- Published: Tuesday, 08 October 2019 07:02
Everything radiates. Whether it's a car door, a pair of shoes or the cover of a book, anything hotter than absolute zero (i.e., pretty much everything) is constantly shedding radiation in the form of photons, the quantum particles of light.
A twin process—absorption—is usually also present. As photons carry away energy, passers-by from the environment can be absorbed to replenish it. When absorption and emission occur at the same rate, scientists say that an object is in equilibrium with its environment. This often means that object and environment share the same temperature.
Far away from equilibrium, new behaviors can emerge. In a paper published August 1, 2019 as an Editors’ Suggestion in the journal Physical Review Letters, scientists at JQI and Michigan State University suggest that certain materials may experience a spontaneous twisting force if they are hotter than their surroundings.
"The fact that a material might feel a torque due to a temperature difference with the environment is very unusual," says lead author Mohammad Maghrebi, a former JQI postdoctoral researcher who is now an assistant professor at Michigan State University.
The effect, which hasn't yet been observed in an experiment, is predicted to arise in a thin ribbon of a material called a topological insulator (TI)—something that allows electrical current to flow on its surface but not through its innards.
In this case, the researchers made two additional assumptions about the TI. One is that it is hotter than its environment. And another is that the TI has some magnetic impurities that affect the behavior of electrons on its surface.
These magnetic impurities interact with a quantum property of the electrons called spin. Spin is part of the basic character of an electron, much like electric charge, and it describes the particle’s intrinsic angular momentum—the tendency of an object to continue rotating. Photons, too, can carry angular momentum.
Although electrons don’t physically rotate, they can still gain and lose angular momentum, albeit only in discrete chunks. Each electron has two spin values—up and down—and the magnetic impurities ensure that one value sits at a higher energy than the other. In the presence of these impurities, electrons can flip their spin from up to down and vice versa by emitting or absorbing a photon that carries the right amount of energy and angular momentum.
Maghrebi and two colleagues, JQI Fellows Jay Deep Sau and Alexey Gorshkov, showed that radiation emanating from this kind of TI carries angular momentum skewed in one rotational direction, like a corkscrew that twists clockwise. The material gets left with a deficit of angular momentum, causing it to feel a torque in the opposite direction (in this example, counterclockwise).
The authors say that TIs are ideal for spotting this effect because they play host to the right kind of interaction between electrons and light. TIs already link electron spin with the momentum of their motion, and it's through this motion that electrons in the material ordinarily absorb and emit light.
If an electron on the surface of this particular kind of TI starts with its spin pointing up, it can shed energy and angular momentum by changing its spin from up to down and emitting a photon. Since the TI is hotter than its environment, electrons will flip from up to down more often than the reverse. That’s because the environment has a lower temperature and lacks the energy to replace the radiation coming from the TI. The result of this imbalance is a torque on the thin TI sample, driven by the random emission of radiation.
Future experiments might observe the effect in one of two ways, the authors say. The most likely method is indirect, requiring experimenters to heat up a TI by running a current through it and collecting the emitted light. By measuring the average angular momentum of the radiation, an experiment might detect the asymmetry and confirm one consequence of the new prediction.
A more direct—and likely more difficult—observation would involve actually measuring the torque on the thin film by looking for tiny rotations. Maghrebi says that he's brought up the idea to several experimentalists. "They were not horrified by having to measure something like a torque, but, at the same time, I think it really depends on the setup," he says. "It certainly didn't sound like it was impossible."
Story by Chris Cesare: https://jqi.umd.edu/news/corkscrew-photons-may-leave-behind-spontaneous-twist
In 1969, University of Maryland physicist Doug Currie helped design three still-in-use lunar instruments placed on the moon by Apollo 11, 14 and 15. Fifty years later, Currie is lead scientist for a just-approved NASA project to place next-generation versions of these instruments on the Moon.
Known as lunar retroreflectors, the instruments reflect laser pulses sent from Earth back to their exact origin point, allowing precise measurements of the Earth-moon distance; providing data to better understand aspects of the moon’s interior, including its liquid core; testing questions of fundamental physics, and allowing better mapping and navigation of the lunar surface.
According to Currie, a senior research scientist and professor emeritus at the University of Maryland, the new UMD-led project can lead to improvements in all of these research areas: (1) because of the hundred fold improvement in the accuracy of individual ranges using the new retroreflectors and (2) by the increased accuracy produced by a larger number of reflectors with a wider lunar area covered by the network. Currently, there are five retroreflectors on the moon: the three placed by Apollo missions and two French-designed instruments placed by Soviet lunar missions. The Currie-led proposal would add three Next Generation Retroreflectors for a total of eight lunar retroreflector arrays.
“Our Next Generation Lunar Retroreflector is a 21st Century version of the instruments currently on the Moon. Each placement of a Next Generation lunar laser ranging array will greatly enhance the scientific and navigational capabilities of retroreflector network,” said Currie. “These additions improve the mapping and navigation capabilities important for NASA’s plans to return to the Moon and by 2028 establish a sustained human presence.”
“And these also will significantly boost scientists’ ability to use the network to conduct important science, such as new tests of general relativity and other theories of gravity. Such studies may help us understand the nature of mysterious dark matter, which appears to constitute almost 27 percent of the Universe,” he said.
According to a NASA release, the Next Generation Lunar Retroreflectors (NGLR) is one of 12 new science and technology payloads selected by the agency to help humans study the Moon and explore more of its surface as part of the NASA’s Artemis lunar program. The agency says the retroreflector and the other 11 investigations and demonstrations “will help the agency to send astronauts to the Moon by 2024 as a way to prepare to send humans to Mars for the first time.”
The selected investigations will go to the Moon on future flights through NASA's Commercial Lunar Payload Services (CLPS) project. According to the agency, the CLPS project allows rapid acquisition of lunar delivery services for payloads like these that advance capabilities for science, exploration, or commercial development of the Moon.
NASA has selected the first three commercial Moon landing service providers that will deliver science and technology payloads to the lunar surface. According to Currie, the Next Generation Retroreflectors are not currently scheduled by NASA to be among the payloads carried on those three commercial Moon landings. “However, we believe because of the low size and weight of these retroreflectors, each mission could safely add one to their planned payloads for each of these commercial delivery missions,” he said.
"The selected lunar payloads represent cutting-edge innovations, and will take advantage of early flights through our commercial services project,” said Thomas Zurbuchen, associate administrator of the agency's Science Mission Directorate in Washington. "Each demonstrates either a new science instrument or a technological innovation that supports scientific and human exploration objectives, and many have broader applications for Mars and beyond.”
The NGLR team consists of: Principal Investigator Douglas Currie Professor Emeritus, Department of Physics, University of Maryland College Park (UMD); Co-I/Co-PI: Simone Dell’Agnello Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Frascati, Italy; and Co-Investigators Professor Christopher Davis Electrical Engineering Department (UMD); James Williams Jet Propulsion Laboratory Technical Personnel; John Rzasa (UMD); and Dennis Wellnitz Department of Astronomy (UMD). UMD Assistant Research Scientist Chensheng Wu, a member of Professor Chris Davis’s Maryland Optical Group in Electrical and Computer Engineering has also done crucial work on the design of the Next Generation Lunar Retroreflectors.
(Currie was interviewed by ABC News about the 50th anniversary of Apollo 11.)
Links for more information:
UMD Professor Dr. Douglas Currie next to the Orbit Beyond lander. Credit Udit Shah of Orbit Beyond
Pictured (L-R) in 1969 are University of Maryland (UMD) physicist Doug Currie, University of Texas (UT) McDonald Observatory Director Harlan J. Smith, NASA Scientist-Astronaut Philip Chapman, UMD Professor of Physics Carroll Alley and NASA Scientist-Astronaut Don L. Lind discussing use of the McDonald Lunar Laser Ranging Observatory (MLLRO) to send short laser pulses to the first Lunar Laser Ranging (LLR) Retroreflector left on the lunar surface by the Apollo 11 Astronauts. The MLLRO program was developed and initially operated by Professor Currie (then associate professor) with the support of the Godard Space Craft Center and the University of Texas. UMD’s Professor Alley was the Principle Investigator of the project to place LLRs on the moon to address lunar physics, gravitation, General Relativity and Earth physics. The LLR Retroreflectors were designed and developed by a team that in addition to UMD physicists Alley and Currie, included scientists from universities and federal institutions that included Princeton University, NASA Goddard Space Craft Center, the National Bureau of Standards, Wesleyan University and the University of California. NASA’s Scientist-Astronaut program trained Ph.D. scientists & engineers as astronauts. Image courtesy Doug Currie.