GPS technology can do much more than guide drivers and smartphone users on unfamiliar streets. The Global Positioning System’s satellites carry expensive atomic clocks that also provide synchronized timekeeping for cell phone networks, major financial institutions, and power grids across the world. But a report by the U.S. government’s “master clock” keepers finds that a ground-based “TimeLoc” technology can provide even better timekeeping accuracy within crowded cities and indoor spaces—places where GPS signals have trouble reaching.
Earlier this year we told you about what might be a solution to a big problem in wireless communications: In order for an antenna to transmit sufficient power you need it to be comparable in size to the wavelength its sending. That’s hard when you’re constrained by the size of a smartphone or smaller gadget.
The Cambridge engineers connected only one contact of a piezoelectric filter to the transmitter circuitry, while the other contact was left floating. That is, the piezoelectric filter was excited in an asymmetric mode. The authors of the paper explained the fact that the small antenna could deliver a lot of power by assuming that a concept called “broken symmetry” caused the filter to resonate strongly.
The concept of symmetry breaking, now more than 60 years old, has been successfully used to explain or describe phenomena in particle physics. But its application to electromagnetism was not appreciated by some.
"Asymmetry is not required for efficient radiation, there are symmetric antennas which radiate quite nicely," says Pavel Belov, a physicist at ITMO University and one of the authors of the Comment. "In this case, symmetry breaking is not meaningful, and there are classical explanations. There is a simple explanation of better impedance matching between waveguides and antenna," says Belov.
Sinha and Amaratunga disagree. They argue that the authors of the Comment have misunderstood the idea of asymmetry, confusing geometrical asymmetry of an antenna with an asymmetric electromagnetic excitation fed to the device. The latter results in the symmetry breaking of the electric field and in radiation.
"We have completely different viewpoints, they have just isolated the concept of geometric symmetry, and they say that geometric symmetry is not needed for radiation,” Amaratunga quotes from the paper they have published as a reply in PRL. “The concept of explicit symmetry breaking is not limited to geometrical symmetries, a point on which Simovksi et al. have based their arguments on.”
Sinha compares the dielectric antenna, excited by an RF signal at one contact with a bell, hit by a hammer on one side: "You might have observed that hitting a suspended bell with a hammer creates sound, but hitting it with a hammer while holding the surface of the bell at the other side with the palm of hand, you will not generate a strong acoustic signal as a closed circuit is established limiting the outflow of energy into free space."
Sinha further explains that symmetry breaking does not mean destroying the geometrical symmetry of the antenna. "In fact, symmetry breaking is just the lowering of the order of symmetry, so symmetry is not lost completely. For example, the symmetry of a sphere can be broken to create a circle. Here, the 3-d symmetry is lowered down to 2-d or it is broken. But the structure still has symmetry," says Sinha.
The battle of minds is not over, “We are preparing a reply,” says Belov.
On Friday, the hunt for gravitational waves began again for the Laser Interferometer Gravitational-Wave Observatory (LIGO)—the largest instrument of its kind. The restart follows a five-year-long, US $200-million project to overhaul the experiment’s detectors.
Many physicists believe the revamped experiment, dubbed Advanced LIGO, will be the first to find direct evidence of gravitational waves: ripples in the fabric of space-time that can be created by, among other things, a pair of neutron stars or black holes orbiting each other.
Editors Note: James Oberg is a regular contributor to IEEE Spectrum and a former NASA mission controller at Johnson Space Center who specialized in orbital rendezvous maneuvers.
The Martian movie is a dream come true, a faithful elucidation of the central technical dramas that made the Andy Weir novel it’s based on the surprise ‘space geek hit’ of the decade. The tale is something of a Robinson Crusoe in space: Astronaut Mark Watney is stranded on the surface of Mars when his crew mates are forced to evacuate their habitat and blast off for Earth. Watney must improvise ways to eke out his limited resources, as the next mission to the red planet isn’t scheduled to land for another four years. Like the book, the movie’s rhythm is a steady cycle of crisis, recovery, humor, determination, and new crisis. On screen, directed by Ridley Scott, these cycles are delivered with satisfying characterization and eye-grabbing visuals of Martian landscapes and mouth-gaping zero-G gymnastics in space.
As a backdrop to the human drama (and comedy), the movie’s hardware is based on real engineering, not magical miracle machines. The Hermes mother ship, for example, is a modified version of the sound design strategy of a ‘cycling spacecraft’ that repeatedly makes the Earth-Mars-Earth loop with crew and small landing craft. (Buzz Aldrin and I wrote up the concept for Scientific American a decade ago.)
As the left-for-dead Watney, played by Matt Damon, faces doom in a dozen guises, he defiantly boosts his courage through styling himself as everything from the “best botanist on Mars” (as he ingeniously grows potatoes in carefully-created garden soil) to a “space pirate” (for his goal of commandeering a rescue rocket emplaced automatically elsewhere on the planet). He expresses his thoughts to his laptop log and later in messages to people on earth campaigning to rescue him. When humans finally reach other planets, we will need real “Watneys”—those smart enough to repurpose on-hand assets in an emergency, and lucky enough not to be killed by their own inevitable oversights.
Space geeks will enjoy identifying and debating technical flaws in the film. The Martian follows its source material remarkably closely, but unlike say, certain installments in the Harry Potter movie franchise, it will still make perfect sense to those who haven’t read the book. Aside from the monster sandstorm that sets up the crisis (Andy Weir, the author of The Martian, is very upfront about choosing to gloss over the realities of Martian atmospheric pressure in favor of a dramatic opening), there are few noticeable factual flubs in the plot. Unlike 2013’s Gravity and its magical space maneuvers, or 2014’s Interstellar with its mystical time continuum tricks, this movie is a remarkably authentic exposition of what space travelers need to be able to do under crisis conditions.
Run time constraints forced the movie’s producers to condense and combine characters, with no real detriment to the story—although viewers might be surprised to see the same set of key NASA experts showing up at every launch or space center on the planet as the action shifts between continents. Similar story-telling imperatives probably compelled the most striking departure (spoiler alert!) from the book—changing who actually goes to rescue Watney in the end—but again, without any significant detour from the plot.
There’s no cheating on even highly-technical spaceflight topics, as shown in the treatment of the so-called “Rich Purnell maneuver,” wherein the Hermes slingshots past Earth back to Mars for a desperate pickup attempt. NASA’s navigation geek Purnell—played with effective nervous energy by Donald Glover— demonstrates the “orbitology” of his maneuver, with two NASA officials as the planets, and a stapler and pens as spacecraft. The result is coherent, comprehensive, and surprisingly comic. The basic strategy of the Rich Purnell maneuver is not fictional—a crippled Japanese Mars probe named Nozomi actually used a similar Earth-flyby scheme to set up a second chance for its own faltering unmanned Mars mission a dozen years ago.
There’s one cringeworthy trope in The Martian that has apparently become an unavoidable cultural myth: the scenes where teams in Mission Control erupt into leaping cheers a few seconds after a rocket blasts off. This imaginary practice would not only be utterly unprofessional but also an indicator of near-criminal negligence. In the screening that I attended following an event at NASA’s Johnson Space Center, these scenes were negatively noticed by many NASA folks in the audience. But I only can suppose it was sympathetically meant to humanize the rocket scientists, and it was the only exception to the admiration shown for all the other portrayals. (It’s also possible this trope has been influenced by footage of the controllers of deep space unmanned missions in recent years, where data about events can take hours to cross the solar system, leaving controllers mostly in the position of spectators during key mission milestones.)
After faithfully reproducing technically-sound crises and responses for two hours, the movie did briefly run off the track into fantasy mode at the very end, by by violating the physics of action-reaction in portraying an over-dramatized dramatic rescue attempt that really didn't need any such cheating. But if that was the price to pay for producing this otherwise magnificent space adventure story, the math checked out. Mother Nature is relentlessly unforgiving in space, but a movie audience can show a lot more mercy to one major technical transgression. I do.
3-D printing has turned prototyping from something that takes days or weeks into something that takes hours. This is great, if the prototype that you end up with does what you want it to do, which it probably won’t, if you’re concerned with interactivity. You can certainly take a dremel to whatever you’ve just printed and try and stuff some sensors into it, but that sounds like a lot of work, which is why researchers at Queen’s University (the one in Ontario) have combined conductive filament with a dual-extruder 3-D printer to create a way to print objects with sensors in them all in one shot.
Yesterday (15 September 2015), The Dallas Morning News published a story about Ahmed Mohamed, a 14-year-old student in Irving, Texas, who was arrested when be brought a home-brew electronic gadget to MacArthur High School.
Ahmed said it was a clock and showed it to one of his teachers, but another teacher thought it looked suspicious and reported it to the school administration. Next thing, Ahmed was taken from class by the school principal and a police officer and taken to a room where he was interrogated by five officers about his electronic project, which the adults decided was a “hoax bomb.” Ahmed, wearing a NASA T-shirt no less, was handcuffed and brought to a juvenile-detention center.
The Dallas Morning News quoted police spokesperson James McLellan. “We have no information that he claimed it was a bomb,” McLellan said. “He kept maintaining it was a clock, but there was no broader explanation.” How about this one: It was a clock.
Seems that nobody has informed the Irving police department or public school system that many young people like to tinker with electronics. There was a time when that was lauded. Now, though, if you live in Irving and can solder you’re considered suspicious, particularly if you have brown skin and are a Muslim.
At a press conference today, Irving police Chief Larry Boyd announced no charges would be filed against Ahmed, confirming that his homebrew contraption was not a bomb and referring to what The Dallas Morning News quotes Boyd as calling “a naïve accident.”
Boyd’s words rang rather hollow, though, as social media simultaneously exploded with support for Ahmed and scorn for school administrators and police officers who can’t distinguish a cobbled-together clock from a dangerous explosive.
President Obama tweeted, “Cool clock, Ahmed. Want to bring it to the White House? We should inspire more kids like you to like science. It’s what makes America great.”
Hillary Clinton tweeted her support as well with, “Assumptions and fear don’t keep us safe—they hold us back. Ahmed, stay curious and keep building.”
On Twitter, Ahmed received offers of membership in a nearby hackerspace and invitations to visit JPL, MIT, and Science Friday, among the outpouring of support from the world’s technical and scientific community.
Irving police stress that Islamophobia had nothing to do with Ahmed’s arrest. Perhaps so, but whether this episode reflects prejudice or stupidity, it speaks very poorly of the school administrators and police officers involved. Let’s just hope that Ahmed receives a scholarship to another high school where the adults are more enlightened—and more interested in nurturing fledgling engineers.
Want to send your total number of steps from your Fitbit without eating up battery life, or communicate between your iPhone and your Apple watch with no fear of eavesdropping? Engineers in California say the best way for wearable devices to talk amongst themselves is by sending magnetic fields through the wearer’s body.
Terahertz waves, which span a frequency range of 300 to 3000 gigahertz, promise a hundred times faster data transmission than today’s cellular and wireless networks. Researchers now face the daunting task of designing and building a communication system with all new components that can work with terahertz radiation. Many efforts have focused on building compact terahertz sources, transmitters and detectors.
Now a group at Brown University in Providence, R.I., has built another key component of any wireless communication system: a multiplexer. Multiplexers combine separate data streams, typically each at a different frequency, into a single combined stream that is sent over optical fibers, TV cables, or telephone lines, making it possible for that medium to carry thousands of phone calls or tens of TV channels at the same time. A demultiplexer separates the signals at the receiver end.
The new device, which acts as both a multiplexer and demultiplexer, is reported in the journal Nature Photonics. It consists of two smooth aluminum plates (any metal should work) placed in parallel, with a few-millimeter gap between them. The plates act like a waveguide for terahertz waves, which travel between them.
To explain how the multiplexer works, it’s helpful to first describe how signals are separated at the receiving end. As the waves travel through the waveguide, some of the radiation leaks out of the slit. The angle at which it emerges depends on the frequency of the wave. So if an entering wave contains multiple frequencies, each carrying a separate data channel, they would each come out a different angle, effectively separated or demultiplexed.
The converse occurs with a multiplexer; the device accepts a signal of a certain frequency only if it comes in at a certain angle.
“We’re envisioning free-space wireless terahertz communication networks,” says Daniel Mittleman, a professor of engineering at Brown University. “But the important difference between a terahertz network and the existing cellular network is that the antenna is broadcasting in all directions. With terahertz, there’s 100 times less diffraction so it’s much more reasonable to think of it as a beam with some directionality.”
The researchers demonstrated a simple multiplexing scheme in which they sent two independent THz-frequency channels to the multiplexer at two different angles. For the source, they used a commercial femtosecond laser, which delivers short pulses of terahertz radiation and is used for spectroscopy. In a practical multiplexer, specially-shaped waveguides and optics could send hundreds of different channels into the multiplexer at different angles.
The device is completely passive right now. But Mittleman says he and his team could make it dynamic by varying the spacing between the plates. The researchers found that this spacing affects the frequency, angle, and bandwidth of the signal that leaks out of the slit. This could, for instance, be used to tune the bandwidth of the channels, Mittleman says.
“In the waveguide, imagine that the lower plate has a trench dug into it parallel to the slot and directly underneath,” he says. “You could use a silicon microelectromechanical switch to dynamically tune the depth of the trench at any location, so that the plate separation varies underneath slot. That would change the bandwidth of that channel.”
The researchers hope to make and demonstrate an active device of that type that changes positions on the timescale of a few milliseconds.
Visible light communication (VLC) is a method of transmitting data using LEDs. It's a simple idea: since it's possible to turn an LED on and off much faster than the human eye can resolve, you can use lights to transmit binary data in the form of light pulses without any detectable flickering that would drive a human nuts. This data transmission is ambient, detectable by other LEDs (acting as photodiodes), and it seems like a promising way to Internet-of-Thingsify household objects without having to rely on Wi-Fi.
Using modulated ambient light to transmit data isn't a new idea; this "Li-Fi" is already being used commercially, with GE and Philips (among others) deploying data transmitting LEDs in places like stores and warehouses. However, it's difficult to encourage wider adoption of the technology because Li-Fi is generally used in specific (and isolated) situations where it operates alongside existing communications technologies rather than being integrated with them. In other words, you can have Wi-Fi, and you can have Li-Fi, but they don't work together all that well.
For easy and convenient Internet-of-Things-scale adoption, Li-Fi needs to be able to run on the same Internet Protocol (IP) that all of our other Web-connected stuff runs on. Taking the lead on this is Disney Research, which has a visible light communication system that uses a friendly Linux-based networking backbone to, among other things, let you use a magic wand to light up a princess dress. Sign me up.
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