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🔗 Physics 🔗 Geology 🔗 Physics/Acoustics

Singing sand, also called whistling sand, barking sand or singing dune, is sand that produces sound. The sound emission may be caused by wind passing over dunes or by walking on the sand.

Certain conditions have to come together to create singing sand:

1. The sand grains have to be round and between 0.1 and 0.5 mm in diameter.
2. The sand has to contain silica.
3. The sand needs to be at a certain humidity.

The most common frequency emitted seems to be close to 450 Hz.

There are various theories about the singing sand mechanism. It has been proposed that the sound frequency is controlled by the shear rate. Others have suggested that the frequency of vibration is related to the thickness of the dry surface layer of sand. The sound waves bounce back and forth between the surface of the dune and the surface of the moist layer, creating a resonance that increases the sound's volume. The noise may be generated by friction between the grains or by the compression of air between them.

Other sounds that can be emitted by sand have been described as "roaring" or "booming".

🔗 Technology 🔗 Physics 🔗 Transport 🔗 Trains 🔗 Engineering

A Hyperloop is a proposed mode of passenger and freight transportation, first used to describe an open-source vactrain design released by a joint team from Tesla and SpaceX. Hyperloop is a sealed tube or system of tubes through which a pod may travel free of air resistance or friction conveying people or objects at high speed while being very efficient, thereby drastically reducing travel times over medium-range distances.

Elon Musk's version of the concept, first publicly mentioned in 2012, incorporates reduced-pressure tubes in which pressurized capsules ride on air bearings driven by linear induction motors and axial compressors.

The Hyperloop Alpha concept was first published in August 2013, proposing and examining a route running from the Los Angeles region to the San Francisco Bay Area, roughly following the Interstate 5 corridor. The Hyperloop Genesis paper conceived of a hyperloop system that would propel passengers along the 350-mile (560 km) route at a speed of 760 mph (1,200 km/h), allowing for a travel time of 35 minutes, which is considerably faster than current rail or air travel times. Preliminary cost estimates for this LA–SF suggested route were included in the white paper—US$6 billion for a passenger-only version, and US$7.5 billion for a somewhat larger-diameter version transporting passengers and vehicles—although transportation analysts had doubts that the system could be constructed on that budget; some analysts claimed that the Hyperloop would be several billion dollars overbudget, taking into consideration construction, development, and operation costs.

The Hyperloop concept has been explicitly "open-sourced" by Musk and SpaceX, and others have been encouraged to take the ideas and further develop them. To that end, a few companies have been formed, and several interdisciplinary student-led teams are working to advance the technology. SpaceX built an approximately 1-mile-long (1.6 km) subscale track for its pod design competition at its headquarters in Hawthorne, California.

🔗 Russia 🔗 Physics 🔗 Russia/science and education in Russia

The tennis racket theorem or intermediate axis theorem is a result in classical mechanics describing the movement of a rigid body with three distinct principal moments of inertia. It is also dubbed the Dzhanibekov effect, after Russian cosmonaut Vladimir Dzhanibekov who noticed one of the theorem's logical consequences while in space in 1985 although the effect was already known for at least 150 years before that.

The theorem describes the following effect: rotation of an object around its first and third principal axes is stable, while rotation around its second principal axis (or intermediate axis) is not.

This can be demonstrated with the following experiment: hold a tennis racket at its handle, with its face being horizontal, and try to throw it in the air so that it will perform a full rotation around the horizontal axis perpendicular to the handle, and try to catch the handle. In almost all cases, during that rotation the face will also have completed a half rotation, so that the other face is now up. By contrast, it is easy to throw the racket so that it will rotate around the handle axis (the third principal axis) without accompanying half-rotation around another axis; it is also possible to make it rotate around the vertical axis perpendicular to the handle (the first principal axis) without any accompanying half-rotation.

The experiment can be performed with any object that has three different moments of inertia, for instance with a book, remote control or smartphone. The effect occurs whenever the axis of rotation differs only slightly from the object's second principal axis; air resistance or gravity are not necessary.

🔗 Spaceflight 🔗 Physics 🔗 Rocketry

The Tsiolkovsky rocket equation, classical rocket equation, or ideal rocket equation is a mathematical equation that describes the motion of vehicles that follow the basic principle of a rocket: a device that can apply acceleration to itself using thrust by expelling part of its mass with high velocity can thereby move due to the conservation of momentum.

${\displaystyle \Delta v=v_{\text{e}}\ln {\frac {m_{0}}{m_{f}}}=I_{\text{sp}}g_{0}\ln {\frac {m_{0}}{m_{f}}}}$

where:

${\displaystyle \Delta v\ }$ is delta-v – the maximum change of velocity of the vehicle (with no external forces acting).

${\displaystyle m_{0}}$ is the initial total mass, including propellant, also known as wet mass.

${\displaystyle m_{f}}$ is the final total mass without propellant, also known as dry mass.

${\displaystyle v_{\text{e}}=I_{\text{sp}}g_{0}}$ is the effective exhaust velocity, where:

${\displaystyle I_{\text{sp}}}$ is the specific impulse in dimension of time.

${\displaystyle g_{0}}$ is standard gravity.

${\displaystyle \ln }$ is the natural logarithm function.

🔗 Physics 🔗 Astronomy

Rømer's determination of the speed of light was the demonstration in 1676 that light has a finite speed and so does not travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer, who was working at the Royal Observatory in Paris at the time.

By timing the eclipses of the Jovian moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun. This would give light a velocity of about 220,000 kilometres per second, about 26% lower than the true value of 299,792 km/s.

Rømer's theory was controversial at the time that he announced it and he never convinced the director of the Paris Observatory, Giovanni Domenico Cassini, to fully accept it. However, it quickly gained support among other natural philosophers of the period such as Christiaan Huygens and Isaac Newton. It was finally confirmed nearly two decades after Rømer's death, with the explanation in 1729 of stellar aberration by the English astronomer James Bradley.

🔗 Physics 🔗 Astronomy 🔗 Chemistry 🔗 Water 🔗 Astronomy/Solar System

Superionic water, also called superionic ice or ice XVIII, is a phase of water that exists at extremely high temperatures and pressures. In superionic water, water molecules break apart and the oxygen ions crystallize into an evenly spaced lattice while the hydrogen ions float around freely within the oxygen lattice. The freely mobile hydrogen ions make superionic water almost as conductive as typical metals, making it a superionic conductor. It is one of the 19 known crystalline phases of ice. Superionic water is distinct from ionic water, which is a hypothetical liquid state characterized by a disordered soup of hydrogen and oxygen ions.

While theorized for decades, it was not until the 1990s that the first experimental evidence emerged for superionic water. Initial evidence came from optical measurements of laser-heated water in a diamond anvil cell, and from optical measurements of water shocked by extremely powerful lasers. The first definitive evidence for the crystal structure of the oxygen lattice in superionic water came from x-ray measurements on laser-shocked water which were reported in 2019.

If it were present on the surface of the Earth, superionic ice would rapidly decompress. In May 2019, scientists at the Lawrence Livermore National Laboratory (LLNL) were able to synthesize superionic ice, confirming it to be almost four times as dense as normal ice and black in color.

Superionic water is theorized to be present in the mantles of giant planets such as Uranus and Neptune.

🔗 Physics

🔗 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.

🔗 Biography 🔗 Soviet Union 🔗 Military history 🔗 Physics 🔗 Italy 🔗 Socialism 🔗 Biography/science and academia 🔗 Military history/Military biography 🔗 Biography/military biography 🔗 Physics/Biographies 🔗 Military history/Russian, Soviet and CIS military history 🔗 Soviet Union/Russian, Soviet and CIS military history

Bruno Pontecorvo (Italian: [ponteˈkɔrvo]; Russian: Бру́но Макси́мович Понтеко́рво, Bruno Maksimovich Pontecorvo; 22 August 1913 – 24 September 1993) was an Italian and Soviet nuclear physicist, an early assistant of Enrico Fermi and the author of numerous studies in high energy physics, especially on neutrinos. A convinced communist, he defected to the Soviet Union in 1950, where he continued his research on the decay of the muon and on neutrinos. The prestigious Pontecorvo Prize was instituted in his memory in 1995.

The fourth of eight children of a wealthy Jewish-Italian family, Pontecorvo studied physics at the University of Rome La Sapienza, under Fermi, becoming the youngest of his Via Panisperna boys. In 1934 he participated in Fermi's famous experiment showing the properties of slow neutrons that led the way to the discovery of nuclear fission. He moved to Paris in 1934, where he conducted research under Irène and Frédéric Joliot-Curie. Influenced by his cousin, Emilio Sereni, he joined the French Communist Party, as did his sisters Giuliana and Laura and brother Gillo. The Italian Fascist regime's 1938 racial laws against Jews caused his family members to leave Italy for Britain, France and the United States.

When the German Army closed in on Paris during the Second World War, Pontecorvo, his brother Gillo, cousin Emilio Sereni and Salvador Luria fled the city on bicycles. He eventually made his way to Tulsa, Oklahoma, where he applied his knowledge of nuclear physics to prospecting for oil and minerals. In 1943, he joined the British Tube Alloys team at the Montreal Laboratory in Canada. This became part of the Manhattan Project to develop the first atomic bombs. At Chalk River Laboratories, he worked on the design of the nuclear reactor ZEEP, the first reactor outside of the United States that went critical in 1945, followed by the NRX reactor in 1947. He also looked into cosmic rays, the decay of muons, and what would become his obsession, neutrinos. He moved to Britain in 1949, where he worked for the Atomic Energy Research Establishment at Harwell.

After his defection to the Soviet Union in 1950, he worked at the Joint Institute for Nuclear Research (JINR) in Dubna. He had proposed using chlorine to detect neutrinos. In a 1959 paper, he argued that the electron neutrino (
ν
_e) and the muon neutrino (
ν
_μ) were different particles. Solar neutrinos were detected by the Homestake Experiment, but only between one third and one half of the predicted number were found. In response to this solar neutrino problem, he proposed a phenomenon known as neutrino oscillation, whereby electron neutrinos became muon neutrinos. The existence of the oscillations was finally established by the Super-Kamiokande experiment in 1998. He also predicted in 1958 that supernovae would produce intense bursts of neutrinos, which was confirmed in 1987 when Supernova SN1987A was detected by neutrino detectors.

🔗 Physics 🔗 Weather 🔗 Weather/Weather 🔗 Weather/General meteorology

A Fata Morgana (Italian: [ˈfaːta morˈɡaːna]) is a complex form of superior mirage visible in a narrow band right above the horizon. The term Fata Morgana is the Italian translation of "Morgan the Fairy" (Morgan le Fay of Arthurian legend). These mirages are often seen in the Italian Strait of Messina, and were described as fairy castles in the air or false land conjured by her magic.

Fata Morgana mirages significantly distort the object or objects on which they are based, often such that the object is completely unrecognizable. A Fata Morgana may be seen on land or at sea, in polar regions, or in deserts. It may involve almost any kind of distant object, including boats, islands, and the coastline. Often, a Fata Morgana changes rapidly. The mirage comprises several inverted (upside down) and upright images stacked on top of one another. Fata Morgana mirages also show alternating compressed and stretched zones.

The optical phenomenon occurs because rays of light bend when they pass through air layers of different temperatures in a steep thermal inversion where an atmospheric duct has formed. In calm weather, a layer of significantly warmer air may rest over colder dense air, forming an atmospheric duct that acts like a refracting lens, producing a series of both inverted and erect images. A Fata Morgana requires a duct to be present; thermal inversion alone is not enough to produce this kind of mirage. While a thermal inversion often takes place without there being an atmospheric duct, an atmospheric duct cannot exist without there first being a thermal inversion.