Carl Sagan (1934 – 1996) was an American astronomer, influential popularize of science, and author. His scientific achievements and contributions are vast. He published over 600 scientific papers, authored over 20 books, and co-wrote the popular 1980 television series Cosmos.
Early Life
Carl Sagan (Credit: NASA)
Carl Sagan was born in 1934 in Brooklyn, New York. As he recalls in his books, he acquired an interest in science at a young age. One influential experience was a visit to the 1939 New York World Fair at the age of four. After that experience he frequently read books on astronomy from his local library. Sagan attended the University of Chicago where he was associated with or taught by many notable scientists and Nobel Laureates. He completed his B.S. in 1955, M.S. in physics in 1955, and in 1960 was awarded his Doctorate in astronomy and astrophysics.
Scientific Career
Carl Sagan had a distinguished career. He first worked as a fellow at the University of Berkley, California for two years before being offered a lecture position at Harvard University. After working at Harvard for a few years he was surprisingly denied tenure. He quickly moved on to Cornell University where he became their director of Laboratory for Planetary Studies. He later was promoted to associate director of the Center for Radiophysics and Space Research. In 1977, he was named the David Duncan Professor of Astronomy and Space Science, a position he held for the rest of his life.
He worked with NASA several of their space missions. He helped determine possible landing sites on Mars for the Viking Program. He was instrumental in sending out the first physical messages sent into space in the form of the Pioneer Plaque and the more famous Voyager Golden Record.
Carl Sagan Standing Next to a Model of a Viking Lander (Credit: Wikimedia Commons)
While he was always a respected scientists among his colleagues, but it was the highly successful Mariner missions to Mars launched Sagan’s public career. People wanted to know more about Mars and Sagan became a sort of spokesperson about the topic. He was selected to appear in several educational films and his exceptional style and clear delivery quickly earned him notoriety. His expertise was in high demand and he soon began appearing on television talk shows. Carl Sagan had become a celebrity scientist.
Bringing Science to the People
Carl Sagan had a burning passion to share his love of science with everyone. He was a prolific author and who made reading about science enjoyable. In 1978 he won a Pulitzer Prize for The Dragons of Eden. The following year he co-wrote the popular PBS televisions series Cosmos. The series was a smashing success and more than 600 million people have viewed the series before the century was over. Sagan died of pneumonia in 1996 at the age of 62.
The atomic nucleus is the tiny, dense, center of the atom. It’s surprising discovery was announced to the world by the physicist Ernest Rutherford at a meeting of the Manchester Literary and Philosophical Society in March 1911. Two months later he published a scientific paper reporting his findings.
The Rutherford Experiment
Nuclear Structure of the Atom
Prior to Rutherford’s discovery of the atomic nucleus, the prevailing atomic model was the “plum pudding” model devised by J. J. Thomson, who discovered the electron in 1897. Thomson proposed a model of the atom that consisted of a diffuse cloud of positive charge with the negatively charged electrons scattered within it. However, there was a lack of experimental evidence to support this model, along with other issues such as the problem of atomic stability, and conflictions with observations of atomic behavior.
Rutherford, along with his colleagues at the University of Manchester, set out to further investigate the structure of the atom using alpha particles. Alpha particles are a form of radioactive decay that have a positive charge. Rutherford’s team fired a beam of alpha particles at a thin sheet of gold foil. To create his beam Rutherford placed radium inside a lead box with a small pinhole directed at the sheet of gold foil. The lead box absorbed most of the alpha decay from the radium except for the small beam that escaped through the pinhole. The foil was completely surrounded with a detector that could locate where they alpha particles ended up after they passed through the foil. Rutherford specifically used gold for its malleable properties. Gold can be hammered into incredibly thin layers which was needed so that the alpha beam could pass through it.
Most of the alpha particles passed through the foil as if it was going through empty space. Occasionally a few alpha particles – about 1 in 20,000 – were reflected straight back towards the source. This was a highly unexpected result that could not be explained by the plum pudding model. Rutherford concluded that most of the mass of an atom must be concentrated in a tiny, dense region of the atom which he called the nucleus. He proposed that the atom had a central nucleus where all the positive charge and most of the mass was concentrated, and the negatively charged electrons orbit this nucleus. His model of the atom resembled the planets in the way that they orbit the Sun in the Solar System.
Rutherford’s model of the atom contained the concept of the nucleus, a significant departure from the plum pudding model. It consists of positive electrically charged protons and the slightly heavier electrically neutral neutrons. Although Rutherford failed to discover the neutrons himself he predicted their existence and his student James Chadwick discovered them in 1932. Orbiting the nucleus are the negatively charged electrons.
One of the most striking implications of this model is that atoms are mostly empty space. The atomic nucleus occupies a space of 1/100,000 of the atom, with electrons occupying the vast region around the nucleus. This poses the question: what fills the region between the nucleus and the orbiting electrons? The answer is empty space. This representation challenges our perception of solid matter, and underscores the weird, mysterious, and counter-intuitive nature of the atomic realm.
Although Rutherford’s proposed a model of that atom was a significant improvement over the plum pudding model it still had limitations of its own. Critically, it still could not explain the stability of the electrons’ orbits around the nucleus. According to classical physics, electrons moving in an orbit would radiate energy and soon – within one second – spiral into the nucleus. Obviously, atoms are stable and do not collapse in this way.
The solution to this problem came from Niels Bohr and the quantum mechanical model of the atom. Bohr introduced the concept of quantized orbits for electrons, where electrons could only exist at certain discrete energy levels without radiating energy. According to Bohr’s theory, electrons moving between orbits actually disappear from one orbit and reappear in the new orbit, without traveling in the space between. This resolved much of the stability issues despite reinforcing the confusing and unusual ways that atoms behave at the subatomic scale.
The discovery of the atomic nucleus had profound implications on atomic physics and led to the development of an entirely new field of research, nuclear physics. It paved the way for the harnessing of nuclear energy, a technology that has the potential to alter the course of human civilization.
Language is one of the special characteristics that distinguishes humans from other animals. It allows us to communicate complex concepts and ideas to other people, undoubtedly providing us with a remarkable evolutionary advantage over other species. Writing is a set of markings used to represent a language. It augments the benefits of language by making it permanent, allowing the message to travel further and persist through time. It is why the invention of writing systems often distinguish history from prehistory.
The History of Writing
Cuneiform Tablet
The history of writing systems traces a complicated journey and much of its detail is lost to time. Written language emerged around 3400 BCE in Sumer, southern Mesopotamia. These same industrious people also invented number systems and the wheel. Their writing form is known as cuneiform (cunea, Latin for “wedge”) and consisted of making wedges on clay tablets. It is derived from their proto-writing system of using clay tokens of various shapes as counters for various goods that were produced and exchanged, essentially an accounting system.
The original tokens date back to around 8500 BCE and the system evolved over the millennia in several stages of abstract symbolism. The earliest tokens were of the most basic geometric shapes such as a cone or a square, and later tokens took the shape of more abstract shapes such as miniature tools and fruit. Despite its complexity, each token was a unique geometric shape, such as a cone, and each one representing, with a one-to-one correspondence, a certain type of good. Two cones mean two baskets of grain. No matter your language, if you understood that a cone token meant a basket of grain you could account for the transaction. These tokens were most likely accounts of debt and were stored inside clay envelopes.
This brings us to the original purpose of the Sumerian writing system: accounting – in the recording amounts of grains, numbers of livestock, and various other goods. As the civilization grew in population size the number of debts increased. Since the tokens were stored inside envelopes their contents could not quickly be known until you opened the envelopes and counted the tokens. Some accountants solved this problem by making wedges on top of the envelopes representing the contents of the envelope. The transition from token to script begun and the worlds first writing system emerged. Eventually clay tablets with markings representing the tokens completely replaced the token system since the impression of the cone on the tablet was identical to the cone token itself.
Egyptian Heiroglyphs, Temple of Kom Ombo (Credit: Wikimedia Commons)
It took around another 400 years until the Sumerian writing system made the shift to create the phonetic signs of speech. This was moving from a clear one to one representation to a more abstract for of representing sounds. This created a big problem for a society inventing a writing system. It has to agree upon a system of symbols or makings to represent spoken sounds. This agreement would take some time. Pictorial notations such as a picture of a bird or a tree were easiest to agree upon. Eventually consensuses were built and writing formats gradually became more formalized, arranging itself it to standardized rows and columns. The full development of the Sumerian writing system took at least 1,000 years.
The Chronological Development of Writing (Credit: Wikimedia Commons)
It is not certain whether writing originated in a single geographic area (Sumeria) and spread throughout the world by cultural diffusion or if it was invented in a few areas independently. The discovery of scripts in ancient Mesoamerica certainly seems to indicate that it was invented at least more than once. In the Old World it is very likely that only the Sumerians and a few centuries later the Egyptians independently invented their own writing system. It is also possible that the Egyptians borrowed the idea from the Sumerians, nobody knows for sure. The Egyptian writing system is called hieroglyphics (meaning “sacred engravings” in Greek) and are pictorial in form. There are about 1000 distinct characters. It is the most famous and well known ancient form of writing.
Good Ideas Like Writing Spread, Now This Good Idea Spreads Other Good Ideas
Due to the difficulty in inventing writing systems, it is likely that all writing systems have been borrowed and altered from early Mesopotamian writing systems with the exception of the Egyptian, Chinese and Mesoamerican writing systems. Writing systems also require a long time to fully develop, probably at least a thousand years. Other rudimentary writing systems may have been invented but they were either absorbed, aborted, or replaced due more the established writing systems rapid diffusion.
A Brief Video on the Spread of Writing Systems Across the Globe
In the 16th century BCE the Canaanites simplified the Sumerian and Egyptian pictographic scripts by creating an alphabet of 22 consonants. All of our modern alphabets are derived from this script. Eventually the Greeks introduced characters for vowels, establishing the alphabet to be used for Western Civilization. Once writing spread across the globe itself became the means for spreading other good ideas. A fitting destiny for one of humanity’s most impactful ideas.
The human domestication of plants was the single most influential event in modern human history. It is the demarcation of the nomadic lifestyle to the settled, urban lifestyle. Its impacts can be summarized by the agricultural revolution, resulting in a tremendous spike in food production. The spike in food production led to larger and larger populations, the birth of city states, which marked the dawn of civilization. The domestication of plants, coinciding with the domestication of animals, has profoundly changed the course of humanity.
When and Where did the Domestication of Plants Happen?
Cereal Crops
Plant domestication is the alteration of wild plant species into crop plants by human, what can be called artificial selection. The original techniques were likely stumbled upon by accident and the process leading to agriculture was certainly a very slow and gradual one. The earliest domestication of plants followed by the transition to agriculture can be thought of as an evolutionary process rather than an intentional discovery.
We have only fragmentary evidence of its beginnings since it began thousands of years before writing was invented. The earliest evidence suggests that plant domestication began around 12000 BCE with cereal crops. The location was in the area between the Tigris and Euphrates rivers in the Middle East. Other area’s of the globe soon independently domesticated other crops. Rice was domesticated in China and maize (corn) was domesticated in America’s all by around 10000 BCE. Herbs such as coriander, vegetables such as sweet potatoes and lentils, and fruits such as figs and plums were also being domesticated by around the same time.
How Did it all Happen?
The process of plant domestication was a complex, slow, and gradual process. In a few places it happened independently but this was a fairly rare occurrence. The most recent evidence suggests agriculture began in no more than ten places independently. The exact number is still debated due to incomplete and inconclusive evidence. Mostly it spread to other areas of the globe through cultural diffusion.
The road to the domestication of plants was long and curvy, full of cliffs and dead ends. It involved centuries of trial and error and was subject to local climate, geography, and available plant species. However a few notable factors seem to have been important in its evolutionary process.
The decline of wild animal populations – By around 13000 BCE humans were becoming extremely proficient hunters and large game was beginning to thin out. This made hunting increasingly less rewarding and alternative food strategies increasingly more rewarding.
An increasing abundance of wild edible plants due to a change in climate – Around 13000 BCE the Earth began warming resulting in increased plant life. This made eating plants increasingly more rewarding and provided more opportunities for learning by trial and error by peoples in locations with the highest proportions of these plants.
The cumulative development of food production technologies – In some area’s edible plants were so abundant that people could abandon a nomadic was of life and establish permanent settlements. This provided the opportunity to develop food storage, tools, and production methods.
Population growth led to new food production strategies – The abundance of wild plants led to a surge in populations. This demanded new ways to feed the population. This creates what is none as an auto-catalytic process.
Completing the Process and Establishing a New Way of Life
Egyptian Agricultural Calendar
When humans began moving around less they noticed changes in plant life much better. Some plants were evidently dropped on the trip back to camp. It wasn’t long before people noticed that new plants soon began growing along these well-worn trails. Also, the garbage dumps of food became breeding grounds for plant growth in the following seasons. Some plants require seeds to be spat out and plants began growing in these spots also.
Soon the connection was made between planting these seeds and the growth of crops. Over time this process was refined and improved, and new species of plants were tested. Some species of plants were more easily domesticated than other species but these weren’t distributed across the world evenly, which is why some societies invented agriculture and others didn’t. This new process of controlling nature to grown your own food soon allowed societies to grown a population so large that only agriculture could support them. Life started revolving around agriculture. These larger society with a greater population were able to conquer or assimilate their neighbors spreading the domestication of plants through cultural diffusion. The age of the hunter-gather was ending and the rise of civilization had begun!
Domestication is the process of selective breeding for human use. The domestication of animals began with the now-lovable dog by at least around 14000 BCE and possibly thousands of years earlier. As it so often happens with much of prehistory, the archeological record is simply unclear as to the exact time and location of the dogs domestication. It may have happened as early as 40000 BCE and it also may have happened several times independently. What is clear is that the domestication of the dog did happen, followed by the goat, pig, sheep, cattle, cat, chicken, horse, and a few other important and well known animals.
The domestication of animals, along with the domestication of plants, played a key role in the agricultural revolution and in the beginnings of civilization. Aside from the dog, animal domestication happened slightly later than plant domestication since enormous quantities of plant food was needed to feed the animals. Domesticated animals provided humans with several benefits of enormous value and was essential on humanity’s path towards urban civilization.
Painting Depicting Beasts of Burden (Credit: Winnifred Neeler, Royal Ontario Museum)
An Increase in Food Production
Grazing Sheep and Cattle (Credit: www.agupdate.com)
Prior to the domestication of animals all food provided by animals had to be obtained from hunting. This changed after domestication. Each of the domesticated animals could be used for their meat in times of food scarcity or after an unsuccessful hunt. However providing a source of meat was not their only addition to food production. In addition to meat, cattle, sheep, and goats provided a steady supply of milk and other dairy products. Once farming became widespread draft animals such as cattle, ox, and water buffalo provided an unprecedented addition of muscle power.
The increase in food production from first domesticated plants and then animals resulted in radical changes to the human condition. A sustainable and predictable source of food lead to a rapid increase in population density. People were able to disband their nomadic hunter and gather lifestyle and establish permanent settlements. The dawn of civilization was underway.
Additional Uses of Domesticated Animals
Egyptian Bronze Statue of a Cat, University of Pennsylvania Museum of Archaeology (Credit: Mary Harrsch, Wikimedia Commons)
In addition to the increase in food production, domesticated animals provided a variety of additional benefits to humans. Around 4000 BCE horses became domesticated allowing for significant improvements in transportation. A person riding a horse could travel double or more the distance and speed of what a person walking or running could travel in a day. The use of the horse was eventually applied to combat leading to superiority in warfare for those civilizations who were able to successfully utilize them.
There were also many other animals that provided significant benefits to human civilizations. Today most people revere cats for being cute and cuddly house companions. But in ancient Egypt cats were revered for their pest control qualities and for their ability to hunt venomous snakes, scorpions, and rodents. An unusually high volume of statues and paintings were dedicated to cats in this culture. Hides of a variety of animals were used for clothing, storage, or shelter. Sheep were prized for their wool that could be spun into clothing, rugs, and a variety of luxury goods.
A Rare Combination of Traits
Not all animals can be domesticated. Of the world’s roughly 150 large, wild, terrestrial, herbivorous mammals – the ideal candidates for animal domestication – only 14 have been domesticated. This indicates that there is a specific mix of traits an animal must possess in order to be successfully domesticated. These traits are:
An efficient diet – Herbivores are much more efficient than carnivores. The conversion of food biomass into the consumer’s biomass is typically around 10 percent. This means if you want to raise 1,000 lb cow you have to grow 10,000 lbs of corn. Large carnivores would be extremely difficult and costly to domesticate because it would take 100,000 lbs of corn to make the 10,000 lbs of herbivore needed to feed the 1,000 lb carnivore. The food preference of the herbivores must also not be finicky.
A quick growth rate – Some herbivore animals such as elephants take decades to reach their full adult size. Cattle on the other hand can reach 1,000 to 2,000 lbs by age three.
A willingness to breed in captivity – Animals such as the cheetah refuse to breed in captivity. In the case of the cheetah it is due to a lengthy and elaborate courtship ritual that cannot take place in a cage.
A friendly disposition towards humans – Large, vicious animals like the grizzly bear will instinctively maul humans making it suicidal to try to domesticate them
A tendency to stay calm or not panic – Nervous species that have a tendency to fight or flight when they precede danger are difficult to domesticate.
A manageable social and herding structure – Living in herds, having a well developed dominance hierarchy that have an overlapping home range is the ideal structure. This rules out solitary animals who are not instinctively submissive.
Bayes’ theorem is a fundamental concept in probability theory. It was formulated in 1763 by the English statistician and Presbyterian minister, Reverend Thomas Bayes.
History of Bayes Theorem
Thomas Bayes was born in London in 1702 and studied at the University of Edinburgh. During his time at Edinburgh he was exposed to some of the leading mathematicians and philosophers of his time. He was elected as a Fellow of the Royal Society where he may have served as a mediator of intellectual debates. He later returned to London to become a minister, but he continued to pursue an interest in mathematics, specifically in probability theory.
Heading for Bayes Doctrine of Chances (1764)
Bayes wrote his theorem in order to address the question of how to revise beliefs in the light of new evidence. However, more interestingly it appears that he likely wrote it as a mathematical means to defend Christianity and to combat an argument by David Hume in his 1748 essay Of Miracles, from his book An Enquiry Concerning Human Understanding. In this essay, Hume made the case for dismissing miracles, such as the resurrection of Christ, on the grounds of probability. In effect he argued that the probability for miracles (a violation of the laws of nature) was much more improbable than the probability that miracle was accurately reported. While there is no absolute or direct evidence that Bayes sole motivation to compose his work was to refute Hume’s essay, there is extremely good circumstantial evidence he did at least in part, given the details surrounding the events of his later life and the eventual publication of his work.
Whatever the real motivation for his work may have been, Bayes’ work was published two years after his death, when his friend Richard Price brought it to the attention of the Royal Society and read on December 23, 1763. It was published to following year both in the Philosophical Transactions, the journal of the Royal Society, and as an offshoot. The now famous essay was titled An Essay towards solving a problem in the Doctrine of Chances. It should be noted that in 1767, Prince published a book titled Four Dissertations, where he explicitly took on the work of Hume and challenged his probabilistic arguments in Of Miracles. He used Bayes results in an attempt to show that Hume failed to recognize the significance of multiple independent witnesses to a miracle, and that the accumulation of even imperfect evidence could overcome the statistical improbability of an event.
As things sometimes happen in the history of science, the theorem initially was largely forgotten, until it was independently rediscovered by the brilliant French mathematician Pierre-Simon LaPlace in 1774. The theorem is used to describe the conditional probability of an event. Conditional probability tells us the probability ofa hypothesis if some event has happened.
The Goal of Getting Closer and Closer to the Truth
Bayes’ Theorem involves beginning with an educated guess, called a prior probability, and then revising your prediction when new evidence comes in. As the new evidence is considered the probability of the event is updated give you the posterior probability. Bayes’ Theorem provides a useful way of thinking by approximation, getting closer and closer to the truth as we accumulate new and relevant evidence. This is an important point to consider because we are always working with incomplete information in nearly all situations.
Formula of Bayes’ Theorem
A Bayesian way of thinking requires us to constantly update our probabilities as new evidence becomes available to us. This revision does not happen just once but can continually happen. We may never know the truth with 100% certainty, for example we can never be 100% certain the sun will rise tomorrow. But with Bayesian thinking we can be 99.999999% sure which tells us we’re getting very close to the truth and gives us a high degree of confidence in the proposition. Bayes theorem helped to revolutionize probability theory by introducing the idea of conditional probability – probability conditioned by evidence. If you have an extraordinary hypothesis, it should require extraordinary evidence to convince you that it’s true.
Practical Uses of Bayes Theorem
Bayes Theorem has relevance in any avenue of life because it is a form of probabilistic thinking. If you think about it, everything you and happens to you in life is probabilistic in nature. The theory’s flexibility and versatility provide the ability to make both life and business decisions under conditions of uncertainty. Here are a few examples of Bayesian theory used in the real word. In biology it is used for medical diagnosis, genetics, and the spread of infectious diseases. In computer science it is used in speech recognition, search engine algorithms, spam filtering, and weather forecasting. Its practical examples are almost limitless. Ultimately, it is a learning process, with more observations and evidence leading to better certainty. Lets take a look at one interesting application of Bayes theorem in a real word setting.
The theorem was used to crack the Nazi Enigma code during WWII. The Enigma code was an encryption machine that the Germans used to send secure messages. Its effectiveness was that its cipher system was changed daily. Alan Turing, the brilliant British mathematician, used Bayes Theorem to break down an almost infinite number of translations based on messages that were most likely to be transmitted. For example, messages from German U-boats were most likely to transmit messages containing information about the weather or allied shipping. The strong priors thus greatly reduced the possible translations to be deciphered and sped up to time to crack the code. Eventually he and his staff invented a machine known as The Bombe, which ultimately cracked the German Enigma Code. The use of Bayes’ theorem in cracking the Enigma code was a monumental breakthrough for the Allies, as it provided them access to critical information about German military operations. It provided a significant strategic advantage in the war effort and played a key role in their eventual victory.
Bayes’ theorem continues to impact statistics and society to this day. In recognition of Bayes’ contribution to the development of probability theory, the Bayesian Analysis journal was established in 2006 as a peer-reviewed academic journal dedicated to Bayesian statistics. Additionally, they Thomas Bayes Award is awarded every two years by the Royal Statistical Society to recognize outstanding contributions to the field of Bayesian statistics. The continuing relevance of Bayes’ theorem is a testament to the enduring legacy of Thomas Bayes and his contribution to the field of probability theory.
