HOME
The Info List - Time In Physics


--- Advertisement ---



Time
Time
in physics is defined by its measurement: time is what a clock reads.[1] In classical, non-relativistic physics it is a scalar quantity and, like length, mass, and charge, is usually described as a fundamental quantity. Time
Time
can be combined mathematically with other physical quantities to derive other concepts such as motion, kinetic energy and time-dependent fields. Timekeeping
Timekeeping
is a complex of technological and scientific issues, and part of the foundation of recordkeeping.

Contents

1 Markers of time 2 The unit of measurement of time: the second

2.1 The state of the art in timekeeping

3 Conceptions of time

3.1 Regularities in nature

3.1.1 Mechanical clocks

3.2 Galileo: the flow of time 3.3 Newton's physics: linear time 3.4 Thermodynamics and the paradox of irreversibility 3.5 Electromagnetism
Electromagnetism
and the speed of light 3.6 Einstein's physics: spacetime 3.7 Time
Time
in quantum mechanics

4 Dynamical systems 5 Signalling 6 Technology for timekeeping standards 7 Time
Time
in cosmology 8 Reprise 9 See also 10 References 11 Further reading

Markers of time[edit] Main article: History
History
of timekeeping devices Before there were clocks, time was measured by those physical processes[2] which were understandable to each epoch of civilization:[3]

the first appearance (see: heliacal rising) of Sirius
Sirius
to mark the flooding of the Nile each year[3] the periodic succession of night and day, seemingly eternally[4] the position on the horizon of the first appearance of the sun at dawn[5] the position of the sun in the sky[6] the marking of the moment of noontime during the day[7] the length of the shadow cast by a gnomon[8]

Eventually,[9][10] it became possible to characterize the passage of time with instrumentation, using operational definitions. Simultaneously, our conception of time has evolved, as shown below.[11] The unit of measurement of time: the second[edit] In the International System of Units
International System of Units
(SI), the unit of time is the second (symbol:

s

displaystyle mathrm s

). It is a SI base unit, and it is currently defined as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". [12] This definition is based on the operation of a caesium atomic clock. The state of the art in timekeeping[edit]

Prerequisites

Measurement Scientific notation Natural units

The UTC timestamp in use worldwide is an atomic time standard. The relative accuracy of such a time standard is currently on the order of 10−15[13] (corresponding to 1 second in approximately 30 million years). The smallest time step considered observable is called the Planck time, which is approximately 5.391×10−44 seconds - many orders of magnitude below the resolution of current time standards. Conceptions of time[edit] Main article: Time

Andromeda galaxy (M31) is two million light-years away. Thus we are viewing M31's light from two million years ago,[14] a time before humans existed on Earth.

Both Galileo
Galileo
and Newton and most people up until the 20th century thought that time was the same for everyone everywhere. This is the basis for timelines, where time is a parameter. Our modern conception of time is based on Einstein's theory of relativity, in which rates of time run differently depending on relative motion, and space and time are merged into spacetime, where we live on a world line rather than a timeline. In this view time is a coordinate. According to the prevailing cosmological model of the Big Bang
Big Bang
theory time itself began as part of the entire Universe
Universe
about 13.8 billion years ago. Regularities in nature[edit] Main article: History
History
of science In order to measure time, one can record the number of occurrences (events) of some periodic phenomenon. The regular recurrences of the seasons, the motions of the sun, moon and stars were noted and tabulated for millennia, before the laws of physics were formulated. The sun was the arbiter of the flow of time, but time was known only to the hour for millennia, hence, the use of the gnomon was known across most of the world, especially Eurasia, and at least as far southward as the jungles of Southeast Asia.[15] In particular, the astronomical observatories maintained for religious purposes became accurate enough to ascertain the regular motions of the stars, and even some of the planets. At first, timekeeping was done by hand by priests, and then for commerce, with watchmen to note time as part of their duties. The tabulation of the equinoxes, the sandglass, and the water clock became more and more accurate, and finally reliable. For ships at sea, boys were used to turn the sandglasses and to call the hours. Mechanical clocks[edit] Richard of Wallingford
Richard of Wallingford
(1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.[16][17] By the time of Richard of Wallingford, the use of ratchets and gears allowed the towns of Europe to create mechanisms to display the time on their respective town clocks; by the time of the scientific revolution, the clocks became miniaturized enough for families to share a personal clock, or perhaps a pocket watch. At first, only kings could afford them. Pendulum clocks were widely used in the 18th and 19th century. They have largely been replaced in general use by quartz and digital clocks. Atomic clocks
Atomic clocks
can theoretically keep accurate time for millions of years. They are appropriate for standards and scientific use. Galileo: the flow of time[edit] Main article: reproducibility In 1583, Galileo
Galileo
Galilei (1564–1642) discovered that a pendulum's harmonic motion has a constant period, which he learned by timing the motion of a swaying lamp in harmonic motion at mass at the cathedral of Pisa, with his pulse.[18] In his Two New Sciences
Two New Sciences
(1638), Galileo
Galileo
used a water clock to measure the time taken for a bronze ball to roll a known distance down an inclined plane; this clock was

"a large vessel of water placed in an elevated position; to the bottom of this vessel was soldered a pipe of small diameter giving a thin jet of water, which we collected in a small glass during the time of each descent, whether for the whole length of the channel or for a part of its length; the water thus collected was weighed, after each descent, on a very accurate balance; the differences and ratios of these weights gave us the differences and ratios of the times, and this with such accuracy that although the operation was repeated many, many times, there was no appreciable discrepancy in the results."[19]

Galileo's experimental setup to measure the literal flow of time, in order to describe the motion of a ball, preceded Isaac Newton's statement in his Principia:

I do not define time, space, place and motion, as being well known to all.[20]

The Galilean transformations assume that time is the same for all reference frames. Newton's physics: linear time[edit] Main article: classical physics In or around 1665, when Isaac Newton
Isaac Newton
(1643–1727) derived the motion of objects falling under gravity, the first clear formulation for mathematical physics of a treatment of time began: linear time, conceived as a universal clock.

Absolute, true, and mathematical time, of itself, and from its own nature flows equably without regard to anything external, and by another name is called duration: relative, apparent, and common time, is some sensible and external (whether accurate or unequable) measure of duration by the means of motion, which is commonly used instead of true time; such as an hour, a day, a month, a year.[21]

The water clock mechanism described by Galileo
Galileo
was engineered to provide laminar flow of the water during the experiments, thus providing a constant flow of water for the durations of the experiments, and embodying what Newton called duration. In this section, the relationships listed below treat time as a parameter which serves as an index to the behavior of the physical system under consideration. Because Newton's fluents treat a linear flow of time (what he called mathematical time), time could be considered to be a linearly varying parameter, an abstraction of the march of the hours on the face of a clock. Calendars and ship's logs could then be mapped to the march of the hours, days, months, years and centuries.

Prerequisites

differential equations partial differential equations

Thermodynamics and the paradox of irreversibility[edit] Main article: arrow of time By 1798, Benjamin Thompson
Benjamin Thompson
(1753–1814) had discovered that work could be transformed to heat without limit - a precursor of the conservation of energy or

1st law of thermodynamics

In 1824 Sadi Carnot (1796–1832) scientifically analyzed the steam engines with his Carnot cycle, an abstract engine. Rudolf Clausius (1822–1888) noted a measure of disorder, or entropy, which affects the continually decreasing amount of free energy which is available to a Carnot engine in the:

2nd law of thermodynamics

Thus the continual march of a thermodynamic system, from lesser to greater entropy, at any given temperature, defines an arrow of time. In particular, Stephen Hawking
Stephen Hawking
identifies three arrows of time:[22]

Psychological arrow of time - our perception of an inexorable flow. Thermodynamic arrow of time - distinguished by the growth of entropy. Cosmological arrow of time - distinguished by the expansion of the universe.

Entropy
Entropy
is maximum in an isolated thermodynamic system, and increases. In contrast, Erwin Schrödinger
Erwin Schrödinger
(1887–1961) pointed out that life depends on a "negative entropy flow".[23] Ilya Prigogine
Ilya Prigogine
(1917–2003) stated that other thermodynamic systems which, like life, are also far from equilibrium, can also exhibit stable spatio-temporal structures. Soon afterward, the Belousov-Zhabotinsky reactions[24] were reported, which demonstrate oscillating colors in a chemical solution.[25] These nonequilibrium thermodynamic branches reach a bifurcation point, which is unstable, and another thermodynamic branch becomes stable in its stead.[26] Electromagnetism
Electromagnetism
and the speed of light[edit] Main article: Maxwell's equations In 1864, James Clerk Maxwell
James Clerk Maxwell
(1831–1879) presented a combined theory of electricity and magnetism. He combined all the laws then known relating to those two phenomenon into four equations. These vector calculus equations which use the del operator (

displaystyle nabla

) are known as Maxwell's equations
Maxwell's equations
for electromagnetism. In free space (that is, space not containing electric charges), the equations take the form (using SI units):[27]

Prerequisites

vector notation partial differential equations

∇ ×

E

= −

B

∂ t

displaystyle nabla times mathbf E =- frac partial mathbf B partial t

∇ ×

B

=

μ

0

ε

0

E

∂ t

=

1

c

2

E

∂ t

displaystyle nabla times mathbf B =mu _ 0 varepsilon _ 0 frac partial mathbf E partial t = frac 1 c^ 2 frac partial mathbf E partial t

∇ ⋅

E

= 0

displaystyle nabla cdot mathbf E =0

∇ ⋅

B

= 0

displaystyle nabla cdot mathbf B =0

where

ε0 and μ0 are the electric permittivity and the magnetic permeability of free space; c =

1

/

ϵ

0

μ

0

displaystyle 1/ sqrt epsilon _ 0 mu _ 0

is the speed of light in free space, 299 792 458 m/s; E is the electric field; B is the magnetic field.

These equations allow for solutions in the form of electromagnetic waves. The wave is formed by an electric field and a magnetic field oscillating together, perpendicular to each other and to the direction of propagation. These waves always propagate at the speed of light c, regardless of the velocity of the electric charge that generated them. The fact that light is predicted to always travel at speed c would be incompatible with Galilean relativity if Maxwell's equations
Maxwell's equations
were assumed to hold in any inertial frame (reference frame with constant velocity), because the Galilean transformations predict the speed to decrease (or increase) in the reference frame of an observer traveling parallel (or antiparallel) to the light. It was expected that there was one absolute reference frame, that of the luminiferous aether, in which Maxwell's equations
Maxwell's equations
held unmodified in the known form. The Michelson-Morley experiment
Michelson-Morley experiment
failed to detect any difference in the relative speed of light due to the motion of the Earth
Earth
relative to the luminiferous aether, suggesting that Maxwell's equations
Maxwell's equations
did, in fact, hold in all frames. In 1875, Hendrik Lorentz
Hendrik Lorentz
(1853–1928) discovered Lorentz transformations, which left Maxwell's equations
Maxwell's equations
unchanged, allowing Michelson and Morley's negative result to be explained. Henri Poincaré (1854–1912) noted the importance of Lorentz's transformation and popularized it. In particular, the railroad car description can be found in Science and Hypothesis,[28] which was published before Einstein's articles of 1905. The Lorentz transformation
Lorentz transformation
predicted space contraction and time dilation; until 1905, the former was interpreted as a physical contraction of objects moving with respect to the aether, due to the modification of the intermolecular forces (of electric nature), while the latter was thought to be just a mathematical stipulation.[citation needed] Einstein's physics: spacetime[edit] Main articles: Special relativity
Special relativity
and General relativity Albert Einstein's 1905 special relativity challenged the notion of absolute time, and could only formulate a definition of synchronization for clocks that mark a linear flow of time:

If at the point A of space there is a clock, an observer at A can determine the time values of events in the immediate proximity of A by finding the positions of the hands which are simultaneous with these events. If there is at the point B of space another clock in all respects resembling the one at A, it is possible for an observer at B to determine the time values of events in the immediate neighbourhood of B. But it is not possible without further assumption to compare, in respect of time, an event at A with an event at B. We have so far defined only an "A time" and a "B time." We have not defined a common "time" for A and B, for the latter cannot be defined at all unless we establish by definition that the "time" required by light to travel from A to B equals the "time" it requires to travel from B to A. Let a ray of light start at the "A time" tA from A towards B, let it at the "B time" tB be reflected at B in the direction of A, and arrive again at A at the “A time” t′A. In accordance with definition the two clocks synchronize if

t

B

t

A

=

t

A

t

B

.

displaystyle t_ text B -t_ text A =t'_ text A -t_ text B text . ,!

We assume that this definition of synchronism is free from contradictions, and possible for any number of points; and that the following relations are universally valid:—

If the clock at B synchronizes with the clock at A, the clock at A synchronizes with the clock at B. If the clock at A synchronizes with the clock at B and also with the clock at C, the clocks at B and C also synchronize with each other.

— Albert Einstein, "On the Electrodynamics of Moving Bodies" [29]

Einstein showed that if the speed of light is not changing between reference frames, space and time must be so that the moving observer will measure the same speed of light as the stationary one because velocity is defined by space and time:

v

=

d

r

d t

,

displaystyle mathbf v = dmathbf r over dt text ,

where r is position and t is time.

Indeed, the Lorentz transformation
Lorentz transformation
(for two reference frames in relative motion, whose x axis is directed in the direction of the relative velocity)

Prerequisites

algebra trigonometry

t ′

= γ ( t − v x

/

c

2

)

 where 

γ = 1

/

1 −

v

2

/

c

2

x ′

= γ ( x − v t )

y ′

= y

z ′

= z

displaystyle begin cases t'&=gamma (t-vx/c^ 2 ) text where gamma =1/ sqrt 1-v^ 2 /c^ 2 \x'&=gamma (x-vt)\y'&=y\z'&=zend cases

can be said to "mix" space and time in a way similar to the way a Euclidean rotation around the z axis mixes x and y coordinates. Consequences of this include relativity of simultaneity.

Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame.

More specifically, the Lorentz transformation
Lorentz transformation
is a hyperbolic rotation

(

c

t ′

x ′

)

=

(

cosh ⁡ ϕ

− sinh ⁡ ϕ

− sinh ⁡ ϕ

cosh ⁡ ϕ

)

(

c t

x

)

 where 

ϕ = artanh

v c

,

displaystyle begin pmatrix ct'\x'end pmatrix = begin pmatrix cosh phi &-sinh phi \-sinh phi &cosh phi end pmatrix begin pmatrix ct\xend pmatrix text where phi =operatorname artanh , frac v c text ,

which is a change of coordinates in the four-dimensional Minkowski space, a dimension of which is ct. (In Euclidean space
Euclidean space
an ordinary rotation

(

x ′

y ′

)

=

(

cos ⁡ θ

− sin ⁡ θ

sin ⁡ θ

cos ⁡ θ

)

(

x

y

)

displaystyle begin pmatrix x'\y'end pmatrix = begin pmatrix cos theta &-sin theta \sin theta &cos theta end pmatrix begin pmatrix x\yend pmatrix

is the corresponding change of coordinates.) The speed of light c can be seen as just a conversion factor needed because we measure the dimensions of spacetime in different units; since the metre is currently defined in terms of the second, it has the exact value of 299 792 458 m/s. We would need a similar factor in Euclidean space if, for example, we measured width in nautical miles and depth in feet. In physics, sometimes units of measurement in which c = 1 are used to simplify equations. Time
Time
in a "moving" reference frame is shown to run more slowly than in a "stationary" one by the following relation (which can be derived by the Lorentz transformation
Lorentz transformation
by putting ∆x′ = 0, ∆τ = ∆t′):

Δ t =

Δ τ

1 −

v

2

/

c

2

displaystyle Delta t= Delta tau over sqrt 1-v^ 2 /c^ 2

where:

∆τ is the time between two events as measured in the moving reference frame in which they occur at the same place (e.g. two ticks on a moving clock); it is called the proper time between the two events; ∆t is the time between these same two events, but as measured in the stationary reference frame; v is the speed of the moving reference frame relative to the stationary one; c is the speed of light.

Moving objects therefore are said to show a slower passage of time. This is known as time dilation. These transformations are only valid for two frames at constant relative velocity. Naively applying them to other situations gives rise to such paradoxes as the twin paradox. That paradox can be resolved using for instance Einstein's General theory of relativity, which uses Riemannian geometry, geometry in accelerated, noninertial reference frames. Employing the metric tensor which describes Minkowski space:

[

( d

x

1

)

2

+ ( d

x

2

)

2

+ ( d

x

3

)

2

− c ( d t

)

2

)

]

,

displaystyle left[(dx^ 1 )^ 2 +(dx^ 2 )^ 2 +(dx^ 3 )^ 2 -c(dt)^ 2 )right],

Einstein developed a geometric solution to Lorentz's transformation that preserves Maxwell's equations. His field equations give an exact relationship between the measurements of space and time in a given region of spacetime and the energy density of that region. Einstein's equations predict that time should be altered by the presence of gravitational fields (see the Schwarzschild metric):

T =

d t

(

1 −

2 G M

r

c

2

)

d

t

2

1

c

2

(

1 −

2 G M

r

c

2

)

− 1

d

r

2

r

2

c

2

d

θ

2

r

2

c

2

sin

2

⁡ θ

d

ϕ

2

displaystyle T= frac dt sqrt left(1- frac 2GM rc^ 2 right)dt^ 2 - frac 1 c^ 2 left(1- frac 2GM rc^ 2 right)^ -1 dr^ 2 - frac r^ 2 c^ 2 dtheta ^ 2 - frac r^ 2 c^ 2 sin ^ 2 theta ;dphi ^ 2

Where:

T

displaystyle T

is the gravitational time dilation of an object at a distance of

r

displaystyle r

.

d t

displaystyle dt

is the change in coordinate time, or the interval of coordinate time.

G

displaystyle G

is the gravitational constant

M

displaystyle M

is the mass generating the field

(

1 −

2 G M

r

c

2

)

d

t

2

1

c

2

(

1 −

2 G M

r

c

2

)

− 1

d

r

2

r

2

c

2

d

θ

2

r

2

c

2

sin

2

⁡ θ

d

ϕ

2

displaystyle sqrt left(1- frac 2GM rc^ 2 right)dt^ 2 - frac 1 c^ 2 left(1- frac 2GM rc^ 2 right)^ -1 dr^ 2 - frac r^ 2 c^ 2 dtheta ^ 2 - frac r^ 2 c^ 2 sin ^ 2 theta ;dphi ^ 2

is the change in proper time

d τ

displaystyle dtau

, or the interval of proper time.

Or one could use the following simpler approximation:

d t

d τ

=

1

1 −

(

2 G M

r

c

2

)

.

displaystyle frac dt dtau = frac 1 sqrt 1-left( frac 2GM rc^ 2 right) .

That is, the stronger the gravitational field (and, thus, the larger the acceleration), the more slowly time runs. The predictions of time dilation are confirmed by particle acceleration experiments and cosmic ray evidence, where moving particles decay more slowly than their less energetic counterparts. Gravitational time dilation
Gravitational time dilation
gives rise to the phenomenon of gravitational redshift and Shapiro signal travel time delays near massive objects such as the sun. The Global Positioning System
System
must also adjust signals to account for this effect. According to Einstein's general theory of relativity, a freely moving particle traces a history in spacetime that maximises its proper time. This phenomenon is also referred to as the principle of maximal aging, and was described by Taylor and Wheeler as:[30]

"Principle of Extremal Aging: The path a free object takes between two events in spacetime is the path for which the time lapse between these events, recorded on the object's wristwatch, is an extremum."

Einstein's theory was motivated by the assumption that every point in the universe can be treated as a 'center', and that correspondingly, physics must act the same in all reference frames. His simple and elegant theory shows that time is relative to an inertial frame. In an inertial frame, Newton's first law
Newton's first law
holds; it has its own local geometry, and therefore its own measurements of space and time; there is no 'universal clock'. An act of synchronization must be performed between two systems, at the least. Time
Time
in quantum mechanics[edit] See also: quantum mechanics There is a time parameter in the equations of quantum mechanics. The Schrödinger equation[31] is

Prerequisites

physics quantum mechanics

H ( t )

ψ ( t )

= i ℏ

∂ t

ψ ( t )

displaystyle H(t)leftpsi (t)rightrangle =ihbar partial over partial t leftpsi (t)rightrangle

One solution can be

ψ

e

( t ) ⟩ =

e

− i H t

/

ψ

e

( 0 ) ⟩

displaystyle psi _ e (t)rangle =e^ -iHt/hbar psi _ e (0)rangle

.

where

e

− i H t

/

displaystyle e^ -iHt/hbar

is called the time evolution operator, and H is the Hamiltonian. But the Schrödinger picture
Schrödinger picture
shown above is equivalent to the Heisenberg picture, which enjoys a similarity to the Poisson brackets of classical mechanics. The Poisson brackets are superseded by a nonzero commutator, say [H,A] for observable A, and Hamiltonian H:

d

d t

A = ( i ℏ

)

− 1

[ A , H ] +

(

∂ A

∂ t

)

c l a s s i c a l

.

displaystyle frac d dt A=(ihbar )^ -1 [A,H]+left( frac partial A partial t right)_ mathrm classical .

This equation denotes an uncertainty relation in quantum physics. For example, with time (the observable A), the energy E (from the Hamiltonian H) gives:

Δ E Δ T ≥

ℏ 2

displaystyle Delta EDelta Tgeq frac hbar 2

where

Δ E

displaystyle Delta E

is the uncertainty in energy

Δ T

displaystyle Delta T

is the uncertainty in time

displaystyle hbar

is Planck's constant

The more precisely one measures the duration of a sequence of events, the less precisely one can measure the energy associated with that sequence, and vice versa. This equation is different from the standard uncertainty principle, because time is not an operator in quantum mechanics. Corresponding commutator relations also hold for momentum p and position q, which are conjugate variables of each other, along with a corresponding uncertainty principle in momentum and position, similar to the energy and time relation above. Quantum mechanics
Quantum mechanics
explains the properties of the periodic table of the elements. Starting with Otto Stern's and Walter Gerlach's experiment with molecular beams in a magnetic field, Isidor Rabi
Isidor Rabi
(1898–1988), was able to modulate the magnetic resonance of the beam. In 1945 Rabi then suggested that this technique be the basis of a clock[32] using the resonant frequency of an atomic beam. Dynamical systems[edit] See dynamical systems and chaos theory, dissipative structures One could say that time is a parameterization of a dynamical system that allows the geometry of the system to be manifested and operated on. It has been asserted that time is an implicit consequence of chaos (i.e. nonlinearity/irreversibility): the characteristic time, or rate of information entropy production, of a system. Mandelbrot introduces intrinsic time in his book Multifractals and 1/f noise. Signalling[edit]

Prerequisites

electrical engineering signal processing

Signalling is one application of the electromagnetic waves described above. In general, a signal is part of communication between parties and places. One example might be a yellow ribbon tied to a tree, or the ringing of a church bell. A signal can be part of a conversation, which involves a protocol. Another signal might be the position of the hour hand on a town clock or a railway station. An interested party might wish to view that clock, to learn the time. See: Time
Time
ball, an early form of Time
Time
signal.

Evolution
Evolution
of a world line of an accelerated massive particle. This world line is restricted to the timelike top and bottom sections of this spacetime figure; this world line cannot cross the top (future) or the bottom (past) light cone. The left and right sections (which are outside the light cones) are spacelike.

We as observers can still signal different parties and places as long as we live within their past light cone. But we cannot receive signals from those parties and places outside our past light cone. Along with the formulation of the equations for the electromagnetic wave, the field of telecommunication could be founded. In 19th century telegraphy, electrical circuits, some spanning continents and oceans, could transmit codes - simple dots, dashes and spaces. From this, a series of technical issues have emerged; see Category:Synchronization. But it is safe to say that our signalling systems can be only approximately synchronized, a plesiochronous condition, from which jitter need be eliminated. That said, systems can be synchronized (at an engineering approximation), using technologies like GPS. The GPS
GPS
satellites must account for the effects of gravitation and other relativistic factors in their circuitry. See: Self-clocking signal. Technology for timekeeping standards[edit] The primary time standard in the U.S. is currently NIST-F1, a laser-cooled Cs fountain,[33] the latest in a series of time and frequency standards, from the ammonia-based atomic clock (1949) to the caesium-based NBS-1 (1952) to NIST-7 (1993). The respective clock uncertainty declined from 10,000 nanoseconds per day to 0.5 nanoseconds per day in 5 decades.[34] In 2001 the clock uncertainty for NIST-F1
NIST-F1
was 0.1 nanoseconds/day. Development of increasingly accurate frequency standards is underway. In this time and frequency standard, a population of caesium atoms is laser-cooled to temperatures of one microkelvin. The atoms collect in a ball shaped by six lasers, two for each spatial dimension, vertical (up/down), horizontal (left/right), and back/forth. The vertical lasers push the caesium ball through a microwave cavity. As the ball is cooled, the caesium population cools to its ground state and emits light at its natural frequency, stated in the definition of second above. Eleven physical effects are accounted for in the emissions from the caesium population, which are then controlled for in the NIST-F1 clock. These results are reported to BIPM. Additionally, a reference hydrogen maser is also reported to BIPM
BIPM
as a frequency standard for TAI (international atomic time). The measurement of time is overseen by BIPM
BIPM
(Bureau International des Poids et Mesures), located in Sèvres, France, which ensures uniformity of measurements and their traceability to the International System
System
of Units (SI) worldwide. BIPM
BIPM
operates under authority of the Metre
Metre
Convention, a diplomatic treaty between fifty-one nations, the Member States of the Convention, through a series of Consultative Committees, whose members are the respective national metrology laboratories. Time
Time
in cosmology[edit] Main article: physical cosmology The equations of general relativity predict a non-static universe. However, Einstein accepted only a static universe, and modified the Einstein field equation to reflect this by adding the cosmological constant, which he later described as the biggest mistake of his life. But in 1927, Georges LeMaître
Georges LeMaître
(1894–1966) argued, on the basis of general relativity, that the universe originated in a primordial explosion. At the fifth Solvay conference, that year, Einstein brushed him off with "Vos calculs sont corrects, mais votre physique est abominable."[35] (“Your math is correct, but your physics is abominable”). In 1929, Edwin Hubble
Edwin Hubble
(1889–1953) announced his discovery of the expanding universe. The current generally accepted cosmological model, the Lambda-CDM model, has a positive cosmological constant and thus not only an expanding universe but an accelerating expanding universe. If the universe were expanding, then it must have been much smaller and therefore hotter and denser in the past. George Gamow (1904–1968) hypothesized that the abundance of the elements in the Periodic Table of the Elements, might be accounted for by nuclear reactions in a hot dense universe. He was disputed by Fred Hoyle (1915–2001), who invented the term 'Big Bang' to disparage it. Fermi and others noted that this process would have stopped after only the light elements were created, and thus did not account for the abundance of heavier elements.

WMAP
WMAP
fluctuations of the cosmic microwave background radiation.[36]

Gamow's prediction was a 5–10 kelvin black body radiation temperature for the universe, after it cooled during the expansion. This was corroborated by Penzias and Wilson in 1965. Subsequent experiments arrived at a 2.7 kelvin temperature, corresponding to an age of the universe of 13.8 billion years after the Big Bang. This dramatic result has raised issues: what happened between the singularity of the Big Bang
Big Bang
and the Planck time, which, after all, is the smallest observable time. When might have time separated out from the spacetime foam;[37] there are only hints based on broken symmetries (see Spontaneous symmetry breaking, Timeline
Timeline
of the Big Bang, and the articles in Category:Physical cosmology). General relativity
General relativity
gave us our modern notion of the expanding universe that started in the Big Bang. Using relativity and quantum theory we have been able to roughly reconstruct the history of the universe. In our epoch, during which electromagnetic waves can propagate without being disturbed by conductors or charges, we can see the stars, at great distances from us, in the night sky. (Before this epoch, there was a time, 300,000 years after the big bang, during which starlight would not have been visible.) Reprise[edit] Ilya Prigogine's reprise is " Time
Time
precedes existence". In contrast to the views of Newton, of Einstein, and of quantum physics, which offer a symmetric view of time (as discussed above), Prigogine points out that statistical and thermodynamic physics can explain irreversible phenomena,[38] as well as the arrow of time and the Big Bang. See also[edit]

Relativistic dynamics Category:systems of units

References[edit]

^ Considine, Douglas M.; Considine, Glenn D. (1985). Process instruments and controls handbook (3 ed.). McGraw-Hill. pp. 18–61. ISBN 0-07-012436-1.  ^ For example, Galileo
Galileo
measured the period of a simple harmonic oscillator with his pulse. ^ a b Otto Neugebauer The Exact Sciences in Antiquity. Princeton: Princeton University Press, 1952; 2nd edition, Brown University Press, 1957; reprint, New York: Dover publications, 1969. Page 82. ^ See, for example William Shakespeare
William Shakespeare
Hamlet: " ... to thine own self be true, And it must follow, as the night the day, Thou canst not then be false to any man." ^ "Heliacal/Dawn Risings". Solar-center.stanford.edu. Retrieved 2012-08-17.  ^ Farmers have used the sun to mark time for thousands of years, as the most ancient method of telling time. ^ Eratosthenes
Eratosthenes
used this criterion in his measurement of the circumference of Earth ^ Fred Hoyle
Fred Hoyle
(1962), Astronomy: A history of man's investigation of the universe, Crescent Books, Inc., London
London
LC 62-14108, p.31 ^ The Mesopotamian (modern-day Iraq) astronomers recorded astronomical observations with the naked eye, more than 3500 years ago. P. W. Bridgman defined his operational definition in the twentieth c. ^ Naked eye astronomy became obsolete in 1609 with Galileo's observations with a telescope. Galileo
Galileo
Galilei Linceo, Sidereus Nuncius (Starry Messenger) 1610. ^ http://tycho.usno.navy.mil/gpstt.html http://www.phys.lsu.edu/mog/mog9/node9.html Today, automated astronomical observations from satellites and spacecraft require relativistic corrections of the reported positions. ^ " Unit of time
Unit of time
(second)". SI brochure. International Bureau of Weights and Measures (BIPM). pp. Section 2.1.1.3. Retrieved 2008-06-08.  ^ S. R. Jefferts et al., "Accuracy evaluation of NIST-F1". ^ Fred Adams and Greg Laughlin (1999), Five Ages of the Universe ISBN 0-684-86576-9 p.35. ^ Charles Hose and William McDougall (1912) The Pagan Tribes of Borneo, Plate 60. Kenyahs measuring the Length
Length
of the Shadow at Noon to determine the Time
Time
for sowing PADI p. 108. This photograph is reproduced as plate B in Fred Hoyle
Fred Hoyle
(1962), Astronomy: A history of man's investigation of the universe, Crescent Books, Inc., London
London
LC 62-14108, p.31. The measurement process is explained by: Gene Ammarell (1997), "Astronomy in the Indo-Malay Archipelago", p.119, Encyclopaedia of the history of science, technology, and medicine in non-western cultures, Helaine Selin, ed., which describes Kenyah Tribesmen of Borneo measuring the shadow cast by a gnomon, or tukar do with a measuring scale, or aso do. ^ North, J. (2004) God's Clockmaker: Richard of Wallingford
Richard of Wallingford
and the Invention of Time. Oxbow Books. ISBN 1-85285-451-0 ^ Watson, E (1979) "The St Albans Clock
Clock
of Richard of Wallingford". Antiquarian Horology
Horology
372-384. ^ Jo Ellen Barnett, Time's Pendulum ISBN 0-306-45787-3 p.99. ^ Galileo
Galileo
1638 Discorsi e dimostrazioni matematiche, intorno á due nuoue scienze 213, Leida, Appresso gli Elsevirii (Louis Elsevier), or Mathematical discourses and demonstrations, relating to Two New Sciences, English translation by Henry Crew and Alfonso de Salvio 1914. Section 213 is reprinted on pages 534-535 of On the Shoulders of Giants:The Great Works of Physics
Physics
and Astronomy (works by Copernicus, Kepler, Galileo, Newton, and Einstein). Stephen Hawking, ed. 2002 ISBN 0-7624-1348-4 ^ Newton 1687 Philosophiae Naturalis Principia Mathematica, Londini, Jussu Societatis Regiae ac Typis J. Streater, or The Mathematical Principles of Natural Philosophy, London, English translation by Andrew Motte 1700s. From part of the Scholium, reprinted on page 737 of On the Shoulders of Giants:The Great Works of Physics
Physics
and Astronomy (works by Copernicus, Kepler, Galileo, Newton, and Einstein). Stephen Hawking, ed. 2002 ISBN 0-7624-1348-4 ^ Newton 1687 page 738. ^ pp. 182-195. Stephen Hawking
Stephen Hawking
1996. The Illustrated Brief History
History
of Time: updated and expanded edition ISBN 0-553-10374-1 ^ Erwin Schrödinger
Erwin Schrödinger
(1945) What is Life? ^ G. Nicolis and I. Prigogine (1989), Exploring Complexity ^ R. Kapral and K. Showalter, eds. (1995), Chemical Waves and Patterns ^ Ilya Prigogine
Ilya Prigogine
(1996) The End of Certainty pp. 63-71 ^ Clemmow, P. C. (1973). An introduction to electromagnetic theory. CUP Archive. pp. 56–57. ISBN 0-521-09815-7. , Extract of pages 56, 57 ^ Henri Poincaré, (1902). Science and Hypothesis
Science and Hypothesis
Eprint ^ Einstein 1905, Zur Elektrodynamik bewegter Körper [On the electrodynamics of moving bodies] reprinted 1922 in Das Relativitätsprinzip, B.G. Teubner, Leipzig. The Principles of Relativity: A Collection of Original Papers on the Special
Special
Theory of Relativity, by H.A. Lorentz, A. Einstein, H. Minkowski, and W. H. Weyl, is part of Fortschritte der mathematischen Wissenschaften in Monographien, Heft 2. The English translation is by W. Perrett and G.B. Jeffrey, reprinted on page 1169 of On the Shoulders of Giants:The Great Works of Physics
Physics
and Astronomy (works by Copernicus, Kepler, Galileo, Newton, and Einstein). Stephen Hawking, ed. 2002 ISBN 0-7624-1348-4 ^ Taylor (2000). "Exploring Black Holes: Introduction to General Relativity" (PDF). Addison Wesley Longman.  ^ E. Schrödinger, Phys. Rev. 28 1049 (1926) ^ A Brief History
History
of Atomic Clocks at NIST ^ D. M. Meekhof, S. R. Jefferts, M. Stepanovíc, and T. E. Parker (2001) "Accuracy Evaluation of a Cesium Fountain Primary Frequency Standard at NIST", IEEE Transactions on Instrumentation and Measurement. 50, no. 2, (April 2001) pp. 507-509 ^ James Jespersen and Jane Fitz-Randolph (1999). From sundials to atomic clocks : understanding time and frequency. Washington, D.C. : U.S. Dept. of Commerce, Technology Administration, National Institute of Standards and Technology. 308 p. : ill. ; 28 cm. ISBN 0-16-050010-9 ^ John C. Mather and John Boslough (1996), The Very First Light ISBN 0-465-01575-1 p.41. ^ George Smoot
George Smoot
and Keay Davidson (1993) Wrinkles in Time ISBN 0-688-12330-9 A memoir of the experiment program for detecting the predicted fluctuations in the cosmic microwave background radiation ^ Martin Rees
Martin Rees
(1997), Before the Beginning ISBN 0-201-15142-1 p.210 ^ Prigogine, Ilya (1996), The End of Certainty: Time, Chaos and the New Laws of Nature. ISBN 0-684-83705-6 On pages 163 and 182.

Further reading[edit]

Boorstein, Daniel J., The Discoverers. Vintage. February 12, 1985. ISBN 0-394-72625-1 Dieter Zeh, H., The physical basis of the direction of time. Springer. ISBN 978-3-540-42081-1 Kuhn, Thomas S., The Structure of Scientific Revolutions. ISBN 0-226-45808-3 Mandelbrot, Benoît, Multifractals and 1/f noise. Springer Verlag. February 1999. ISBN 0-387-98539-5 Prigogine, Ilya (1984), Order out of Chaos. ISBN 0-394-54204-5 Serres, Michel, et al., "Conversations on Science, Culture, and Time (Studies in Literature and Science)". March, 1995. ISBN 0-472-06548-3 Stengers, Isabelle, and Ilya Prigogine, Theory Out of Bounds. University of Minnesota Press. November 1997. ISBN 0-8166-2517-4

v t e

Time

Key concepts

Past

history deep time

Present Future Futures studies Far future in religion Far future in science fiction and popular culture Timeline
Timeline
of the far future Eternity Eternity
Eternity
of the world

Measurement
Measurement
and standards

Chronometry

UTC UT TAI Unit of time Planck time Second Minute Hour Day Week Month Season Year Decade Century Millennium Tropical year Sidereal year Samvatsara

Measurement systems

Time
Time
zone Six-hour clock 12-hour clock 24-hour clock Daylight saving time Solar time Sidereal time Metric time Decimal time Hexadecimal time

Calendars

Gregorian Julian Hebrew Islamic Lunar Solar Hijri Mayan Intercalation Leap second Leap year

Clocks

Horology History
History
of timekeeping devices Main types

astrarium atomic

quantum

marine sundial sundial markup schema watch water-based

Chronology History

Astronomical chronology Big History Calendar
Calendar
era Chronicle Deep time Periodization Regnal year Timeline

Religion Mythology

Dreamtime Kāla Kalachakra Prophecy Time
Time
and fate deities Wheel of time Immortality

Philosophy of time

A-series and B-series B-theory of time Causality Duration Endurantism Eternal return Eternalism Event Multiple time dimensions Perdurantism Presentism Static interpretation of time Temporal finitism Temporal parts The Unreality of Time

Human
Human
experience and use of time

Accounting period Chronemics Fiscal year Generation time Mental chronometry Music Procrastination Punctuality Temporal database Term Time
Time
discipline Time
Time
management Time
Time
perception

Specious present

Time-tracking software Time-use research Time-based currency
Time-based currency
(time banking) Time
Time
value of money Time
Time
clock Timesheet Yesterday – Today – Tomorrow

Time
Time
in

Geology

Geological time

age chron eon epoch era period

Geochronology Geological history of Earth

Physics

Absolute time and space Arrow of time Chronon Coordinate
Coordinate
time Imaginary time Planck epoch Planck time Proper time Rate Spacetime Theory of relativity Time
Time
dilation

gravitational

Time
Time
domain Time
Time
translation symmetry Time
Time
reversal symmetry

other subject areas

Chronological dating Chronobiology Circadian rhythms Dating methodologies in archaeology Time
Time
geography

Related topics

Carpe diem Clock
Clock
position Space System
System
time Tempus fugit Time
Time
capsule Time
Time
complexity Time
Time
signature Time
Time
travel

Time
Time
portal Category

v t e

Time
Time
measurement and standards

Chronometry Orders of magnitude Metrology

International standards

Coordinated Universal Time

offset

UT ΔT DUT1 International Earth
Earth
Rotation
Rotation
and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 6-hour clock 12-hour clock 24-hour clock Barycentric Coordinate
Coordinate
Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate
Coordinate
Time International Date Line Leap second Solar time Terrestrial Time Time
Time
zone 180th meridian

Obsolete standards

Ephemeris time Greenwich Mean Time Prime meridian

Time
Time
in physics

Absolute time and space Spacetime Chronon Continuous signal Coordinate
Coordinate
time Cosmological decade Discrete time and continuous time Planck time Proper time Theory of relativity Time
Time
dilation Gravitational time dilation Time
Time
domain Time
Time
translation symmetry T-symmetry

Horology

Clock Astrarium Atomic clock Complication History
History
of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch Water clock Sundial Dialing scales Equation of time History
History
of sundials Sundial
Sundial
markup schema

Calendar

Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names

Archaeology
Archaeology
and geology

Chronological dating Geologic time scale International Commission on Stratigraphy

Astronomical chronology

Galactic year Nuclear timescale Precession Sidereal time

Other units of time

Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium

Related topics

Chronology Duration

music

Mental chronometry Metric time System
System
time Time
Time
value of mone

.