Violet Skies Are for the Birds

I love the XKCD webcomic, especially when it features a science-related theme. Comic number 1145 poses the question “Why isn’t the sky violet?” which I will attempt to answer this week. But first we need to understand why the sky is blue: it’s because of Rayleigh scattering.
   Rayleigh scattering (named after the British scientist Lord Rayleigh) occurs when sunlight passes through the atmosphere and is scattered by air molecules. The light from the sun is a mixture of all the colors of the rainbow, each with its own characteristic wavelength. Sir Isaac Newton demonstrated this nearly 350 years ago by using a prism to separate white light into its different spectral colors.

Rayleigh scattering: blue light is scattered
more strongly than red light as it passes
through the atmosphere and is why the sky
is blue during the day.

   The amount of scattering is inversely proportionate to the fourth power of the light’s wavelength. That means that the shorter-wavelength components of sunlight (blue and violet) are about ten times more strongly scattered than the longer-wavelength components at the red end of the visible spectrum. Rayleigh scattering is responsible for the blue color of the sky during the day and the orange color during sunrise and sunset. It’s also the reason that the sun itself is yellow when overhead and red at sunrise and sunset.
   Now back to the original question: since violet light has an even shorter wavelength than blue light, why does the sky appear blue instead of violet? 
   First, the sun produces a lot more blue light than violet light. The Sun’s spectral peak is in the green range and as the wavelength decreases from blue to violet there is a steep drop-off in intensity.

Solar emission intensity compared to human cone cell responsivity. Both are shown
as a function of wavelength.

   Second and more importantly, even though blue and violet both have short wavelengths, our eyes don’t see violet as well as blue. We have three types of color receptors in our retina, called cone cells. There are short-, medium- and long-wavelength cone cells that respond most strongly to blue, green and red light, respectively. Cone cells are stimulated in different proportions and our brain uses this information to construct the colors we see. Across the visible spectrum, it turns out that blue provides the maximum responsivity.

This image demonstrates how
yellow light can be perceived
as a mixture of red and green
light. Take a few step back from
the monitor to see the effect.

   Since the cone cells are sensitive over broad, overlapping ranges of wavelengths, many colors can be seen by mixing other colors. Take yellow for example. There’s a good reason why caution signs are yellow—yellow light lies right between green and red on the spectrum and causes a large response in both the medium- and long-wavelength cone cells. Regardless of whether you see pure yellow or a mixture of red and green, your eyes can’t tell the difference. When two colors can be created with different spectral distributions they are called metamers. In this same way, the sky’s combination of violet and blue triggers the same cone response as pure blue plus white light, which yields the pale blue color that we see.
   It’s no coincidence that we see things the way we do. Human evolution is shaped by our environment—the ability to separate the colors around us provides an evolutionary advantage. 
   Even though humans don’t see violet in the sky, some birds might because they have an extra type of cone cell that extends their color vision into the ultraviolet range. The male Blue Grosbeak appears mostly blue to humans but has plumage shifted to the UV range that he uses to his advantage during courtship. The Common Kestrel uses its UV-enhanced vision to find voles by following their scent trail which reflects UV light, making it visible to this clever hunter. So maybe violet skies are for the birds.

Big Brains

There’s probably no single event more significant in the history of human evolution than the harnessing of fire. Many species make and use tools, but only humans control fire. Fire provided early humans a means to protect themselves from predators. Fire provided humans with warmth and light, and expanded productivity into the night. Socializing around a campfire may have been an essential part of human development.
   Perhaps the biggest benefit of fire was for cooking. Cooking food provided better nutrition and made food safer to eat. Cooked meat was easier to digest because less energy was spent digesting the tougher proteins and connective tissues.
   Cooking plants that contained starches made the complex carbohydrates they contained more digestible so that more energy could be absorbed from them. The human digestive system has evolved after we started eating cooked foods: our teeth, jaws and digestive tract have all gotten smaller, allowing our developing brain to have a greater share of the food energy taken in. Eating cooked food helped provide the extra energy required to support a hunter-gatherer lifestyle.
   The earliest known evidence for the controlled use of fire comes in the form of ash and charred bone excavated from a South African cave that is known from previous digs to have been occupied by early man. These materials were found alongside stone tools in a layer dating back about a million years.
   Although modern humans are the only human species alive today, originating about 200,000 years ago, other human species once roamed the Earth, such as Homo erectus, which arose about 1.9 million years ago.
   Some anthropologists think that Homo erectus was cooking as far back as 1.9 million years ago and was the reason that they experienced major brain expansion at that time. Others think that brains got bigger just by the introduction of meat into their diets and that while there was the opportunistic use of natural fire, it was not until about 300,000 to 400,000 years ago that early humans fully mastered the use of fire.
   One thing is certain—our brains have tripled in size over that last two million years. But evolution doesn’t say anything about whether larger brains are good or bad, just that it happened. Author Kurt Vonnegut believes that our brains have over-evolved: “Our brains are much too large. We are much too busy. Our brains have proved to be terribly destructive.”
   Vonnegut explored this theory in his 1985 book Galapagos where our big brains have brought civilization to the brink of destruction. The last humans ironically survive because they get stranded and isolated on the Galapagos Islands made famous by Charles Darwin. They spend the next million years de-evolving.
   As evidence for his theory, Vonnegut says that big brains invented nuclear weapons and the Third Reich. Even Einstein noted that “He who joyfully marches to music rank and file, has already earned my contempt. He has been given a large brain by mistake, since for him the spinal cord would surely suffice,” indicating his belief that war is a huge step backwards in human evolution. And while I’m more optimistic, I didn’t witness the bombing of Dresden firsthand as did Vonnegut, nor did I have to flee my home and country out of fear for my life as did Einstein. Food for thought…

Dark Lightning

Artist’s conception of the Earth’s magnetic field (in pink)
funneling positrons (in yellow) and sending them to the Fermi
Gamma-ray Space Telescope where they were observed.

The Fermi Gamma-ray Space Telescope is an Earth-orbiting space observatory that is being used to perform high-energy gamma-ray astronomy. Launched in June of 2008, this telescope is probing the cosmos for gamma rays and high-energy events. And while it is finding many sources for these events, such as supernova explosions and distant, supermassive black holes from other galaxies, it has also found an unlikely source closer to home.
   In 2009, the telescope was hit by a stream of high-energy positrons—the antimatter version of electrons—coming from a thunderstorm on Earth. 100 trillion positrons had been funneled into a tight pulse by the Earth’s magnetic field and hurled straight to the observatory at nearly the speed of light. To put that number into perspective, it’s more than what hits the Earth’s atmosphere from all other cosmic sources combined. Somehow, antimatter had been produced in the clouds above Earth and the best theory we have to explain it is dark lightning.
   Earth-orbiting satellites have been observing terrestrial gamma ray flashes (TGFs) from thunderstorms as far back as 1994. And it is also known that gamma-rays at the right energy can produce electron-positron pairs.
   Normal lightning occurs when unbalanced electrostatic charges in the atmosphere trigger a massive discharge between a cloud and the ground or between two clouds. A light flash traces the path of the charged particles which heat the air to 30,000°C, nearly six times hotter than the surface of the Sun. 

Feynman diagram for a gamma ray photon
decaying into an electron-positron pair.

   Dark lighting may seem crazy, but there is mounting evidence that it’s real. Like ordinary lightning, dark lightning also tries to neutralize the unbalanced electric fields in a thunderstorm. Under the right conditions, the thunderstorm creates a powerful avalanche of electrons shooting away from Earth at nearly the speed of light. The electrons collide with air molecules in the atmosphere to produce gamma rays. Next, the gamma ray energy transforms into electron-positron particle pairs. Further collisions between these particles and other air molecules creates a repeating cycle—a self-generating, self-sustaining particle accelerator. Once the loop gets started, it can discharge the thundercloud as fast as lightning. And because the cascading electrons and positrons generate more gamma rays than visible light, the process is practically invisible to the human eye. 
   Researchers once thought the gamma ray flashes from thunderstorms were a weird by-product of ordinary lightning. Now many think it is dark lightning instead. The gamma ray burst monitor onboard the Fermi Gamma-ray Space Telescope is perfectly suited to record these flashes and new data processing techniques have improved the burst monitor’s performance. In mid-2010, a testing a mode was initiated which allows for the detection of faint gamma ray flashes that had previously gone undetected. Now Fermi should be able to catch nearly 1,000 flashes each year. With an abundance of new data, researches hope to gain new insights on the mysteries of dark lightning.

A Hundred Authors Against Einstein

The book “Hundert Autoren Gegen Einstein”
(A Hundred Authors Against Einstein) was
published in 1931.

The old adage “there is strength in numbers” is not always true, especially when it comes to science. Science is not advanced through polls or consensus. Observation and experimental evidence is what matters. Thankfully, being in the minority does not necessarily mean one is wrong.
   Case in point: The book Hundert Autoren Gegen Einstein (A Hundred Authors Against Einstein), a collection of various criticisms of Einstein’s theory of relativity. Published in 1931, it contains short essays from 28 authors, and published excerpts from 19 more. The balance was a list of 53 people who were also opposed to relativity for various reasons.
   The book was not a reaction against Einstein from the physics community—only one physicist had contributed. Nor was it supported by the younger generation—only two of the contributors were much younger than Einstein. It was a dying cry from the old guard of science that felt left behind by the new physics and incompetent because they didn’t know what to do with it. Before Einstein published his work, Newton’s theories were gospel among the scientific community. Einstein had the temerity to use space and time as a way to think of our Universe, not just an a priori condition in which we lived.

Before relativity, space was thought
to be best represented by Euclidean
geometry (above). Relativity requires
the extra dimension of time be
considered when representing
space (below).

   Many had a philosophic objection to relativity, based on Immanuel Kant’s assertion that space was intuitive and could not be perceived by observation or experience. Newton’s view that space was absolute and existed independently of what it contained, as defined by Euclidean geometry, had ruled for over two centuries unchallenged. 
   When asked about the book, Einstein retorted by saying “Why 100 authors? If I were wrong, then one would have been enough!”
   Einstein’s fame from the success of his ground-breaking theories had created a backlash. Even thought the book contains no outright anti-Semitism, six of the authors were either anti-Semitic and/or Nazi sympathizers. The rising Nazi movement denounced Einstein, calling relativity “Jewish physics”. Einstein left Germany in 1932 out of fear for his safety and never returned. The Nazis had put a price on his head, publishing his photo on the cover of one of their magazines with the caption “Not Yet Hanged”. Einstein moved to the United States, settling at the Institute for Advanced Study at Princeton in New Jersey.
   Of all the contributors to the book, the one that I found the most distressing was Emanuel Lasker. Lasker was a German mathematician, philosopher and the World Chess Champion for an incredible 27 years. Einstein and Lasker had met through a mutual friend in Berlin in 1927, and over the course of many walks together they exchanged opinions about a variety of topics. According to Einstein it was a somewhat lopsided exchange, in that Einstein received more than he gave. Nonetheless, they developed a close friendship. Given that Lasker was also Jewish and had been forced to leave Germany after the Nazis took power, it’s disheartening that he had gone against his friend, but apparently it didn’t bother Einstein.
   To Lasker, the notion that no matter how fast you travelled the observed speed of that light was constant was ridiculous. Einstein claimed to have “never considered in detail, either in writing or in our conversations, Emanuel Lasker’s critical essay on the theory of relativity” and thought of Lasker as a Renaissance man and uniquely independent. Many years later, when asked to write the forward to a posthumous biography on Lasker, Einstein was forced to address this reproach to relativity, saying that “…chess playing of a master ties him to the game, fetters his mind and shapes it to a certain extent so that his internal freedom and ease, no matter how strong he is, must inevitably be affected”. In other words, Lasker—while brilliant—lacked the capacity to think outside the box.