An Etymological Dictionary of Astronomy and Astrophysics

1. The time that must be added to the lunar year (12 lunations) to make it coincide with the solar year (about 11 days).
   
2. The moon’s age at the beginning of the calendar year.

Etymology (EN): From Fr. épacte, from L. epacta, from Gk. epaktos, verbal adj. of epagein “to intercalate, add, bring forward,” from epi “on” + ag-, from agein “to bring, to lead;” cf. L. agere “to drive, set in motion,” → act.

Etymology (PE): Barafzâ, from bar- “on, upon, up” (Mid.Pers. abar; O.Pers. upariy “above; over, upon, according to;” Av. upairi “above, over,” upairi.zəma- “located above the earth;” cf. Gk. hyper- “over, above;” L. super-; O.H.G. ubir “over;” PIE base *uper “over”)

• afzâ, afzudan “to add, increase” (Mid.Pers. abzudan “to increase, grow;” O.Pers. abijav- “to increase, add to, promote,” from abi-, aiby- “in addition to; to; against” + root jav- “to press forward;” Av. gav- “to hasten, drive;” Sk. jav- “to press forward, impel quickly, excite,” javate “hastens”).

1. The time that must be added to the lunar year (12 lunations) to make it coincide with the solar year (about 11 days).
   
2. The moon’s age at the beginning of the calendar year.

Etymology (EN): From Fr. épacte, from L. epacta, from Gk. epaktos, verbal adj. of epagein “to intercalate, add, bring forward,” from epi “on” + ag-, from agein “to bring, to lead;” cf. L. agere “to drive, set in motion,” → act.

Etymology (PE): Barafzâ, from bar- “on, upon, up” (Mid.Pers. abar; O.Pers. upariy “above; over, upon, according to;” Av. upairi “above, over,” upairi.zəma- “located above the earth;” cf. Gk. hyper- “over, above;” L. super-; O.H.G. ubir “over;” PIE base *uper “over”)

• afzâ, afzudan “to add, increase” (Mid.Pers. abzudan “to increase, grow;” O.Pers. abijav- “to increase, add to, promote,” from abi-, aiby- “in addition to; to; against” + root jav- “to press forward;” Av. gav- “to hasten, drive;” Sk. jav- “to press forward, impel quickly, excite,” javate “hastens”).

In Old Iranian and Egyptian calendars and much later in the → French Republican Calendar, one of five (or six) days placed between the 30th of the last month and the first day of the new year to result in a fixed year of 365 (366) days every year; plural epagomenae. Same as → epagomenal day. See also → sansculottide.

Etymology (EN): From Gk. epagomenos “added,” from epagein “to add, to intercalate,” from → epi- “on” + agein “to bring, to lead,” → act.

Etymology (PE): Andargâh “intercalary,” literally “time between,” from andar “between, among,” → inter-, + gâh “time;” Mid.Pers. gâh; O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot” (Skt. gátu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come”).
Tarufté “intercalary,” literally “stolen (day);” Mid.Pers. truftag, from taruftan “to steal,” traft “stolen;” Mod.Pers. Lârestâni dialect toftak “spy;” Av. tarəp- “to steal,” tarəfiiāt- “he would steal;” cf. Skt. tarp- “to steal, rob,” paśu.trp- “stealing cattle.”
Dozdidé “intercalary,” literally “stolen (day),” p.p. of dozdidan “to steal,” Mid.Pers. duz(d)itan, from duzd “thief,” from Av. duždāo- “miscreant, villain.”

In Old Iranian and Egyptian calendars and much later in the → French Republican Calendar, one of five (or six) days placed between the 30th of the last month and the first day of the new year to result in a fixed year of 365 (366) days every year; plural epagomenae. Same as → epagomenal day. See also → sansculottide.

Etymology (EN): From Gk. epagomenos “added,” from epagein “to add, to intercalate,” from → epi- “on” + agein “to bring, to lead,” → act.

Etymology (PE): Andargâh “intercalary,” literally “time between,” from andar “between, among,” → inter-, + gâh “time;” Mid.Pers. gâh; O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot” (Skt. gátu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come”).
Tarufté “intercalary,” literally “stolen (day);” Mid.Pers. truftag, from taruftan “to steal,” traft “stolen;” Mod.Pers. Lârestâni dialect toftak “spy;” Av. tarəp- “to steal,” tarəfiiāt- “he would steal;” cf. Skt. tarp- “to steal, rob,” paśu.trp- “stealing cattle.”
Dozdidé “intercalary,” literally “stolen (day),” p.p. of dozdidan “to steal,” Mid.Pers. duz(d)itan, from duzd “thief,” from Av. duždāo- “miscreant, villain.”

Same as → epagomena.

See also: → epagomena + → -al; → day.

Same as → epagomena.

See also: → epagomena + → -al; → day.

A table of computed positions occupied by a celestial body over successive intervals of time such as daily; plural ephemerides.

Etymology (EN): From L. ephemeris “day book, diary,” from Gk. ephemeris “diary, account book,” from ephemeros “short-lived, lasting but a day,” from → epi “on, upon”

• hemerai, dative of hemera “day.”

Etymology (PE): Ruzij, from ruz, → day + zij “astronomical table,” from Mid.Pers. zig “astronomical table,” originally “string,” since the lines of a table were compared to strings used on a weaver’s instrument, variant zih, meaning “cord, string” (Modern Persian zeh “cord, string”); Av. jiiā- “bow-string;” cf. Skt. jiyā- “bow-string;” PIE base *g^whi- “thread, tendon” (from which derive also Gk. bios “bow;” L. filum “thread;” Russ. žca “thread”).

A table of computed positions occupied by a celestial body over successive intervals of time such as daily; plural ephemerides.

Etymology (EN): From L. ephemeris “day book, diary,” from Gk. ephemeris “diary, account book,” from ephemeros “short-lived, lasting but a day,” from → epi “on, upon”

• hemerai, dative of hemera “day.”

Etymology (PE): Ruzij, from ruz, → day + zij “astronomical table,” from Mid.Pers. zig “astronomical table,” originally “string,” since the lines of a table were compared to strings used on a weaver’s instrument, variant zih, meaning “cord, string” (Modern Persian zeh “cord, string”); Av. jiiā- “bow-string;” cf. Skt. jiyā- “bow-string;” PIE base *g^whi- “thread, tendon” (from which derive also Gk. bios “bow;” L. filum “thread;” Russ. žca “thread”).

86,400 → ephemeris seconds.

See also: → ephemeris; → day.

86,400 → ephemeris seconds.

See also: → ephemeris; → day.

A fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by
→ Terrestrial Dynamical Time (TDT).

See also: → ephemeris; → meridian.

A fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by
→ Terrestrial Dynamical Time (TDT).

See also: → ephemeris; → meridian.

The length of a tropical second (1/31,556,925.97474 of the tropical year) on 1900 January 0.5 → ephemeris time.

See also: → ephemeris; → second.

The length of a tropical second (1/31,556,925.97474 of the tropical year) on 1900 January 0.5 → ephemeris time.

See also: → ephemeris; → second.

The uniform time-scale used as the independent variable
to calculate the orbits in the solar system prior to 1984. Ephemeris Time was adopted in 1960 to deal with irregularities in the → Earth’s rotation
that had been found to affect the course of mean solar time. The definition of Ephemeris Time is based on Newcomb’s analytical theory of the Earth’s motion around the Sun (Newcomb 1898), according to which the geometric mean longitude of the Sun with respect to the Earth-Moon barycenter is expressed by:
L = 279° 41’ 48".04 + 129 602 768".13 T + 1’’.089 T²,

where L refers to the → mean equinox of date while T measures time from noon 1900 January 0 GMT in Julian centuries of 36525 days.
Ephemeris Time is therefore defined as
the instant near the beginning of the calendar year A.D. 1900 when the mean longitude of the Sun was 279° 41’ 48’’.04, at which instant the measure of ET was 1900 January 0, 12h precisely. In this system the fundamental unit was the → ephemeris second, which was defined so that the → tropical year at the epoch 1900.0 should be exactly 31 556 925,9747 seconds of ephemerides. Ephemeris Time was inconvenient in many ways and
was supeseded with the → Terrestrial Dynamical Time (TDT), whose fundamental unit is the SI second.

See also: → ephemeris; → time.

The uniform time-scale used as the independent variable
to calculate the orbits in the solar system prior to 1984. Ephemeris Time was adopted in 1960 to deal with irregularities in the → Earth’s rotation
that had been found to affect the course of mean solar time. The definition of Ephemeris Time is based on Newcomb’s analytical theory of the Earth’s motion around the Sun (Newcomb 1898), according to which the geometric mean longitude of the Sun with respect to the Earth-Moon barycenter is expressed by:
L = 279° 41’ 48".04 + 129 602 768".13 T + 1’’.089 T²,

where L refers to the → mean equinox of date while T measures time from noon 1900 January 0 GMT in Julian centuries of 36525 days.
Ephemeris Time is therefore defined as
the instant near the beginning of the calendar year A.D. 1900 when the mean longitude of the Sun was 279° 41’ 48’’.04, at which instant the measure of ET was 1900 January 0, 12h precisely. In this system the fundamental unit was the → ephemeris second, which was defined so that the → tropical year at the epoch 1900.0 should be exactly 31 556 925,9747 seconds of ephemerides. Ephemeris Time was inconvenient in many ways and
was supeseded with the → Terrestrial Dynamical Time (TDT), whose fundamental unit is the SI second.

See also: → ephemeris; → time.

The passage of a celestial body or point across the → ephemeris meridian.

See also: → ephemeris; → transit.

The passage of a celestial body or point across the → ephemeris meridian.

See also: → ephemeris; → transit.

Prefix meaning “upon, at, close upon (in space or time), on the occasion of, in addition.”

Etymology (EN): Gk. epi- “upon, at, close upon (in space or time), on the occasion of, in addition,” cognate with O.Pers./Av. apiy-, aipi- “upon, toward, along; also; however;” Skt. api “also, besides.”

Etymology (PE): Prefix api-, from O.Pers./Av. apiy-, aipi-, as above.

Prefix meaning “upon, at, close upon (in space or time), on the occasion of, in addition.”

Etymology (EN): Gk. epi- “upon, at, close upon (in space or time), on the occasion of, in addition,” cognate with O.Pers./Av. apiy-, aipi- “upon, toward, along; also; however;” Skt. api “also, besides.”

Etymology (PE): Prefix api-, from O.Pers./Av. apiy-, aipi-, as above.

1. In → Ptolemaic system, a circular → orbit of a body around a point that itself orbits circularly another point. Such a system was formulated to explain some → planetary orbits in terms of → circular motions in a → geocentric cosmology.
   

2a) Math.: A circle that rolls, externally or internally on another circle, generating an → epicycloid or → hypocycloid.

2b) In → galactic dynamics models describing the → spiral arms, a → perturbation of simple circular orbits. → epicyclic theory.

Etymology (EN): → epi-; → cycle.

Etymology (PE): 1) Falak-e tadvir, from Ar. falak al-tadwir, from falak “sphere” + tadwir “causing to turn in a circle.”

2. → epi-; → cycle.

1. In → Ptolemaic system, a circular → orbit of a body around a point that itself orbits circularly another point. Such a system was formulated to explain some → planetary orbits in terms of → circular motions in a → geocentric cosmology.
   

2a) Math.: A circle that rolls, externally or internally on another circle, generating an → epicycloid or → hypocycloid.

2b) In → galactic dynamics models describing the → spiral arms, a → perturbation of simple circular orbits. → epicyclic theory.

Etymology (EN): → epi-; → cycle.

Etymology (PE): 1) Falak-e tadvir, from Ar. falak al-tadwir, from falak “sphere” + tadwir “causing to turn in a circle.”

2. → epi-; → cycle.

Of or pertaining to an → epicycle.

See also: → epicycle; → -ic.

Of or pertaining to an → epicycle.

See also: → epicycle; → -ic.

In the → epicyclic theory of Galactic rotation, the frequency at which a star in the → Galactic disk describes an ellipse around its mean circular orbit. The epicyclic frequency relates to the → Oort’s constants. In the solar neighborhood the epicyclic frequency is about 32 km s^-1 kpc^-1.

See also: → epicyclic; → frequency.

In the → epicyclic theory of Galactic rotation, the frequency at which a star in the → Galactic disk describes an ellipse around its mean circular orbit. The epicyclic frequency relates to the → Oort’s constants. In the solar neighborhood the epicyclic frequency is about 32 km s^-1 kpc^-1.

See also: → epicyclic; → frequency.

In a → disk galaxy, the motion of a star about the orbital → guiding center when it is displaced radially. See also → epicyclic frequency, → epicyclic theory.

See also: → epicyclic; → oscillation.

In a → disk galaxy, the motion of a star about the orbital → guiding center when it is displaced radially. See also → epicyclic frequency, → epicyclic theory.

See also: → epicyclic; → oscillation.

The theory that describes the Galactic dynamics, that is the orbits of stars and gas clouds in the → Galactic disk, as well as the spiral → density wave. Formulated by Bertil Lindblad (1895-1965), the epicyclic theory assumes that orbits are circular with small deviations. Star orbits are described by the superposition of two motions: i) a rotation of the star (epicenter) around the Galactic center at the circular angular velocity, Ω, and ii) a retrograde elliptical motion at → epicyclic frequency, κ. The epicyclic motion in the Galactic plane occurs in a retrograde sense to conserve → angular momentum. In general Ω and κ are different and, therefore, orbits do not close. However, seen by an
observer who rotates with the epicenter, orbits are closed ellipses.

See also: → epicyclic; → theory.

The theory that describes the Galactic dynamics, that is the orbits of stars and gas clouds in the → Galactic disk, as well as the spiral → density wave. Formulated by Bertil Lindblad (1895-1965), the epicyclic theory assumes that orbits are circular with small deviations. Star orbits are described by the superposition of two motions: i) a rotation of the star (epicenter) around the Galactic center at the circular angular velocity, Ω, and ii) a retrograde elliptical motion at → epicyclic frequency, κ. The epicyclic motion in the Galactic plane occurs in a retrograde sense to conserve → angular momentum. In general Ω and κ are different and, therefore, orbits do not close. However, seen by an
observer who rotates with the epicenter, orbits are closed ellipses.

See also: → epicyclic; → theory.

A curve traced by a point of a circle that rolls on the outside of a fixed circle. This curve was described by the Gk. mathematicians and astronomer Hipparchus, who made use of it to account for the apparent movement of many of the heavenly bodies.

See also: → epi-; → cycloid.

A curve traced by a point of a circle that rolls on the outside of a fixed circle. This curve was described by the Gk. mathematicians and astronomer Hipparchus, who made use of it to account for the apparent movement of many of the heavenly bodies.

See also: → epi-; → cycloid.

The fifth of → Saturn’s known satellites. It has a
mean radius of 55 x 69 km and orbits its planet at a mean distance of 151,422 km. It shares the same → horseshoe orbit with → Janus. Epimetheus was discovered by Richard L. Walker in 1966. Also known as Saturn XI.

See also: In Gk. mythology, brother of → Prometheus and → Atlas, and husband of → Pandora. His task was to populate the Earth with animals.

The fifth of → Saturn’s known satellites. It has a
mean radius of 55 x 69 km and orbits its planet at a mean distance of 151,422 km. It shares the same → horseshoe orbit with → Janus. Epimetheus was discovered by Richard L. Walker in 1966. Also known as Saturn XI.

See also: In Gk. mythology, brother of → Prometheus and → Atlas, and husband of → Pandora. His task was to populate the Earth with animals.

A → morphism f : Y → X if, for any two morphisms u,v : X → Z, u f = v f  implies u = v.

See also: → epi-; → morphism.

A → morphism f : Y → X if, for any two morphisms u,v : X → Z, u f = v f  implies u = v.

See also: → epi-; → morphism.

1. An incident in the course of a series of events.
   

2. An incident, scene, etc., within a narrative, usually fully developed and either integrated within the main story or digressing from it (Dictionary.com).

Etymology (EN): From Fr. épisode from Gk. epeisodion “addition,” noun use of neuter of epeisodios “coming in besides,” from → epi- “in addition” + eisodos “a coming in, entrance” (from eis“into” + hodos “way,” → period).

Etymology (PE): Apyâ, literally “coming in besides,” from api-, → epi-,

• â- present stem of âmadan “to come,” → rise.

1. An incident in the course of a series of events.
   

2. An incident, scene, etc., within a narrative, usually fully developed and either integrated within the main story or digressing from it (Dictionary.com).

Etymology (EN): From Fr. épisode from Gk. epeisodion “addition,” noun use of neuter of epeisodios “coming in besides,” from → epi- “in addition” + eisodos “a coming in, entrance” (from eis“into” + hodos “way,” → period).

Etymology (PE): Apyâ, literally “coming in besides,” from api-, → epi-,

• â- present stem of âmadan “to come,” → rise.

1. Pertaining to or of the nature of an episode.
   

2. Divided into separate or tenuously related parts or sections.
   

3. Occurring sporadically or incidentally (Dictionary.com).

See also: → episode; → -ic.

1. Pertaining to or of the nature of an episode.
   

2. Divided into separate or tenuously related parts or sections.
   

3. Occurring sporadically or incidentally (Dictionary.com).

See also: → episode; → -ic.

A branch of philosophy that investigates the possibility, origins, nature, methods, and limits of human knowledge.

Etymology (EN): From Gk. episteme “knowledge,” from Ionic Gk. epistasthai “to understand,” literally “overstand,” from → epi- “over, near” + histasthai “to stand;” cognate with Pers. istâdan “to stand,” → standard; PIE base *sta- “to stand.”

Etymology (PE): From šenaxt, → knowledge, +
-šenâsi, → -logy.

A branch of philosophy that investigates the possibility, origins, nature, methods, and limits of human knowledge.

Etymology (EN): From Gk. episteme “knowledge,” from Ionic Gk. epistasthai “to understand,” literally “overstand,” from → epi- “over, near” + histasthai “to stand;” cognate with Pers. istâdan “to stand,” → standard; PIE base *sta- “to stand.”

Etymology (PE): From šenaxt, → knowledge, +
-šenâsi, → -logy.

1. The date for which → orbital elements or the positions of celestial objects are calculated. Specifying the epoch is important because the apparent positions of objects in the sky change gradually due to → precession and → nutation, while orbital elements change due to the gravitational effects of the → planets. The → standard epoch used in ephemerides (→ ephemeris) and stellar catalogues at present is January 1, 2000, 12h (written also as 2000.0). See also: → Julian epoch.
   

2. Same as → cosmological epoch, such as → current cosmological epoch, → electroweak epoch, → epoch of thermalization, → recombination epoch, → reionization epoch.
   

3. A period of time usually marked by some distinctive development or series of events. See also: → polarity epoch, → epoch angle.

Etymology (EN): From M.L. epocha, from Gk. epokhe “pause, cessation, fixed point,” from epekhein “to pause, take up a position,” from epi- “on” + ekhein “to hold, to have;” cf. Av. hazah- “power, violence, superiority;” Skt. sahate “he masters,” sáhas- “power, violence, might;” Goth. sigis; O.H.G. sigu; O.E. sige “victory;” PIE base *segh- “to hold.”

Etymology (PE): Zime, from Mid.Pers. zim “time, year, winter,” from Av. zyam-, zayan- “winter,” probably related to zaman “time” + nuance suffix -é.

1. The date for which → orbital elements or the positions of celestial objects are calculated. Specifying the epoch is important because the apparent positions of objects in the sky change gradually due to → precession and → nutation, while orbital elements change due to the gravitational effects of the → planets. The → standard epoch used in ephemerides (→ ephemeris) and stellar catalogues at present is January 1, 2000, 12h (written also as 2000.0). See also: → Julian epoch.
   

2. Same as → cosmological epoch, such as → current cosmological epoch, → electroweak epoch, → epoch of thermalization, → recombination epoch, → reionization epoch.
   

3. A period of time usually marked by some distinctive development or series of events. See also: → polarity epoch, → epoch angle.

Etymology (EN): From M.L. epocha, from Gk. epokhe “pause, cessation, fixed point,” from epekhein “to pause, take up a position,” from epi- “on” + ekhein “to hold, to have;” cf. Av. hazah- “power, violence, superiority;” Skt. sahate “he masters,” sáhas- “power, violence, might;” Goth. sigis; O.H.G. sigu; O.E. sige “victory;” PIE base *segh- “to hold.”

Etymology (PE): Zime, from Mid.Pers. zim “time, year, winter,” from Av. zyam-, zayan- “winter,” probably related to zaman “time” + nuance suffix -é.

Same as the → initial phase angle.

See also: → epoch; → angle.

Same as the → initial phase angle.

See also: → epoch; → angle.

→ reionization epoch.

See also: → epoch; → reionization.

→ reionization epoch.

See also: → epoch; → reionization.

The period during the → early Universe before the → recombination era when the photons were hot enough to ionize hydrogen. The density was so high that the interactions between → matter and → radiation were very numerous. Therefore, matter and photons were in constant contact and their → temperatures were the same. As a result, the radiation became → thermalized, i.e. the → electromagnetic spectrum of the radiation became that of a → blackbody, a process called → thermalization. Since the time of recombination the photons of → cosmic background radiation have been free to travel uninhibited by interactions with matter. Thus, their distribution of energy is a perfect → blackbody curve, as predicted by the → Big Bang theory and shown by several observations, such as → Cosmic Background Explorer (COBE), → Wilkinson Microwave Anisotropy Probe (WMAP), and → Planck Satellite.

See also: → epoch; → thermalization.

The period during the → early Universe before the → recombination era when the photons were hot enough to ionize hydrogen. The density was so high that the interactions between → matter and → radiation were very numerous. Therefore, matter and photons were in constant contact and their → temperatures were the same. As a result, the radiation became → thermalized, i.e. the → electromagnetic spectrum of the radiation became that of a → blackbody, a process called → thermalization. Since the time of recombination the photons of → cosmic background radiation have been free to travel uninhibited by interactions with matter. Thus, their distribution of energy is a perfect → blackbody curve, as predicted by the → Big Bang theory and shown by several observations, such as → Cosmic Background Explorer (COBE), → Wilkinson Microwave Anisotropy Probe (WMAP), and → Planck Satellite.

See also: → epoch; → thermalization.

A thought experiment developed in 1935 by A. Einstein (1879-1955), Boris Podolsky (1896-1966), and Nathan Rosen (1909-1995) to demonstrate that there is a fundamental inconsistency in → quantum mechanics.

They imagined two physical systems that are allowed to interact initially so that they will subsequently be defined by a single quantum mechanical state. For example, a neutral → pion at rest which decays into a pair of → photons. The pair of photons is described by a single two-particle → wave function. Once separated, the two photons are still described by the same wave function, and a measurement of one → observable of the first system will determine the measurement of the corresponding observable of the second system. For example, if photon 1 is found to have → spin up along the x-axis, then photon 2 must have spin down along the x-axis, since the final total → angular momentum of the two-photon system must be the same as the angular momentum of the initial state. This means that we know the spin of photon 2 even without measuring it. Likewise, the measurement of another observable of the first system will determine the measurement of the corresponding observable of the second system, even though the systems are no longer physically linked in the traditional sense of local coupling (→ quantum entanglement).

So, EPR argued that quantum mechanics was not a complete theory, but it could be corrected by postulating the existence of → hidden variables that furthermore would be “local”. According to EPR, the specification of these local hidden parameters would predetermine the result of measuring any observable of the physical system. However, in 1964 John S. Bell developed a theorem, → Bell’s inequality, to test for the existence of these hidden variables.
He showed that if the inequality was satisfied, then no local hidden variable theory can reproduce the predictions of quantum mechanics. → Aspect experiment.

See also: A. Einstein, B. Podolsky, N. Rosen: “Can quantum-mechanical description of physical reality be considered complete?” Phys. Rev. 41, 777 (15 May 1935); → paradox.

A thought experiment developed in 1935 by A. Einstein (1879-1955), Boris Podolsky (1896-1966), and Nathan Rosen (1909-1995) to demonstrate that there is a fundamental inconsistency in → quantum mechanics.

They imagined two physical systems that are allowed to interact initially so that they will subsequently be defined by a single quantum mechanical state. For example, a neutral → pion at rest which decays into a pair of → photons. The pair of photons is described by a single two-particle → wave function. Once separated, the two photons are still described by the same wave function, and a measurement of one → observable of the first system will determine the measurement of the corresponding observable of the second system. For example, if photon 1 is found to have → spin up along the x-axis, then photon 2 must have spin down along the x-axis, since the final total → angular momentum of the two-photon system must be the same as the angular momentum of the initial state. This means that we know the spin of photon 2 even without measuring it. Likewise, the measurement of another observable of the first system will determine the measurement of the corresponding observable of the second system, even though the systems are no longer physically linked in the traditional sense of local coupling (→ quantum entanglement).

So, EPR argued that quantum mechanics was not a complete theory, but it could be corrected by postulating the existence of → hidden variables that furthermore would be “local”. According to EPR, the specification of these local hidden parameters would predetermine the result of measuring any observable of the physical system. However, in 1964 John S. Bell developed a theorem, → Bell’s inequality, to test for the existence of these hidden variables.
He showed that if the inequality was satisfied, then no local hidden variable theory can reproduce the predictions of quantum mechanics. → Aspect experiment.

See also: A. Einstein, B. Podolsky, N. Rosen: “Can quantum-mechanical description of physical reality be considered complete?” Phys. Rev. 41, 777 (15 May 1935); → paradox.