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🔗 Mathematics 🔗 Statistics

In probability theory, the birthday problem or birthday paradox concerns the probability that, in a set of n randomly chosen people, some pair of them will have the same birthday. By the pigeonhole principle, the probability reaches 100% when the number of people reaches 367 (since there are only 366 possible birthdays, including February 29). However, 99.9% probability is reached with just 70 people, and 50% probability with 23 people. These conclusions are based on the assumption that each day of the year (excluding February 29) is equally probable for a birthday.

Actual birth records show that different numbers of people are born on different days. In this case, it can be shown that the number of people required to reach the 50% threshold is 23 or fewer. For example, if half the people were born on one day and the other half on another day, then any two people would have a 50% chance of sharing a birthday.

It may well seem surprising that a group of just 23 individuals is required to reach a probability of 50% that at least two individuals in the group have the same birthday: this result is perhaps made more plausible by considering that the comparisons of birthday will actually be made between every possible pair of individuals = 23 × 22/2 = 253 comparisons, which is well over half the number of days in a year (183 at most), as opposed to fixing on one individual and comparing his or her birthday to everyone else's. The birthday problem is not a "paradox" in the literal logical sense of being self-contradictory, but is merely unintuitive at first glance.

Real-world applications for the birthday problem include a cryptographic attack called the birthday attack, which uses this probabilistic model to reduce the complexity of finding a collision for a hash function, as well as calculating the approximate risk of a hash collision existing within the hashes of a given size of population.

The history of the problem is obscure. W. W. Rouse Ball indicated (without citation) that it was first discussed by Harold Davenport. However, Richard von Mises proposed an earlier version of what is considered today to be the birthday problem.

🔗 Amateur radio

Olivia MFSK is an amateur radioteletype protocol, using multiple frequency-shift keying (MFSK) and designed to work in difficult (low signal-to-noise ratio plus multipath propagation) conditions on shortwave bands. The signal can be accurately received even if the surrounding noise is 10 dB stronger. It is commonly used by amateur radio operators to reliably transmit ASCII characters over noisy channels using the high frequency (3–30 MHz) spectrum. The effective data rate of the Olivia MFSK protocol is 150 characters/minute.

Olivia modes are commonly referred to as Olivia X / Y (or, alternatively, Olivia Y / X ), where X refers to the number of different audio tones transmitted and Y refers to the bandwidth in hertz over which these signals are spread. Examples of common Olivia modes are 16/500, 32/1000 and 8/250.

🔗 Systems 🔗 Politics 🔗 Socialism 🔗 Sociology 🔗 Globalization

World-systems theory (also known as world-systems analysis or the world-systems perspective) is a multidisciplinary, macro-scale approach to world history and social change which emphasizes the world-system (and not nation states) as the primary (but not exclusive) unit of social analysis.

"World-system" refers to the inter-regional and transnational division of labor, which divides the world into core countries, semi-periphery countries, and the periphery countries. Core countries focus on higher skill, capital-intensive production, and the rest of the world focuses on low-skill, labor-intensive production and extraction of raw materials. This constantly reinforces the dominance of the core countries. Nonetheless, the system has dynamic characteristics, in part as a result of revolutions in transport technology, and individual states can gain or lose their core (semi-periphery, periphery) status over time. This structure is unified by the division of labour. It is a world-economy rooted in a capitalist economy. For a time, certain countries become the world hegemon; during the last few centuries, as the world-system has extended geographically and intensified economically, this status has passed from the Netherlands, to the United Kingdom and (most recently) to the United States.

World-systems theory has been examined by many political theorists and sociologists to explain the reasons for the rise and fall of nations, income inequality, social unrest, and imperialism.

🔗 Telecommunications

The Wheatstone system was an automated telegraph system that replaced a human operator with machines capable of sending and recording Morse code at a consistent fast rate. The system included a perforator, which prepared punched paper tape called a Wheatstone slip, a transmitter that read the tape and converted the symbols into dots and dashes encoded as mark and space electric currents on the telegraph line, and a receiver at the other end of the telegraph line that printed the Morse symbols. The system was invented by Charles Wheatstone. Enhancements could be made so that it was a duplex system, able to send and receive on the same line simultaneously.

The Wheatstone slip was a paper tape that contained holes in a pattern to control the mark and space signals on the telegraph line. The paper tape was from 0.46 to 0.48 inches in width, (but the standard width is from 0.472 to 0.475 inches) and a standard thickness of 0.004 to 0.0045 inches. Olive oil coating lubricated the punch process. There were three rows of holes. The middle row forms a rack so that a star wheel can move the paper forward. Every used position on the tape has a middle hole punched. The top hole indicates when to turn on the mark signal on the line, and the bottom hole says to turn off the mark signal. Each vertical column represents a time interval in the Morse code, including the spacing between the holes. The holes are spaced 0.1 inches apart. A column of three holes turns on the mark at the beginning of the interval, and turns it off at the end making a dot. If there is a top hole without a bottom, and then the next column has a bottom without a top hole, mark is on for three intervals, and a dash is represented. If there is only a centre hole, then nothing changes, and this would normally be used to put in space between letters and words.

The Wheatstone perforator was a manually operated hole punch machine to produce Wheatstone slips. It had three buttons (or keys) labelled "A", "A1" and "A2". "A" punched the pattern for dot, "A1" punched the pattern for space, and "A2" punched the dash pattern in two columns. The keys were so difficult to press that fist-held rubber-tipped mallets were used to depress them and operate the punches. Using this, invalid combinations of holes could not be produced. The blank paper tape was fed in from the right over a roller and came out the left side. It was oriented in a vertical plane. The paper punches were labelled with numbers: 1 for the top hole of the dot, 2 for the sprocket hole for dot, and 3 for the bottom hole for dot. When a dash was punched, extra hole punches to the right punched a centre hole with number 4 and a bottom hole with number 5. The perforator was introduced in 1867. It enabled transmission speeds on a telegraph line to increase to 70 words per minute. The very first message ever punched onto a tape was "SOS EIOS". The manual perforator was subsequently replaced by keyboard perforators like the Gell keyboard perforator or Kleinschmidt keyboard perforator.

Each of the keys had a spring to restore its position after pressing. Each key moved a corresponding lever underneath the instrument. The other end of the levers protruded up into the back of the mechanism. Each punch rod also had a spring to put it back in place after punching a hole. For space and dot keying (A or A1) the star wheel was only allowed to turn one position by a pawl, and the paper tape only moved forward one position. However, when key A2 was hit, the corresponding lever B2 raised a bar (h) which allowed another lever attached to the pawl to move further back when the star wheel rotated, and the wheel could turn two positions, for a dash. The distance the paper tape moved for each position was determined by how far lever k moved, and its range of movement had to be set by adjusting screws i and t. A flat spring g stored energy from the punch to move the paper. The force of the spring was determined by adjusting screws n and n'. A guide roller (r) with a groove was pressed by an adjustable spring to press the pawl against the star wheel. The star wheel was on a frame with a piece sticking out the left hand side as a lever. When the operator wanted to insert paper tape, this lever was pulled, and the star wheel retracted from the paper.

The Wheatstone transmitter read a paper tape (Wheatstone slip) and converted the dot pattern into mark and space symbols on the telegraph line. It worked by two rods alternately rising up to sample the holes in the tape. First of all the top hole was probed, and if the rod could go through, it moved a compound lever that connected the mark signal to the line. With no hole the lever remained unmoved. Next the top hole rod dropped and the bottom hole rod checked whether there was a bottom hole in the tape. If there was, the compound lever was moved back to connect the space signal on the line. If there was no hole, the compound lever was left alone as it was. An extra switch enabled the transmitter to be bypassed so that a Morse key could be used instead.

The Wheatstone receiver converted the signal on the telegraph line to an inked pattern on a paper strip. An electromagnet electrically connected to the telegraph line moved an inking wheel to press against the paper. A clockwork mechanism advanced the paper tape, and turned the inking wheel, and an ink supply wheel. The paper advance speed could be adjusted between 7 and 60 feet per minute. Power to the clockwork had three sources: it could be a coiled spring, a weight, or an electric motor. Paper spools were stored in drawers beneath the reader to allow quick change when one was exhausted. The ink supply wheel turned in an inkwell. The machine was started and stopped by use of a lever. In electrical characteristics, the electromagnet had two windings, each of 100 ohms resistance. These could be connected in parallel or series to achieve a 50 or 200 ohm resistance, to better match the telegraph line. Other maintenance that might have been required was cleaning of the marker and supply wheels, adjusting the armature-coil spacing to avoid a marking or spacing bias, and cleaning the sounding tongue and contact points.

The Wheatstone telegram consisted of strips of paper tape with the Morse code printed on it, pasted on a form. The telegram would later be retyped to make a final presentable message for the recipient.

🔗 Video games 🔗 Video games/Nintendo

Link: The Faces of Evil, Zelda: The Wand of Gamelon and Zelda's Adventure are action-adventure games produced by Philips for their CD-i format as part of Nintendo's The Legend of Zelda video game series. Not designed for Nintendo platforms, the games owe their existence to negotiations related to Nintendo's decision not to have Philips create a CD add-on to the Super NES. During these negotiations, Philips secured the rights to use Nintendo characters in CD-i third-party developer games. The Faces of Evil and The Wand of Gamelon were developed by Animation Magic and were both released in North America on October 10, 1993, and Zelda's Adventure was developed by Viridis and was released in North America on June 5, 1994. The games were given little funding or development time, and Nintendo provided only cursory input. None of the games are canonical to the Zelda franchise.

CD-i players did not sell well and the games saw relatively small sales figures. Though the games initially received largely positive reviews, they have been universally criticized since the mid-2000s. This is attributed to the reaction of many gamers to the obscure games' full motion video cutscenes when they first became widely available through video-sharing websites such as YouTube. The cutscenes are perceived to be of poor quality. Because the aging early 1990s visual effects of the titles failed to live up to the graphic effects of the 2000s, and because for many fans this was their first experience of the games, the CD-i Zelda titles have developed a critical reputation as particularly poor based largely on animation quality and to an extent awkward controls. In the eyes of "devout" hardcore gamers, according to Edge, the games are now considered "tantamount to blasphemy".

Faces of Evil and Wand of Gamelon are played using the side-scrolling view introduced in Zelda II: The Adventure of Link, while Zelda's Adventure has a top-down view reminiscent of the original The Legend of Zelda. All the CD-i Zelda games begin with animated FMVs to illustrate the capabilities of the CD-ROM format, save Zelda's Adventure, which begins with a live-action video.

🔗 Polynesia 🔗 Anthropology 🔗 Norway 🔗 Archaeology 🔗 South America 🔗 Sailing

The Kon-Tiki expedition was a 1947 journey by raft across the Pacific Ocean from South America to the Polynesian islands, led by Norwegian explorer and writer Thor Heyerdahl. The raft was named Kon-Tiki after the Inca god Viracocha, for whom "Kon-Tiki" was said to be an old name. Kon-Tiki is also the name of Heyerdahl's book, the Academy Award-winning 1950 documentary film chronicling his adventures, and the 2012 dramatized feature film nominated for the Academy Award for Best Foreign Language Film.

Heyerdahl believed that people from South America could have reached Polynesia during pre-Columbian times. His aim in mounting the Kon-Tiki expedition was to show, by using only the materials and technologies available to those people at the time, that there were no technical reasons to prevent them from having done so. Although the expedition carried some modern equipment, such as a radio, watches, charts, sextant, and metal knives, Heyerdahl argued they were incidental to the purpose of proving that the raft itself could make the journey.

Heyerdahl's hypothesis of a South American origin of the Polynesian peoples, as well as his "drift voyaging" hypothesis, is generally rejected by scientists today. Archaeological, linguistic, cultural, and genetic evidence tends to support a western origin for Polynesians, from Island Southeast Asia, using sophisticated multihull sailing technologies and navigation techniques during the Austronesian expansion. However, there is evidence of some gene flow from South America to Easter Island.

The Kon-Tiki expedition was funded by private loans, along with donations of equipment from the United States Army. Heyerdahl and a small team went to Peru, where, with the help of dockyard facilities provided by the Peruvian authorities, they constructed the raft out of balsa logs and other native materials in an indigenous style as recorded in illustrations by Spanish conquistadores. The trip began on April 28, 1947. Heyerdahl and five companions sailed the raft for 101 days over 6,900 km (4,300 miles) across the Pacific Ocean before smashing into a reef at Raroia in the Tuamotus on August 7, 1947. The crew made successful landfall and all returned safely.

Thor Heyerdahl's book about his experience became a bestseller. It was published in Norwegian in 1948 as The Kon-Tiki Expedition: By Raft Across the South Seas, later reprinted as Kon-Tiki: Across the Pacific in a Raft. It appeared with great success in English in 1950, also in many other languages. A documentary motion picture about the expedition, also called Kon-Tiki, was produced from a write-up and expansion of the crew's filmstrip notes and won an Academy Award in 1951. It was directed by Heyerdahl and edited by Olle Nordemar. The voyage was also chronicled in the documentary TV-series The Kon-Tiki Man: The Life and Adventures of Thor Heyerdahl, directed by Bengt Jonson.

The original Kon-Tiki raft is now on display in the Kon-Tiki Museum at Bygdøy in Oslo.

🔗 Biography 🔗 Medicine 🔗 COVID-19 🔗 China 🔗 Biography/science and academia

Li Wenliang (Chinese: 李文亮; pinyin: Lǐ Wénliàng; 12 October 1986 – 7 February 2020) was a Chinese ophthalmologist who worked as a physician at Wuhan Central Hospital. Li warned his colleagues in December 2019 about a possible outbreak of an illness that resembled severe acute respiratory syndrome (SARS), later acknowledged as COVID-19. He became a whistleblower when his warnings were later shared publicly. On 3 January 2020, Wuhan police summoned and admonished him for "making false comments on the Internet". Li returned to work, later contracted the virus from an infected patient (who had been originally treated for glaucoma) and died from the disease on 7 February 2020, at age 33. A subsequent Chinese official inquiry exonerated him and the Communist Party formally offered a "solemn apology" to his family and revoked its admonishment of him.

🔗 Russia 🔗 Russia/technology and engineering in Russia 🔗 Spaceflight 🔗 Russia/science and education in Russia 🔗 Russia/history of Russia

Soyuz 11 (Russian: Союз 11, Union 11) was the only crewed mission to board the world's first space station, Salyut 1 (Soyuz 10 had soft-docked but had not been able to enter due to latching problems). The crew, Georgy Dobrovolsky, Vladislav Volkov, and Viktor Patsayev, arrived at the space station on 7 June 1971 and departed on 29 June. The mission ended in disaster when the crew capsule depressurized during preparations for reentry, killing the three-man crew. The three crew members of Soyuz 11 are the only humans known to have died in space.

🔗 Biology 🔗 Neuroscience 🔗 Physiology 🔗 Science

A microtome (from the Greek mikros, meaning "small", and temnein, meaning "to cut") is a cutting tool used to produce extremely thin slices of material known as sections, with the process being termed microsectioning. Important in science, microtomes are used in microscopy for the preparation of samples for observation under transmitted light or electron radiation.

Microtomes use steel, glass or diamond blades depending upon the specimen being sliced and the desired thickness of the sections being cut. Steel blades are used to prepare histological sections of animal or plant tissues for light microscopy. Glass knives are used to slice sections for light microscopy and to slice very thin sections for electron microscopy. Industrial grade diamond knives are used to slice hard materials such as bone, teeth and tough plant matter for both light microscopy and for electron microscopy. Gem-quality diamond knives are also used for slicing thin sections for electron microscopy.

Microtomy is a method for the preparation of thin sections for materials such as bones, minerals and teeth, and an alternative to electropolishing and ion milling. Microtome sections can be made thin enough to section a human hair across its breadth, with section thickness between 50 nm and 100 μm.

🔗 Climate change 🔗 Environment 🔗 Geology

Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term is named for Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypothesized that variations in eccentricity, axial tilt, and precession resulted in cyclical variation in the solar radiation reaching the Earth, and that this orbital forcing strongly influenced climatic patterns on Earth.

Similar astronomical hypotheses had been advanced in the 19th century by Joseph Adhemar, James Croll and others, but verification was difficult because there was no reliably dated evidence, and because it was unclear which periods were important.

Now, materials on Earth that have been unchanged for millennia (obtained via ice, rock, and deep ocean cores) are being studied to indicate the history of Earth's climate. Though they are consistent with the Milankovitch hypothesis, there are still several observations that the hypothesis does not explain.