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time dilation

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, Ashby, Neil

, 2003

, Relativity in the Global Positioning System

,weblink

, Living Reviews in RelativityLiving Reviews in Relativity

According to the theory of relativity, , 2003

, Relativity in the Global Positioning System

,weblink

, Living Reviews in RelativityLiving Reviews in Relativity

**time dilation**is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field. As a result of the nature of spacetime,WEB, Toothman, Jessika,weblink HowStuffWorks, How Do Humans age in space?, 2018-04-08, 2010-09-28, a clock that is moving relative to an observer will be measured to tick slower than a clock that is at rest in the observer's own frame of reference. A clock that is under the influence of a stronger gravitational field than an observer's will also be measured to tick slower than the observer's own clock. Such time dilation has been repeatedly demonstrated, for instance by small disparities in a pair of atomic clocks after one of them is sent on a space trip, or by clocks on the Space Shuttle running slightly slower than reference clocks on Earth, or clocks on GPS and Galileo satellites running slightly faster.WEB, Lu, Ed,weblink Ed's Musing from Space, NASA, Expedition 7: Relativity, 2018-04-08, Time dilation has also been the subject of science fiction works, as it technically provides the means for forward time travel.WEB,weblink Is time travel possible?, NASA Space Place, 2018-08-03,

## History

Time dilation by the Lorentz factor was predicted by several authors at the turn of the 20th century.BOOK, Miller, Arthur I., 1981, Albert Einstein's Special Theory of Relativity: Emergence (1905) and Early Interpretation (1905â€“1911), Reading, Massachusetts, Addisonâ€“Wesley, 978-0-201-04679-3, .BOOK, Darrigol, Olivier, The Genesis of the Theory of Relativity, 2005, SÃ©minaire PoincarÃ©, 1, 1â€“22,weblink 10.1007/3-7643-7436-5_1, 978-3-7643-7435-8, Joseph Larmor (1897), at least for electrons orbiting a nucleus, wrote "... individual electrons describe corresponding parts of their orbits in times shorter for the [rest] system in the ratio :scriptstyle sqrt{1 - frac{v^2}{c^2}}".JOURNAL, Larmor, Joseph, 1897, On a Dynamical Theory of the Electric and Luminiferous Medium, Part 3, Relations with Material Media, Philosophical Transactions of the Royal Society, 190, 205â€“300, 10.1098/rsta.1897.0020, 1897RSPTA.190..205L, s:Dynamical Theory of the Electric and Luminiferous Medium III, Emil Cohn (1904) specifically related this formula to the rate of clocks.{{Citation|last=Cohn, Emil|year=1904|title=Zur Elektrodynamik bewegter Systeme II|trans-title=s:Translat|journal= Sitzungsberichte der KÃ¶niglich Preussischen Akademie der Wissenschaften|volume=1904/2|issue=43|pages =1404â€“1416}} In the context of special relativity it was shown by Albert Einstein (1905) that this effect concerns the nature of time itself, and he was also the first to point out its reciprocity or symmetry.JOURNAL, Einstein, Albert, 1905, Zur Elektrodynamik bewegter KÃ¶rper, Annalen der Physik, 322, 10, 891â€“921, 10.1002/andp.19053221004, 1905AnP...322..891E,weblink . See also: English translation. Subsequently, Hermann Minkowski (1907) introduced the concept of proper time which further clarified the meaning of time dilation.{{Citation|author=Minkowski, Hermann|year=1908|orig-year=1907|title=Die Grundgleichungen fÃ¼r die elektromagnetischen VorgÃ¤nge in bewegten KÃ¶rpern|trans-title=s:Translat|journal=Nachrichten von der Gesellschaft der Wissenschaften zu GÃ¶ttingen, Mathematisch-Physikalische Klasse|pages=53â€“111|title-link=s:Die Grundgleichungen fÃ¼r die elektromagnetischen VorgÃ¤nge in bewegten KÃ¶rpern}}## Velocity time dilation

File:Nonsymmetric velocity time dilation.gif|thumb|right|256px|From the local frame of reference of the blue clock, the red clock, being in motion, is perceived as ticking slowerBOOK, Basic Relativity: An Introductory Essay, illustrated, PÃ©ter, HraskÃ³, Springer Science & Business Media, 2011, 978-3-642-17810-8, 60,weblink
( Extract of page 60 (Exaggerated))

Special relativity indicates that, for an observer in an inertial frame of reference, a clock that is moving relative to him will be measured to tick slower than a clock that is at rest in his frame of reference. This case is sometimes called special relativistic time dilation. The faster the relative velocity, the greater the time dilation between one another, with the rate of time reaching zero as one approaches the speed of light (299,792,458 m/s). This causes massless particles that travel at the speed of light to be unaffected by the passage of time.Theoretically, time dilation would make it possible for passengers in a fast-moving vehicle to advance further into the future in a short period of their own time. For sufficiently high speeds, the effect is dramatic. For example, one year of travel might correspond to ten years on Earth. Indeed, a constant 1 g acceleration would permit humans to travel through the entire known Universe in one human lifetime.BOOK, Calder, Nigel, 2006, Magic Universe: A grand tour of modern science,weblink 378, Oxford University Press, 978-0-19-280669-7, .With current technology severely limiting the velocity of space travel, however, the differences experienced in practice are minuscule: after 6 months on the International Space Station (ISS) (which orbits Earth at a speed of about 7,700 m/s) an astronaut would have aged about 0.005 seconds less than those on Earth. The current human time travel record holder is Russian cosmonaut Sergei Krikalev.NEWS,weblink A Trip Forward in Time. Your Travel Agent: Einstein., Overbye, Dennis, The New York Times, 2005-06-28, 2015-12-08, He is 22.68 milliseconds younger than he would have been had he stayed stationary on Earth. He beat the previous record of about 20 milliseconds by cosmonaut Sergei Avdeyev.BOOK, Time Travel in Einstein's Universe, Gott, J., Richard, 2002, 75, ### Simple inference of velocity time dilation

thumb|upright=3|**Left**: Observer at rest measures time 2

*L*/

*c*between co-local events of light signal generation at A and arrival at A.

**Right**: Events according to an observer moving to the left of the setup: bottom mirror A when signal is generated at time

*t'=*0, top mirror B when signal gets reflected at time

*t'=D/c*, bottom mirror A when signal returns at time

*t'=2D/c*Time dilation can be inferred from the observed constancy of the speed of light in all reference frames dictated by the second postulate of special relativity.BOOK

, Cassidy, David C.

, Holton, Gerald James

, Rutherford, Floyd James

, 2002

, Understanding Physics

,weblink

, Springer-Verlag

, 422

, 978-0-387-98756-9

, BOOK
, Holton, Gerald James

, Rutherford, Floyd James

, 2002

, Understanding Physics

,weblink

, Springer-Verlag

, 422

, 978-0-387-98756-9

, Cutner, Mark Leslie

, 2003

, Astronomy, A Physical Perspective

,weblink

, 128

, Cambridge University Press

, 978-0-521-82196-4

, BOOK
, 2003

, Astronomy, A Physical Perspective

,weblink

, 128

, Cambridge University Press

, 978-0-521-82196-4

, Lerner, Lawrence S.

, 1996

, Physics for Scientists and Engineers, Volume 2

, 1051â€“1052

, Jones and Bartlett

, 978-0-7637-0460-5

,weblink

, BOOK
, 1996

, Physics for Scientists and Engineers, Volume 2

, 1051â€“1052

, Jones and Bartlett

, 978-0-7637-0460-5

,weblink

, Ellis, George F. R.

, Ruth M., Williams

, 2000

, Flat and Curved Space-times

,weblink

, 2n, 28â€“29

, Oxford University Press

, 978-0-19-850657-7

, This constancy of the speed of light means that, counter to intuition, speeds of material objects and light are not additive. It is not possible to make the speed of light appear greater by moving towards or away from the light source.Consider then, a simple clock consisting of two mirrors {{math|A}} and {{math|B}}, between which a light pulse is bouncing. The separation of the mirrors is {{math|, Ruth M., Williams

, 2000

, Flat and Curved Space-times

,weblink

, 2n, 28â€“29

, Oxford University Press

, 978-0-19-850657-7

*L*}} and the clock ticks once each time the light pulse hits either of the mirrors.In the frame in which the clock is at rest (diagram on the left), the light pulse traces out a path of length {{math|2

*L*}} and the period of the clock is {{math|2

*L*}} divided by the speed of light:

Delta t = frac{2 L}{c}.

From the frame of reference of a moving observer traveling at the speed {{math|*v*}} relative to the resting frame of the clock (diagram at right), the light pulse is seen as tracing out a longer, angled path. Keeping the speed of light constant for all inertial observers, requires a lengthening of the period of this clock from the moving observer's perspective. That is to say, in a frame moving relative to the local clock, this clock will appear to be running more slowly. Straightforward application of the Pythagorean theorem leads to the well-known prediction of special relativity:The total time for the light pulse to trace its path is given by

Delta t' = frac{2 D}{c}.

The length of the half path can be calculated as a function of known quantities as
D = sqrt{left (frac{1}{2}v Delta t'right )^2 + L^2}.

Elimination of the variables {{math|*D*}} and {{math|

*L*}} from these three equations results in

Delta t' = frac{Delta t}{sqrt{1 - frac{v^2}{c^2}}},

which expresses the fact that the moving observer's period of the clock Delta t' is longer than the period Delta t in the frame of the clock itself.### Reciprocity

right|thumb|Time UV of a clock in S is shorter compared to Uxâ€² in Sâ€², and time UW of a clock in Sâ€² is shorter compared to Ux in S.Given a certain frame of reference, and the "stationary" observer described earlier, if a second observer accompanied the "moving" clock, each of the observers would perceive the other's clock as ticking at a*slower*rate than their own local clock, due to them both perceiving the other to be the one that's in motion relative to their own stationary frame of reference.Common sense would dictate that, if the passage of time has slowed for a moving object, said object would observe the external world's time to be correspondingly sped up. Counterintuitively, special relativity predicts the opposite. When two observers are in motion relative to each other, each will measure the other's clock slowing down, in concordance with them being moving relative to the observer's frame of reference.While this seems self-contradictory, a similar oddity occurs in everyday life. If two persons A and B observe each other from a distance, B will appear small to A, but at the same time A will appear small to B. Being familiar with the effects of perspective, there is no contradiction or paradox in this situation.BOOK, Adams, Steve, 1997, Relativity: An introduction to space-time physics,weblink 54, CRC Press, 978-0-7484-0621-0, The reciprocity of the phenomenon also leads to the so-called twin paradox where the aging of twins, one staying on Earth and the other embarking on a space travel, is compared, and where the reciprocity suggests that both persons should have the same age when they reunite. On the contrary, at the end of the round-trip, the traveling twin will be younger than his brother on Earth. The dilemma posed by the paradox, however, can be explained by the fact that the traveling twin must markedly accelerate in at least three phases of the trip (beginning, direction change, and end), while the other will only experience negligible acceleration, due to rotation and revolution of Earth. During the acceleration phases of the space travel, time dilation is not symmetric.

### Experimental testing

{{See also|Tests of special relativity}}#### Doppler effect

- Ives and Stilwell (1938, 1941) stated purpose of these experiments was to verify the time dilation effect, predicted by Larmorâ€“Lorentz ether theory, due to motion through the ether using Einstein's suggestion that Doppler effect in canal rays would provide a suitable experiment. These experiments measured the Doppler shift of the radiation emitted from cathode rays, when viewed from directly in front and from directly behind. The high and low frequencies detected were not the classically predicted values

frac{f_0}{1 - v/c} qquad text{and} qquad frac{f_0}{1+v/c}. ,

The high and low frequencies of the radiation from the moving sources were measured as
sqrt{ frac{1 + v/c}{1 - v/c} } f_0 = gamma left(1 + v/cright) f_0 qquad text{and} qquad sqrt{ frac{1 - v/c}{1 + v/c} } f_0 = gamma left(1 - v/cright) f_0, ,

as deduced by Einstein (1905) from the Lorentz transformation, when the source is running slow by the Lorentz factor.
- Hasselkamp, Mondry, and Scharmann

, Hasselkamp, D.

, Mondry, E.

, Scharmann, A.

, 1979

, Direct observation of the transversal Doppler-shift

, Zeitschrift fÃ¼r Physik A

, 289, 2, 151â€“155

, 1979ZPhyA.289..151H

, 10.1007/BF01435932

, (1979) measured the Doppler shift from a source moving at right angles to the line of sight. The most general relationship between frequencies of the radiation from the moving sources is given by:
, Mondry, E.

, Scharmann, A.

, 1979

, Direct observation of the transversal Doppler-shift

, Zeitschrift fÃ¼r Physik A

, 289, 2, 151â€“155

, 1979ZPhyA.289..151H

, 10.1007/BF01435932

f_mathrm{detected} = f_mathrm{rest}{left(1 - frac{v}{c} cosphiright)/sqrt{1 - {v^2}/{c^2}} }

as deduced by Einstein (1905).
*Ï•*{{=}} 90Â°}} ({{nowrap|cos

*Ï•*{{=}} 0}}) this reduces to {{nowrap|

*f*detected {{=}}

*f*restÎ³}}. This lower frequency from the moving source can be attributed to the time dilation effect and is often called the transverse Doppler effect and was predicted by relativity.

- In 2010 time dilation was observed at speeds of less than 10 meters per second using optical atomic clocks connected by 75 meters of optical fiber.

, Chou, C. W.

, Hume, D. B.

, Rosenband, T.

, Wineland, D. J.

, 2010

, Optical Clocks and Relativity

, Science (journal), Science

, 329, 5999, 1630â€“1633

, 2010Sci...329.1630C

, 10.1126/science.1192720

, 20929843

, , Hume, D. B.

, Rosenband, T.

, Wineland, D. J.

, 2010

, Optical Clocks and Relativity

, Science (journal), Science

, 329, 5999, 1630â€“1633

, 2010Sci...329.1630C

, 10.1126/science.1192720

, 20929843

#### Moving particles

- A comparison of muon lifetimes at different speeds is possible. In the laboratory, slow muons are produced; and in the atmosphere, very fast moving muons are introduced by cosmic rays. Taking the muon lifetime at rest as the laboratory value of 2.197 Î¼s, the lifetime of a cosmic ray produced muon traveling at 98% of the speed of light is about five times longer, in agreement with observations. An example is Rossi and Hall (1941), who compared the population of cosmic-ray-produced muons at the top of a mountain to that observed at sea level.BOOK

, Stewart, J. V.

, 2001

, Intermediate electromagnetic theory

,weblink

, 705

, World Scientific

, 978-981-02-4470-5

, , 2001

, Intermediate electromagnetic theory

,weblink

, 705

, World Scientific

, 978-981-02-4470-5

- The lifetime of particles produced in particle accelerators appears longer due to time dilation. In such experiments the "clock" is the time taken by processes leading to muon decay, and these processes take place in the moving muon at its own "clock rate", which is much slower than the laboratory clock. This is routinely taken into account in particle physics, and many dedicated measurements have been performed. For instance, in the muon storage ring at CERN the lifetime of muons circulating with Î³ = 29.327 was found to be dilated to 64.378 Î¼s, confirming time dilation to an accuracy of 0.9 Â± 0.4 parts per thousand.Bailey, J. et al. Nature 268, 301 (1977)

### Proper time and Minkowski diagram

{{Multiple images|align=right|width=200|direction=horizontal|background-color = white|image1=Zeitdilatation3.svg|caption1=Clock C in relative motion between two synchronized clocks A and B. C meets A at*d*, and B at

*f*.|image2=EigenzeitZwill.svg|caption2=Twin paradox. One twin has to change frames, leading to different proper times in the twin's world lines.|header=Minkowski diagram and twin paradox}}In the Minkowski diagram from the second image on the right, clock C resting in inertial frame Sâ€² meets clock A at

*d*and clock B at

*f*(both resting in S). All three clocks simultaneously start to tick in S. The worldline of A is the ct-axis, the worldline of B intersecting

*f*is parallel to the ct-axis, and the worldline of C is the ctâ€²-axis. All events simultaneous with

*d*in S are on the x-axis, in Sâ€² on the xâ€²-axis.The proper time between two events is indicated by a clock present at both events.BOOK, Edwin F. Taylor, John Archibald Wheeler, Spacetime Physics: Introduction to Special Relativity, 1992, W. H. Freeman, New York, 978-0-7167-2327-1, It is invariant, i.e., in all inertial frames it is agreed that this time is indicated by that clock. Interval

*df*is therefore the proper time of clock C, and is shorter with respect to the coordinate times

*ef=dg*of clocks B and A in S. Conversely, also proper time

*ef*of B is shorter with respect to time

*if*in Sâ€², because event

*e*was measured in Sâ€² already at time

*i*due to relativity of simultaneity, long before C started to tick.From that it can be seen, that the proper time between two events indicated by an unaccelerated clock present at both events, compared with the synchronized coordinate time measured in all other inertial frames, is always the

*minimal*time interval between those events. However, the interval between two events can also correspond to the proper time of accelerated clocks present at both events. Under all possible proper times between two events, the proper time of the unaccelerated clock is

*maximal*, which is the solution to the twin paradox.

### Derivation and formulation

Image:Time dilation.svg|thumb|300px|Lorentz factorLorentz factorIn addition to the light clock used above, the formula for time dilation can be more generally derived from the temporal part of the Lorentz transformation.{{Citation|author=Born, Max|authorlink=Max Born|title = Einstein's Theory of Relativity|publisher=Dover Publications|year =1964|isbn=978-0-486-60769-6 }} Let there be two events at which the moving clock indicates t_{a} and t_{b}, thus
t_{a}^{prime}=frac{t_{a}-frac{vx_{a}}{c^{2}}}{sqrt{1-frac{v^{2}}{c^{2}}}}, t_{b}^{prime}=frac{t_{b}-frac{vx_{b}}{c^{2}}}{sqrt{1-frac{v^{2}}{c^{2}}}}.

Since the clock remains at rest in its inertial frame, it follows x_{a}=x_{b}, thus the interval Delta t^{prime}=t_{b}^{prime}-t_{a}^{prime} is given by
Delta t' = gamma , Delta t = frac{Delta t}{sqrt{1-frac{v^2}{c^2}}} ,

where Î”*t*is the time interval between

*two co-local events*(i.e. happening at the same place) for an observer in some inertial frame (e.g. ticks on his clock), known as the

*proper time*, Î”tâ€² is the time interval between those same events, as measured by another observer, inertially moving with velocity

*v*with respect to the former observer,

*v*is the relative velocity between the observer and the moving clock,

*c*is the speed of light, and the Lorentz factor (conventionally denoted by the Greek letter gamma or γ) is

gamma = frac{1}{sqrt{1-frac{v^2}{c^2}}} , .

Thus the duration of the clock cycle of a moving clock is found to be increased: it is measured to be "running slow". The range of such variances in ordinary life, where {{nowrap|1=*v*â‰ª

*c*,}} even considering space travel, are not great enough to produce easily detectable time dilation effects and such vanishingly small effects can be safely ignored for most purposes. It is only when an object approaches speeds on the order of 30,000 km/s (1/10 the speed of light) that time dilation becomes important.BOOK, Relativity and the Nature of Spacetime, 2nd, illustrated, Vesselin, Petkov, Springer Science & Business Media, 2009, 978-3-642-01962-3, 87,weblink Extract of page 87

### Hyperbolic motion

In special relativity, time dilation is most simply described in circumstances where relative velocity is unchanging. Nevertheless, the Lorentz equations allow one to calculate proper time and movement in space for the simple case of a spaceship which is applied with a force per unit mass, relative to some reference object in uniform (i.e. constant velocity) motion, equal to*g*throughout the period of measurement.Let

*t*be the time in an inertial frame subsequently called the rest frame. Let

*x*be a spatial coordinate, and let the direction of the constant acceleration as well as the spaceship's velocity (relative to the rest frame) be parallel to the

*x*-axis. Assuming the spaceship's position at time {{nowrap|

*t*{{=}} 0}} being {{nowrap|

*x*{{=}} 0}} and the velocity being

*v*0 and defining the following abbreviation

gamma_0 = frac{1}{sqrt{1-v_0^2/c^2}},

the following formulas hold:See equations 3, 4, 6 and 9 of JOURNAL
, Lorenzo, Iorio

, 2005

, An analytical treatment of the Clock Paradox in the framework of the Special and General Theories of Relativity

, Foundations of Physics Letters

, 18, 1, 1â€“19

, physics/0405038

, 2005FoPhL..18....1I

, 10.1007/s10702-005-2466-8

, Position:
, 2005

, An analytical treatment of the Clock Paradox in the framework of the Special and General Theories of Relativity

, Foundations of Physics Letters

, 18, 1, 1â€“19

, physics/0405038

, 2005FoPhL..18....1I

, 10.1007/s10702-005-2466-8

x(t) = frac {c^2}{g} left( sqrt{1 + frac{left(gt + v_0gamma_0right)^2}{c^2}} -gamma_0 right).

Velocity:
v(t) =frac{gt + v_0gamma_0}{sqrt{1 + frac{ left(gt + v_0gamma_0right)^2}{c^2}}}.

Proper time as function of coordinate time:
tau(t) = tau_0 + int_0^t sqrt{ 1 - left( frac{v(t')}{c} right)^2 } dt'.

In the case where *v*(0) =

*v*0 = 0 and

*Ï„*(0) =

*Ï„*0 = 0 the integral can be expressed as a logarithmic function or, equivalently, as an inverse hyperbolic function:

tau(t) = frac{c}{g} ln left( frac{gt}{c} + sqrt{ 1 + left( frac{gt}{c} right)^2 } right) = frac{c}{g} operatorname {arsinh} left( frac{gt}{c} right) .

As functions of the proper time tau of the ship, the following formulae hold:BOOK, Rindler, W., Essential Relativity, 1977, 49â€“50, Springer, 978-3540079705, Position:
x(tau) = frac{c^2}{g} left( cosh frac{g tau}{c}-1 right).

Velocity:
v(tau) = c tanh frac{g tau}{c}.

Coordinate time as function of proper time:
t(tau) = frac{c}{g} sinh frac{g tau}{c}.

### Clock hypothesis

The**clock hypothesis**is the assumption that the rate at which a clock is affected by time dilation does not depend on its acceleration but only on its instantaneous velocity. This is equivalent to stating that a clock moving along a path P measures the proper time, defined by:

d tau = int_P sqrt {dt^2 - dx^2/c^2 - dy^2/c^2 - dz^2/c^2}.

The clock hypothesis was implicitly (but not explicitly) included in Einstein's original 1905 formulation of special relativity. Since then, it has become a standard assumption and is usually included in the axioms of special relativity, especially in the light of experimental verification up to very high accelerations in particle accelerators.JOURNAL, Bailey, H., Borer, K., Combley F., Drumm H., Krienen F., Lange F., Picasso E., Ruden W. von, Farley F. J. M., Field J. H., Flegel W., Hattersley P. M., yes, 1977, Measurements of relativistic time dilatation for positive and negative muons in a circular orbit, Nature, 268, 5618, 301â€“305, 10.1038/268301a0, 1977Natur.268..301B, JOURNAL, Roos, C. E., Marraffino, J., Reucroft, S., Waters, J., Webster, M. S., Williams, E. G. H., 1980, Ïƒ+/- lifetimes and longitudinal acceleration, Nature, 286, 5770, 244â€“245, 10.1038/286244a0, 1980Natur.286..244R, ## Gravitational time dilation

(File:The Earth seen from Apollo 17.jpg|thumb|right|140px|Time passes more quickly further from a center of gravity, as is witnessed with massive objects (like the Earth))Gravitational time dilation is experienced by an observer that, being under the influence of a gravitational field, will measure his own clock to slow down, compared to another that is under a weaker gravitational field.Gravitational time dilation is at play e.g. for ISS astronauts. While the astronauts' relative velocity slows down their time, the reduced gravitational influence at their location speeds it up, although at a lesser degree. Also, a climber's time is theoretically passing slightly faster at the top of a mountain compared to people at sea level. It has also been calculated that due to time dilation, the core of the Earth is 2.5 years younger than the crust.WEB,weblink New calculations show Earth's core is much younger than thought, Phys.org, 26 May 2016, "A clock used to time a full rotation of the earth will measure the day to be approximately an extra 10 ns/day longer for every km of altitude above the reference geoid." JOURNAL, Measurement of gravitational time dilation: An undergraduate research project, American Journal of Physics, 85, 10, 757â€“762, 18 September 2017, 10.1119/1.5000802, Burns, M. Shane, Leveille, Michael D., Dominguez, Armand R., Gebhard, Brian B., Huestis, Samuel E., Steele, Jeffrey, Patterson, Brian, Sell, Jerry F., Serna, Mario, Gearba, M. Alina, Olesen, Robert, O'Shea, Patrick, Schiller, Jonathan, 1707.00171, Travel to regions of space where extreme gravitational time dilation is taking place, such as near a black hole, could yield time-shifting results analogous to those of near-lightspeed space travel.Contrarily to velocity time dilation, in which both observers measure the other as aging slower (a reciprocal effect), gravitational time dilation is not reciprocal. This means that with gravitational time dilation both observers agree that the clock nearer the center of the gravitational field is slower in rate, and they agree on the ratio of the difference.### Experimental testing

- In 1959 Robert Pound and Glen A. Rebka measured the very slight gravitational red shift in the frequency of light emitted at a lower height, where Earth's gravitational field is relatively more intense. The results were within 10% of the predictions of general relativity. In 1964, Pound and J. L. Snider measured a result within 1% of the value predicted by gravitational time dilation.JOURNAL, Pound, R. V., Snider J. L., November 2, 1964, Effect of Gravity on Nuclear Resonance, Physical Review Letters, 13, 18, 539â€“540, 10.1103/PhysRevLett.13.539, 1964PhRvL..13..539P, (See Poundâ€“Rebka experiment)
- In 2010 gravitational time dilation was measured at the earth's surface with a height difference of only one meter, using optical atomic clocks.

## Combined effect of velocity and gravitational time dilation

Image:Daily satellite time dilation.png|thumb|360px|Daily time dilation (gain or loss if negative) in microseconds as a function of (circular) orbit radius*r*=

*rs*/

*re*, where

*rs*is satellite orbit radius and

*re*is the equatorial Earth radius, calculated using the Schwarzschild metric. At

*r*â‰ˆ 1.497{{refn|group="Note"|Average time dilation has a weak dependence on the orbital inclination angle (Ashby 2003, p.32). The

*r*â‰ˆ 1.497 result corresponds toJOURNAL

, Ashby, Neil

, 2002

, Relativity in the Global Positioning System

,weblink

, Physics Today

, 55, 5, 45

, 2002PhT....55e..41A

, 10.1063/1.1485583 Physics Today(File:Time.dilation.to.orbital.height.svg|thumb|360px|Daily Time Dilation over Circular Orbit height split into its components.)High accuracy timekeeping, low earth orbit satellite tracking, and pulsar timing are applications that require the consideration of the combined effects of mass and motion in producing time dilation. Practical examples include the International Atomic Time standard and its relationship with the Barycentric Coordinate Time standard used for interplanetary objects.Relativistic time dilation effects for the solar system and the earth can be modeled very precisely by the Schwarzschild solution to the Einstein field equations. In the Schwarzschild metric, the interval dt_text{E} is given bySee equations 2 & 3 (combined here and divided throughout by

,

, 2002

, Relativity in the Global Positioning System

,weblink

, Physics Today

, 55, 5, 45

, 2002PhT....55e..41A

, 10.1063/1.1485583 Physics Today(File:Time.dilation.to.orbital.height.svg|thumb|360px|Daily Time Dilation over Circular Orbit height split into its components.)High accuracy timekeeping, low earth orbit satellite tracking, and pulsar timing are applications that require the consideration of the combined effects of mass and motion in producing time dilation. Practical examples include the International Atomic Time standard and its relationship with the Barycentric Coordinate Time standard used for interplanetary objects.Relativistic time dilation effects for the solar system and the earth can be modeled very precisely by the Schwarzschild solution to the Einstein field equations. In the Schwarzschild metric, the interval dt_text{E} is given bySee equations 2 & 3 (combined here and divided throughout by

*c*2) at pp. 35â€“36 in JOURNAL, Moyer, T. D.

, 1981

, Transformation from proper time on Earth to coordinate time in solar system barycentric space-time frame of reference

, Celestial Mechanics (journal), Celestial Mechanics

, 23, 1, 33â€“56

, 1981CeMec..23...33M

, 10.1007/BF01228543, 2060/19770007221

, A version of the same relationship can also be seen at equation 2 in

JOURNAL
, 1981

, Transformation from proper time on Earth to coordinate time in solar system barycentric space-time frame of reference

, Celestial Mechanics (journal), Celestial Mechanics

, 23, 1, 33â€“56

, 1981CeMec..23...33M

, 10.1007/BF01228543, 2060/19770007221

, A version of the same relationship can also be seen at equation 2 in

, Ashbey, Neil

, 2002

,weblink

, Relativity and the Global Positioning System

, Physics Today

, 55, 5, 45

, 2002PhT....55e..41A

, 10.1063/1.1485583

, , 2002

,weblink

, Relativity and the Global Positioning System

, Physics Today

, 55, 5, 45

, 2002PhT....55e..41A

, 10.1063/1.1485583

dt_text{E}^2 = left( 1-frac{2GM_text{i}}{r_text{i} c^2} right) dt_text{c}^2 - left( 1-frac{2GM_text{i}}{r_text{i} c^2} right)^{-1} frac{dx^2+dy^2+dz^2}{c^2} ,

where
dt_text{E} is a small increment of proper time t_text{E} (an interval that could be recorded on an atomic clock),
dt_text{c} is a small increment in the coordinate t_text{c} (coordinate time),
dx, dy, dz are small increments in the three coordinates x, y, z of the clock's position,
frac{G M_i}{r_i} represents the sum of the Newtonian gravitational potentials due to the masses in the neighborhood, based on their distances r_i from the clock. This sum includes any tidal potentials.

The coordinate velocity of the clock is given by
v^2 = frac{dx^2+dy^2+dz^2}{dt_text{c}^2}. ,

The coordinate time t_c is the time that would be read on a hypothetical "coordinate clock" situated infinitely far from all gravitational masses (U=0), and stationary in the system of coordinates (v=0). The exact relation between the rate of proper time and the rate of coordinate time for a clock with a radial component of velocity is
frac{dt_text{E}}{dt_text{c}} = sqrt{ 1 - frac{2U}{c^2} - frac{v^2}{c^2} - left( frac{c^2}{2U} - 1 right)^{-1} frac{{v_shortparallel}^2}{c^2} } = sqrt{ 1 - left( beta^2 + beta_e^2 + frac{beta_shortparallel^2 beta_e^2}{1 - beta_e^2} right) } ,

where
v_shortparallel is the radial velocity,
v_e = sqrt{ frac{2 G M_i}{r_i} } is the escape velocity,
beta = v/c, beta_e = v_e/c and beta_shortparallel = v_shortparallel/c are velocities as a percentage of speed of light c,
U = frac{G M_i}{r_i} is the Newtonian potential, equivalent to half of the escape velocity squared.

The above equation is exact under the assumptions of the Schwarzschild solution. It reduces to velocity time dilation equation in the presence of motion and absence of gravity, i.e. beta_e = 0. It reduces to gravitational time dilation equation in the absence of motion and presence of gravity, i.e. beta = 0 = beta_shortparallel.### Experimental testing

- Hafele and Keating, in 1971, flew caesium atomic clocks east and west around the earth in commercial airliners, to compare the elapsed time against that of a clock that remained at the U.S. Naval Observatory. Two opposite effects came into play. The clocks were expected to age more quickly (show a larger elapsed time) than the reference clock, since they were in a higher (weaker) gravitational potential for most of the trip (c.f. Poundâ€“Rebka experiment). But also, contrastingly, the moving clocks were expected to age more slowly because of the speed of their travel. From the actual flight paths of each trip, the theory predicted that the flying clocks, compared with reference clocks at the U.S. Naval Observatory, should have lost 40Â±23 nanoseconds during the eastward trip and should have gained 275Â±21 nanoseconds during the westward trip. Relative to the atomic time scale of the U.S. Naval Observatory, the flying clocks lost 59Â±10 nanoseconds during the eastward trip and gained 273Â±7 nanoseconds during the westward trip (where the error bars represent standard deviation).

, 2005

, Einstein

,weblink

, 18

, 1â€“4

, Metromnia

, National Physical Laboratory (United Kingdom), National Physical Laboratory

, The NPL experiment differed from the original in that the caesium clocks were sent on a shorter trip (Londonâ€“Washington, D.C. return), but the clocks were more accurate. The reported results are within 4% of the predictions of relativity, within the uncertainty of the measurements. , Einstein

,weblink

, 18

, 1â€“4

, Metromnia

, National Physical Laboratory (United Kingdom), National Physical Laboratory

- The Global Positioning System can be considered a continuously operating experiment in both special and general relativity. The in-orbit clocks are corrected for both special and general relativistic time dilation effects as described above, so that (as observed from the earth's surface) they run at the same rate as clocks on the surface of the Earth.BOOK, Understanding GPS: Principles and Applications, Elliott, Kaplan, Christopher, Hegarty, Artech House, 2005, 978-1-58053-895-4, 306,weblink Extract of page 306

## Footnotes

{{Reflist|group=Note}}## References

{{Reflist|30em}}## Further reading

- BOOK

,

- JOURNAL

, Einstein, A.

, 1905

, Zur Elektrodynamik bewegter KÃ¶rper

,weblink

, Annalen der Physik

, 322, 10, 891

, 10.1002/andp.19053221004

, 1905AnP...322..891E

, , 1905

, Zur Elektrodynamik bewegter KÃ¶rper

,weblink

, Annalen der Physik

, 322, 10, 891

, 10.1002/andp.19053221004

, 1905AnP...322..891E

- JOURNAL

, Einstein, A.

, 1907

, Ãœber die MÃ¶glichkeit einer neuen PrÃ¼fung des RelativitÃ¤tsprinzips

, Annalen der Physik

, 328, 6, 197â€“198

, 1907AnP...328..197E

, 10.1002/andp.19073280613

, , 1907

, Ãœber die MÃ¶glichkeit einer neuen PrÃ¼fung des RelativitÃ¤tsprinzips

, Annalen der Physik

, 328, 6, 197â€“198

, 1907AnP...328..197E

, 10.1002/andp.19073280613

- JOURNAL

, Hasselkamp, D.

, Mondry, E.

, Scharmann, A.

, 1979

, Direct Observation of the Transversal Doppler-Shift

, Zeitschrift fÃ¼r Physik A

, 289, 2, 151â€“155

, 1979ZPhyA.289..151H

, 10.1007/BF01435932

, , Mondry, E.

, Scharmann, A.

, 1979

, Direct Observation of the Transversal Doppler-Shift

, Zeitschrift fÃ¼r Physik A

, 289, 2, 151â€“155

, 1979ZPhyA.289..151H

, 10.1007/BF01435932

- JOURNAL

, Ives, H. E.

, Stilwell, G. R.

, 1938

, An experimental study of the rate of a moving clock

, Journal of the Optical Society of America

, 28, 7, 215â€“226

, 10.1364/JOSA.28.000215

, , Stilwell, G. R.

, 1938

, An experimental study of the rate of a moving clock

, Journal of the Optical Society of America

, 28, 7, 215â€“226

, 10.1364/JOSA.28.000215

- JOURNAL

, Ives, H. E.

, Stilwell, G. R.

, 1941

, An experimental study of the rate of a moving clock. II

, Journal of the Optical Society of America

, 31, 5, 369â€“374

, 10.1364/JOSA.31.000369

, , Stilwell, G. R.

, 1941

, An experimental study of the rate of a moving clock. II

, Journal of the Optical Society of America

, 31, 5, 369â€“374

, 10.1364/JOSA.31.000369

- BOOK

, Joos, G.

, 1959

, Bewegte Bezugssysteme in der Akustik. Der Doppler-Effekt

, Lehrbuch der Theoretischen Physik, Zweites Buch

, 11th

, , 1959

, Bewegte Bezugssysteme in der Akustik. Der Doppler-Effekt

, Lehrbuch der Theoretischen Physik, Zweites Buch

, 11th

- JOURNAL

, Larmor, J.

, 1897

, On a dynamical theory of the electric and luminiferous medium

, Philosophical Transactions of the Royal Society

, 190, 205â€“300

, 1897RSPTA.190..205L

, 10.1098/rsta.1897.0020

, (third and last in a series of papers with the same name). , 1897

, On a dynamical theory of the electric and luminiferous medium

, Philosophical Transactions of the Royal Society

, 190, 205â€“300

, 1897RSPTA.190..205L

, 10.1098/rsta.1897.0020

- JOURNAL

, PoincarÃ©, H.

, 1900

, La thÃ©orie de Lorentz et le principe de RÃ©action

, Archives NÃ©erlandaises

, 5, 253â€“78

, , 1900

, La thÃ©orie de Lorentz et le principe de RÃ©action

, Archives NÃ©erlandaises

, 5, 253â€“78

- JOURNAL

, Puri, A.

, 2015

, Einstein versus the simple pendulum formula: does gravity slow all clocks?

,weblink

, Physics Education

, 50, 4, 431

, 10.1088/0031-9120/50/4/431

, 2015PhyEd..50..431P,

, 2015

, Einstein versus the simple pendulum formula: does gravity slow all clocks?

,weblink

, Physics Education

, 50, 4, 431

, 10.1088/0031-9120/50/4/431

, 2015PhyEd..50..431P,

- JOURNAL

, Reinhardt

, S.

, etal

, 2007

, Test of relativistic time dilation with fast optical atomic clocks at different velocities

,weblink

, Nature Physics

, 3

, 12

, 861â€“864

, 2007NatPh...3..861R

, 10.1038/nphys778

, yes

,weblink" title="web.archive.org/web/20090712195322weblink">weblink

, 2009-07-12

,

, , S.

, etal

, 2007

, Test of relativistic time dilation with fast optical atomic clocks at different velocities

,weblink

, Nature Physics

, 3

, 12

, 861â€“864

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, 10.1038/nphys778

, yes

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, 2009-07-12

,

- JOURNAL

, Rossi, B.

, Hall, D. B.

, 1941

, Variation of the Rate of Decay of Mesotrons with Momentum

, Physical Review

, 59, 3, 223

, 1941PhRv...59..223R

, 10.1103/PhysRev.59.223

, , Hall, D. B.

, 1941

, Variation of the Rate of Decay of Mesotrons with Momentum

, Physical Review

, 59, 3, 223

, 1941PhRv...59..223R

, 10.1103/PhysRev.59.223

- WEB

, Weiss

, M.

, Two way time transfer for satellites

,weblink

, National Institute of Standards and Technology

, yes

,weblink" title="web.archive.org/web/20170529153938weblink">weblink

, 2017-05-29

,

, , M.

, Two way time transfer for satellites

,weblink

, National Institute of Standards and Technology

, yes

,weblink" title="web.archive.org/web/20170529153938weblink">weblink

, 2017-05-29

,

- JOURNAL

, Voigt, W.

, 1887

, Ãœber das Doppler'sche princip

, Nachrichten von der KÃ¶niglicher Gesellschaft der Wissenschaften zu GÃ¶ttingen

, 2, 41â€“51

,

,

, , 1887

, Ãœber das Doppler'sche princip

, Nachrichten von der KÃ¶niglicher Gesellschaft der Wissenschaften zu GÃ¶ttingen

, 2, 41â€“51

,

,

## External links

- Online Time Dilation Calculator
- Proper Time
- WEB, Merrifield, Michael, Lorentz Factor (and time dilation),weblink Sixty Symbols, Brady Haran for the University of Nottingham,

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