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Temperature is a way to describe how hot or cold something is on any of a number of different scales. It also shows the direction that heat energy will naturally flow, from something hotter (higher temperature) to something cooler (lower temperature). A lit match is at a much higher temperature than an iceberg, but the total heat energy in an iceberg is much greater than the energy in a match. Temperature is not the same thing as energy in a thermodynamic system. Temperature, like pressure or density, is an intensive property—one that doesn’t depend on how much matter is being considered—as opposed to widespread properties like mass or volume.

Today, most people use three different temperature scales. In the US and a few other English-speaking countries, the Fahrenheit (°F) scale is used to measure temperature. A lot of people in the sciences use the Celsius (°C) temperature scale, which is common in almost all countries that use the metric system. The Kelvin (K) scale is used to measure absolute temperatures. It is created by shifting the Celsius scale by -273.15° so that absolute zero is the same as 0 K. It is the usual way for scientists around the world to measure temperatures.

Other than the Kelvin scale, the Rankine scale (see William Rankine) is sometimes used instead of the Kelvin scale in engineering. In the same way that one Celsius degree is equal to one degree Rankine (°R), one Fahrenheit degree is equal to one degree Rankine.

A lot of people in Europe used the Réaumur (°Re) temperature scale, also known as the octogesimal division, in the 18th and 19th centuries. It was mostly used to check the temperature of mixtures when making beer, syrups when making certain foods, and milk when making cheese.

Other than the Kelvin scale, the Rankine scale (see William Rankine) is sometimes used instead of the Kelvin scale in engineering. In the same way that one Celsius degree is equal to one degree Rankine (°R), one Fahrenheit degree is equal to one degree Rankine.

Dark Matter

Dark matter is a part of the universe that can be detected by its gravitational pull rather than its brightness. There is 30.1 percent dark matter in the world, and 69.4 percent dark energy. The rest is made up of “normal” visible matter and dark energy.

First called the “missing mass,” dark matter was first suggested to exist by Fritz Zwicky, a Swiss-American astronomer who found in 1933 that the mass of all the stars in the Coma cluster of galaxies was only 1% of the mass needed to keep the galaxies from escaping the cluster’s gravitational pull. For many years, people didn’t believe in this missing mass. But in the 1970s, American astronomers Vera Rubin and W. Kent Ford observed a similar event that proved it existed: the stars visible in a typical galaxy only have about 10% of the mass that is needed to keep them orbiting the galaxy’s center. In general, the speed at which stars circle the center of their galaxy doesn’t depend on how far away they are from it. In fact, orbital velocity either stays the same or slightly increases with distance, which is the opposite of what you’d expect. To explain this, the mass of the galaxy around the stars must rise in a straight line as the stars move farther away from the galaxy’s center. But there is no light coming from this mass inside, which is why it is called “dark matter.”

Since it was proven that dark matter exists, gravitational lensing has shown that there is a lot of it in galaxies and groups of galaxies. Gravitational lensing is the effect of matter stretching space and changing the way background light passes through it. Another way to figure out if this missing matter is in the centers of galaxies and groups of galaxies is to look at the motion and heat of the gas that produces X-rays. The Bullet cluster is made up of two galaxy clusters that are merging. The Chandra X-ray Observatory has seen that the hot gas (normal visible matter) is slowing down as one cluster passes through the other. The groups’ mass, on the other hand, doesn’t change, which means that dark matter makes up most of the mass.

Three-quarters of the universe’s matter-energy make up is matter. A very small amount—0.5%—makes up the mass of stars, and only 0.03% of that mass is made up of elements heavier than hydrogen. Dark matter makes up the rest. There are two kinds of dark matter that have been found. As you may know, the first type makes up about 4.5% of the universe and is made up of baryons like protons, neutrons, and atomic nuclei. These are the same baryons that make up stars and galaxies. For the most part, this baryonic dark matter is likely to be gas in and between galaxies. Researchers have found this common, or baryonic, part of dark matter by counting how many elements heavier than hydrogen were made in the first few minutes after the big bang 13.8 billion years ago.

It’s not known what form dark matter takes, but it makes up the other 26.1% of the universe’s mass. It’s clear that the nonbaryonic dark matter is relatively “cold” or “nonrelativisitic” because of how quickly galaxies and large structures made up of galaxies came together after changes in the universe’s density. This means that the backbones of galaxies and clusters of galaxies are made of heavy particles that move slowly. The fact that these particles don’t give off any light is another sign that they are electromagnetically neutral. Weakly interacting massive particles (WIMPs) is the name given to these particles because of their features. We don’t know what these particles are made of yet, and the normal model of particle physics doesn’t say what they are. The standard model, on the other hand, could be expanded in a number of ways. For example, supersymmetric theories suggest that WIMPs could be made up of imaginary elementary particles like axions or neutralinos….

There are amazing efforts being made to find and measure the properties of these invisible WIMPs, either by watching them hit a lab detector or by watching them break apart when they hit each other. Some people also think that tests at new particle accelerators like the Large Hadron Collider might help us figure out if they exist and how heavy they are.

Many people have suggested that changes to gravity could be the cause of the perceived presence of “missing matter.” These changes make it seem like the pulling power of ordinary matter might be stronger in situations that only happen on the size of galaxies. However, most of the ideas are not good enough from an academic point of view because they don’t explain why gravity changes. These ideas also can’t explain why dark matter and normal matter have been seen to be physically split in the Bullet cluster. The fact that dark matter can be separated from normal matter shows that it exists in the real world.

Physical Stability

A physical constant is any of a set of fundamentally constant quantities observed in nature and appearing in the fundamental equations of physics. It is essential to accurately evaluate these constants in order to verify the correctness of the theories and to develop beneficial applications based on those theories.

The speed of light in a vacuum (c) is mentioned in electromagnetic theory and relativity theory; in the latter, it relates energy to mass via the equation E = mc2. Its value is independent of specific experimental conditions, such as those that affect the speed of a sound wave in air (for which air temperature, wind direction, and wind speed are relevant). It is a natural universal constant.

The electron’s charge is a fundamental property of a physical particle and the smallest unit of pure electric charge found in nature. In many areas of physics and chemistry, its numerical value is required, such as when calculating the mass of an element or compound liberated by a certain quantity of current passing through an electrochemical cell.

Planck’s constant (h) is not a fundamental particle property, but rather a constant that appears in the equations of quantum mechanics. Equation E = h relates the energy (E) of a photon (a quantum of electromagnetic radiation) to its frequency (v).

The universal gravitational constant (G) relates the gravitational attractive force between two bodies to their masses and separation. It is exceedingly difficult to experimentally determine its value. It has been hypothesized that G has changed over the course of the universe’s history and that it is dependent on scale. If this were the case, laboratory-determined values would not be applicable to terrestrial or astronomical problems, but there is currently no evidence to support this assertion.

Various laboratories around the world, such as the U.S. National Institute of Standards and Technology (NIST; formerly the National Bureau of Standards), determine and refine the values of physical constants as experimental methods and techniques advance.

The numerical values of physical constants are dependent on the unit system used to express them. The speed of light, for instance, can be expressed as 30,000,000,000,000 cm per second or 186,000 miles per second. In recent years, however, units have become increasingly defined in terms of physical constants. Consequently, the metre is now defined as the distance light travels in a specific amount of time. Such definitions are determined through international cooperation.

The table provides a listing of essential physical constants.

constant of gravitationG6.67384 × 10−11 cubic metre per second squared per kilogram
speed of light (in a vacuum)c2.99792458 × 108 metres per second
Planck’s constanth6.626070040 × 10−34 joule second
Boltzmann constantk1.38064852 × 10−23 joule per kelvin
Faraday constantF9.648533289 × 104 coulombs per mole
electron rest massme9.10938356 × 10−31 kilogram
proton rest massmp1.672621898 × 10−27 kilogram
neutron rest massmn1.674927471 × 10−27 kilogram
charge on electrone1.6021766208 × 10−19 coulomb
Rydberg constantR∞1.0973731568508 × 107 per metre
Stefan-Boltzmann constantσ5.670367 × 10−8 watt per square metre per kelvin4
fine-structure constantα7.2973525664 × 10−3

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