told Science News. He’s a bird researcher at Sovon, the Dutch Center for Field Ornithology in Nijmegen.
Farmers spray neonicotinoids on crops. They also buy seeds coated with these insecticides. Rains can then wash these treatments off farm fields and into nearby lakes and rivers.
Sampling these waters can be a good way to track the use of pesticides in an area. And that’s what Foppen and his colleagues did. They looked at Dutch water pollution data from 2003 to 2009. They focused on imidacloprid (ih-MEED-uh-KLOH-prid). It’s the most commonly used neonicotinoid in that area. They also examined surveys of 15 insect-eating birds. Nine of the species eat nothing but insects. The rest feed insects to their babies. Scientists tallied those birds from 2003 to 2010.
Foppen’s group was able to link the bird and pesticide data. After imidacloprid showed up in the water — which would have happened only after farmers started using the chemical on their crops — bird numbers began to fall. And the more imidacloprid in local streams, the bigger a decline in birds compared to the last year.
The scientists looked for other possible explanations on why bird numbers fell. Only imidacloprid seemed to explain the yearly decrease seen in local birds.
Foppen and his team published their findings July 9 in Nature.
Bird numbers may be falling because the bug killers are leaving fewer insects for them to eat, the researchers say. Or it’s possible that the chemicals may poison the birds directly when they dine on treated bugs.
Other ecologists are taking note of the findings. Scientists had suspected that neonicotinoids might affect species other than bees. But the new study is the first to find direct evidence for this, says Dave Goulson. He's an ecologist at the University of Sussex in England.
“This moves the debate on quite a bit,” he told Science News.
Use also widespread in United States
Neonicotinoids also are being widely used in the U.S. Midwest, according to new data by scientists with the U.S. Geological Survey. Many cereal grains and soybeans are grown throughout the Midwest. Not surprisingly, it has come to be known as the American “breadbasket.”
In the United States, now, almost all corn sold for planting as seed comes coated with a neonicotinoid insecticide. It protects the young seed from being eaten by bugs. But the chemical can wash off into the environment.
USDA
Michelle Hladik of USGS in Sacramento, Calif., worked with agency scientists in two other states. They sampled water during last year’s growing season from nine streams in Iowa. It’s a prime corn- and soy-growing region. The researchers screened the water for six neonic pesticides. These included imidacloprid.
The chemicals showed up in every stream. The levels varied, however, over time. “We noticed higher levels of these insecticides after rain storms during crop planting,” Hladik says. And this makes sense. Today, most U.S. corn and soybeans come from seeds that had been coated with neonicotinoids. However, she adds, “the insecticides also were detected prior to their first use during the growing season.” That suggests, she says, “that they can persist” in the environment from at least the past year.
Long-term exposure to even low levels of these chemicals can poison fish and other aquatic animals. Concentrations of just 10 to 100 nanograms (billionths of a gram) per liter of imidacloprid can prove toxic to such organisms. The new study found levels of up to 42.7 nanograms per liter of this insecticide in Iowa rivers. Clothianidin (Klo-thee-AN-ih-din) was the most commonly detected neonic in these rivers. It showed up in 75 percent of the 79 water samples. The highest level seen: 257 nanograms per liter of water. The findings will appear in the October issue of the journal Environmental Pollution.
Hladik’s team concludes that the “near universal” use of neonic-coated seeds by U.S. farmers now “needs to be closely examined in relation to environmental impacts of these insecticides.”
Power Words
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.
insecticide A poison applied to kill insects.
ornithology The scientific study of birds.
pesticide A chemical or mix of compounds used to kill insects, rodents or other organisms harmful to cultivated plants, pet or livestock, or unwanted organisms that infest homes, offices, farm buildings and other protected structures.
pollinate To transport male reproductive cells — pollen — to female parts of a flower. This allows fertilization, the first step in plant reproduction.
U.S. Geological Survey, or USGS This is the largest nonmilitary U.S. agency charged with mapping water, Earth and biological resources. It collects information to help monitor the health of ecosystems, natural resources and natural hazards. It also studies the impacts of climate and land-use changes. A part of the U.S. Department of the Interior, USGS is headquartered in Reston, Va.
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