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Monday, August 13th, 2018
| Time | Event |
| 12:00a | 3Q: A bold mission to touch the sun On Sunday, NASA launched a bold mission to fly directly into the sun’s atmosphere, with a spacecraft named the Parker Solar Probe, after solar astrophysicist Eugene Parker. The incredibly resilient vessel, vaguely shaped like a lightbulb the size of a small car, was launched early in the morning from Cape Canaveral Air Force Station in Florida. Its trajectory will aim straight for the sun, where the probe will come closer to the solar surface than any other spacecraft in history. The probe will orbit the blistering corona, withstanding unprecedented levels of radiation and heat, in order to beam back to Earth data on the sun’s activity. Scientists hope such data will illuminate the physics of stellar behavior. The data will also help to answer questions about how the sun’s winds, eruptions, and flares shape weather in space, and how that activity may affect life on Earth, along with astronauts and satellites in space. Several researchers from MIT are collaborating on the mission, including co-principal investigators John Belcher, the Class of 1992 Professor of Physics, and John Richardson, a principal research scientist in MIT’s Kavli Institute for Astrophysics and Space Research. MIT News spoke with Belcher about the historic mission and its roots at the Institute. Q: This has to be one extreme vehicle to withstand the sun’s radiation at such close range. What kind of effects will the probe experience as it orbits the sun, and what about the spacecraft will help it stay on course? A: The spacecraft will come as close as 3.9 million miles to the sun, well within the orbit of Mercury and more than seven times closer than any spacecraft has come before. This distance is about 8.5 solar radii, very close to the region where the solar wind is accelerated. At these distances the sun will be over 500 times brighter than it appears to Earth, and particle radiation from solar activity will be harsh. In order to survive, the spacecraft folds its solar panels into the shadows of its protective solar shade, leaving just enough of the specially angled panels in sunlight to provide power closer to the sun. To perform these unprecedented investigations, the spacecraft and instruments will be protected from the sun’s heat by a 4.5-inch-thick carbon-composite shield, which will need to withstand temperatures outside the spacecraft that reach nearly 2,500 degrees Fahrenheit. Q: What data will the probe be collecting, and what insights are scientists ultimately hoping to gain from these data? A: There will be a variety of instruments to measure solar particles and fields near the sun, including a low-energy plasma instrument, a magnetometer, and a suite of energetic particle instruments. These will help determine the structure and dynamics of the magnetic fields at the sources of solar wind, trace the flow of energy that heats the corona and accelerates the solar wind, and determine what mechanisms accelerate and transport energetic particles. The acceleration of the solar wind is still an outstanding question, mostly because all of the acceleration is over by [the time the wind has traveled] 25 solar radii. The Earth sits at 215 solar radii, so we have never made the most crucial observations close to the sun. It is only by getting this close to the sun that we have a chance of answering definitely what accelerates the wind. The major question is whether thermal processes or wave acceleration processes are most important, or both. Q: What is MIT’s role in this endeavor?
The SWEAP Investigation is the set of instruments on the spacecraft that will directly measure the properties of the plasma in the solar atmosphere during these encounters. A special component of SWEAP is a small instrument that will look around the protective heat shield of the spacecraft directly at the sun, the only instrument on the spacecraft to do so. This will allow SWEAP to sweep up a sample of the atmosphere of the sun, our star, for the first time at these distances. This small instrument looking around the heat shield is a Faraday cup, and is a direct descendant of the first instrument to measure the existence of the supersonic solar wind expansion. That measurement was carried out by Professor Herb Bridge, Dr. Al Lazarus, and Professor Bruno Rossi, [all of MIT], on Explorer 10 in 1961. At the same time the solar probe Faraday cup is measuring the properties of the solar wind close to the sun at 8 solar radii, a sister Faraday cup on Voyager (launched in 1977) will probably be measuring plasma in the local interstellar space, totally outside the solar atmosphere, beyond 100 astronomical units, or 20,000 solar radii. This Voyager 2 instrument has been in space for more than 40 years, consistently returning data to Earth. Thus two probes which trace their lineage to MIT Professor Herb Bridge will be making measurements at opposite ends of the solar system, from as close as you can get to the sun to as far away as the local interstellar medium. |
| 11:00a | In neutron stars, protons may do the heavy lifting Neutron stars are the smallest, densest stars in the universe, born out of the gravitational collapse of extremely massive stars. True to their name, neutron stars are composed almost entirely of neutrons — neutral subatomic particles that have been compressed into a small, incredibly dense celestial package. A new study in Nature, co-led by MIT researchers, suggests that some properties of neutron stars may be influenced not only by their multitude of densely packed neutrons, but also by a substantially smaller fraction of protons — positively charged particles that make up just 5 percent of a neutron star. Instead of gazing at the stars, the researchers came to their conclusion by analyzing the microscopic nuclei of atoms on Earth. The nucleus of an atom is packed with protons and neutrons, though not quite as densely as in neutron stars. Occasionally, if they are close enough in distance, a proton and a neutron will pair up and streak through an atom’s nucleus with unusually high energy. Such “short-range correlations,” as they are known, can contribute significantly to the energy balance and overall properties of a given atomic nucleus. The researchers looked for signs of proton and neutron pairs in atoms of carbon, aluminum, iron, and lead, each with a progressively higher ratio of neutrons to protons. They found that, as the relative number of neutrons in an atom increased, so did the probability that a proton would form an energetic pair. The likelihood that a neutron would pair up, however, stayed about the same. This trend suggests that, in objects with high densities of neutrons, the minority protons carry a disproportionally large part of the average energy. “We think that when you have a neutron-rich nucleus, on average, the protons move faster than the neutrons, so in some sense, protons carry the action,” says study co-author Or Hen, assistant professor of physics at MIT. “We can only imagine what might happen in even more neutron-dense objects like neutron stars. Even though protons are the minority in the star, we think the minority rules. Protons seem to be very active, and we think they might determine several properties of the star.” Digging through data Hen and his colleagues based their study on data collected by CLAS — the CEBAF (Continuous Electron Beam Accelerator Facility) Large Acceptance Spectrometer, a particle accelerator and detector based at Jefferson Laboratory in Virginia. CLAS, which operated from 1998 to 2012, was designed to detect and record the multiple particles that are emitted when beams of electrons impinge on atomic targets. “Having this property of a detector that sees everything and also keeps everything for offline analysis is extremely rare,” Hen says. “It even has kept what people considered ‘noise,’ and we’re now learning that one person’s noise is another person’s signal.” The team chose to mine CLAF’s archived data for signs of short-range correlations — interactions that the detector was not necessarily meant to produce, but that it captured nonetheless. “People were using the detector to look at specific interactions, but meanwhile, it also measured in parallel a bunch of other reactions that took place,” says collaborator Larry Weinstein, a professor of physics at Old Dominion University. “So we thought, ‘Let’s dig into this data and see if there’s anything interesting there.’ We want to squeeze as much science as we can out of experiments that have already run.” A full dance card The team chose to mine CLAS data collected in 2004, during an experiment in which the detector aimed beams of electrons at carbon, aluminum, iron, and lead atoms, with the goal of observing how particles produced in nuclear interactions travel through each atom’s respectively larger volume. Along with their varying sizes, each of the four types of atoms have different ratios of neutrons to protons in their nuclei, with carbon having the fewest neutrons and lead having the most. The reanalysis of the data was done by graduate student Meytal Duer from Tel Aviv University in a collaboration with MIT and Old Dominion University, and was led by Hen. The overall study was conducted by an international consortium called the CLAS Collaboration, made up of 182 members from 42 institutions in 9 countries. The group studied the data for signs of high-energy protons and neutrons — indications that the particles had paired up — and whether the probability of this pairing changed as the ratio of neutrons to protons increased. “We wanted to start from a symmetric nucleus and see, as we add more neutrons, how things evolve,” Hen says. “We would never get to the symmetries of neutron stars here on Earth, but we could at least see some trend and understand from that, what could be going on in the star.” In the end, the team observed that as the number of neutrons in an atom’s nucleus increased, the probability of protons having high energies (and having paired up with a neutron) also increased significantly, while the same probability for neutrons remained the same. “The analogy we like to give is that it’s like going to a dance party,” Hen says, invoking a scenario in which boys who might pair up with girls on the dance floor are vastly outnumbered. “What would happen is, the average boy would … dance a lot more, so even though they were a minority in the party, the boys, like the protons, would be extremely active.” Hen says this trend of energetic protons in neutron-rich atoms may extend to even more neutron-dense objects, such as neutron stars. The role of protons in these extreme objects may then be more significant than people previously suspected. This revelation, Hen says, may shake up scientists’ understanding of how neutron stars behave. For instance, as protons may carry substantially more energy than previously thought, they may contribute to properties of a neutron star such as its stiffness, its ratio of mass to size, and its process of cooling. “All these properties then affect how two neutron stars merge together, which we think is one of the main processes in the universe that create nuclei heavier than iron, such as gold,” Hen says. “Now that we know the small fraction of protons in the star are very highly correlated, we will have to rethink how [neutron stars] behave.” This research was supported, in part, by the U.S. Department of Energy, the National Science Foundation, the Israel Science Foundation, the Chilean Comisión Nacional de Investigación Científica y Tecnológica, the French Centre National de la Recherche Scientifique and Commissariat a l’Energie Atomique, the French-American Cultural Exchange, the Italian Istituto Nazionale di Fisica Nucleare, the National Research Foundation of Korea, and the UK’s Science and Technology Facilities Council. |
| 11:00a | Novel optics for ultrafast cameras create new possibilities for imaging MIT researchers have developed novel photography optics that capture images based on the timing of reflecting light inside the optics, instead of the traditional approach that relies on the arrangement of optical components. These new principles, the researchers say, open doors to new capabilities for time- or depth-sensitive cameras, which are not possible with conventional photography optics. Specifically, the researchers designed new optics for an ultrafast sensor called a streak camera that resolves images from ultrashort pulses of light. Streak cameras and other ultrafast cameras have been used to make a trillion-frame-per-second video, scan through closed books, and provide depth map of a 3-D scene, among other applications. Such cameras have relied on conventional optics, which have various design constraints. For example, a lens with a given focal length, measured in millimeters or centimeters, has to sit at a distance from an imaging sensor equal to or greater than that focal length to capture an image. This basically means the lenses must be very long. In a paper published in this week’s Nature Photonics, MIT Media Lab researchers describe a technique that makes a light signal reflect back and forth off carefully positioned mirrors inside the lens system. A fast imaging sensor captures a separate image at each reflection time. The result is a sequence of images — each corresponding to a different point in time, and to a different distance from the lens. Each image can be accessed at its specific time. The researchers have coined this technique “time-folded optics.” “When you have a fast sensor camera, to resolve light passing through optics, you can trade time for space,” says Barmak Heshmat, first author on the paper. “That’s the core concept of time folding. … You look at the optic at the right time, and that time is equal to looking at it in the right distance. You can then arrange optics in new ways that have capabilities that were not possible before.” The new optics architecture includes a set of semireflective parallel mirrors that reduce, or “fold,” the focal length every time the light reflects between the mirrors. By placing the set of mirrors between the lens and sensor, the researchers condensed the distance of optics arrangement by an order of magnitude while still capturing an image of the scene. In their study, the researchers demonstrate three uses for time-folded optics for ultrafast cameras and other depth-sensitive imaging devices. These cameras, also called “time-of-flight” cameras, measure the time that it takes for a pulse of light to reflect off a scene and return to a sensor, to estimate the depth of the 3-D scene. Co-authors on the paper are Matthew Tancik, a graduate student in the MIT Computer Science and Artificial Intelligence Laboratory; Guy Satat, a PhD student in the Camera Culture Group at the Media Lab; and Ramesh Raskar, an associate professor of media arts and sciences and director of the Camera Culture Group. Folding the optical path into time The researchers’ system consists of a component that projects a femtosecond (quadrillionth of a second) laser pulse into a scene to illuminate target objects. Traditional photography optics change the shape of the light signal as it travels through the curved glasses. This shape change creates an image on the sensor. But, with the researchers’ optics, instead of heading right to the sensor, the signal first bounces back and forth between mirrors precisely arranged to trap and reflect light. Each one of these reflections is called a “round trip.” At each round trip, some light is captured by the sensor programed to image at a specific time interval — for example, a 1-nanosecond snapshot every 30 nanoseconds. A key innovation is that each round trip of light moves the focal point — where a sensor is positioned to capture an image — closer to the lens. This allows the lens to be drastically condensed. Say a streak camera wants to capture an image with the long focal length of a traditional lens. With time-folded optics, the first round-trip pulls the focal point about double the length of the set of mirrors closer to the lens, and each subsequent round trip brings the focal point closer and closer still. Depending on the number of round trips, a sensor can then be placed very near the lens. By placing the sensor at a precise focal point, determined by total round trips, the camera can capture a sharp final image, as well as different stages of the light signal, each coded at a different time, as the signal changes shape to produce the image. (The first few shots will be blurry, but after several round trips the target object will come into focus.) In their paper, the researchers demonstrate this by imaging a femtosecond light pulse through a mask engraved with “MIT,” set 53 centimeters away from the lens aperture. To capture the image, the traditional 20-centimeter focal length lens would have to sit around 32 centimeters away from the sensor. The time-folded optics, however, pulled the image into focus after five round trips, with only a 3.1-centimeter lens-sensor distance. This could be useful, Heshmat says, in designing more compact telescope lenses that capture, say, ultrafast signals from space, or for designing smaller and lighter lenses for satellites to image the surface of the ground. Multizoom and multicolor The researchers next imaged two patterns spaced about 50 centimeters apart from each other, but each within line of sight of the camera. An “X” pattern was 55 centimeters from the lens, and a “II” pattern was 4 centimeters from the lens. By precisely rearranging the optics — in part, by placing the lens in between the two mirrors — they shaped the light in a way that each round trip created a new magnification in a single image acquisition. In that way, it’s as if the camera zooms in with each round trip. When they shot the laser into the scene, the result was two separate, focused images, created in one shot — the X pattern captured on the first round trip, and the II pattern captured on the second round trip. The researchers then demonstrated an ultrafast multispectral (or multicolor) camera. They designed two color-reflecting mirrors and a broadband mirror — one tuned to reflect one color, set closer to the lens, and one tuned to reflect a second color, set farther back from the lens. They imaged a mask with an “A” and “B,” with the A illuminated the second color and the B illuminated the first color, both for a few tenths of a picosecond. When the light traveled into the camera, wavelengths of the first color immediately reflected back and forth in the first cavity, and the time was clocked by the sensor. Wavelengths of the second color, however, passed through the first cavity, into the second, slightly delaying their time to the sensor. Because the researchers knew which wavelength would hit the sensor at which time, they then overlaid the respective colors onto the image — the first wavelength was the first color, and the second was the second color. This could be used in depth-sensing cameras, which currently only record infrared, Heshmat says. One key feature of the paper, Heshmat says, is it opens doors for many different optics designs by tweaking the cavity spacing, or by using different types of cavities, sensors, and lenses. “The core message is that when you have a camera that is fast, or has a depth sensor, you don’t need to design optics the way you did for old cameras. You can do much more with the optics by looking at them at the right time,” Heshmat says. This work “exploits the time dimension to achieve new functionalities in ultrafast cameras that utilize pulsed laser illumination. This opens up a new way to design imaging systems,” says Bahram Jalali, director of the Photonics Laboratory and a professor of electrical and computer engineering at the University of California at Berkeley. “Ultrafast imaging makes it possible to see through diffusive media, such as tissue, and this work hold promise for improving medical imaging in particular for intraoperative microscopes.” |
| 3:00p | MIT mathematicians solve age-old spaghetti mystery If you happen to have a box of spaghetti in your pantry, try this experiment: Pull out a single spaghetti stick and hold it at both ends. Now bend it until it breaks. How many fragments did you make? If the answer is three or more, pull out another stick and try again. Can you break the noodle in two? If not, you’re in very good company. The spaghetti challenge has flummoxed even the likes of famed physicist Richard Feynman ’39, who once spent a good portion of an evening breaking pasta and looking for a theoretical explanation for why the sticks refused to snap in two. Feynman’s kitchen experiment remained unresolved until 2005, when physicists from France pieced together a theory to describe the forces at work when spaghetti — and any long, thin rod — is bent. They found that when a stick is bent evenly from both ends, it will break near the center, where it is most curved. This initial break triggers a “snap-back” effect and a bending wave, or vibration, that further fractures the stick. Their theory, which won the 2006 Ig Nobel Prize, seemed to solve Feynman’s puzzle. But a question remained: Could spaghetti ever be coerced to break in two? The answer, according to a new MIT study, is yes — with a twist. In a paper published this week in the Proceedings of the National Academy of Sciences, researchers report that they have found a way to break spaghetti in two, by both bending and twisting the dry noodles. They carried out experiments with hundreds of spaghetti sticks, bending and twisting them with an apparatus they built specifically for the task. The team found that if a stick is twisted past a certain critical degree, then slowly bent in half, it will, against all odds, break in two. The researchers say the results may have applications beyond culinary curiosities, such as enhancing the understanding of crack formation and how to control fractures in other rod-like materials such as multifiber structures, engineered nanotubes, or even microtubules in cells. “It will be interesting to see whether and how twist could similarly be used to control the fracture dynamics of two-dimensional and three-dimensional materials,” says co-author Jörn Dunkel, associate professor of physical applied mathematics at MIT. “In any case, this has been a fun interdisciplinary project started and carried out by two brilliant and persistent students — who probably don’t want to see, break, or eat spaghetti for a while.” The two students are Ronald Heisser ’16, now a graduate student at Cornell University, and Vishal Patil, a mathematics graduate student in Dunkel’s group at MIT. Their co-authors are Norbert Stoop, instructor of mathematics at MIT, and Emmanuel Villermaux of Université Aix Marseille.
Experiments (above) and simulations (below) show how dry spaghetti can be broken into two or more fragments, by twisting and bending. A deep dish dive Heisser, together with project partner Edgar Gridello, originally took up the challenge of breaking spaghetti in the spring of 2015, as a final project for 18.354 (Nonlinear Dynamics: Continuum Systems), a course taught by Dunkel. They had read about Feynman’s kitchen experiment, and wondered whether spaghetti could somehow be broken in two and whether this split could be controlled. “They did some manual tests, tried various things, and came up with an idea that when he twisted the spaghetti really hard and brought the ends together, it seemed to work and it broke into two pieces,” Dunkel says. “But you have to twist really strongly. And Ronald wanted to investigate more deeply.” So Heisser built a mechanical fracture device to controllably twist and bend sticks of spaghetti. Two clamps on either end of the device hold a stick of spaghetti in place. A clamp at one end can be rotated to twist the dry noodle by various degrees, while the other clamp slides toward the twisting clamp to bring the two ends of the spaghetti together, bending the stick. Heisser and Patil used the device to bend and twist hundreds of spaghetti sticks, and recorded the entire fragmentation process with a camera, at up to a million frames per second. In the end, they found that by first twisting the spaghetti at almost 360 degrees, then slowly bringing the two clamps together to bend it, the stick snapped exactly in two. The findings were consistent across two types of spaghetti: Barilla No. 5 and Barilla No. 7, which have slightly different diameters. Experiments (above) and simulations (below) show how dry spaghetti can be broken into two or more fragments, by twisting and bending. Noodle twist In parallel, Patil began to develop a mathematical model to explain how twisting can snap a stick in two. To do this, he generalized previous work by the French scientists Basile Audoly and Sebastien Neukirch, who developed the original theory to describe the “snap-back effect,” in which a secondary wave caused by a stick’s initial break creates additional fractures, causing spaghetti to mostly snap in three or more fragments. Patil adapted this theory by adding the element of twisting, and looked at how twist should affect any forces and waves propagating through a stick as it is bent. From his model, he found that, if a 10-inch-long spaghetti stick is first twisted by about 270 degrees and then bent, it will snap in two, mainly due to two effects. The snap-back, in which the stick will spring back in the opposite direction from which it was bent, is weakened in the presence of twist. And, the twist-back, where the stick will essentially unwind to its original straightened configuration, releases energy from the rod, preventing additional fractures. “Once it breaks, you still have a snap-back because the rod wants to be straight,” Dunkel explains. “But it also doesn’t want to be twisted.” Just as the snap-back will create a bending wave, in which the stick will wobble back and forth, the unwinding generates a “twist wave,” where the stick essentially corkscrews back and forth until it comes to rest. The twist wave travels faster than the bending wave, dissipating energy so that additional critical stress accumulations, which might cause subsequent fractures, do not occur. “That’s why you never get this second break when you twist hard enough,” Dunkel says. The team found that the theoretical predictions of when a thin stick would snap in two pieces, versus three or four, matched with their experimental observations. “Taken together, our experiments and theoretical results advance the general understanding of how twist affects fracture cascades,” Dunkel says. For now, he says the model is successful at predicting how twisting and bending will break long, thin, cylindrical rods such as spaghetti. As for other pasta types? “Linguini is different because it’s more like a ribbon,” Dunkel says. “The way the model is constructed it applies to perfectly cylindrical rods. Although spaghetti isn’t perfect, the theory captures its fracture behavior pretty well,” The research was supported, in part, by the Alfred P. Sloan Foundation and the James S. McDonnell Foundation. |
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