Science
Discovery of new mechanisms to control the flow of sound – Phys.org
Using a network of vibrating nano-strings controlled with light, researchers from AMOLF have made sound waves move in a specific irreversible direction and attenuated or amplified the waves in a controlled manner for the first time. This gives rise to a lasing effect for sound. To their surprise, they discovered new mechanisms, so-called “geometric phases,” with which they can manipulate and transmit sound in systems where that was thought to be impossible. “This opens the way to new types of (meta)materials with properties that we do not yet know from existing materials,” says group leader Ewold Verhagen who, together with shared first authors Javier del Pino and Jesse Slim, publishes the surprising results on June 2 in Nature.
The response of electrons and other charged particles to magnetic fields leads to many unique phenomena in materials. “For a long time, we have wanted to know whether an effect similar to a magnetic field on electrons could be achieved on sound, which has no charge,” says Verhagen. “The influence of a magnetic field on electrons has a wide impact: for example, an electron in a magnetic field cannot move along the same path in the opposite direction. This principle lies at the basis of various exotic phenomena at the nanometer scale, such as the quantum Hall effect and the functioning of topological insulators (materials that conduct current perfectly at their edges and not in their bulk). For many applications, it would be useful if we could achieve the same for vibrations and sound waves and therefore break the symmetry of their propagation, so it is not time-reversal symmetric anymore.”
Magnetic field for sound
Unlike electrons, mechanical vibrations have no charge, and so they do not respond to magnetic fields. However, they are sensitive to the radiation pressure of light. Verhagen’s group therefore used laser light to influence mechanical nano-resonators. In 2020, they used these same vibrating strings to demonstrate that time-reversal symmetry could be broken for sound that jumps from one resonator to another: sound transfer from one string to another is different than in the opposite direction. See also the news item of February 3rd, 2020. “We have now shown that if we make a network of multiple vibrating nano-strings, we can realize a range of unconventional vibrational patterns by illuminating the strings with laser light,” states Verhagen. “For example, we managed to get sound particles (phonons) to move in a single direction in the same way as electrons in the quantum Hall effect.”
Amplification
The researchers realized that they could also use the radiation pressure to control the amplification and attenuation of the sound. “That works in a similar manner to a child on a swing extending or pulling back its legs at the right moment,” explains Verhagen. “Such amplification or attenuation is not possible for electrons in a magnetic field.”
The researchers were the first to conduct experiments in which the driving light amplifies the sound waves while at the same time ensuring that these experience an effect similar to that of a magnetic field. “We discovered that the combination of amplification and breaking the time-reversal symmetry leads to a range of new and unexpected physical effects,” says Verhagen. “First of all, laser light determines the direction in which the sound is amplified. In the other direction, the sound is blocked. This is caused by a geometric phase: a quantity that indicates the extent to which the sound wave is shifted as it passes through the network of nano-strings, which in this case is caused by the radiation pressure. Our experiment allowed us to fully control and alter that geometric phase. In addition, we used the radiation pressure of the laser light to amplify the sound. That sound can even spontaneously start to oscillate, like light in a laser. We discovered that the geometric phase we apply determines whether that happens or not, and with what strength of amplification.”
New materials
The researchers discovered that new geometric phases could be realized in systems where that was not considered to be possible. In all of these, the phases influence the amplification, direction, and pitch of the sound waves. “Geometric phases are important in many branches of physics, describing the behavior of different systems and materials. When combined with magnetic fields, they can lead to a topological insulator for electrons, but which properties a ‘sound’ variant based on the discovered principles could have is something we still need to learn. However, we do know that this will not be similar to anything we know,” states Verhagen. “We could further investigate the effects by linking more nano-strings in acoustic ‘metamaterials’ that we control with light. But the effects that we have observed should apply to a range of waves without charge, including light, microwaves, cold atoms, et cetera. We expect that with the new mechanisms we have discovered, it will be possible to produce new (meta)materials with properties that we do not yet know from existing materials.”
Such materials and systems have unusual properties that might have useful applications. Verhagen: “It is still too early to provide a complete overview of the possibilities. However, we can already recognize some potential directions. For example, a unidirectional amplifier for waves could have useful applications in quantum communication. We could also make sensors far more sensitive by breaking the time-reversal symmetry.”
Explore further
Ewold Verhagen, Non-Hermitian chiral phononics through optomechanically induced squeezing, Nature (2022). DOI: 10.1038/s41586-022-04609-0. www.nature.com/articles/s41586-022-04609-0
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The moon has a new double crater after a rocket body collided with its surface on March 4.
New images shared by NASA’s Lunar Reconnaissance Orbiter, which has been circling the moon since 2009, have revealed the location of the unusual crater.
The impact created two craters that overlap, an eastern crater measuring 59 feet (18 metres) across and a western crater spanning 52.5 feet (16 metres). Together, they create a depression that is roughly 91.8 feet (28 metres) wide in the longest dimension.
Although astronomers expected the impact after discovering that the rocket part was on track to collide with the moon, the double crater it created was a surprise.
Typically, spent rockets have the most mass at the motor end because the rest of the rocket is largely just an empty fuel tank. But the double crater suggests that this object had large masses at both ends when it hit the moon.
The exact origin of the rocket body, a piece of space junk that had been careening around for years, is unclear, so the double crater could help astronomers determine what it was.
The moon lacks a protective atmosphere, so it’s littered with craters created when objects like asteroids regularly slam into the surface.
This was the first time a piece of space junk unintentionally hit the lunar surface that experts know of. But craters have resulted from spacecraft being deliberately crashed into the moon.
For example, four large moon craters attributed to the Apollo 13, 14, 15 and 17 missions are all much larger than each of the overlapping craters created during the March 4 impact. However, the maximum width of the new double crater is similar to the Apollo craters.
UNCLEAR ORIGIN
Bill Gray, an independent researcher focused on orbital dynamics and the developer of astronomical software, was first to spot the trajectory of the rocket booster.
Gray had initially identified it as the SpaceX Falcon rocket stage that launched the US Deep Space Climate Observatory, or DSCOVR, in 2015 but later said he’d gotten that wrong and it was likely from a 2014 Chinese lunar mission — an assessment NASA agreed with.
However, China’s Ministry of Foreign Affairs denied the booster was from its Chang’e-5 moon mission, saying that the rocket in question burned up on reentry to Earth’s atmosphere.
No agencies systematically track space debris so far away from Earth, and the confusion over the origin of the rocket stage has underscored the need for official agencies to monitor deep-space junk more closely, rather than relying on the limited resources of private individuals and academics.
However, experts say that the bigger challenge is the space debris in low-Earth orbit, an area where it can collide with functioning satellites, create more junk and threaten human life on crewed spacecraft.
There are at least 26,000 pieces of space junk orbiting Earth that are the size of a softball or larger and could destroy a satellite on impact; over 500,000 objects the size of a marble — big enough to cause damage to spacecraft or satellites; and over 100 million pieces the size of a grain of salt, tiny debris that could nonetheless puncture a spacesuit, according to a NASA report issued last year.
Can’t afford to join a commercial space mission offered by Jeff Bezos or Richard Branson? Consider the next best thing: seeing a starry, starry night in a sea of darkness, unimpeded by artificial light, at one of the International Dark Sky Parks in the U.S. It’s a rare treat, since light pollution prevents nearly 80 percent of Americans from seeing the Milky Way from their homes.
The International Dark-Sky Association (IDSA) has certified 14 of the nation’s 63 national parks as dark sky destinations. So visitors can take full advantage of such visibility, many of them offer specialized after-dark programs, from astronomy festivals and ranger-led full-moon walks to star parties and astrophotography workshops. If you prefer to stargaze on your own at a park, the National Park Service recommends bringing a pair of 7-by-50 binoculars, a red flashlight, which enhances night vision, and a star chart, which shows the arrangement of stars in the sky.
Here are seven of the IDSA-certified parks where you can appreciate how the heavens looked from the Earth before the dawn of electric light.
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Award-winning travel writer Veronica Stoddart is the former travel editor of USA Today. She has written for dozens of travel publications and websites.
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While we think of the moon as a static place, sometimes an event happens that reminds us that things can change quickly.
On March 4, a human-made object (a rocket stage) slammed into the moon and left behind a double crater, as seen by NASA’s Lunar Reconnaissance Orbiter (LRO) mission.
Officials announced June 23 that they spotted a double crater associated with the event. But what’s really interesting is there’s no consensus about what kind of rocket caused it.
China has denied claims that the rocket was part of a Long March 3 rocket that launched the country’s Chang’e-5 T1 mission in October 2014, although the orbit appeared to match. Previous speculation suggested it might be from a SpaceX rocket launching the DISCOVR mission, but newer analysis has mostly discredited that.
Here’s what the March 4 rocket crash site looked like from a wider field of view.
NASA/GSFC/Arizona State University
On a broader scale, the value of LRO observations like this is showing how the moon can change even over a small span of time. The spacecraft has been in orbit there since 2009 and has spotted numerous new craters since its arrival.
It’s also a great spacecraft scout, having hunted down the Apollo landing sites from orbit and also having tracked down a few craters from other missions that slammed into the moon since the dawn of space exploration.
It may be that humans return to the moon for a closer-up look in the coming decade, as NASA is developing an Artemis program to send people to the surface no earlier than 2025.
LRO will also be a valuable scout for that set of missions, as the spacecraft’s maps will be used to develop plans for lunar bases or to help scout safe landing sites for astronauts.
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