Talk:Time dilation/Archive 3

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Origins of time dilation

I apologize if I am supposed to write this in a different section. Please forgive my ignorance.

The introduction states that Time Dilation originated out of Einstein's Special Theory of Relativity. This is incorrect. Time Dilation was one of two parts of the Fitzgerald-Lorentz Contraction introduced in 1904, whereas Einstein's theory, introduced in 1905, was merely expounding on that —Preceding unsigned comment added by 24.10.164.230 (talk • contribs)

Sectioning added; welcome aboard! You may wish to create a user account, to avoid confusion with other users of your IP address (I count three vandalism warnings given to it). --Christopher Thomas 06:13, 20 December 2005 (UTC)

On re-read, only two warnings. I've also added the "welcome aboard" template spiel, which contains links to useful tutorials about editing. --Christopher Thomas 06:18, 20 December 2005 (UTC)

There are a number of errors in this anon's statements:

1. The Lorentz-Fitzgerald contraction dates from 1889, not 1904 [1].
2. Time dilation is not a part of it at all.
3. Time dilation is a prediction of the Lorentz transformations, first derived in 1887 and rediscovered by Lorentz in 1899 - E4mmacro 21:44, 21 December 2005 (UTC) What is the reference for 1887? see Joseph Larmor for a 1897 reference? Is 1887 a misprint? E4mmacro 21:44, 21 December 2005 (UTC) - I see that 1887 is probably a reference to W. Voigt's paper. Voigt's transformation involves a time dilation of $\gamma ^{2}$, rather than $\gamma$. All the experiments seem to confirm the Lorentz transformations, not the Voigt transformation (which also gives a universal speed of light. See Woldemar Voigt page E4mmacro 06:37, 29 December 2005 (UTC)

Both time dilation and the Lorentz-Fitzgerald contraction arise from the effects of the Lorentz Transformations. In 1904, Lorentz published a famous article on how the Lorentz Transformations of an electromagnetic field correctly account for their observed properties, but even at that time failed to grasp the significance of the time transformation for time itself. (The view at the time was that the time transformation was a mathematical artifact.)

From the number of times the words, "seems", "appears", and variants are used, I get the distinction impression that many contributors still consider the time transformation as a mathematical artifact. E4mmacro 10:40, 30 December 2005 (UTC)

1. Einstein was not merely expounding on the known. Instead he introduced a truly radical way of looking at and dealing with the Lorentz transformations. Before Einstein, everyone assummed that Newtonian physics underlaid everything, and that even the Lorentz Transformations had a classical cause. Einstein instead said that they are the real unerlying rules of physics, and that their descriptions of space and time were real.

So it was Einstein who took the admitedly known Lorentz transformations and its admitedly known time transformation into his theory, but used them to predict the previously unknown phenomenon that someone traveling in a fast enough spaceship will return having aged less than those who stayed at the home planet. --EMS | Talk 15:48, 20 December 2005 (UTC)

That is all more or less correct (undisputed), I think, until the last paragraph: time dilation is more general than the twin effect, it's foremostly the apparent slowing down of a clock that is moving relative to one's own clock. And that has, as anonymous correctly claimed, already been proposed by Lorentz (and next apparently forgotten by him!) as well as by ... (now I have a blank, sorry); only they didn't realise the reciprocity of those effects, but that was published by Poincare before Einstein finished writing his paper about it. In any case, it originated out of electromagnetic theory, and is used in special relativity, which BTW isn't exclusively of Einstein: metaphysical time and space are not official part of the principle theory that is called SRT. But that doesn't make, IMO, the intro really wrong; why can't one say that it's (also) a consequence of SRT and GRT? It's just slightly inaccurate. Harald88 17:25, 20 December 2005 (UTC)

I can't see any inaccuracy in the intro as written, unless you want to argue that time dilation is not a part of the theories of relativity. Also, even if others were aware of the time effect of the Lorentz transformations, they certainly did not give it the physical significance that Einstein did. Poincare was headed in the direction of SR, but did not quite get there, or at least not as I understand it. The view that I have encountered is that Einstein, although he was not the first to work with the Lorentz transformations, was the first to interpret them correctly as the underlying rules of the universe. --EMS | Talk 17:40, 20 December 2005 (UTC)

That definitely was Poincare, despite the nonsense that is told about him; nevertheless, we agree that that that doesn't make the intro wrong; and it's certainly counterproductive to start the article with talk about a dispute. However, the way it is stated now (I had not noticed it had changed) looks even better to me.Harald88 15:06, 24 December 2005 (UTC)

I think it is better if this article does not express opinions about who "really understood" time dilation, i.e. Einstein was smarter than his pre-decessors. So what? The article now states the facts, that Voigt, Larmor and Lorentz thought that moving clocks ran slow. The place to argue (if you must) about who was smarter is in the page on special relativuty? E4mmacro 09:16, 30 December 2005 (UTC)

If someone thinks that it is a fact that according to Voigt (what year?) moving clocks run slow, please cite him on that. I think that he didn't think so, at least not before the others came with that. It is a feact however that accoring to Einstein (1905), moving clocks run slow. Harald88 12:54, 12 February 2006 (UTC)

Operational definition of gravitational time dilation

I copy and paste from above:

[...] to me a gravitational field is defined by the result and not the cause. --EMS | Talk 05:15, 21 December 2005 (UTC)

It is also my opinion that the concept of gravitational field should be defined in terms of unambiguous result, that is: an operational definition.

First an example of no gravitational field present (Minkowski space-time): Let there be an empty (no air inside), rotating space-station, situated in Minkowski space-time. The space-station is just an emtpy hull. If you are inside that space-station, and you are not in contact with the hull, then as long as your center of mass does not move relative to the space-station's center of mass you will float indefinitely. As long as you float there will be no time dilation between you and a reference clock outside the station (A reference clock that does not move relative to the station's center of mass.)

Second example: gravitational field present (curved space-time): If you are orbiting a planet then you are following an inertial path in space-time; you are floating, and if your orbit is circular, then your distance to the planet remains constant. The operational definition of gravitational time dilation is that it occurs even in the absence of any mechanical force. There is gravitational time dilation, both in the case of resting on the surface of a planet, and in the case of orbiting a planet.

I agree with EMS that definitions should be stated in terms of observable results. Certainly a definition should never invoke some or other hypothesized cause. --Cleonis | Talk 02:39, 23 December 2005 (UTC)

Very right. But that's what the article does, isn't it? Harald88 15:00, 24 December 2005 (UTC)

Clarification sought

quote: "Time dilation occurs with respect to temporal coordinate systems set up in this manner."

A non-physicist reader might be forgiven for thinking this means time dilation does not occur unless the clock synchronisation procedure is used. He could then ask, "So clocks never ran at different rates until sometime in the late 19th century? It is just an illusuon?". I guess the time dilation always occured, but it is always difficult to write about this without making it sound like a mere "appearance" on the one hand and a "reality" on the other.

E4mmacro 22:51, 21 December 2005 (UTC)

It's not just "mere appearance", but it's difficult to explain. Maybe something like "it's a real effect, but the measured quantity depends on the measurement"... Harald88 15:14, 24 December 2005 (UTC)

request on editing

Hi please try not to save endless times small incremental edits; it makes for a horrible mess on the article spaces -- see for example: http://en.wikipedia.org/w/index.php?title=Time_dilation&action=history If you have internet problems or computer problems, you can use a text editor for the purpose of saveguarding your edits. Thanks, Harald88 15:22, 2 January 2006 (UTC)

www.wbabin.net?

There are more and more links to articles on wbabin.net and it has some interesting articles; but apparently those are in no way peer reviewed nor handed in with peer recommandation -- perhaps the editor decides if he likes the looks of it? Thus I think that that site is not a good source, so that any link to an article it should only be included in the exceptional case that there is consensus about that link being helpful. Any other ideas? Harald88 22:06, 16 January 2006 (UTC)

Animation of nonsymmetric velocity time dilation

Asymmetric velocity time dilation

I have created an animation showing asymmetric velocity time dilation. This applies for example in the case of particles following a circular trajectory in a particle accelerator. Historially, the asymmetrical case is interesting, because it served to suggest the equivalence principle. Any object that is being accelerated by a force is pulling G's.

The acceleration with respect to the inertial frame has a symmetry breaking effect. The operative factor for the amount of difference in lapse of proper time is the difference in spatial distance travelled.

A possible disadvantage of this animation is its size: 114 KB. I think that is very large. --Cleonis | Talk 15:17, 28 January 2006 (UTC)

I like the animation, it's very simple. But it definitely needs a simple explanation...what kind of a caption would you put to go along with it?Pkeck 05:02, 31 January 2006 (UTC)

If you click on the image you get the description page. What is written there is pretty much what I would add as accompanying text if the animation is included in the article. --Cleonis | Talk 05:22, 31 January 2006 (UTC)

Time Dilation Equation

Shouldn't the equation at the top of the page be: $T_{1}=T_{0}{\sqrt {1-v^{2}/c^{2}}}$

Not: $T_{1}={T_{0} \over {\sqrt {1-v^{2}/c^{2}}}}$ [unsigned by 86.0.228.241]

No it's correct, but the way it is presented is confusing, and even not sufficiently defined. I'll try to rephrase it. Harald88 12:57, 12 February 2006 (UTC)

Oh, I see, got the T's mixed up. But I would think the equation should be in terms of $T_{0}$, since that would probably be the given (just like relativistic mass is in terms of $m_{0}$, and length contraction is in terms of $L_{0}$). Plus, it's interesting to note that ${\begin{matrix}{\frac {L_{1}}{T_{1}}}\end{matrix}}={\begin{matrix}{\frac {L_{0}}{T_{0}}}\end{matrix}}$. Well, I suppose it doesn't matter. Thanks for the response. --24.236.173.129 18:02, 12 February 2006 (UTC)

Actually, this thing L1/T1 = L0/T0 is not interesting at all. See http://en.wikipedia.org/wiki/Special_relativity_for_beginners under 'Caveats and Warnings' - DVdm 6-May-2006

You are right that that can be confusing. Note that T has memory. We can replace T_0 by a more standard notation with primed for co-moving, and unprimed for rest frame; but I think that your suggestion is the clearest. OK I change that. Harald88 22:53, 12 February 2006 (UTC)

Wow, now the equation is really confusing, especially with that explanation of the variables "where $T$ is the time period as measured by a stationary observer of the time $T_{0}$ between two ticks on a moving clock, traveling at velocity $v$ relative to the stationary frame." I see what you're saying, but why be so confusing. Ok, how about this:

$T_{1}=T_{0}{\sqrt {1-v^{2}/c^{2}}}$

Where $T_{0}$ is the time experienced by a stationary observer, $T_{1}$ is the time experienced by an observer moving at velocity $v$, and $c$ is the speed of light in a vacuum. --Magnus VII 22:14, 13 February 2006 (UTC)

I have no clue what could be confusing about it. Perhaps you want to first explain in one sentence that the time between two ticks is (by definition) the time period? If you like, go ahead! But by all means avoid to use without definition the ambiguous word "time", and define about time on which clock you are talking! Usually time means coordinate time, but not here. And no, the indice "0" is commonly used for "proper", but the indice "1" is not commonly used for the frame of observation/stationary frame. Often used alternative is one version back (T and T'). People who simplify it to such an extreme level of ambiguity usually completely misunderstand time dilation... Harald88 22:01, 14 February 2006 (UTC)

Voigt and SR

Just so that you know where the Voigt business came from: [2]. BTW - I did not put that reference into this article. --EMS | Talk 19:24, 12 February 2006 (UTC)

It looks correct about Voigt: "Voigt who was writing on the Doppler shift when he wrote down the transformations." The Doppler shift is not about time dilation. Harald88 22:00, 12 February 2006 (UTC)

To my knowledge the transformations that Voigt wrote didn't play a part in the exchange of ideas surrounding relativity.

What I think is interesting is that Voigt's transformations are generally applicable in the context of wavemechanical equations; the paper by Voigt was "Über das Doppler'sche princip".

I just found this webpage that discusses connections between doppler shift and Lorentz transformations, suggesting a connection between the Lorentz transformations and wave mechanics. --Cleonis | Talk 20:20, 12 February 2006 (UTC)

Oops, the site above is an anti-relativity site. I should have read it more carefully first. --Cleonis | Talk 20:27, 12 February 2006 (UTC)

A quick look at the first page yields: "We also know that Relativity is true and that it involves the Lorentz transformation" Thus that site may be anything, but hardly an anti-relativity site. Harald88 22:00, 12 February 2006 (UTC)

Gabriel LaFreniere opposes relativity in the sense that he assumes that every object has a particular velocity with respect to the ether. It is one thing to accept the Lorentz transformations, and another thing to accept the axiom of special relativity that there is full blown symmetry of all inertial frames of reference. Unfortunately, many people who accept the Lorentz transformations, but reject the full blown symmetry, claim to be supportive of special relativity, since they feel that the full blown symmetry shouldn't have been incorporated in the first place. It is very awkward that this mis-appropriation of the expression 'special relativity' occurs, it leads to a lot of babylonian confusion.

Apart from that, I think Gabriel LaFreniere's site is very interesting, I hope to gain more understanding of the connection between Doppler shift and the Lorentz transformations through his animations. --Cleonis | Talk 10:55, 13 February 2006 (UTC)

Cleonis, on the symmetry question and those who believe in an absolute rest frame. If, as Lorentz appeared to think and Ives too, one might say that there is an aether, or absolute rest frame; the speed of light is isotropic in this rest frame, and the length and time rates change for moving objects (as a function of velocity relatibve to this rest frame). One can accept the clock synchronisation procedure in moving frames leading to Lorentz's local time as shown by clocks in moving frames when so synchronised. A new measured-velocity addition law follows. Then measured-cordinates in all other reference frames are related to the mesured-coordinates in the rest frame by the Lorentz transformations. It then follows that the rest frame is identical to any inertial frame of special relativity. The symmtery properties of the measured-coordinates follow from the equations. Thus, I think for Lorentz the symmetry is a symmetry of the coordinates defined by a certain convention. It is not a symmetry of real contractions, which Lorentz woudl say was illogical. He can still believe that for the real lengths and real times (i.e. defined relative to the rest frame) there is no symmtery; objects moving faster are contracted more. That is, he says "one of these rest frames (I don't claim to know which one) is the rest frame", the length contrations and time dilation predicted by special relativity in frames moving wrt this rest frame are real, the symmetry lies only in the way the coordinates are defined, a symmetry between measured-lengths and indicated-times". He hasn't said anything wrong, and can't be contradicted. He can just be criticised for the style of expression (i.e. you could say it is not necessary to assume a rest frame, it adds nothing important). Or am I missing something? E4mmacro 22:40, 11 April 2006 (UTC)

Senselessness

I know, it makes no sense, anyway! If universe is a closed system, conservation of momentum takes place. So, at, before, during, after the big bang, momentum should be the same, what ever happend. Therefore there exists a point of view (and so a lot of frames TypeX), where momentum is zero. But, universe contents energy, therefore objects are moving relative. But, an object with zero momentum in a frame TypeX "has speed zero" and therefore the highest rate of time. In this sense, there is an absolute frame. But, all macroscopic objects in the universe are very close to rest and all object with relativistic speed, we observe, are moving relativ to TypeX and, therefore are timedeleated. Still the laws of physics are the same for all inertial systems and there is no contradiction between a fixes frame and RT. ErNa 09:30, 12 April 2006 (UTC)

Gravitational redshift/time dilation is now crappy

It is an often heard misconception that photon's "fall" so that light speed increases and the frequency increases. Instead, light speed decreases but in steady state that's irrelevant: the only thing that matters is the difference in local clock rate. I have papers about that somewhere (starting with Einstein 1911) but I won't have much time to bother about this in the coming weeks. Anyway it's well explained elsewhere on this page as well as in the redshift article, if it hasn't been messed up of course.Harald88 22:13, 12 February 2006 (UTC)

I'm pretty sure that "light speed" stays constant. Thats one of the postulates of relativity. What are you trying to say about light speed decreasing? User:fresheneesz 128.111.95.147 21:51, 10 April 2006 (UTC)

This actually depends on how you define your coordinate system. The problem is especially noticeable when trying to build a coordinate system to describe a black hole. While my personal preference is for coordinate systems where the speed of light is treated as constant, you can certainly construct ones where it isn't that end up being mathematically equivalent. --Christopher Thomas 03:52, 11 April 2006 (UTC)

It's not a postulate of GRT. Einstein's basic calculations of bending of light (1911, improved in 1915/1916) rely on Huygens bending - which, as every physicist knows, depends on local differences in the speed of light. In fact he explains it very clearly in his 1916 paper which is linked to from the general relativity page. Light speed is lower near the earth, quite the opposite of what "falling" suggests! Harald88 06:27, 21 April 2006 (UTC)

One might add that if the speed of light were not lower near the earth we wouldn't even existed since there wouldn't be any gravitation to hold things together.

It's really funny how little people (physicists including) know about basic physics. Most of them don't even know that the reason for gravitation is this variable speed of light. They "assume" that speed of light is "constant". It happens 90 years after Einstein's discovery of the mechanism of gravitation.

But if one tries to explain in Wikipedia the mechanism that Einstein proposed then your page gets deleted by someone who never understood Einstein's idea and so he is against it. As Einstein said: "There are two infinite things ...". Jim 06:23, 3 May 2006 (UTC)

I think you misunderstood that. According to GR light travels at constant speed in a straight line - it's space itself that's curved. Catch up on your reading, folks. Shinobu 17:39, 13 June 2006 (UTC)

Revert by Harald88

I was wondering why you reverted the explanation of "lower potential in a gravitational field". First of all, that is a long drawn out way of saying "stronger gravitational field" as i put in parens. My version (i think) is easier to understand, and shorter at that. Secondly, accellerating reference frames are postulated to be the same as reference frames in a gravitational field, so why did you remove the addition of accelleration to the "gravitational time dialation" part? User:fresheneesz 128.111.95.147 21:51, 10 April 2006 (UTC)

It's not the field strength but the potential that matters - just as in electricity it's not correct to replace potential by strength. And what is regarded to be an accelerated frame in SRT can be regarded as an inertial frame with gravitation in GRT - which isn't at all what you made of it! When we assume an accelerating frame (without gravitation) at low speed, then we have no time dilation but only a Doppler effect instead. Harald88 12:55, 12 April 2006 (UTC)

Hmm. But doesn't field strength increase directly proportional to gravitational potential? I don't see the difference here. In electronics it would be just as correct to replace potential with Efield strength, but.. it would be mathematically much harder. Also, when you say "what is regarded to be an accelerated frame in SRT can be regarded as an inertial frame with gravitation", that is true - but accelleration is not gravity - especially when talking to people who may not be well versed in general relativity. Gravity does not mean accelleration, but they have equivalent effects, which is why I added both into the explanation. Also, at low speeds you have low time dialation - not none. Fresheneesz 10:29, 20 April 2006 (UTC)

It's mathematically just as erroneous to confuse gravitational field strength with potential as in its electrical equivalent: one can make a zero field strength zone at high potential. Apart of that, acceleration is indeed not gravity; most important difference is that acceleration does *not* directly relate to time dilation, while gravitation and speed do (just look at the equations, or read the article on Twin paradox). Harald88 06:37, 21 April 2006 (UTC)

? I was under the impression that accelleration *does* directly relate to time dilation, and I would have cited the twin paradox myself. In the twin paradox, one twin is older than the other *because of his accelleration* - not because of his speed.

As for potential, you're saying that if one person was directly between two large point masses (ie he had no net force on him), then he'd be in a different frame than someone infintely far away from those masses (also with no net force on him)? Fresheneesz 09:04, 21 April 2006 (UTC)

That's wrong: time dilation is a function of speed v, not of acceleration dv/dt - as also explained in Twin paradox. It's a similar error as mixing potential with field.

And I'm saying that GRT's time dilation is a function of gravitational potential, and not of gravitational field strength (an easily verifyable fact). That implies a redshift even of light that is emitted from gas between two stars (double star). Harald88 13:25, 21 April 2006 (UTC)

Isn't potential a function of the strength?

I'm pretty sure that ones time dilation *does* depend on their acceleration. How else do you explain the twin paradox? I thought that one twin accelerates away, then deccelerates and accellerates back - finnally deccelerating again and stoping at earth. Both had the same speed relative to eachother, but the one that underwent acceleration had a bit of time dilation that *does not* depend on one's inertial reference frame. Am I wrong here? Fresheneesz 22:46, 22 April 2006 (UTC)

Acceleration only indirectly affects time dilation, as explained in twin paraodox. But twins can have had equal accelerations during equal times and still end up with a time difference, because time dilation is a direct function of speed and not of its derivative. If that article isn't clear (indeed it's an often misunderstood subject), please use the corresponding Talk page. Harald88 11:55, 23 April 2006 (UTC)

Let me see if I can straighten this discussion out a bit, but I would start off by saying the this is about gravitational time dilation, which is a different topic. Fresheneesz wrote:

As for potential, you're saying that if one person was directly between two large point masses (ie he had no net force on him), then he'd be in a different frame than someone infintely far away from those masses (also with no net force on him)?

The simple answer to that is "yes". At the least, the clock of the observer between the masses would tick slower that that of an observer distant from the masses.

Gravitational time dilation appears in accelerated frames of reference. In this case, the people being accelerated are constantly shifting from one intertial frame of reference to the next. When you work througth he math, you find that the magnitude of the effect is proportional to the sqaure of the change in velocity for a freefalling object as it goes between the positions, which is itself given the gravitational potential, or $\Phi =gh$. I hope this settles the issue. --EMS | Talk 00:42, 23 April 2006 (UTC)

Ok, thanks EMS. The issue isn't quite solved tho, since I would like to make it clear that acceleration causes "gravitational time dilation" - not just difference in potentials. Fresheneesz 11:18, 23 April 2006 (UTC)

Precision: Gravitational time dilation appears when one uses an accelerated frames of reference and pretends that it's an inertial frame of reference. It disappears when one takes into account that it's an accelerated frame, as it's fully covered by the Doppler effect. In fact, that's how Einstein, using the equivalence principle, predicted gravitational time dilation in the first place! He assumed no time dilation effect from acceleration. In agreement with that, acceleration in ultra centrifuges has been found to have no measurable time dilation effect on radioactive particles - only their speed matters. That's why I revert such erroneous statements. Harald88 11:55, 23 April 2006 (UTC)

Hmm, thats very interesting. However, I wouldn't say its "fully" covered by teh Doppler effect - just as much as inertial time dilation isn't "fully" covered by the invariance of c. We can derive the dilation from the invariance of c and the doppler effect, but that doesn't mean that we shouldn't say that accelleration causes time dilation.

I'm quite interested in the effects of gravity and acceleration in relativity, but I can't find anything on it here on wikipedia. I'm simply trying to give the accelerative effects of relativity their fair spot. Fresheneesz 21:11, 23 April 2006 (UTC)

Theory as well as experiment can't be denied in Wikipedia by editors who are "saying something else". In relativity, acceleration has no other effect on time dilation than by means of speed change - that's all there is to it.

About acceleration, see twin paradox (I already told you, couldn't you find it?); and about gravitation, see general relativity. See also redshift (soon to be a featured article candidate). Harald88 22:48, 23 April 2006 (UTC)

I've read over some of the twin paradox a couple times now. Its still very unclear what you mean. You say "acceleration has no other effect on time dilation than by means of speed change" this is obviously true because acceleration is only a speed change. However, from what I've gleaned from all this, acceleration *does* have an effect on time dilation, but only if you choose an accelerating reference frame. Obviously, one twin must choose an accelerating reference frame. Its also unclear why " physicists have increasingly treated general relativity as a theory of gravitation, rejecting its aspirations to be a theory about relative motion including acceleration" (quote at end of twin paradox). Obviously there are effects of accelleration that aren't explained by inertial time dilation - despite the fact that they come from the same principles. Fresheneesz 03:21, 26 April 2006 (UTC)

What I meant is relevant for your last sentence: there is no known (measured or predicted) "time dilation" effect of acceleration that is not already accounted for by what you here call inertial time dilation. Cheers, Harald88 21:20, 28 April 2006 (UTC)

Hi Fresheneesz, don't worry. Physics is a very vast field and no physicist can be an expert in all areas. So they fight against those, that say, RT is principally wrong. But, they do not spend efforts to find out, what RT really means. RT is old fashioned! They main point is: laws of physic to not depend on the related inertial frame. That is clear. But, this does not say: there is no inertial frame of absolute rest! What is true: When to particles are in relative rest, gaining speed is equal to acceleration. Therefore it is clear, which ones time will be dilated. Particles know about there history. The twin paradox -in the sense of symmetrical timedilation- arises from the fact, that each twin "forget" his history. And most people say, there is no experiment, that can proof, which one was accelerated. But, this experiment is very simple, but not well known. ErNa 07:19, 28 April 2006 (UTC)

The problem here is mixing math with physics. The "velocity" has no physical existence (one can't even meassure "velocity" because it is always relative to a ref frame which means that it is only a mathematical idea connected with masurement. The "acceleration" on the other hand has physical (absolute) existence (measured absolutely by an accelerometer). Time dilation, to have physical meaning, must therefore depend on acceleration. And it does, since it is the time integral of acceleration times the distance at which the observation is made. Since at the distance zero, regardless of acceleration, this time integral is zero our "time dilation" is zero as well. But at a non zero distance the "time dilation" is non zero and so integrating acceleration at this distance we accumulate dilated time. And that's why the time dilation looks like depending on velocity because the velocity is also the time integral of acceleration.

I hope that now you understand why one is right saying that "time dilation" does not depend on acceleration (when at zero distance) and why one is right saying that it depends on acceleration (as in the twin paradox when there is non zero distance to the point where twin's acceleration is relevant and why accelerations at start and end points don't matter). Jim 07:42, 3 May 2006 (UTC)

If speed has physical existence has been the subject of endless debate, and it has never been solved officially (that is, different authors reached opposite conclusions in peer reviewed publications). However, that acceleration has no direct effect on time dilation is well known and also explained in the article twin paradox. If it's not clear, please comment there and explain what is wromg or not made sufficiently clear according to you so that the article may be improved. Note also that the distance to an ultracentrifuge doesn't affect the amount of time dilation - thus it's at best misleading to present time dilation as due to acceleration at a distance. Harald88 10:15, 3 May 2006 (UTC)

Time periode

ΔT is the time period as measured by a stationary observer,

ΔT₀ is the time period of the moving object as seen by the stationary observer,

I miss the definition of "time periode" ErNa 15:24, 13 April 2006 (UTC)

Perhaps clock cycle is clearer. But, on second thought, "time interval" is probably more standard. Harald88 10:31, 6 May 2006 (UTC)

Urashima effect

Dear Harald88, perhaps you were not familiar with this term. For clarification, the term "Urashima effect" is not used to suggest that there is really anything magical, or fairy-tale like going on.

What is meant is the relativistic time dilatation, and it's often called Urashima effect due to the parallels of the physical effect of diverging eigentimes, and the old fairy tale. Shinobu 20:20, 23 May 2006 (UTC)

Dear Gerbrant, perhaps it's sometimes called so in Japan?

For sure, I've never seen it called like that in the English scientific literature; it certainly isn't common enough for the intro. And for possible inclusion in the article, please provide references.

Harald88 21:07, 23 May 2006 (UTC)

In my experience scientific literature tends not to contain any kind of colourful wording whatsoever. At my university we are even trained not to use any kind of language that is not dry as sand, and from the books and papers I've read I'd say this practice must be commonplace - the horror... but I digress.

Doing a websearch for terms like "Urashima-effect", "ウラシマ効果" and "浦島効果" yield plenty of hits. A lot of those from Japan (most certainly those with the Japanese characters), which stands to reason as this metaphor is based on a Japanese fairy tale, but not all.

These searches even revealed that this article has a sister-article in the Japanese language version, to which I'll add an interwiki link straight away.

As for the location of the addition, I chose it because "Urashima effect" should redirect here, and hence the term should be boldfaced, and I don't like it when boldfaced definitions are somewhere at the bottom of the article. But maybe that's just me.

As for wording, language and detail, that can, perhaps, be improved upon - a much more sensible reaction than blindly reverting in any case. Have a bit of good faith in your fellow editors - we're all in it together. Shinobu 22:47, 23 May 2006 (UTC)

I don't think this it merits enough significance to be mentioned in the introduction, if indeed it should be mentioned at all. This is the first I've ever come across it. Perhaps the claim could be included somewhere, properly sourced, but I would favor its removal. — Knowledge Seeker দ 23:10, 23 May 2006 (UTC)

First off, I'm surprised you don't know the term - but then again, people have different backgrounds. I've researched this matter some more, and it seems that this is the term for "time dilatation" in Japanese, as in the most commonly used. I'm not Japanese, so it has at least some penetration outside of Japan as well. Maybe I'm too much of an SF fan? Maybe.

But I still think it's notable enough to warrant inclusion, although possibly not in the lead paragraph. I also think it's a good metaphor, providing the reader with an interesting bit of general knowledge. Shinobu 00:09, 24 May 2006 (UTC)

I have moved it down to "Time dilation in popular culture", which, although not a perfect match, seemed the most fitting for now. Shinobu 00:13, 24 May 2006 (UTC)

Suggested Subdivision of Article for Clarity

It strikes me that this article has three audiences: (1) the inquiring layman who's just looking for a very general overview in everyday language (ie, "The results of a great number of experiments have been interpreted as confirming Einstein's theory: that time runs more slowly for objects moving at speeds approaching that of light."); (2) nonexperts with some significant scientific background who find themselves "unclear on the concept" and would like a discussion of apparent inconsistencies, objections, and misconceptions; (3) persons looking for the math. Couldn't the article be restructured accordingly, in three labelled sections? -S. Dickerson

Anyone can edit Wikipedia. So I advise getting to work. As a practial matter, it is best to create an account, draft the revised article in one's user space, and when it is ready tell people about it here and get their opinions. --EMS | Talk 05:30, 10 June 2006 (UTC)