What Does E=mc^2 Mean?

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What Does E=mc^2 Mean?

By: Life’s Little Mysteries Staff
Date: 20 December 2012 Time: 05:38 PM ET
E=mc^2 is a version of Einstein’s famous relativity equation. Specifically, it means that energy is equal to mass multiplied by the speed of light squared. While seemingly simple, this equation has many profound implications, chief among them being that matter and energy are actually the same stuff. Pure energy in the form of motion can be converted into matter, through the creation of a particle, which has mass. However, as the equation implies, it takes a huge amount of energy to create a tiny bit of mass.

The Sun: Formation, Facts and Characteristics

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The Sun: Formation, Facts and Characteristics


The sun lies at the heart of the solar system, where it is by far the largest object. It holds 99.8 percent of the solar system’s mass and is roughly 109 times the diameter of the Earth — about one million Earths could fit inside the sun.

The visible part of the sun is roughly 10,000 degrees F (5,500 degrees C), while temperatures in the core reach more than 27 million degrees F (15 million degrees C), driven by nuclear reactions. One would need to explode 100 billion tons of dynamite every second to match the energy produced by the Sun.

The sun is one of more than 100 billion stars in the Milky Way. It orbits some 25,000 light years from the galactic core, completing a revolution once every 250 million years or so. The sun is relatively young, part of a generation of stars known as Population I, which are relatively rich in elements heavier than helium. An older generation of stars is called Population II, and an earlier generation of Population III may have existed, although no members of this generation are known yet.

A huge solar filament snakes around the southwestern horizon of the sun in this full disk photo taken by NASA's Solar Dynamics Observatory on Nov. 17, 2010.

A huge solar filament snakes around the southwestern horizon of the sun in this full disk photo taken by NASA’s Solar Dynamics Observatory on Nov. 17, 2010. 

Formation & Evolution

The sun was born roughly 4.6 billion years ago. Many scientists think the sun and the rest of the solar system formed from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun.

The sun has enough nuclear fuel to stay much as it is now for another 5 billion years. After that, it will swell to become a red giant. Eventually, it will shed its outer layers, and the remaining core will collapse to become a white dwarf. Slowly, this will fade, to enter its final phase as a dim, cool object sometimes known as a black dwarf.


  • Internal structure and atmosphere
See how solar flares, sun storms and huge eruptions from the sun work in this SPACE.com infographic. 
CREDIT: Karl Tate/SPACE.com

The sun and its atmosphere are divided into several zones and layers. The solar interior, from the inside out, is made up of the core, radiative zone and the convective zone. The solar atmosphere above that consists of the photosphere, chromosphere, a transition region and the corona. Beyond that is the solar wind, an outflow of gas from the corona.

The core extends from the sun’s center to about a quarter of the way to its surface. Although it only makes up roughly 2 percent of the sun’s volume, it isalmost 15 times the density of lead and holds nearly half of the sun’s mass. Next is the radiative zone, which extends from the core to 70 percent of the way to the sun’s surface, making up 32 percent of the sun’s volume and 48 percent of its mass. Light from the core gets scattered in this zone, so that a single photon often may take a million years to pass through. The convection zone reaches up to the sun’s surface, and makes up 66 percent of the sun’s volume but only a little more than 2 percent of its mass. Roiling “convection cells” of gas dominate this zone. Two main kinds of solar convection cells exist — granulation cells about 600 miles (1,000 kilometers) wide and supergranulation cells about 20,000 miles (30,000 kilometers) in diameter.

The photosphere is the lowest layer of the sun’s atmosphere, and emits the light we see. It is about 300 miles (500 kilometers) thick, although most of the light comes from its lowest third. Temperatures there range from 11,000 degrees F (6,125 degrees C) at bottom to 7,460 degrees F (4,125 degrees C) at top. Next up is the chromosphere, which is hotter at up to 35,500 degrees F (19,725 degrees C) and is apparently made up entirely of spiky structures known as spicules typically some 600 miles (1,000 kilometers) across and up to 6,000 miles (10,000 kilometers) high. After that is the transition region a few hundred to a few thousand miles or kilometers thick, which is heated by the corona above it and sheds most of its light as ultraviolet rays. At the top is the super-hot corona, which is made of structures such as loops and streams of ionized gas. The corona generally ranges from 900,000 degrees F (500,000 degrees C) to 10.8 million degrees F (6 million degrees C) and can even reach tens of millions of degrees when a solar flare occurs. Matter from the corona is blown off as the solar wind.

  • Magnetic Field

The strength of the sun’s magnetic field is typically only about twice as strong as Earth’s field. However, it becomes highly concentrated in small areas, reaching up to 3,000 times stronger than usual. These kinks and twists in the magnetic field develop because the sun spins more rapidly at the equator than at the higher latitudes and because the inner parts of the sun rotate more quickly than the surface. These distortions create features ranging from sunspots to spectacular eruptions known as flares and coronal mass ejections. Flares are the most violent eruptions in the solar system, while coronal mass ejections are less violent but involve extraordinary amounts of matter — a single ejection can spout roughly 20 billion tons (18 billion metric tons) of matter into space.

Chemical Composition

Just like most other stars, the sun is made up mostly of hydrogen, followed by helium. Nearly all the remaining matter consists of seven other elements — oxygen, carbon, neon, nitrogen, magnesium, iron and silicon. For every 1 million atoms of hydrogen in the sun, there are 98,000 of helium, 850 of oxygen, 360 of carbon, 120 of neon, 110 of nitrogen, 40 of magnesium, 35 of iron, and 35 of silicon. Still, hydrogen is the lightest of all elements, so it only accounts for roughly 72 percent of the sun’s mass, while helium makes up about 26 percent.

Sunspots & Solar Cycle

Sunspots are relatively cool, dark features on the sun’s surface that are often roughly circular. They emerge where dense bundles of magnetic field lines from the sun’s interior break through the surface. The number of sunspots varies as solar magnetic activity does — the change in this number, from a minimum of none to a maximum of roughly 250 sunspots or clusters of sunspots and then back to a minimum, is known as the solar cycle, and averages about 11 years long. At the end of a cycle, the magnetic field rapidly reverses its polarity.


Ancient cultures often modified natural rock formations or built stone monuments to mark the motions of the sun and moon, charting the seasons, creating calendars and monitoring eclipses. Many believed the sun revolved around the Earth, with ancient Greek scholar Ptolemy formalizing this “geocentric” model in 150. Then, in 1543, Copernicus described a heliocentric, sun-centered model of the solar system, and in 1610, Galileo’s discovery of Jupiter’s moons revealed that not all heavenly bodies circled the Earth.

To learn more about how the sun and other stars work, after early observations using rockets, scientists began studying the sun from Earth orbit. NASA launched a series of eight orbiting observatories known as the Orbiting Solar Observatory between 1962 and 1971. Seven of them were successful, and analyzed the sun at ultraviolet and X-ray wavelengths and photographed the super-hot corona, among other achievements.

In 1990, NASA and the European Space Agency launched the Ulysses probe to make the first observations of its polar regions. In 2004, NASA’s Genesis spacecraft returned samples of the solar wind to Earth for study. In 2007, NASA’s double-spacecraft Solar Terrestrial Relations Observatory (STEREO) mission returned the first three-dimensional images of the Sun.

One of the most important solar missions to date has been the Solar and Heliospheric Observatory (SOHO), which was designed to study the solar wind, as well as the sun’s outer layers and interior structure. It has imaged the structure of sunspots below the surface, measured the acceleration of the solar wind, discovered coronal waves and solar tornadoes, found more than 1000 comets, and revolutionized our ability to forecast space weather. Recently, NASA’s Solar Dynamics Observatory (SDO), the most advanced spacecraft yet designed to study the sun, has returned never-before-seen details of material streaming outward and away from sunspots, as well as extreme close-ups of activity on the sun’s surface and the first high-resolution measurements of solar flares in a broad range of extreme ultraviolet wavelengths.

RELATED: See our overview of Solar System Facts or learn more about the Solar System Planets.

What is the Sun Made Of?

The sun is a big ball of hot gases. The gases are converted into energy in the sun’s core. The energy moves outward through the interior layers, into the sun’s atmosphere, and is released into the solar system as heat and light.

NASA’s Solar Dynamics Observatory saw sunspot AR 1520 before the solar flare erupted from it on July 12, 2012.

Most of the gas — about 72 percent — is hydrogen. Nuclear fusion converts hydrogen into other elements. The sun is also composed of about 26 percent helium and trace amounts of other elements — oxygen, carbon, neon, nitrogen, magnesium, iron and silicon.

These elements are created in the sun’s core, which makes up 25 percent of the sun. Gravitational forces create tremendous pressure and temperatures in the core. The temperature of the sun in this layer is about 27 million degrees F (15 million degrees C). Hydrogen atoms are compressed and fuse together, creating helium and a lot of energy. This process is called nuclear fusion.

The energy, mostly in the form of gamma-ray photons and neutrinos, is carried into the radiative zone. Photons can bounce around in this zone for about a million years before passing through the interface layer, or tachocline. Scientists think the sun’s magnetic field is generated by a magnetic dynamo in this layer.

The convection zone is the outermost layer of the sun’s interior. It extends from about 125,000 miles (200,000 km) deep up to the visible surface or the sun’s atmosphere. Temperatures cool in this zone, enough for heavier ions — such as carbon, nitrogen, oxygen, calcium and iron — to hold onto their electrons. This makes the material more opaque and traps heat, causing the plasma to boil or “convect.”

The convective motions carry heat quite rapidly to the surface, which is the bottom layer of the sun’s atmosphere, or photosphere. This is the layer where the energy is released as sunlight. The light passes through the outer layers of the sun’s atmosphere — the chromosphere and the corona — before reaching Earth eight minutes later.

Abundance of elements

Astronomers who have studied the composition of the sun have catalogued 67 chemical elements in the sun. There may be more, but in amounts too small for instruments to detect. Here is a table of the 10 most common elements in the sun:

Element Abundance (pct.
of total number
of atoms)
(pct. of total mass)
Hydrogen 91.2         71.0        
Helium 8.7         27.1        
Oxygen 0.078         0.97        
Carbon 0.043         0.40        
Nitrogen 0.0088         0.096        
Silicon 0.0045         0.099        
Magnesium 0.0038         0.076        
Neon 0.0035         0.058        
Iron 0.030         0.014        
Sulfur 0.015         0.040        

— Tim Sharp, Reference Editor


What Does the Sun Burn?

By: Benjamin Radford, Life’s Little Mysteries Contributor

Date: 20 December 2012 Time: 04:31 PM ET

For millennia, people have looked up to the sky and wondered about celestial bodies. The sparkling stars and fiery sun hold mystery and wonder. To astronomers, the sun is just another dying star, but to everyone else it’s a huge burning ball that gives heat, light, and life. So far so good.

But what is it burning? We all know that there is no air in space, and therefore no oxygen to burn. In our everyday experience, the only burning most of us are familiar with is fire combustion. But that is not the only type of reaction; the sun is indeed burning, but it is a nuclear reaction, not a chemical one.

The sun burns hydrogen — a lot of it, several hundred million tons per second. But don’t worry; there’s plenty more where that came from; by most estimates, the sun has enough fuel for about another five billion years.

Most Amazing Earth Images of 2012

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Most Amazing Earth Images of 2012

OurAmazingPlanet Staff – Dec 26, 2012 10:18 AM ET
Amazing Images

Auroras Over AntarcticaCredit: ESA/A. Kumar & E. Bondoux.These bright green auroras dancing the sky brought a dose of cheer to the bleak, perpetually dark Antarctic winter. Below the stunning scene in the sky are the lonely lights of Concordia Station, situated in the middle of the East Antarctic Ice Sheet. The photo was snapped on July 18, during the austral winter

New 'Blue Marble'
New ‘Blue Marble’Credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman KuringJust four days into the year, the Suomi NPP satellite sent back a breathtaking “Blue Marble” image of the Earth from its perch in orbit. The image was compiled from shots taken on multiple passes of the planet Jan. 4. This newest Blue Marble image is one of many iconic portraits of the planet, including the iconic one taken by the crew of Apollo 17 in 1972 and views taken by retreating space probes such as Voyagers 1 and 2.
Stunning Cloud Formation
Stunning Cloud FormationCredit: Richard H. HahnThis sight in the sky may look otherworldly, but it’s a terrestrial phenomenon known as a lenticular cloud that was caught by professional photographer Richard H. Hahn during sunset on Jan. 5. Lenticular clouds form when waves of moist, fast-moving air are pushed upward by winds and ascend over high mountains. At the higher altitudes, the water vapor in the air condenses. When the air moves over the mountaintop and descends to uniformly humid conditions, lenticular clouds form. They can look like one large, lens-shaped cloud, or several waves of moist air can result in lenticular clouds that resemble pancakes stacked atop each other, like the ones in this photo.
Sneaky Snow Leopards

Sneaky Snow LeopardsCredit: Panthera/FFI.Researchers got quite a surprise when one of the most elusive creatures on Earth, the snow leopard, was caught by motion-sensing cameras in a remote part of Tajikistan — and then stole one of the cameras! The photos taken by the cameras showed that the culprits were two young cubs. Other photos showed researchers that at least five snow leopards dwell in the region, as well as other rare creatures found in the area.

Crazy Cloud

 Crazy CloudCredit: Capt. Andreas M. van der WurffTotally tubular! This photo of what is known as a “roll cloud” was taken from a ship near Brazil on Feb. 6. Roll clouds sometimes form along with thunderstorms as the cold, sinking air of a downdraft causes warm moist air at the surface to rise. The moisture in that warm air condenses out as the air cools to form a cloud as winds from the storm “roll” the cloud parallel to the horizon.
World's Tiniest Chameleon
World’s Tiniest ChameleonCredit: PLoS One.Yes, that’s an actual, real chameleon perched on a human finger. The chameleon (Brookesia micra) was found on the biologically rich island of Madagascar and is the tiniest chameleon ever discovered. Adults grow to about 1 inch (30 millimeters) in length from nose to the tip of their tail.
Starry Sky
Starry SkyCredit: Tunc Tezel / The World At NightThis window the starry heavens was photographed in Utah’s Canyonlands National Park by astrophotographer Tunc Tezel on May 23. The image looks through a mysterious, manmade feature in the park called the False Kiva. Visible in the night sky are the planet Jupiter and the band of the Milky Way.
Solar Eclipse Shadow

Solar Eclipse ShadowCredit: NASAThousands of skywatchers peered up into the skies on May 20 to catch an annular lunar eclipse that was visible from Asia to the western United States. At the same time, NASA’s Terra satellite was looking down at the Earth and took a spectacular image of the moon’s shadow over the Pacific Ocean. Annular eclipses occur when the moon is at a point in its orbit that is too far from Earth to completely block the sun’s disk. The result is a ringlike, or annulus, effect.

Big Bull Shark

Big Bull SharkCredit: Emma Smith/333productionsShark researchers caught and tagged this whopper of a bull shark with a satellite tag to learn more about where the shark swims in an effort to conserve it and other species. The female shark tipped the scales at about 1,000 pounds (450 kilograms), the largest that Neil Hammerschlag, the researcher pulling the shark up in the photo, has ever caught, he said. Like other shark species, bull sharks are threatened by the shark fin trade, which cuts off shark fins to use in delicacies like shark fin soup.

Glories and Swirls

 Glories and SwirlsCredit: Jeff Schmaltz, LANCE MODIS Rapid ResponseThere are so many amazing features in this NASA satellite photo, taken on June 20, that it’s hard to know where to begin. What looks like a double rainbow streaking down the middle of the image is actually an optical phenomenon called a glory that is created by waves of light being scattered by water droplets in the atmosphere. The swirls to the right of the glories are so-called von Karman vortices, caused by the Pacific island of Guadalupe disrupting the southern flow of clouds, like the wake of a ship.
Astounding Rain Shaft
Astounding Rain ShaftCredit: Dhani Jones. Twitter: @dhanijonesWhile New Yorkers on the ground were busying scurrying through the streets with umbrellas in hand, former NFL player Dhani Jones was 10,000 feet in the air on his Delta flight and snapped a picture of a rain shaft, a term meteorologists use to refer to a heavy downpour coming from a single thunderstorm. One weather station in Queens measuring 2.83 inches (7 centimeters) of rain from storms that rolled through on July 18.
Stunning Supermoon Shot

Stunning Supermoon ShotCredit: Sven Lidstrom, National Science Foundation.Another stunning sight in the sky brought a little light to the United States’ Amundsen-Scott Station at the South Pole. When this photo was taken in early May, the large, bright supermoon was visible above the station. The supermoon occurs when the full moon stage coincides with the moon’s perigee, or closest monthly pass of the Earth. The bright of the moon gave researchers wintering over a little does of light.

Aurora Borealis from Above

 Aurora Borealis from AboveCredit: NASA Earth ObservatoryMost images of auroras come from the ground looking up, but the Suomi NPP satellite caught this spectacular image of an aurora from its aerie looking down on the planet. The auroras were generated by a powerful solar flare, known as a coronal mass ejection, hit Earth’s magnetic field on Oct. 8. The image shows the aurora dancing over the night lights of Canada’s Quebec and Ontario provinces.
Growing Glacier Crack
Growing Glacier CrackCredit: NASA Earth ObservatoryA giant fissure was discovered cracking across Antarctica’s Pine Island Glacier. A NASA satellite image taken on Sept. 14 showed that the crack was widening. Ultimately, the crack should extend across the glacier and spawn a new iceberg.
Sandy Smashes Shore

Sandy Smashes ShoreCredit: Carlos AyalaCarlos Ayala snapped this image of waves crashing ashore near the Verrazano Bridge in Brooklyn during Hurricane Sandy on Oct. 29. Sandy’s waves broke records, with a 32.5-foot (9.9 meters) detected southeast of Breezy Point, NY, and a 31-foot-high (9.4 m) wave recorded at a buoy located 30 nautical miles (55 km) south of Islip, Long Island.

'Black Marble'

Black Marble’Credit: NASA Earth Observatory.Bookending our look back at the year comes another entry from the Suomi NPP satellite. The satellite’s team recently released a set of images they are calling the “Black Marble,” because they are shots of the Earth taken at night. The images were taken in April and October and span the globe, showing city lights at night, the nocturnal glow produced by Earth’s atmosphere (called air glow), and even lights from ships at sea.

Phantom Eye Syndrome: When People Without Eyes Can Still See

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Phantom Eye Syndrome: When People Without Eyes Can Still See

 Phantom Eye Syndrome: When People Without Eyes Can Still See

We’ve all heard of phantom limb syndrome, but what if you lose something less mechanical? A much more complicated syndrome out there – one that produces a phantom eye.

With phantom limb syndrome, people with amputated limbs experience sensations in areas that no longer exist. These can be vague senses that their body part is still there even when they can see it’s gone, or they can be acute pain – like a clenched fist that can’t be unclenched even though all the muscles ache. All this sensation is illusory, but understandable. We all have a sense of where our body is and can feel how it moves, and we can conceptualize what such a phantom sensation would feel like. But there is a version of this syndrome that isn’t as well-known. About thirty to fifty percent of people who have had an eye removed experience phantom eye syndrome. This has some things in common with general phantom limb syndrome. People feel sensations in the missing eye. Sometimes it’s the sense that they need to blink, or that they’ve stayed up too long and their eye aches. For some it’s more serious, with people feeling real and immediate pain in the nonexistent eye.

A smaller percentage of people have phantom visions, as if they eye is still there. For the most part, they are basic geometrical shapes or colored lights. They’re described as tiles or fireworks, and occur thirty to forty percent of the time. One percent of cases have complex hallucinations. They can see objects or faces in the are of space usually seen by the eye. Occasionally these are vivid enough to be mistaken for the real world. And so they have a phantom eye that sees phantom objects.

Painful phantom limb syndrome has a strange, but simple, treatment. Set up a box with a diagonal mirror inside. Have the person put their existing limb in the box. The mirror will make it look, to the viewer, like the opposite limb. Have them relax or wriggle their limb. They’ll see that the missing limb appears undamaged, and often the pain goes away. Doctors think that the brain has a system meant to monitor where the body parts are, but which needs visual confirmation to check that its sense of them is correct. When doctors remove the limb, they don’t remove the part of the brain that both oversees the limb and checks up on it. If the limb disappears, the brain can conjure up sensations meant to get a person to pay more attention to their body – like pain. It’s a distress signal that the person can’t respond to. As soon as they “confirm” via the mirrored box, that the amputated limb is fine, the brain relaxes and gives up the idea of phantom sensations.

This isn’t possible with something like phantom eye syndrome. Some doctors prescribe tranquilizers or have the patient seek therapy. Strenuous exercise is also a very good way making the person forget to focus on the missing eye. Those who can’t be helped this way are given something much stranger. Some phantom eye syndrome sufferers receive a medical device that runs an electric current through the eye socket. This constant sensation either distracts from, or masks, the phantom sensations. They taser their phantom eye into submission. If only it were as simple as a mirrored box.

Image: Howie Le

Via NCBI three times.

Why do we blink more than we need to?

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Why do we blink more than we need to?

 Why do we blink more than we need to?

Human blinking is somewhat of a mystery to scientists. While it’s well known that eye-blinking is done to lubricate the cornea, these seemingly spontaneous flashes happen at a rate that’s greater than what’s needed. But now, as new research from Japan suggests, our blinking patterns may serve an unexpected purpose — one that works to release our attention and mentally prepare us for the next task.

Indeed, we blink a lot — about 15 to 20 times every minute. And it all adds up. Studies show that400 milliseconds of visual time is lost every time we blink, which amounts to a surprising 10 percent of our total viewing time. Given that these blinking rates happen at a level several times higher than what’s required for adequate ocular lubrication, scientists have had good reason to suspect that something else is going on — something that’s clearly important.


A recent study published in the Proceedings of the National Academy of Sciences offers some potential answers. Researchers Tamami Nakano, Shigeru Kitazawa, and colleagues now theorize that eye-blinks are actively involved in the resetting and delivery of attention. But to reach this conclusion, the researchers had to rely on two very important tools: An fMRI scanner and a Mr. Bean video. 

For the experiment, Nakano et al recruited several volunteers who were asked to watchMr. Bean episodes while hooked up to an fMRI scanner. Previous studies by the same researchers showed that human eye-blinks become synchronized while watching these videos (i.e. eyeblinks tend to occur at implicit breakpoints) — so they had good reason to continue their research; it was clear that something was happening from a neurological perspective.

During the Mr. Bean episodes, the scientists observed that the participants were spontaneously blinking an average of 17.4 times per minute. But while the blinking was happening, there was observable activity occurring in two competing anatomical brain networks responsible for attention.

Specifically, they noticed spikes of mental activity in areas related to the default network — an area of the brain that allows us to enter into a kind of ‘idling’ mode when we’re in a state of wakeful rest (as opposed to focused attention). And at the same time, they noticed decreased cortical activity in the dorsal attention network (a sensory orienting system that helps us know where we should focus our attention).

Consequently, the researchers hypothesize that eye-blinks — because they activate the default network — are a way for us to take a super-quick mental break before renewing our attention on a new task or activity — and they tend to occur at logical transition points (e.g. the end of a scene, or the end of a sentence…like right now).

They speculate that this serves an important cognitive function, what gives us an increasedcapacity for focused attention after the cognitive reset.

The entire study can be read at Proceedings of the National Academy of Sciences.

H/t: Smithsonian.

Image: Nejron Photo/Shutterstock.

Meet the cheap facial-recognition system that will identify you everywhere you go

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Meet the cheap facial-recognition system that will identify you everywhere you go

 Meet the cheap facial-recognition system that will identify you everywhere you go

You are being watched — especially when you go to the airport. But now you might be recognized, too. A company called Flight Display Systems has been demonstrating a $9,275 facial recognition device (pictured here) that can be installed inside airplane doors to check the identity of every person entering the plane, and alert staff if there is an “unauthorized” person. Basically it’s just a souped-up CCTV camera, and it can be installed anywhere fairly unobtrusively.

The biggest problem? It’s only 75-90% accurate. Get ready for some serious civil liberties violations.

According to Aviation International News:

See3 is based on Linus Fast Access facial-recognition software but adds Flight Display’s own proprietary and expanding set of algorithms. The hardware consists of two main components–the camera and computer–both of which already have FAA parts manufacturer approval.

Placed at the entrance to the aircraft, the system elevates aircraft security by comparing the faces of those entering the airplane with a known database and alerting the crew of the entry of any unauthorized person.

See3 uses nearly 100,000 values to code a face image. Among the less complex of these are the obvious inter-ocular distance, distance between nose tip and eyes and the ratio of dimensions of the bounding box of the face. At this point, accuracy is between 75 and 90 percent, but Flight Display continues to add algorithms to improve on this.

So after going through two ID check points in the airport, 10-25% of people who are mis-identified as “unauthorized” are going to have to suffer the indignities of being kicked off a flight or searched or worse? Sounds like a great system.

Read more at Aviation International News. (Spotted on Evgeny Morozov’s Twitter feed)

Why do mirrors reverse left and right, but not top and bottom?

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Why do mirrors reverse left and right, but not top and bottom?

Position yourself in front of a mirror and you’ll notice it immediately. The text on your sweatshirt is reversed. The part in your hair has switched to the other side of your reflection’s head. The mole on your left ear stares back at you from your mirror image’s right earlobe. Before you stands a bauplan reversed; what was once left is now right, and vice versa. And yet, up remains up and down is still down — as though the mirror knows to switch left and right, but not top and bottom.

This, of course, is not the case. The mirror doesn’t “know” anything about your position; it simply reflects the light that hits it, doing so as objectively as any inanimate object knows how. Why, then, when that reflected light reaches the photoreceptors in your eyes, has your mirror image been reversed from left-to-right?

The short answer is that it hasn’t. In fact, the question of what makes the horizontal axis so special in the context of mirrors is itself flawed. That’s because a mirror does not reverse images left-to-right or top-to-bottom, but from front-to-back. In other words, your mirror image hasn’t been swapped, but inverted along a third dimension, like a glove being turned inside out.

Here’s a thought experiment to help illustrate the concept of front-to-back reversal. Assume, for a second, that you are capable of squeezing your body perfectly flat. Imagine, also, that your body is able to pass through itself, without damaging any of its various tissues. When you stand with the tip of your nose pressed gently against a mirror, it’s easy to assume that the image you see looking back at you is the result of non-mirror you turning in-place 180 degrees and stepping backwards, through the mirror, into mirror-land. This is not the case.

In actuality, the back half of non-mirror you has been pressed flat in the direction of the mirror. As your form began to pancake, the front half of your body (that is, all parts of your body situated behind the tip of your nose, but still in front of the back half of your body), the back half of your body and the tip of your nose all came to reside within the same plane (i.e., the plane occupied by the mirror). But then your back half kept pushing, continuing on its journey through the plane of the mirror and passing right through your body’s front half before re-acquiring its “normal” shape on the other side of the mirror (probably with a satisfying *POP* sound). This new, inverted you is symmetrical to you, but your two bodies cannot be superimposed. In chemistry, such entities are said to be “chiral.”





Here’s another way to think of it, widely popularized by physicist Richard Feynman (see the interview response featured here). Stand in front of a mirror, and note which direction you’re facing. For the sake of this thought experiment, let’s assume you’re facing North. Point due East with your right hand, and your reflection points East as well. Point due west with your left hand, and your reflection gestures in the same direction. That’s because these directions both lie along a plane parallel with the mirror. Similarly, point up or down and your reflection will follow suit, motioning in the same direction. 

But deviate from that parallel plane even a little and thinks go wonky. Remember: your image has been reversed along the axis perpendicular to the mirror. Try pointing directly at the mirror, such that your fingertip is now directed due North. Your reflection is now pointing directly at you — not North, like your finger, but South.

For more on the mirror paradox, thinking in three-space, chirality and handedness, see this great explainer, presented in the form of a conversation, by UC Riverside’s Eric Schmidt. See also: The Left Hand of the Electron, by Isaac Asimov.

Top image via Shutterstock