Time dilation in the context of Louis Essen

Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future. Time dictates all forms of action, age, and causality, being a component quantity of various measurements used to sequence events, to compare the duration of events (or the intervals between them), and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.

Time is primarily measured in linear spans or periods, ordered from shortest to longest. Practical, human-scale measurements of time are performed using clocks and calendars, reflecting a 24-hour day collected into a 365-day year linked to the astronomical motion of the Earth. Scientific measurements of time instead vary from Planck time at the shortest to billions of years at the longest. Measurable time is believed to have effectively begun with the Big Bang 13.8 billion years ago, encompassed by the chronology of the universe. Modern physics understands time to be inextricable from space within the concept of spacetime described by general relativity. Time can therefore be dilated by velocity and matter to pass faster or slower for an external observer, though this is considered negligible outside of extreme conditions, namely relativistic speeds or the gravitational pulls of black holes.

The theory of relativity usually encompasses two interrelated physics theories by Albert Einstein: special relativity and general relativity, proposed and published in 1905 and 1915, respectively. Special relativity applies to all physical phenomena in the absence of gravity. General relativity explains the law of gravitation and its relation to the forces of nature. It applies to the cosmological and astrophysical realm, including astronomy.

The theory transformed theoretical physics and astronomy during the 20th century, superseding a 200-year-old theory of mechanics created primarily by Isaac Newton. It introduced concepts including 4-dimensional spacetime as a unified entity of space and time, relativity of simultaneity, kinematic and gravitational time dilation, and length contraction. In the field of physics, relativity improved the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.

Relativistic speed refers to speed at which relativistic effects become significant to the desired accuracy of measurement of the phenomenon being observed. Relativistic effects are those discrepancies between values calculated by models considering and not considering relativity. Related words are velocity, rapidity, and celerity which is proper velocity. Speed is a scalar, being the magnitude of the velocity vector which in relativity is the four-velocity and in three-dimension Euclidean space a three-velocity. Speed is empirically measured as average speed, although current devices in common use can estimate speed over very small intervals and closely approximate instantaneous speed. Non-relativistic discrepancies include cosine error which occurs in speed detection devices when only one scalar component of the three-velocity is measured and the Doppler effect which may affect observations of wavelength and frequency.

Relativistic effects are highly non-linear and for everyday purposes are insignificant because the Newtonian model closely approximates the relativity model. In special relativity the Lorentz factor is a measure of time dilation, length contraction and the relativistic mass increase of a moving object.

A spacetime diagram is a graphical illustration of locations in space at various times, especially in the special theory of relativity. Spacetime diagrams can show the geometry underlying phenomena like time dilation and length contraction without mathematical equations.

The history of an object's location through time traces out a line or curve on a spacetime diagram, referred to as the object's world line. Each point in a spacetime diagram represents a unique position in space and time and is referred to as an event.

Gravitational time dilation is a form of time dilation, an actual difference of elapsed time between two events, as measured by observers situated at varying distances from a gravitating mass. The lower the gravitational potential (the closer the clock is to the source of gravitation), the slower time passes, speeding up as the gravitational potential increases (the clock moving away from the source of gravitation). Albert Einstein originally predicted this in his theory of relativity, and it has since been confirmed by tests of general relativity.

This effect has been demonstrated by noting that atomic clocks at differing altitudes (and thus different gravitational potential) will eventually show different times. The effects detected in such Earth-bound experiments are extremely small, with differences being measured in nanoseconds. Relative to Earth's age in billions of years, Earth's core is in effect 2.5 years younger than its surface. Demonstrating larger effects would require measurements at greater distances from the Earth, or a larger gravitational source.

The Shapiro time delay effect, or gravitational time delay effect, is one of the four classic Solar System tests of general relativity. Radar signals passing near a massive object take slightly longer to travel to a target and longer to return than they would if the mass of the object were not present. The time delay is caused by time dilation, which increases the time it takes light to travel a given distance from the perspective of an outside observer. In a 1964 article entitled Fourth Test of General Relativity, Irwin Shapiro wrote:

Throughout this article discussing the time delay, Shapiro uses c as the speed of light and calculates the time delay of the passage of light waves or rays over finite coordinate distance according to a Schwarzschild solution to the Einstein field equations.

The theory of relativity comprises two physics theories by Albert Einstein: special relativity and general relativity, proposed and published in 1905 and 1915, respectively. Special relativity applies to all physical phenomena in the absence of gravity. General relativity explains the law of gravitation and its relation to the forces of nature. It applies to the cosmological and astrophysical realm, including astronomy.

The theory transformed theoretical physics and astronomy during the 20th century, superseding a 200-year-old theory of mechanics created primarily by Isaac Newton. It introduced concepts including 4-dimensional spacetime as a unified entity of space and time, relativity of simultaneity, kinematic and gravitational time dilation, and length contraction. In the field of physics, relativity improved the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.

Psilocin, also known as 4-hydroxy-N,N-dimethyltryptamine (4-HO-DMT), is a psychedelic drug and fungal alkaloid of the tryptamine and 4-hydroxytryptamine families. Along with its phosphate ester psilocybin, it is found in most species of psilocybin-containing mushrooms, such as Psilocybe cubensis and Psilocybe mexicana, and is the compound responsible for their hallucinogenic effects, although concentrations of psilocin are variably lower than those of psilocybin. The drug is taken orally and its effects include perceptual changes and visual effects, emotional changes, ego dissolution, time dilation, and mystical experiences, among others. Psilocybin, as well as synthetic acyl esters such as 4-AcO-DMT (psilacetin; O-acetylpsilocin) and 4-PrO-DMT (O-propionylpsilocin), are prodrugs of psilocin and have similar properties and effects.

Psilocin acts as a non-selective serotonin receptor agonist, including of the serotonin 5-HT_2A receptor among others. The drug produces its hallucinogenic effects specifically via activation of the serotonin 5-HT_2A receptor. However, other serotonin receptors, such as the serotonin 5-HT_1A and 5-HT_2C receptors, may also contribute to its effects. Notable analogues of psilocin include dimethyltryptamine (DMT), its positional isomer bufotenin (5-HO-DMT), its higher homologue 4-HO-MET (metocin), and others.

The relativistic Doppler effect is the change in frequency, wavelength and amplitude of light, caused by the relative motion of the source and the observer (as in the classical Doppler effect, first proposed by Christian Doppler in 1842), when taking into account effects described by the special theory of relativity.

The relativistic Doppler effect is different from the non-relativistic Doppler effect as the equations include the time dilation effect of special relativity and do not involve the medium of propagation as a reference point. They describe the total difference in observed frequencies and possess the required Lorentz symmetry.

Narnia is a fantasy world created by C. S. Lewis as the primary location for his series of seven fantasy novels for children, The Chronicles of Narnia. The name is derived from the land where much of the Chronicles takes place.

In Narnia, some animals talk, mythical beasts abound, and magic is common. The series tracks the story of Narnia when humans, usually children, enter the Narnian world from Earth. The entire Narnian timeline, from its creation to its end (c. 1–2555), ran parallel to 49 Earth-years (c. 1900–1949), thus on average a year on Earth meant 52.14 time dilation years in Narnia.

Space travel, or space flight (less often, starfaring or star voyaging) is a science fiction theme that has captivated the public and is almost archetypal for science fiction. Space travel, interplanetary or interstellar, is usually performed in space ships, and spacecraft propulsion in various works ranges from the scientifically plausible to the totally fictitious.

While some writers focus on realistic, scientific, and educational aspects of space travel, other writers see this concept as a metaphor for freedom, including "free[ing] mankind from the prison of the solar system". Though the science fiction rocket has been described as a 20th-century icon, according to The Encyclopedia of Science Fiction "The means by which space flight has been achieved in sf – its many and various spaceships – have always been of secondary importance to the mythical impact of the theme". Works related to space travel have popularized such concepts as time dilation, space stations, and space colonization.