Here the Meissner effect is demonstrated by the levitation of a magnet above a liquid nitrogen cooled superconductor. Credit: Mai-Linh Doan.

Physics is a "science that deals with matter and energy and their interactions"[1] and forces and fields such as gravitation, electric and magnetic fields, and the weak and strong nuclear forces.

## Physics

Def. the "basic structural component of the universe"[2] is called matter.

Def. "something that happens"[3] is called an event.

Def. an event that "has already happened"[4] is called a past.

Def. an event "ahead;[5] those [events] yet to be experienced"[6] is called a future.

Def. the "inevitable progression into the future with the passing of present events into the past"[7] or the "inevitable passing of events from future to present then past"[8] is called time, or a time.

Def. an amount "intervening [...] between two points,[9] usually geographical points, usually (but not necessarily) measured along a straight line"[10] is called a distance.

Def. a "three-dimensional measure that consists a length, a width and a height"[11] is called a volume, or volume.

Def.

1. "distance between things"[12],
2. physical "extent across two or three dimensions; area, volume (sometimes for or to do something)"[13],
3. physical "extent in all directions, seen as an attribute of the universe (now usually considered as a part of space-time), or a mathematical model of this"[13],
4. the "near-vacuum in which planets, stars and other celestial objects are situated; the universe beyond the earth's atmosphere"[13],
5. the "physical and psychological area one needs within which to live or operate"[14],
6. a "(chiefly empty) area or volume with set limits or boundaries"[14], or
7. a "set of points, each of which is uniquely specified by a"[15] "number (the dimensionality) of coordinates"[16] is called space, or a space.

Def. "the study of properties and interactions of [space, time,][17] matter and energy"[18] is called physics.

## Astrophysics

A bubbling cauldron of star birth is highlighted in this new image from NASA's Spitzer Space Telescope. Credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA.

Astrophysics at its simplest is the application of laboratory physics, i.e., physics demonstrated in a laboratory and described with logical laws, to natural astronomical entities. This is done to understand these astronomical entities, their origin, history, and current constitution.

Many of the elementary concepts in physics are introduced to students at or before the secondary level so this resource begins there.

As more of the elementary concepts are introduced and applied to natural astronomical entities, the level of description approaches that of an introductory college level course with details included that are sometimes left out of a more traditional course.

To describe some of the more challenging events that are observed by astronomers, concepts from theoretical physics are modeled to help in the interpretation. This laps into research and allows the presentation of fairly recent results from the scholarly literature.

The natural entities are those observed by astronomers, but the interpretations often require additional trips to the laboratory here on Earth to extend traditional physics.

Stars, for example, are quite large. Putting one in a laboratory for examination has not happened. But, by using computer simulation and creative miniatures, a star can be represented.

## Time and conditions

The photograph of various lamps illustrates the effect of color temperature differences (left to right): (1) Compact Fluorescent: General Electric, 13 watt, 6500 K (2) Incandescent: Sylvania 60-Watt Extra Soft White (3) Compact Fluorescent: Bright Effects, 15 watts, 2644 K, and (4) Compact Fluorescent: Sylvania, 14 watts, 3000 K. Credit: Ramjar.

Many early units of time reflect the astronomical conditions surrounding the observers.

Def. a "period of [fourteen nights;][19] two weeks"[20] is called a fortnight.

Def. a "period of seven nights; a week"[21] is called a sennight.

Def.

1. any "period of seven consecutive days"[22],
2. a period of "seven days beginning with Sunday or Monday"[23],
3. a "subdivision of the month into longer periods of work days punctuated by shorter weekend periods of days for markets, rest, or religious observation such as a sabbath"[24], or
4. seven "days after (sometimes before) a specified date"[25]

is called a week.

In physics, a key value on the time axis for collecting physical data is the starting time. Incandescents reach full brightness a fraction of a second after being switched on.

Eventually it occurred to many of the intelligent life forms on Earth that in addition to where the observations of the natural objects or entities in the sky are taken, also when and how the observations are taken is important. The results of observations change with time, temperature, and other atmospheric conditions.

Many of the early laws begin to make sense and increase understanding of the phenomena observed when coupled to experimental and theoretical studies performed in a laboratory here on Earth under controlled conditions. Astronomy benefits from physics.

Laboratory conditions are often expressed in terms of standard temperature and pressure.

Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

In chemistry, IUPAC established standard temperature and pressure (informally abbreviated as STP) as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm, 1 bar),[26] An unofficial, but commonly used standard is standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm). The STP and the SATP should not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs free energy of a reaction.

"Standard conditions for gases: Temperature, 273.15 K [...] and pressure of 105 pascals. The previous standard absolute pressure of 1 atm (equivalent to 1.01325 × 105 Pa) was changed to 100 kPa in 1982. IUPAC recommends that the former pressure should be discontinued."[26]

NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi, 1 atm). The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (59.00 °F, 15.00 °C) and 101.325 kPa.[27]

## Line of sight

This beautiful galaxy is tilted at an oblique angle on to our line of sight, giving a "birds-eye view" of the spiral structure. Credit: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics.

Def. a “straight line along which an observer has a clear view”[28] is called a line of sight.

In the section on 'senses' above is a demonstration of the principle of 'line of sight'; i.e., "a line from an observer's eye to a distant point toward which [the observer] is looking"[1]. In the image on the left of rain beneath a dark cloud, there is a highway with a vehicle on it. The vehicle is further away from the observer than the right turn onto a side road. Is the blue sky behind the dark cloud? Is the line of trees in the background further away than the dark cloud? Many objects in this image and the others can be layered relative to the observer (some are closer by inspection than others). These layers or strata are strata along the line of sight. The principle of line of sight can be used to make deductions about the relative locations (or positions) of objects from the observer's perspective.

By observing many of the wandering lights in the night sky, an occasional occultation of the light of one astronomical object may occur by the intervention of another along a closer astronomical stratum. On April 25, 1838, an occultation of Mercury by the Moon occurred when Mercury was visible to the unaided eye after sunset.[29] An occultation of Venus by the Moon occurred "on the afternoon of October 14", 1874.[29] An earlier such occultation "occurred on May 23, 1587, and is thus recorded by [Tycho Brahe] in his Historia Celestis"[29]. "Thomas Street, in his Astronomia Carolina (A.D. 1661), mentions three occultations by Venus, being two occasions when the planet covered Regulus, and once when there was an occultation of Mars by Venus."[29] "[Thomas Street] describes [the occultation of Mars by Venus] as follows: "1590,. Oct. 2nd, 16h. 24s. Michael Mœstlin observed ♂ eclipsed by ♀.""[29]

## Coordinates

Cartesian coordinate system with a circle of radius 2 centered at the origin marked in red. The equation of a circle is (x - a)2 + (y - b)2 = r2 where a and b are the coordinates of the center (a, b) and r is the radius. Credit: 345Kai.

A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length. Each reference line is called a coordinate axis or just axis of the system, and the point where they meet is its origin, usually at ordered pair (0,0). The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as signed distances from the origin.

## Distances

Distance along a path is compared in this diagram with displacement. Credit: Stannered.

Distance (or farness) is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, or an estimation based on other criteria (e.g. "two counties over"). In mathematics, a distance function or metric is a generalization of the concept of physical distance. A metric is a function that behaves according to a specific set of rules, and provides a concrete way of describing what it means for elements of some space to be "close to" or "far away from" each other.

## Emptiness

The universe within 1 billion light-years (307 Mpc) of Earth is shown to contain the local superclusters, galaxy filaments and voids. Credit: Richard Powell.

In set theory, emptiness is symbolized by the empty set: a set that contains no elements

Def. the state of being "devoid of content;[30] containing nothing"[31] is called empty.

Free space, a perfect vacuum is expressed in the classical physics model. Vacuum state is a perfect vacuum based on the quantum mechanical model. In mathematical physics, the homogeneous equation may correspond to a physical theory formulated in empty space are disambiguations for "empty space".

In astronomy, voids are the empty spaces between filaments (the largest-scale structures in the Universe), which contain very few, or no, galaxies. Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe.[32]

## Forces

Def. a physical quantity that denotes ability to push, pull, twist or accelerate a body which is measured in a unit dimensioned in mass × distance/time² (ML/T²): SI: newton (N); CGS: dyne (dyn) is called force.

Def. a force associated with nuclear decay is called the weak nuclear force.

Def. a fundamental force that is associated with the strong bonds is called the strong nuclear force.

## Energies

Def. a quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent is called energy.

## Fields

Surface magnetic field of Tau Scorpii is reconstructed by means of Zeeman–Doppler imaging. Credit: Pascalou petit.

Def. a region affected by a particular force is called a field.

Def. a region of space around a charged particle, or between two voltages; it exerts a force on charged objects in its vicinity is called an electric field.

Def. a condition in the space around a magnet or electric current in which there is a detectable magnetic force and two magnetic poles are present is called a magnetic field.

## Gravity

Def. the fundamental force of attraction that exists between all particles with mass in the universe is called gravitation.

## Masses

Def. the "quantity of matter which a body contains, irrespective of its bulk or volume"[33] or a "quantity of matter cohering together so as to make one body, or an aggregation of particles or things which collectively make one body or quantity"[34] is called mass, or a mass.

Def. the "basic structural component of the universe"[2] that "usually has mass and volume"[35] is called matter.

In physics, mass, more specifically inertial mass, can be defined as a quantitative measure of an object's resistance to the change of its speed. In addition to this, gravitational mass can be described as a measure of magnitude of the gravitational force which is

1. exerted by an object (active gravitational mass), or
2. experienced by an object (passive gravitational force)

when interacting with a second object. The SI unit of mass is the kilogram (kg).

## Measurements

A typical tape measure with both metric and US units is shown to measure two US pennies. Credit: Stilfehler.

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the meter, second or degree Celsius.

## Astronomical units

Def. "exactly 149,597,870.691 kilometres" is called one AU.[36]

Def. "86,400 SI seconds (s)" is called 1 day (d).[36]

Notation: let the symbol ${\displaystyle R_{\oplus }}$ indicate the Earth's radius.

Notation: let the symbol ${\displaystyle R_{J}}$ indicate the radius of Jupiter.

Notation: let the symbol ${\displaystyle R_{\odot }}$ indicate the solar radius.

Def. "the mass of the Sun" is called the astronomical unit of mass.[36]

Notation: let the symbol ${\displaystyle M_{\odot }}$ indicate the solar mass.

Def. "1.9891 x 1030 kg" is called the mass of the Sun.[36]

Astronomical observations do not necessarily need the number of kilograms in the mass of the Sun, but rather use the Sun in proportions or ratios versus another astronomical entity, source, object, or body.

The same is true for luminosity. The total or bolometric luminosity of the Sun is the sum of a spectral distribution from radio through gamma rays. Such a distribution may be compared to other luminous astronomical entities, sources, or objects. Differences in spectral distributions may be used to characterize stars. Each sum also is characteristic. The sum is subject to distance. The farther away a star is the smaller the sum of its total spectral distribution. To compare such a sum to the Sun a standard distance of 10 parsecs is used.

Usually, observational astronomy uses radiation to obtain information about astronomical entites, sources, and objects. The most prevalent observations use optical or visual astronomy.

Def. "9,460,730,472,580.8 km" is called the light-year (ly).[36]

Units of Physics and Astronomy
Dimension Astronomy Symbol Physics Symbol Conversion
time 1 day d 1 second s 1 d = 86,400 s[36]
time 1 "Julian year"[37] J 1 second s 1 J = 31,557,600 s
distance 1 astronomical unit AU 1 meter m 1 AU = 149,597,870.691 km[36]
mass 1 Sun Mʘ 1 kilogram kg 1 Mʘ = 1.9891 x 1030 kg[36]
luminosity 1 Sun Lʘ 1 watt W 1 Lʘ = 3.846 x 1026 W[38]
angular distance 1 parsec pc 1 meter m 1 pc ~ 30.857 x 1012 km[36]

## Fissions

In nuclear physics and nuclear chemistry, nuclear fission refers to either a nuclear reaction or a radioactive decay process in which the [atomic nucleus] nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays), and releasing a very large amount of energy, even by the energetic standards of radioactive decay. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes.[39][40] Most fissions are binary fissions, but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced in a ternary fission. The smallest of these ranges in size from a proton to an argon nucleus.

## Fusions

Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy. Fusion is the process that powers active stars, the Teller–Ulam design hydrogen bomb and some experimental devices examining fusion power for electrical generation. The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission. This means that fusion generally occurs for lighter elements only, and likewise, that fission normally occurs only for heavier elements. There are extreme astrophysical events that can lead to short periods of fusion with heavier nuclei. This is the process that gives rise to nucleosynthesis, the creation of the heavy elements during events such as supernovas.

A lead castle is built to shield a radioactive sample. Credit: Changlc.
The electromagnetic spectrum. The red line indicates the room temperature thermal energy. Credit: Opensource Handbook of Nanoscience and Nanotechnology.
This diagram illustrates a special version of a "black body" (instrument), used for defining the luminous intensity unit, before its current scientific International Standard (SI) definition. 1=Radiating cavity 2=Crucible 3=Solidifying platinum (2046 K) Credit: Lex Tollenaar.`{{free media}}`

Def. an action or process of throwing or sending out a traveling ray in a line, beam, or stream of small cross section is called radiation, from radiation astronomy.

The term radiation is often used to refer to the ray itself.

Radiation comes in many forms and energies.

Notation: let various International System of Units, SI prefixes, occur before the unit of energy, the electronvolt, abbreviated as eV.

For example, PeV denotes 1015 eV.

Cosmic rays may be upwards of a ZeV (1021 eV). Ultra high energy neutrons are around an EeV (1018 eV). But, X-rays only range up to about 120 keV, while the visible (visual) range is around 2 eV.

Astronomy likely started with visual astronomy. Visual refers to that portion of the electromagnetic spectrum called the visible spectrum. Probing the sky with additional portions of this spectrum is difficult as the atmosphere absorbs over many portions.

This has produced fields of observational astronomy based on some portions of the electromagnetic spectrum:

Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body.[41][42][43][44]

Rotational superradiance[45] is associated with the acceleration or motion of a nearby body (which supplies the energy and momentum for the effect). It is also sometimes described as the consequence of an "effective" field differential around the body (e.g. the effect of tidal forces). This allows a body with concentration of angular or linear momentum to move towards a lower energy state, even when there is no obvious classical mechanism for this to happen.

“The rotating body [black hole] produces spontaneous pair production [and] in the case when the body can absorb one of the particles, ... the other (anti)particle goes off to infinity and carries away energy and angular momentum.”[46] Such superradiance is called Zel'dovich radiation.

## Hypotheses

1. Observers have been watching the skies and recording what they saw for more than 40,000 years.

## References

1. Philip B. Gove, ed (1963). Webster's Seventh New Collegiate Dictionary. Springfield, Massachusetts: G. & C. Merriam Company. pp. 1221.
2. Emperorbma (20 July 2003). "matter". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
3. Widsith (3 October 2010). "event". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
4. Tormod (13 July 2004). "past". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2015-03-28.
5. ILVI (23 April 2003). "future". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
6. Tormod (12 July 2004). "future". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
7. DAVilla (3 January 2009). "time". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 July 2019.
8. 24.13.132.38 (23 September 2005). "time". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 July 2019.
9. Emperorbma (17 August 2003). "distance". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 24 May 2019.
10. Brya (17 January 2006). "distance". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 24 May 2019.
11. Emperorbma (20 July 2003). "volume". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
12. Widsith (10 October 2012). "space". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
13. Widsith (11 October 2012). "space". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
14. Widsith (26 October 2012). "space". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
15. 160.36.157.140 (11 July 2004). "space". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
16. Spinningspark (9 October 2012). "space". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
17. 149.132.103.69 (14 June 2004). "physics". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
18. Zandperl~enwiktionary (20 October 2003). "physics". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
19. 219.173.119.31 (22 November 2004). "fortnight". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2013-12-25.
20. Merphant (10 January 2003). "fortnight". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2013-12-25.
21. Jtle515 (1 July 2012). "sennight". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2013-12-25.
22. Msh210 (15 February 2012). "week". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
23. Davilla (12 March 2006). "week". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
24. Hippietrail (10 September 2009). "week". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
25. SemperBlotto (21 July 2010). "week". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
26. Alan D. McNaught, Andrew Wilkinson (1997). Compendium of Chemical Terminology, The Gold Book (2nd ed.). Blackwell Science. ISBN 0-86542-684-8.
27. Natural gas – Standard reference conditions (ISO 13443). Geneva, Switzerland: International Organization for Standardization. 1996.
28. Mzajac (27 May 2008). "line of sight". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2012-06-16.
29. Samuel J. Johnson (1874). "Occultations of and by Venus". Astronomical register 12: 268-70.
30. Rammer (29 July 2004). "empty". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
31. Equinox (23 October 2011). "empty". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
32. U. Lindner, J. Einasto, M. Einasto, W. Freudling, K. Fricke, E. Tago (1995). The Structure of Supervoids I: Void Hierarchy in the Northern Local Supervoid "The structure of supervoids. I. Void hierarchy in the Northern Local Supervoid". Astron. Astrophys. 301: 329. http://www.uni-sw.gwdg.de/research/preprints/1995/pr1995_14.html/ The Structure of Supervoids I: Void Hierarchy in the Northern Local Supervoid.
33. Eclecticology (12 September 2003). "mass". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2013-08-12.
34. Emperorbma (14 November 2003). "mass". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2014-02-28.
35. 128.101.220.42 (21 December 2006). "matter". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 23 August 2021.
36. P. K. Seidelmann (1976). "Measuring the Universe The IAU and astronomical units". The International Astronomical Union. Retrieved 2011-11-27.
37. International Astronomical Union "SI units" accessed February 18, 2010. (See Table 5 and section 5.15.) Reprinted from George A. Wilkins & IAU Commission 5, "The IAU Style Manual (1989)" (PDF file) in IAU Transactions Vol. XXB
38. David R. Williams (September 2004). Sun Fact Sheet. Greenbelt, MD: NASA Goddard Space Flight Center. Retrieved 20 December 2011.
39. M. G. Arora and M. Singh (1994). Nuclear Chemistry. Anmol Publications. p. 202. ISBN 81-261-1763-X. Retrieved 2011-04-02.
40. Saha, Gopal (2010). Fundamentals of Nuclear Pharmacy. Springer Science+Business Media. p. 11. ISBN 1-4419-5859-2. Retrieved 2011-04-02.
41. Loudon (2000). "1". The Quantum Theory of Light. .
42. Mandel; Wolf (1995). "13". Optical Coherence and Quantum Optics. .
43. Kondepudi; Prigogine (1998). "11". Modern Thermodynamics: From Heat Engines to Dissipative Structures.
44. Peter Theodore Landsberg (1990). Bosons: black-body radiation, In: Thermodynamics and statistical mechanics (Reprint of Oxford University Press 1978 ed.). Courier Dover Publications. pp. 208 ff. ISBN 0486664937.
45. Jacob Bekenstein; Marcelo Schiffer (1998). "The many faces of superradiance". Physical Review D 58 (6). doi:10.1103/PhysRevD.58.064014.
46. Ya. B. Zel’dovich (1971). JETP Letters 14: 180.