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Long Ago, a Rodent as Big as a Bull Lurked in South America

Uruguayan scientists say they have uncovered fossil evidence of the biggest species of rodent ever found, one that scurried across wooded areas of South America about four million years ago, when the continent was not connected to North America.

A herbivore, the beast may have been a contemporary, and possibly prey, of saber-toothed cats — a prehistoric version of Tom and Jerry.

Its huge skull, more than 20 inches long, suggested a beast more than eight feet long and weighing between 1,700 and 3,000 pounds.

Although British newspapers variously described it as a mouse or a rat, researchers say the animal, named Josephoartigasia monesi, actually was more closely related to a guinea pig or porcupine. “These are totally different from the rats and mice we’re accustomed to,” said Bruce Patterson, the curator of mammals at the Field Museum in Chicago, adding that it was the biggest rodent that he had ever heard of.

An artist’s rendering showed a creature that looked like a cross between a hippopotamus and a guinea pig.

The fossil was found in 1987 about 65 miles west of the capital, Montevideo, near the vast River Plate estuary — a muddy waterway separating Uruguay from Argentina that empties into the South Atlantic. That area is the site of ancient riverbanks and other deposits where fossils have been found, Mr. Patterson said.

An Argentine fossil collector identified as Sergio Viera donated the skull to Uruguay’s National History and Anthropology Museum nearly two decades ago, said the museum’s director, Arturo Toscano.

It spent years hidden away in a box at the museum and was rediscovered by a curator, Andrés Rinderknecht, who enlisted the help of a fellow researcher, Ernesto Blanco, to study it.

Mr. Blanco said he was shocked when he first came face to face with the fossil, saying it looked even bigger than a cow skull. “It’s a beautiful piece of nature,” he said in an interview. “You feel the power of a very big animal behind this.”

The research by Mr. Rinderknecht and Mr. Blanco was published Wednesday in this week’s issue of a biological research journal, Proceedings of the Royal Society B.

Mr. Blanco said the skull’s shape and the huge incisors left no doubt that they were dealing with a rodent, but he cautioned that the estimate of the animal’s bulk was imprecise.

The extinct rodent clearly outclassed its nearest rival, the Phoberomys, found in Venezuela and estimated to weigh between 880 and 1,500 pounds.

Mr. Blanco said the rodent was far bigger than any South American rodent alive today, surpassing the present-day capybara, which can weigh up to 110 pounds.

He said the animal’s teeth pointed to a diet of aquatic plants.

“From what we can tell, we know it was a herbivore that lived on the shores of rivers or alongside streams in woodland areas,” Mr. Rinderknecht said. “Possibly it had a behavior similar to other water-faring rodents that exist today, such as beavers, which split their time between land and water.”


PR

What's Behind the Record Price of Gold?

The price of gold continues to hit record highs this week, trading above $900 an ounce, but the precious metal has been highly valued for thousands of years.

The latest high prices for gold are part of an upward trend that began in April 2001. Analysts explain the bull market in gold by pointing to a slowing economy and the metal's increasing scarcity in the ground.

“Gold is inversely correlated to the dollar,” said George Milling-Stanley, an analyst for the World Gold Council, an organization funded by gold mining companies. “Gold is a safe haven in times of political as well as economic turmoil.”

Trouble is, this extremely rare commodity is getting harder to find.

Miners don’t happen upon rich veins of gold today like they used to. Big mining companies nowadays hope to find mere flecks. Although gold is mined in more than 60 countries, it is estimated only 167,600 tons of gold have ever been mined. In comparison, 999 million tons of iron are extracted annually.

Hard-to-reach pots of gold have become harder and harder to find, and not many new gold mines have come into production in recent years. With the absence of big new discoveries, demand for gold continues to grow, as does its price.

Still, with inflation taken into account, the price is nowhere near as high as it seems.

Golden elements

Most of the gold collected today becomes jewelry. According to the U.S. Geological Survey, 84 percent of the gold produced in 2006 was used for jewelry and the arts.

Gold’s chemical symbol Au comes from the Latin word aurum, which means shining dawn. Combining gold with an alloy element such as nickel or palladium turns gold white.

Beyond its charm, gold’s unusual properties have put it to good use.

Pure gold is relatively soft, with the same hardness of a copper penny (try finding a penny made of real copper, however). It is the most malleable and ductile of metals. Only copper and silver are better at transferring heat and electricity than gold. In addition, gold is extremely resistant to corrosion. Only a solution of cyanide can dissolve the hearty metal.

Gold’s properties have made it an essential industrial metal in technologies such as computers, communications equipment, spacecraft, and jet aircraft engines.

The visors of astronauts’ helmets are coated in a thin layer of gold that reduces glare and keeps them cool.

Gold Standard

Artisans of ancient civilizations used the precious metal to decorate tombs, jewelry, figurines, and beads.

The oldest known objects worked from gold were discovered at a burial site in Bulgaria and were made by members of the ancient Thracian civilization in 4400 B.C.

Since then many societies worldwide have used gold for jewelry and as money. Its monetary value shone so brightly that it was a factor in driving Europeans to explore the New World.

During the 1800s, the United States and many other countries relied on a system of money, called the gold standard, which fixed U.S. currency to the price of gold and silver.

The system was rocked when the SS Central America and its three tons of treasure sunk off the coast of South Carolina in 1857. The loss led to the economic depression that lasted until the Civil War.

In 1900 the Gold Standard Act officially set a golden value for the dollar, but the act did not live long. In 1933 President Franklin D. Roosevelt outlawed private ownership of gold, except for jewelry.

The Bretton Woods system of 1946 (which established rules for financial relations among the world's major industrial states) allowed foreign governments to sell gold to the United States treasury for $35 an ounce. But in 1971, President Richard Nixon ended the system, and officially ended the gold standard. Since then world currencies have not been formally linked to gold.

In the money

The latest price surge is not the first driven by economics and politics.

During World War I, a shortage of manpower closed many gold mines. Mines were brought back into production during the Depression. In 1934, the price of gold was raised from $20.67 to $35 an ounce, and production increased to more than 4 million ounces annually.

Although the $1,000-an-ounce mark does have an unfamiliar and ominous ring to it, the World Gold Council’s analyst Milling-Stanley points out that the benchmark is deceiving.

The previous all-time high of $850 in 1980 was the result of “a slew of special circumstances,” Milling-Stanley told LiveScience, such as inflation, 40 years of pent up investing and the perception that Jimmy Carter was a weak president.

After 28 years of inflation and a weak dollar, it will take a big push in the markets to surpass the 80s high in real terms. Gold would have to hit $2,200 an ounce in today’s dollars to match the 1980 price, Milling-Stanley said. 

Big Brain Theory: Have Cosmologists Lost Theirs?

If true, it would mean that you yourself reading this article are more likely to be some momentary fluctuation in a field of matter and energy out in space than a person with a real past born through billions of years of evolution in an orderly star-spangled cosmos. Your memories and the world you think you see around you are illusions.

This bizarre picture is the outcome of a recent series of calculations that take some of the bedrock theories and discoveries of modern cosmology to the limit. Nobody in the field believes that this is the way things really work, however. And so there in the last couple of years there has been a growing stream of debate and dueling papers, replete with references to such esoteric subjects as reincarnation, multiple universes and even the death of spacetime, as cosmologists try to square the predictions of their cherished theories with their convictions that we and the universe are real. The basic problem is that across the eons of time, the standard theories suggest, the universe can recur over and over again in an endless cycle of big bangs, but it’s hard for nature to make a whole universe. It’s much easier to make fragments of one, like planets, yourself maybe in a spacesuit or even — in the most absurd and troubling example — a naked brain floating in space. Nature tends to do what is easiest, from the standpoint of energy and probability. And so these fragments — in particular the brains — would appear far more frequently than real full-fledged universes, or than us. Or they might be us.

Alan Guth, a cosmologist at the Massachusetts Institute of Technology who agrees this overabundance is absurd, pointed out that some calculations result in an infinite number of free-floating brains for every normal brain, making it “infinitely unlikely for us to be normal brains.” Welcome to what physicists call the Boltzmann brain problem, named after the 19th-century Austrian physicist Ludwig Boltzmann, who suggested the mechanism by which such fluctuations could happen in a gas or in the universe. Cosmologists also refer to them as “freaky observers,” in contrast to regular or “ordered” observers of the cosmos like ourselves. Cosmologists are desperate to eliminate these freaks from their theories, but so far they can’t even agree on how or even on whether they are making any progress.

If you are inclined to skepticism this debate might seem like further evidence that cosmologists, who gave us dark matter, dark energy and speak with apparent aplomb about gazillions of parallel universes, have finally lost their minds. But the cosmologists say the brain problem serves as a valuable reality check as they contemplate the far, far future and zillions of bubble universes popping off from one another in an ever-increasing rush through eternity. What, for example is a “typical” observer in such a setup? If some atoms in another universe stick together briefly to look, talk and think exactly like you, is it really you?

“It is part of a much bigger set of questions about how to think about probabilities in an infinite universe in which everything that can occur, does occur, infinitely many times,” said Leonard Susskind of Stanford, a co-author of a paper in 2002 that helped set off the debate. Or as Andrei Linde, another Stanford theorist given to colorful language, loosely characterized the possibility of a replica of your own brain forming out in space sometime, “How do you compute the probability to be reincarnated to the probability of being born?”

The Boltzmann brain problem arises from a string of logical conclusions that all spring from another deep and old question, namely why time seems to go in only one direction. Why can’t you unscramble an egg? The fundamental laws governing the atoms bouncing off one another in the egg look the same whether time goes forward or backward. In this universe, at least, the future and the past are different and you can’t remember who is going to win the Super Bowl next week.

“When you break an egg and scramble it you are doing cosmology,” said Sean Carroll, a cosmologist at the California Institute of Technology.

Boltzmann ascribed this so-called arrow of time to the tendency of any collection of particles to spread out into the most random and useless configuration, in accordance with the second law of thermodynamics (sometimes paraphrased as “things get worse”), which says that entropy, which is a measure of disorder or wasted energy, can never decrease in a closed system like the universe.

If the universe was running down and entropy was increasing now, that was because the universe must have been highly ordered in the past.

In Boltzmann’s time the universe was presumed to have been around forever, in which case it would long ago have stabilized at a lukewarm temperature and died a “heat death.” It would already have maximum entropy, and so with no way to become more disorderly there would be no arrow of time. No life would be possible but that would be all right because life would be excruciatingly boring. Boltzmann said that entropy was all about odds, however, and if we waited long enough the random bumping of atoms would occasionally produce the cosmic equivalent of an egg unscrambling. A rare fluctuation would decrease the entropy in some place and start the arrow of time pointing and history flowing again. That is not what happened. Astronomers now know the universe has not lasted forever. It was born in the Big Bang, which somehow set the arrow of time, 14 billion years ago. The linchpin of the Big Bang is thought to be an explosive moment known as inflation, during which space became suffused with energy that had an antigravitational effect and ballooned violently outward, ironing the kinks and irregularities out of what is now the observable universe and endowing primordial chaos with order.

Inflation is a veritable cosmological fertility principle. Fluctuations in the field driving inflation also would have seeded the universe with the lumps that eventually grew to be galaxies, stars and people. According to the more extended version, called eternal inflation, an endless array of bubble or “pocket” universes are branching off from one another at a dizzying and exponentially increasing rate. They could have different properties and perhaps even different laws of physics, so the story goes.

A different, but perhaps related, form of antigravity, glibly dubbed dark energy, seems to be running the universe now, and that is the culprit responsible for the Boltzmann brains.

The expansion of the universe seems to be accelerating, making galaxies fly away from one another faster and faster. If the leading dark-energy suspect, a universal repulsion Einstein called the cosmological constant, is true, this runaway process will last forever, and distant galaxies will eventually be moving apart so quickly that they cannot communicate with one another. Being in such a space would be like being surrounded by a black hole.

Rather than simply going to black like “The Sopranos” conclusion, however, the cosmic horizon would glow, emitting a feeble spray of elementary particles and radiation, with a temperature of a fraction of a billionth of a degree, courtesy of quantum uncertainty. That radiation bath will be subject to random fluctuations just like Boltzmann’s eternal universe, however, and every once in a very long, long time, one of those fluctuations would be big enough to recreate the Big Bang. In the fullness of time this process could lead to the endless series of recurring universes. Our present universe could be part of that chain.

In such a recurrent setup, however, Dr. Susskind of Stanford, Lisa Dyson, now of the University of California, Berkeley, and Matthew Kleban, now at New York University, pointed out in 2002 that Boltzmann’s idea might work too well, filling the megaverse with more Boltzmann brains than universes or real people.

In the same way the odds of a real word showing up when you shake a box of Scrabble letters are greater than a whole sentence or paragraph forming, these “regular” universes would be vastly outnumbered by weird ones, including flawed variations on our own all the way down to naked brains, a result foreshadowed by Martin Rees, a cosmologist at the University of Cambridge, in his 1997 book, “Before the Beginning.”

The conclusions of Dr. Dyson and her colleagues were quickly challenged by Andreas Albrecht and Lorenzo Sorbo of the University of California, Davis, who used an alternate approach. They found that the Big Bang was actually more likely than Boltzmann’s brain.

“In the end, inflation saves us from Boltzmann’s brain,” Dr. Albrecht said, while admitting that the calculations were contentious. Indeed, the “invasion of Boltzmann brains,” as Dr. Linde once referred, was just beginning.

In an interview Dr. Linde described these brains as a form of reincarnation. Over the course of eternity, he said, anything is possible. After some Big Bang in the far future, he said, “it’s possible that you yourself will re-emerge. Eventually you will appear with your table and your computer.”

But it’s more likely, he went on, that you will be reincarnated as an isolated brain, without the baggage of stars and galaxies. In terms of probability, he said, “It’s cheaper.”

You might wonder what’s wrong with a few brains — or even a preponderance of them — floating around in space. For one thing, as observers these brains would see a freaky chaotic universe, unlike our own, which seems to persist in its promise and disappointment.

Another is that one of the central orthodoxies of cosmology is that humans don’t occupy a special place in the cosmos, that we and our experiences are typical of cosmic beings. If the odds of us being real instead of Boltzmann brains are one in a million, say, waking up every day would be like walking out on the street and finding everyone in the city standing on their heads. You would expect there to be some reason why you were the only one left right side up.

Some cosmologists, James Hartle and Mark Srednicki, of the University of California, Santa Barbara, have questioned that assumption. “For example,” Dr. Hartle wrote in an e-mail message, “on Earth humans are not typical animals; insects are far more numerous. No one is surprised by this.”

In an e-mail response to Dr. Hartle’s view, Don Page of the University of Alberta, who has been a prominent voice in the Boltzmann debate, argued that what counted cosmologically was not sheer numbers, but consciousness, which we have in abundance over the insects. “I would say that we have no strong evidence against the working hypothesis that we are typical and that our observations are typical,” he explained, “which is very fruitful in science for helping us believe that our observations are not just flukes but do tell us something about the universe.”

Dr. Dyson and her colleagues suggested that the solution to the Boltzmann paradox was in denying the presumption that the universe would accelerate eternally. In other words, they said, that the cosmological constant was perhaps not really constant. If the cosmological constant eventually faded away, the universe would revert to normal expansion and what was left would eventually fade to black. With no more acceleration there would be no horizon with its snap, crackle and pop, and thus no material for fluctuations and Boltzmann brains.

String theory calculations have suggested that dark energy is indeed metastable and will decay, Dr. Susskind pointed out. “The success of ordinary cosmology,” Dr. Susskind said, “speaks against the idea that the universe was created in a random fluctuation.”

But nobody knows whether dark energy — if it dies — will die soon enough to save the universe from a surplus of Boltzmann brains. In 2006, Dr. Page calculated that the dark energy would have to decay in about 20 billion years in order to prevent it from being overrun by Boltzmann brains.

The decay, if and when it comes, would rejigger the laws of physics and so would be fatal and total, spreading at almost the speed of light and destroying all matter without warning. There would be no time for pain, Dr. Page wrote: “And no grieving survivors will be left behind. So in this way it would be the most humanely possible execution.” But the object of his work, he said, was not to predict the end of the universe but to draw attention to the fact that the Boltzmann brain problem remains.

People have their own favorite measures of probability in the multiverse, said Raphael Buosso of the University of California, Berkeley. “So Boltzmann brains are just one example of how measures can predict nonsense; anytime your measure predicts that something we see has extremely small probability, you can throw it out,” he wrote in an e-mail message.

Another contentious issue is whether the cosmologists in their calculations could consider only the observable universe, which is all we can ever see or be influenced by, or whether they should take into account the vast and ever-growing assemblage of other bubbles forever out of our view predicted by eternal inflation. In the latter case, as Alex Vilenkin of Tufts University pointed out, “The numbers of regular and freak observers are both infinite.” Which kind predominate depends on how you do the counting, he said..

In eternal inflation, the number of new bubbles being hatched at any given moment is always growing, Dr. Linde said, explaining one such counting scheme he likes. So the evolution of people in new bubbles far outstrips the creation of Boltzmann brains in old ones. The main way life emerges, he said, is not by reincarnation but by the creation of new parts of the universe. “So maybe we don’t need to care too much” about the Boltzmann brains,” he said.

“If you are reincarnated, why do you care about where you are reincarnated?” he asked. “It sounds crazy because here we are touching issues we are not supposed to be touching in ordinary science. Can we be reincarnated?”

“People are not prepared for this discussion,” Dr. Linde said.

G.M. Buys Stake in Ethanol Made From Waste

General Motors, eager to ensure a supply of fuel for the big fleet of flex-fuel ethanol-capable vehicles it is building, has joined the rush into alternative energy and invested in a company that intends to produce ethanol from crop wastes, wood chips, scrap plastic, rubber and even municipal garbage.

Rick Wagoner, G.M.’s chairman and chief executive, announced the investment on Sunday in a speech at the opening of the North American International Auto Show in Detroit. The company purchased an equity stake in Coskata, a start-up company in Warrenville, Ill., that plans to make ethanol without using corn. G.M. would not say how much it paid or how big a stake it took in the company.

Coskata plans to build a pilot-scale plant this year in Warrenville, William Roe, the president and chief executive of Coskata, said in a briefing with reporters last week. It has demonstrated all the phases of its technology but has not linked them together in an operating plant, he acknowledged.

Putting money into the fuel business is new for car companies, said Jeffrey Leetsma, the president of the Automotive Hall of Fame, in Dearborn, Mich., and a car historian. “I think this could be new ground,” he said.

Henry Ford, he said, established rubber plantations in Brazil to try to break the Dutch cartel, but in the modern era the car companies have generally not invested in fuel.

“I don’t really see the logic of it,” said Christopher Flavin, president of the Worldwatch Institute, a Washington environmental group. “It’s not particularly an industry they know well, or have expertise in.” Companies like G.M., he said, could be more effective by concentrating on the fuel efficiency of their products..

But Lee Schipper, a visiting scholar at the transportation center of the University of California, Berkeley, said that a new method to make ethanol “presents them with a way of wiggling the industry out of fuel economy standards.” California is seeking a standard based on how much carbon is added to the atmosphere, he said, and ethanol made from waste materials could result in substantially less carbon per mile.

“If I were that company and I really believed in the process, why wait for someone else to invest?” he said.

Coskata is one of many companies, and far from the leader, in an emerging world of start-up firms that are making alternative fuels with a mix-and-match approach to existing technologies. In Coskata’s case it is a combination of gasification and bacterial action.

The first step is cooking the raw feedstock into synthesis gas, a mixture of hydrogen and carbon monoxide. That gas is cooled and fed to bacteria that consume it and excrete ethanol.

Coskata is not the only company pursuing the gas-to-bacteria-to-fuel route, but claims its process gives more ethanol per ton of raw material — 100 gallons — and uses less water, less than one gallon for each gallon of ethanol.

If it can be done economically, the Coskata process has three large advantages over corn-based ethanol, according to General Motors. First, it uses a cheaper feedstock that would not compete with food production. Second, the feedstock is available all over the country, a crucial point since ethanol cannot be shipped from the corn belt to areas of high gasoline demand in existing pipelines.

In addition, the process appears to require less electricity and natural gas, meaning that making it would not release as much carbon. The product would qualify for a federal tax exemption for ethanol.

Mr. Roe said that “at full production, Coskata ethanol should be 50 cents to $1 cheaper than gasoline at the pump,” and that the total production cost would be under $1 a gallon when the fuel begins flowing in 2010 or 2011. Mary Beth Stanek, G.M.’s director for energy and environment, said the process showed “near-term readiness” and that no scientific work was involved to commercialize it.

“It’s literally just physical building,” she said. Her company intends to help get the fuel into pumps around the country, she said. Many of G.M.’s vehicles are already capable of running on a blend of 85 percent ethanol and 15 percent gasoline, but that fuel has not become widely available. Most ethanol is used in a blend with 90 percent gasoline.

Coskata is financed in part by Vinod Khosla, the computer entrepreneur turned venture capitalist, but is only one of the companies he is backing to produce ethanol without corn.

Choosing the best music for exercise


Fitness magazines and Web sites love to ask readers about their favorite workout music while presenting their playlists or suggestions from celebrities. Self.com features the "80s cardio playlist," which includes the short-shorts video classic "Wake Me Up Before You Go-Go" by Wham! On Fitnessmagazine.com, the singer Rihanna reveals her favorite workout songs - immodestly recommending four of her own for "when you have to pick up the pace on the treadmill."

The playlist fixation has a scientific basis: Studies have shown that listening to music during exercise can improve results, both in terms of being a motivator (people exercise longer and more vigorously to music) and as a distraction from negatives like fatigue. But are certain songs more effective than others?

Generally speaking there is a science to choosing an effective exercise soundtrack, said Costas Karageorghis, an associate professor of sport psychology at Brunel University in England, who has studied the effects of music on physical performance for 20 years.

Karageorghis created the Brunel Music Rating Inventory, a questionnaire that is used to rate the motivational qualities of music in the context of sport and exercise. For nearly a decade, he has been administering the questionnaire to panels representing different demographics, who listen to 90 seconds of a song and rate its motivational qualities for various physical activities.

One of the most important elements, Karageorghis found, is a song's tempo, which should be between 120 and 140 beats per minute, or BPM.

That pace coincides with the range of most commercial dance music, and many rock songs are near that range, which leads people to develop "an aesthetic appreciation for that tempo," he said. It also roughly corresponds to the average person's heart rate during a routine workout - say, 20 minutes on an elliptical trainer by a person who is more casual exerciser than fitness warrior.

Karageorghis said "Push It" by Salt-N-Pepa and "Drop It Like It's Hot" by Snoop Dogg are around that range, as is the dance remix of "Umbrella" by Rihanna (so maybe the pop star was onto something).

For a high-intensity workout like a hard run, he suggested Glenn Frey's "The Heat Is On."

Music preferences are as idiosyncratic as workout routines, of course. Allison Goldberg, a 39-year-old life coach and amateur runner who lives in Texas and who is training for the Houston Marathon on Sunday, has been running to the Green Day CD "American Idiot" because, she said, "there's no way you can run slow to Green Day." (Though she may not be listening on race day; a rule bars runners from using portable music players and headphones.) Haile Gebrselassie, the Olympian from Ethiopia who has won the gold medal at 10,000 meters, often requested that the techno song "Scatman," which has a BPM of around 135, be played over the sound system during his races.

Goldberg also includes on her playlist "Don't Phunk With My Heart" by the Black Eyed Peas (130 BPM), "Mr. Brightside" by the Killers (150 BPM) and "Dancing Queen" by ABBA. The musical style that seems to most reliably contain a high BPM is dance music, said Richard Petty, the founder of Power Music, a company that has produced workout compilations for instructors and fitness enthusiasts for two decades. "A rock song doesn't have that same consistency," said Petty, a former DJ who takes a metronomic approach to making exercise music: He chooses a hit song with a catchy melody - say, "Gold Digger" by Kanye West - and produces a remix whose BPM count is tailored to experience level and type of workout.

For a stroll walker going at a pace of around three miles, or five kilometers, an hour, a remixed track has a count of 115 to 118 BPM; for a power walker going 4.5 miles per hour, the count is 137 to 139 BPM; while the BPM for a runner rises to 147 to 160.

The compilations, aimed largely at women doing cardio, with titles like "Shape Walk - '70s Hits Remixed," contain no pauses between songs. That unwavering beat allows a person to synchronize movements to music, something that Kate Gfeller, a music professor at the University of Iowa, said was crucial.

"Music provides a timing cue," said Gfeller, who after taking an aerobics class several years ago where the teacher picked music whose tempo didn't match the moves, was inspired to study the components of music most important to a gainful workout. "It helps you to move more efficiently, which, in turn, can help you with endurance." (She likes to warm up for figure skating to the Buena Vista Social Club, in particular the songs "Candela" and "El Cuarto de Tula.")

In other words, the best workout songs have both a high BPM count and a rhythm to which you can coordinate your movements. This would seem to eliminate any music with abrupt changes in time signature, like free-form jazz or hard-core punk, as well as music that varies widely in intensity, like much of indie rock. It also rules out what the writer and neurologist Oliver Sacks calls "music which doesn't have adequate rhythmic force."

"Here, I think of Wagner," said Sacks, whose recent book, "Musicophilia," discusses the link between rhythm and movement. "Nietzsche wrote of what he called Wagner's 'degeneration of the sense of rhythm.' "

Sacks is fond of swimming and said the one-two-three cadence of his strokes often leads him to play a waltz in his mind.

"Neurologically, it makes no difference if you're listening to music or imagining it," he said. "Vivid imagining activates motor parts."

Much of the research done on music and exercise is geared toward aerobic workouts like jogging and cardio. But as anyone who has heard Metallica blasting from a weight room stereo knows, music is a motivator in strength training, too. "The vast majority of bodybuilders are fans of heavy metal, if not in their personal life at least in the gym," said Shawn Perine, a senior writer at Flex magazine. Loud, aggressive music, he said, "keeps you elevated, especially in between sets."

Perine prefers to work out to hip-hop. "Let's say you've done a grueling set of squats," he said. "You're out of breath, and L.L. Cool J's 'Mama Said Knock You Out' comes on. Your energy won't flag."

But is there a perfect workout track, a song that transcends exercise forms and personal preferences? One comes up repeatedly: "Gonna Fly Now," the theme from "Rocky." In a forthcoming book on music and sport that he contributed to, Karageorghis writes that the song "evokes a state of optimism and excitement in the listener," and Goldberg said it helped her get through her first marathon. The band from Bishop Loughlin Memorial High School in Brooklyn has set up along the New York City Marathon route and performed the "Rocky" theme for runners each race day for the last 30 years.

Bill Conti, the song's composer, shed light on why it continues to motivate. "I put a da-da! in the beginning," Conti said. "Any kind of da-da! gets your attention. Then it goes into a tune we've heard played so weepily throughout the movie, but now I put a beat behind it and put it in a major key." When Rocky runs up the museum steps, musically, Conti said, "I am milking it as much as I can."

Still, Conti is reluctant to overanalyze it. "Music is anti-intellectual," he said. "We know the Greeks went into battle listening to music in the Dorian mode. I can only imagine some Greek guy said, 'This works.' "

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