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🔗 Mathematics 🔗 Statistics 🔗 Systems 🔗 Robotics 🔗 Systems/Control theory

In statistics and control theory, Kalman filtering, also known as linear quadratic estimation (LQE), is an algorithm that uses a series of measurements observed over time, containing statistical noise and other inaccuracies, and produces estimates of unknown variables that tend to be more accurate than those based on a single measurement alone, by estimating a joint probability distribution over the variables for each timeframe. The filter is named after Rudolf E. Kálmán, one of the primary developers of its theory.

The Kalman filter has numerous applications in technology. A common application is for guidance, navigation, and control of vehicles, particularly aircraft, spacecraft and dynamically positioned ships. Furthermore, the Kalman filter is a widely applied concept in time series analysis used in fields such as signal processing and econometrics. Kalman filters also are one of the main topics in the field of robotic motion planning and control and can be used in trajectory optimization. The Kalman filter also works for modeling the central nervous system's control of movement. Due to the time delay between issuing motor commands and receiving sensory feedback, use of the Kalman filter supports a realistic model for making estimates of the current state of the motor system and issuing updated commands.

The algorithm works in a two-step process. In the prediction step, the Kalman filter produces estimates of the current state variables, along with their uncertainties. Once the outcome of the next measurement (necessarily corrupted with some amount of error, including random noise) is observed, these estimates are updated using a weighted average, with more weight being given to estimates with higher certainty. The algorithm is recursive. It can run in real time, using only the present input measurements and the previously calculated state and its uncertainty matrix; no additional past information is required.

Optimality of the Kalman filter assumes that the errors are Gaussian. In the words of Rudolf E. Kálmán: "In summary, the following assumptions are made about random processes: Physical random phenomena may be thought of as due to primary random sources exciting dynamic systems. The primary sources are assumed to be independent gaussian random processes with zero mean; the dynamic systems will be linear." Though regardless of Gaussianity, if the process and measurement covariances are known, the Kalman filter is the best possible linear estimator in the minimum mean-square-error sense.

Extensions and generalizations to the method have also been developed, such as the extended Kalman filter and the unscented Kalman filter which work on nonlinear systems. The underlying model is a hidden Markov model where the state space of the latent variables is continuous and all latent and observed variables have Gaussian distributions. Also, Kalman filter has been successfully used in multi-sensor fusion, and distributed sensor networks to develop distributed or consensus Kalman filter.

🔗 Physics 🔗 Physics/relativity

In astrophysics, spaghettification (sometimes referred to as the noodle effect) is the vertical stretching and horizontal compression of objects into long thin shapes (rather like spaghetti) in a very strong non-homogeneous gravitational field; it is caused by extreme tidal forces. In the most extreme cases, near black holes, the stretching is so powerful that no object can withstand it, no matter how strong its components. Within a small region the horizontal compression balances the vertical stretching so that small objects being spaghettified experience no net change in volume.

Stephen Hawking described the flight of a fictional astronaut who, passing within a black hole's event horizon, is "stretched like spaghetti" by the gravitational gradient (difference in strength) from head to toe. The reason this happens would be that the gravity force exerted by the singularity would be much stronger at one end of the body than the other. If one were to fall into a black hole feet first, the gravity at their feet would be much stronger than at their head, causing the person to be vertically stretched. Along with that, the right side of the body will be pulled to the left, and the left side of the body will be pulled to the right, horizontally compressing the person. However, the term "spaghettification" was established well before this. Spaghettification of a star was imaged for the first time in 2018 by researchers observing a pair of colliding galaxies approximately 150 million light-years from Earth.

🔗 California 🔗 Military history 🔗 Military history/North American military history 🔗 Military history/United States military history 🔗 Military history/World War II 🔗 Paranormal 🔗 California/Southern California

The Battle of Los Angeles, also known as the Great Los Angeles Air Raid, is the name given by contemporary sources to a rumored attack on the mainland United States by Imperial Japan and the subsequent anti-aircraft artillery barrage which took place from late 24 February to early 25 February 1942, over Los Angeles, California. The incident occurred less than three months after the U.S. entered World War II in response to the Imperial Japanese Navy's surprise attack on Pearl Harbor, and one day after the bombardment of Ellwood near Santa Barbara on 23 February. Initially, the target of the aerial barrage was thought to be an attacking force from Japan, but speaking at a press conference shortly afterward, Secretary of the Navy Frank Knox called the purported attack a "false alarm". Newspapers of the time published a number of reports and speculations of a cover-up.

When documenting the incident in 1949, the United States Coast Artillery Association identified a meteorological balloon sent aloft at 1:00 am as having "started all the shooting" and concluded that "once the firing started, imagination created all kinds of targets in the sky and everyone joined in". In 1983, the U.S. Office of Air Force History attributed the event to a case of "war nerves" triggered by a lost weather balloon and exacerbated by stray flares and shell bursts from adjoining batteries.

🔗 Climate change 🔗 Environment 🔗 Antarctica 🔗 Glaciers

The Western Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. The WAIS is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.

🔗 Viruses 🔗 Biology 🔗 Philosophy 🔗 Philosophy/Contemporary philosophy 🔗 History of Science 🔗 Molecular and Cell Biology 🔗 Philosophy/Ethics 🔗 Genetics 🔗 Evolutionary biology 🔗 Science Policy 🔗 Molecular Biology/Molecular and Cell Biology

HeLa (; also Hela or hela) is an immortal cell line used in scientific research. It is the oldest and most commonly used human cell line. The line was derived from cervical cancer cells taken on February 8, 1951 from Henrietta Lacks, a patient who died of cancer on October 4, 1951. The cell line was found to be remarkably durable and prolific, which gives rise to its extensive use in scientific research.

The cells from Lacks's cancerous cervical tumor were taken without her knowledge or consent, which was common practice at the time. Cell biologist George Otto Gey found that they could be kept alive, and developed a cell line. Previously, cells cultured from other human cells would only survive for a few days. Scientists would spend more time trying to keep the cells alive than performing actual research on them. Cells from Lacks' tumor behaved differently. As was custom for Gey's lab assistant, she labeled the culture 'HeLa', the first two letters of the patient's first and last name; this became the name of the cell line.

These were the first human cells grown in a lab that were naturally "immortal", meaning that they do not die after a set number of cell divisions (i.e. cellular senescence). These cells could be used for conducting a multitude of medical experiments—if the cells died, they could simply be discarded and the experiment attempted again on fresh cells from the culture. This represented an enormous boon to medical and biological research, as previously stocks of living cells were limited and took significant effort to culture.

The stable growth of HeLa enabled a researcher at the University of Minnesota hospital to successfully grow polio virus, enabling the development of a vaccine, and by 1952, Jonas Salk developed a vaccine for polio using these cells. To test Salk's new vaccine, the cells were put into mass production in the first-ever cell production factory.

In 1953, HeLa cells were the first human cells successfully cloned and demand for the HeLa cells quickly grew in the nascent biomedical industry. Since the cells' first mass replications, they have been used by scientists in various types of investigations including disease research, gene mapping, effects of toxic substances on organisms, and radiation on humans. Additionally, HeLa cells have been used to test human sensitivity to tape, glue, cosmetics, and many other products.

Scientists have grown an estimated 50 million metric tons of HeLa cells, and there are almost 11,000 patents involving these cells.

The HeLa cell lines are also notorious for invading other cell cultures in laboratory settings. Some have estimated that HeLa cells have contaminated 10–20% of all cell lines currently in use.

🔗 Maps 🔗 Photography

The Boston Camera, also known as Pie Face and officially classified as the K-42 Camera Model, was a prototype airborne photo reconnaissance camera manufactured for the United States Air Force by Boston University in 1951 and tested on the Convair B-36 and the C-97 Stratofreighter. The model carried on the first ERB-36D (44-92088) had a 6,096-millimetre (240.0 in) focal length, which was achieved using a series of lenses and mirrors. The lens had an f/8 stop and used a 1/400 second shutter speed, and could photograph a golf ball from an altitude of 45,000 feet (14,000 m) feet. The camera used 18-by-36-inch (46 by 91 cm) negatives. The camera was installed aboard Boeing C-97A 49-2592 (not an "RC-97" or "EC-97" as often widely quoted) which was used operationally by the 7405th Support Squadron based at Wiesbaden, West Germany between 1952 and 1962. It was given to the Air Force Museum in 1964, along with a contact print of a golf ball on a course.

In the words of CIA historian Dino Brugioni:

The lens was designed in 1947 by Dr. James Baker for installation in a camera designed by the Boston University Optical Research Laboratory. The camera weighed about three tons, and eight hundred pounds of lead shot were required to balance it. Supposedly, it was first installed and test-flown in an RB-36, then installed as a left-looking oblique camera in an RC-97. The first photo Arthur Lundahl and I saw from this project was of New York City. The aircraft was seventy-two miles away, and yet we could see people in Central Park.

The Boston Camera was plagued with problems that caused it to vibrate and produce smearing on the newspaper-sized negative, so that photo interpreters would see several smeared frames along with several clear ones. It is currently displayed at the National Museum of the US Air Force in Dayton, Ohio.

From the display placard:

This camera, manufactured for the US Air Force by Boston University in 1951, is the largest aerial camera ever built. It was installed in an RB-36D in 1954 and tested for about a year. Later it was used in a C-97 aircraft flying along the air corridor through communist East Germany to Berlin, but a 10,000 ft (3,000 m) altitude restriction imposed by the communists made the camera less useful than at a higher altitude. It was also used on reconnaissance missions along the borders of Eastern European nations. The camera made an 18 x 36 inch negative and was so powerful a photo interpreter could detect a golf ball from an altitude of 45,000 feet (14,000 m). Dr. James Baker of Harvard University designed the camera.

Technical Notes:

Shutter Speed: 1/400 sec

Weight: 6,500 lbs (3 metric ton) (camera and aircraft mount)

🔗 Death 🔗 Greece 🔗 Paranormal

The term Drosoulites (Greek: Δροσουλίτες) refers to a long procession of visions, seen by residents around Frangokastello castle in Sfakia region of Crete (Greece). The phenomenon is rumored to be visible every year, on the anniversary of the Battle of Frangokastello or even in early June near a small village in southern Crete.

The visions, as described by witnesses, consist of a group of human-like shadows dressed in black, walking or riding, armed with weapons, moving from the monastery of Agios Charalambos and advancing towards the old fort, Frangokastello, a 14th-century Venetian fortification. Legend has it that this group of people are Greek fighters that died during the Battle of Frangokastello (17 May 1828) and since then they appear as supernatural beings in the area.

The ghost army is led by Hatzimichalis Dalianis, from Delvinaki in Epirus, the chief of the Greek men, 350 of whom were lost, in the battle. The army took refuge in the fort during the Greek War of Independence against the Turks, where they were killed after a seven-day siege.

The local people named them Drosoulites ("dew shadows") due to the time of day that the phenomenon takes place. The phenomenon is observed when the sea is calm and the atmosphere is moist and before the sun rises too high in the sky. It usually lasts about 10 minutes.

The shadows are visible from the valley at a distance of 1000 m. Many have tried to explain the phenomenon scientifically, and at one time it was explained as a mirage from the coast of North Africa, but still there is no accepted consensus. On the other hand, another, occult, interpretation implies the existence of a psychic phenomenon, clairvoyance, seen in some countries like Britain and Germany, also regarding ghost armies. The appearance of the Drosoulites is documented over the ages. In 1890 a transient Turkish army took the images for rebels and fled away. Even during the Second World War, a German patrol is said to have opened fire on the visions.

🔗 Military history 🔗 Military history/Military science, technology, and theory 🔗 Military history/Weaponry 🔗 Elements 🔗 Military history/Cold War

Red mercury is purportedly a substance of uncertain composition used in the creation of nuclear weapons, as well as other weapons systems. Because of the great secrecy surrounding the development and manufacture of nuclear weapons, there is no proof that red mercury exists. However, all samples of alleged "red mercury" analyzed in the public literature have proven to be well-known, common red substances of no interest to weapons makers.

Red mercury is therefore considered a hoax, perpetrated by con artists who sought to take advantage of gullible buyers on the black market for arms. There have been reports of "red mercury" on the international black markets since 1979, but the hoax seems to have been most common in the early 1990s, with very high asking prices.

🔗 Mathematics 🔗 Percussion

To hear the shape of a drum is to infer information about the shape of the drumhead from the sound it makes, i.e., from the list of overtones, via the use of mathematical theory.

"Can One Hear the Shape of a Drum?" is the title of a 1966 article by Mark Kac in the American Mathematical Monthly which made the question famous, though this particular phrasing originates with Lipman Bers. Similar questions can be traced back all the way to Hermann Weyl . For his paper, Kac was given the Lester R. Ford Award in 1967 and the Chauvenet Prize in 1968.

The frequencies at which a drumhead can vibrate depend on its shape. The Helmholtz equation calculates the frequencies if the shape is known. These frequencies are the eigenvalues of the Laplacian in the space. A central question is whether the shape can be predicted if the frequencies are known; for example, whether a circle-shaped triangle can be recognized in this way. Kac admitted he did not know if it was possible for two different shapes to yield the same set of frequencies. The question of whether the frequencies determine the shape was finally answered in the negative in the early 1990s by Gordon, Webb and Wolpert.

🔗 Military history 🔗 Heraldry and vexillology 🔗 Catholicism 🔗 Military history/Crusades 🔗 Military history/Medieval warfare 🔗 Countries 🔗 Former countries 🔗 Military history/National militaries 🔗 Malta 🔗 Orders, decorations, and medals

The Sovereign Military Order of Malta (SMOM), officially the Sovereign Military Hospitaller Order of Saint John of Jerusalem, of Rhodes and of Malta (Italian: Sovrano Militare Ordine Ospedaliero di San Giovanni di Gerusalemme di Rodi e di Malta; Latin: Supremus Militaris Ordo Hospitalarius Sancti Ioannis Hierosolymitani Rhodiensis et Melitensis), commonly known as the Order of Malta, Malta Order or Knights of Malta, is a Catholic lay religious order, traditionally of military, chivalric and noble nature. Though it possesses no territory, the order is a sovereign entity of international law and maintains diplomatic relations with many countries.

SMOM claims continuity with the Knights Hospitaller, a chivalric order that was founded c. 1099 by the Blessed Gerard in the Kingdom of Jerusalem. The order is led by an elected Prince and Grand Master. Its motto is Tuitio fidei et obsequium pauperum ('defence of the faith and assistance to the poor'). The order venerates the Virgin Mary as its patroness, under the title of Our Lady of Philermos. Its modern-day role is largely focused on providing humanitarian assistance and assisting with international humanitarian relations, for which purpose it has had permanent observer status at the United Nations General Assembly since 1994.