Light bending by a black hole may offer proof of extra dimensions

Most of the work by astrophysicists studying the effects ofgravitational lensing, or light bending, relates togalaxiesand galaxy clusters. New research from Penn makes use of thesupermassive black holebelieved to exist at the center of theMilky Waygalaxy.

The analysis was carried out by Amitai Y. Bin-Nun, a theoretical astrophysics and cosmology graduate student at Penn, with guidance from Justin Khoury, assistant professor, and Ravi K. Sheth, professor, both in the Physics and Astronomy Department in Penn’s School of Arts and Sciences. The article appears in the journalPhysical Review D.

“We found that, if our universe is described by a theory incorporating extra dimensions, light near the black hole at the center of our galaxy may appear brighter than it would if we live in a universe without extra dimensions,” Bin-Nun said.“Detecting images at the brighter intensity would represent evidence ofextra dimensionsand would be an incredibly important development.”

Bin-Nun studied the effect of gravitational lensing on the stars orbiting Sagittarius A*, or Sgr A*, a radio source in the center of the Milky Way. Sgr A* was chosen because it hosts the supermassive black hole hypothesized to exist at the center of the Milky Way. The strong gravitational pull of the black hole distorts the light from Sgr A* before it reaches Earth, creating the illusion of multiple images of the same star.

Bin-Nun simulated the orbits of stars near the black hole and treated each star as a source lensed by the black hole, solving for the location and brightness of the“secondary” image which appears near the black hole. For each individual star, Bin-Nun found that the brightness of the secondary image would change over time and would peak in brightness when the star is nearly aligned with Sgr A*.

Next, he repeated the lensing analysis assuming the black hole was described by a metric coming from the theoretical Randall-Sundrum II braneworld scenario, which prescribes an extra fifth dimension. If that description of the black hole is correct, then the seconday image of the star S2 will be up to 44 percent brighter in early 2018 when it reaches its peak brightness, providing evidence for the presence of a fifth dimension where gravity is severely diluted. If not, then the four dimensional description of the black hole should be seen as more accurate.

Even if the exact universe is not five dimensional, or this analysis breaks down at other points,“we have shown alternative gravity theories have the possibility of creating a large gravitational lensing effect and we should look into lensing as a test for gravity theories,” Bin-Nun said.

The findings come with several caveats.

Certain assumptions on the form of the black hole were made as the shape of space around a five dimensional black hole is not known. Researchers did not take into account the spin of the black hole, which confounds the analysis. It’s also highly probable that, because the image is so close to the black hole and resolution of available ground-based telescopes are limited, the light from larger, nearby objects could obscure the image of the star, meaning observers won’t be able to isolate the effects of this particular iamge.

“These findings illustrate how the opportunities provided by the Penn physics Ph.D. program and its new Center for Particle Cosmology allow its students to make important contributions at the cutting edge of discovery,” Sheth said.

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Cleaner stoves for developing countries, thanks to heat-powered fan design

At an upcoming meeting of the 2nd Pan-American/Iberian Meeting on Acoustics in Cancun, Mexico, he will present a simple heat-powered fan that could help to make these stoves more efficient and combat the serious health problems associated with cooking in unventilated spaces.

How important is this? In a speech in September 2010 U.S. Secretary of State Hillary Clinton made a speech in which she underscored the impact of simple stoves on living standards in many parts of the world. More than 3 billion people use open fire cooking to eat daily, she said. But such cooking is very energy inefficient; finding fuel itself is a laborious; the combustion contributes disproportionately togreenhouse warming; and, worst of all, the fumes (often gathering in unventilated rooms) produce air that often exceeds EPA guidelines for healthful air by a factor of 200.

The fumes kill an estimated 1.9 million people a year, according to the United Nations. TheWorld Health Organizationcites this smoke as one of the five greatest killers indeveloping countries. Ms. Clinton's speech launched a worldwide effort called the Global Alliance for Clean Cookstoves.

Some moderate-sized devices generate combined heat and power, or CHP. The smallest of these highly-efficient machines can make, for example, 2 kilowatts of heat and 1 kilowatt of electricity. But even this is too much for a person in a rural area to use and too expensive, so Montgomery is trying to make a simple appliance that is 100 times smaller still.

His device, still at the experimental stage, captures some of the stove'swaste heatand converts the heat intosound wavesin a simple thermo-acoustic engine. Then theacoustic energyis converted into a tiny bit of electricity in an electro-acoustic transducer. The electricity in turn can partly charge a battery (delivering well-needed lighting after dark) and operate a fan directed at the combustion of the stove's biofuel, making the whole process more energy efficient.

The more efficient combustion, the less biomass must be burned to cook and the less smoke produced.

"Although a thermo-acoustic cogeneration cook stove would produce only on the order of ten watts of electrical power,"he says,"there are probably two billion biomass-fueled cook stoves in use worldwide that might benefit from nano-CHP technology."

The target price for the device that attaches to the stove is $25, says Montgomery, who will report on his ongoing engineering research in Cancun.

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Foucault, revisited: Scientists show how to build a pendulum for any classroom

The only problem, according to Argentinean researcher Horacio Salva, is that the devices are generally large and unwieldy, making them impractical to install in places where space is at a premium. This limitation was something he and his colleagues at the Centro Atómico Bariloche in Argentina wanted to address.

Now in the American Institute of Physics journalReview of Scientific Instruments, Salva and colleagues report success in what he acknowledges was a fun side project -- building two pendulums precisely enough to make measurements of the spinning Earth yet compact enough to fit in a lobby or classroom of just about any science building.

By definition, Foucault pendulums -- which are named after the French physicist Léon Foucault who first conceived of one in the middle of the 19th century -- count as a simple technology. Generally, a metal orb is suspended by a wire and hung from a height that can be dozens and dozens of feet. The orb is pulled back and released, and as it swings back and forth over the course of a day, it appears to slowly rotate in a circle. In fact what's observed is the Earth moving underneath the pendulum, which swings back and forth in a fixed plane, like a gyroscope.

Or rather, it's more accurate to say that pendulums swing in a mostly fixed plane. That's because, as anyone who pushes a child in a swing can attest, it's tough to keep a pendulum swinging in a straight line. Over time, due to the vagaries of friction and other forces, a pendulum will start to travel in an ellipse, an effect that can easily garble evidence of the Earth's rotation, which for generations has been novel enough to astonish when first observed. Here's the beginning of a February 23, 1908 article in The New York Times describing a Foucault pendulum display in the Big Apple:"Perhaps you were one of the crowd of people who saw the great Foucault pendulum experiment last week at Columbia University. Probably you watched it like the rest with openmouthed wonder."

The heavier the suspended orb and the longer the wire, the more limited the elliptical drift. Similarly, older children on taller swings tend to fly straighter than younger children in the shorter toddler swings.

Consider the dimensions of an 80-year-old Foucault pendulum on display at Philadelphia's Franklin Institute: a 180-pound-orb hangs from a wire 85 feet long and swings back and forth once every 10 seconds. The two pendulums built by Salva are kiddie-sized in comparison. In the case of the first, a 27-pound weight swings back and forth on a 16-foot-long piano wire once every 4 and a half seconds. The second pendulum uses the same weight and an even shorter wire. Using a copper ring underneath each orb to damp down the drift, Salva was able to easily observe and measure precession, the technical name for the movement of the Earth relative to the fixed swinging of the pendulums. Indeed his jiggering of the pendulums was able to tune out all but one percent of the elliptical"noise,"at least in the case of his longer pendulum.

Admittedly, says Salva, this new pendulum by no means has the precision necessary to make any groundbreaking new measurements. But the design, he says, is sophisticated enough to be a useful tool for teaching basic physics concepts to physics students and the general public.

"There's obviously no pressure to do work like this,"said Salva, who in his day job studies far more sophisticated"pendulums"involving the elasticity of various materials."It's mostly for fun, though I think it may well help students in the future, too."

Pointing to one possible application, the paper notes that the device was able to detect earthquakes of medium intensity that took place as far away as 765 km."Some earthquakes can be seen, because the seismic wave moves the support of thependulumincreasing the ellipse of the moment and changing the precession speed,"said Salva.

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Study reveals the subtle dynamics underpinning how cats drink (w/ Video)

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Researchers at MIT, Virginia Tech and Princeton University analyzed the way domestic and big cats lap and found thatfelinesof all sizes take advantage of a perfect balance between two physical forces. The results will be published in the November 11 online issue of the journalScience.

It was known that when they lap, cats extend their tongues straight down toward the bowl with the tip of the tongue curled backwards like a capital"J"to form a ladle, so that the top surface of the tongue actually touches the liquid first. We know this because another MIT engineer, the renowned Doc Edgerton, who first used strobe lights in photography to stop action, filmed a domestic cat lapping milk in 1940.

But recent high-speed videos made by this team clearly revealed that the top surface of the cat's tongue is the only surface to touch the liquid. Cats, unlike dogs, aren't dipping their tongues into the liquid like ladles after all. Instead, the cat's lapping mechanism is far more subtle and elegant. The smooth tip of the tongue barely brushes the surface of the liquid before the cat rapidly draws its tongue back up. As it does so, a column of milk forms between the moving tongue and the liquid's surface. The cat then closes its mouth, pinching off the top of the column for a nice drink, while keeping its chin dry.

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The liquid column, it turns out, is created by a delicate balance between gravity, which pulls the liquid back to the bowl, and inertia, which in physics, refers to the tendency of the liquid or any matter, to continue moving in a direction unless another force interferes. The cat instinctively knows just how quickly to lap in order to balance these two forces, and just when to close its mouth. If it waits another fraction of a second, the force of gravity will overtake inertia, causing the column to break, the liquid to fall back into the bowl, and the cat's tongue to come up empty.

While the domestic cat averages about four laps per second, with each lap bringing in about 0.1 milliliters of liquid, the big cats, such as tigers, know to slow down. They naturally lap more slowly to maintain the balance of gravity and inertia.

Analyzing the mechanics

In this research, Roman Stocker of MIT's Department of Civil and Environmental Engineering (CEE), Pedro Reis of CEE and the Department of Mechanical Engineering, Sunghwan Jung of Virginia Tech's Department of Engineering Science and Mechanics, and Jeffrey Aristoff of Princeton's Department of Mechanical and Aerospace Engineering used observational data gathered from high-speed digital videos of domestic cats, including Stocker's family cat, and a range of big cats (tiger, lion and jaguar) from the Boston-area zoos, thanks to a collaboration with Zoo New England's mammal curator John Piazza and assistant curator Pearl Yusuf. And, in what could be a first for a paper published inScience, the researchers also gathered additional data by analyzing existing YouTube.com videos of big cats lapping.

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With these videos slowed way down, the researchers established the speed of the tongue's movement and the frequency of lapping. Knowing the size and speed of the tongue, the researchers then developed a mathematical model involving the Froude number, a dimensionless number that characterizes the ratio between gravity and inertia. For cats of all sizes, that number is almost exactly one, indicating a perfect balance.

To better understand the subtle dynamics of lapping, they also created a robotic version of a cat's tongue that moves up and down over a dish of water, enabling the researchers to systematically explore different aspects of lapping, and ultimately, to identify the mechanism underpinning it.

"The amount of liquid available for the cat to capture each time it closes its mouth depends on the size and speed of thetongue. Our research— the experimental measurements and theoretical predictions— suggests that the cat chooses the speed in order to maximize the amount of liquid ingested per lap,"said Aristoff, a mathematician who studies liquid surfaces."This suggests thatcatsare smarter than many people think, at least when it comes to hydrodynamics."

Aristoff said the team benefitted from the diverse scientific backgrounds of its members: engineering, physics and mathematics.

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The researchers needed to be able to change a cat's lapping speed in order to test their theory. So they developed a robotic version of a cat's tongue— a mechanical column with a 1-inch glass disk at the tip. This device allowed the researchers to study the liquid column for different lapping speeds using a high-speed digital camera. The initial image was taken at 1000 frames per second. The video above is slowed to 15 frames per second. Video /Pedro M. Reis, Sunghwan Jung, Jeffrey M. Aristoff and Roman Stocker

"In the beginning of the project, we weren't fully confident that fluid mechanics played a role in cat's drinking. But as the project went on, we were surprised and amused by the beauty of the fluid mechanics involved in this system,"said Jung, an engineer whose research focuses on soft bodies, like fish, and the fluids surrounding them.

The work began three-and-a-half years ago when Stocker, who studies the fluid mechanics of the movements of ocean microbes, was watching his cat lap milk. That cat, eight-year-old Cutta Cutta, stars in the researchers' best videos and still pictures. And like all movie stars (Cutta Cutta means"stars stars"in an Australian aboriginal language), he likes being waited on. With their cameras trained on Cutta Cutta's bowl, Stocker and Reis said they spent hours at the Stocker home waiting on Cutta Cutta… to drink, that is. But the wait didn't dampen their enthusiasm for the project, which very appropriately originated from a sense of curiosity.

"Science allows us to look at natural processes with a different eye and to understand how things work, even if that's figuring out how my cat laps his breakfast,"Stocker said."It's a job, but also a passion, and this project for me was a high point in teamwork and creativity. We did it without any funding, without any graduate students, without much of the usual apparatus that science is done with nowadays."

"Our process in this work was typical, archetypal really, of any new scientific study of a natural phenomenon. You begin with an observation and a broad question, 'How does the cat drink?' and then try to answer it through careful experimentation and mathematical modeling,"said Reis, a physicist who works on the mechanics of soft solids."To us, this study provides further confirmation of how exciting it is to explore the scientific unknown, especially when this unknown is something that's part of our everyday experiences."

Besides their obvious enthusiasm for the work itself, the researchers are also delighted that it builds on Edgerton's 1940 film of the cat lapping. That film appeared as part of an MGM-released movie called"Quicker'n a Wink,"which won an Academy Award in 1941. Reis and Stocker say they're moving on to other collaborations closer to their usual areas of research. But their feline friend Cutta Cutta might have Oscar hopes.

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