Talk:Time dilation

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Time dilation was a good article nominee, but did not meet the good article criteria at the time. There are suggestions below for improving the article. Once these are addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake.

Reviewed version: December 16, 2006
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[edit] Lack of citation in First section

To DVdm:

The main issue is that there is no citation at all. How do we know it was not written by a crank? For example in A.P. French "Special Relativity" 1968 p.100, in essence, it states that you need 3 clocks to see what is going on, which is discussed to death on the twin paradox page. Anyway, for years this page has contained very cranky stuff. Why do you delete citations of other articles? Jok2000 18:23, 28 October 2007 (UTC)

Hi Jok2000, I deleted your first fact request because the fact is explained in the definitions of the first section, and I undid your second edit because it made the concept frame of reference point to twin paradox, whereas the latter is much narrower a concept than time dilation and even more so than frame of reference. I.m.o. the introduction has sufficient pointers to other wiki articles, although it might be a good idea to have an extra wikilink to the time dilation section in the main article on special relativity.
And of course, if it is written by a crank, it will be quickly (or at best ultimately) reverted :-). I admit it's a while ago since I had a complete read of the article, but do you see cranky stuff in its current state? Cheers, DVdm 18:53, 28 October 2007 (UTC)
Er, the article has been pretty much devoid of citation (using the "ref" tags) for 3 years running. Jok2000 23:46, 28 October 2007 (UTC)
Ah, you mean actual <ref>tags</ref>. Yes, I agree that the article could use a few more of those. But at least the article is stable. :-) DVdm 07:56, 29 October 2007 (UTC)

[edit] Phrasing of 2nd paragraph

In the 1st bullet of the 2nd paragraph of the article itself, where it says, "In special relativity, clocks that are moving with respect to an inertial system of observation (the putatively stationary observer) are found to be running slower", it would be more accurate to replace "are found to be running slower" with "are observed to be running slower". The reasons for this (important) word change are well stated by what follows in the article itself.

In the 2nd bullet, one would not make an analogous change to "In general relativity, clocks at lower potentials in a gravitational field — such as in proximity to a planet — are found to be running slower" as that would be a change for the worse. The reasons for the different wordings is well stated in the 3rd and 4th bullets. Since I have not yet read the history of this article, I will let someone who has been involved in developing this article make the change - or if there is a consensus against and rationale for not changing, please put in the Discussion. Otherwise, I'll assume it hasn't been considered one way or the other and change it, after a week passes, to flush out other views. (Jok2000's comments below have been addressed.) TwPx 04:00, 7 November 2007 (UTC)

They have not been addressed. You have not provided a citation that justifies removing the word "found". Actually, this is ridiculous. I have no stated position aside from WP:V so far. Jok2000 20:46, 9 November 2007 (UTC)
Jok2000, as I have read the history of this article and another, I have been impressed by your politeness and sincere desire to get it right. So my apologies for obviously pushing the wrong button by my "have been addressed" comment above. I was not saying it as arguer AND judge. I was just noting in the main section that I had seen your subsequent comment below and had responded to it and, hence, my intent to do the change had not been subsequently changed by your comment.
TwPx 00:01, 14 November 2007 (UTC)
I disagree, some of the experiments listed involve moving a clock around and bringing it back and looking at the two clocks. That would be a finding. Going through the pain of synchronizing GPS satellites' clocks using transmitted signals, now that's an observation. Jok2000 04:14, 7 November 2007 (UTC)
"Found" is certainly correct for clocks that are brought back, but it's inaccurate for clocks that are in inertial motion - slowness is in that case no absolute. Thus the addition of for example "measured" is certainly an improvement eventhough it makes that sentence a little more complex. I'll be bold about it and make that little change. Harald88 12:47, 11 November 2007 (UTC)
Your 1st sentence is correct (e.g., J.C. Hafele and R.E. Keating, Science 177, 166 (1972)). However, just as in the Twin Paradox debate, there is great debate about what was the cause of that net proper time difference - so your position is assuming that one of a number of different positions is correct. (By the way, the position that you side with is currently a small minority position, if published papers are a good measure - not that that rules it out.) Hence, there are unresolved, to say the least, theoretical problems with your position. In addition, the GPS, VBLI and NASA data sets argue against your position - if need be, I and others can go into greater detail. In addition, Special Relativity says that both A and B will observe the other's clock to be running slow. So that says it is unquestionaly correct to use the suggested "observe".
I didn't understand what you meant by "Going through the pain of synchronizing GPS satellites' clocks ...", but, from the context, I think you were suggesting that saying that gravitational time dilation is on shakier ground than SRT time dilation on the "observed" vs "found" issue. That's incorrect. The physical, asymmetric difference in clock rates as a function of the difference in gravitational potential was shown to virtually everyone's satisfaction (e.g., Pound, R. V.; Rebka Jr. G. A. (November 1, 1959). "Gravitational Red-Shift in Nuclear Resonance", Physical Review Letters 3 (9): 439-441, R.V. Pound and J.L. Snider, Phys. Rev. 140, B788 (1965)) - well before GPS sattelites .
Also, your argument is in direct contraction to bullets 3 & 4 of the article itself. As I look at the rest of the article, I now see that the three references I gave above are also mentioned in the body of the article which is good as they are the classics and quite relevant. Hence, consistent acceptance of your arguments would require a significant re-write of the current article.
TwPx 18:01, 7 November 2007 (UTC)

[edit] Voice of a Distant Star

Time dilation plays an integral part of the story in the short anime film Voices of a Distant Star. Young lovers are separated when one decides to enlist in a six month tour of the galaxy and the other ages many years.

That wasn't time dilation. The girl traveled by way of a wormhole to a distant location, many light-years away. There was no time-dilation, it's just that it took many years for her messages to reach him back on Earth. I removed it. Nik42 06:54, 8 November 2007 (UTC)

[edit] Readability

I think this article is well done. I have just two comments - here and in the next section.

First, I will basically repeat what I said for another physics article. Reading this article, I wonder if the authors were considering who the typical reader was and crafting the article for maximum readability for that audience or if they were sometimes writing for their peers and/or just writing what they know.

Someone once said, “Women make better teachers than men, because women tend naturally to focus on helping the student learn whereas as men are interested in showing what they know.” I don’t know if that’s true, but it makes a good point. As I read, some sections of the current article, I get the impression that the author “knows it and wants to show it” as opposed to being really focused on what the typical reader is looking for.

Since you all are interested in physics, you've probably heard that when Stephen Hawking discussed writing his “A Brief History of Time”, his savvy editor told him that for each equation in the book, expected sales would be cut in half – not just that people would skip those sections, but that when they skimmed the book and saw equations, their eyes would glaze over and they would go elsewhere. I think it’s a rare reader who will come to the Wikipedia Time Dilation article wanting to crawl through a set of equations. Even for that rare reader, the best approach is probably to guide him to some source intended for technical exposition as in “For a more in depth discussion see Jones’ ‘ABC of Relativity’ and for the full treatment see Smith’s ‘A To Z of Relativity’.” The typical reader may come to the article because there was some reference to the "Time Dilation" in a newspaper, magazine, scifi book, etc., but he probably will not be using it to supplement his reading a special relativity text book.

I think it would be good for all the major points to be made in readily understandable English before hitting a mass of equations and/or technical discussion.

I really doubt that any reader will get anything out of the Time dilation at constant acceleration section and strongly suggest it be deleted or at the very least moved to the very end where it will scare off the fewest number of readers. I liked the Simple inference of time dilation section, but I would suggest it follow the Time dilation is symmetric between two inertial observers section which is very good and the The spacetime geometry of velocity time dilation section which is also very good and maybe both of those sections should be moved up. I'll leave it to others to change or not. TwPx 02:52, 14 November 2007 (UTC)

Seriously, it's wikipedia, someone put a laymen explanation summary near the top of the article. -donald duck —Preceding unsigned comment added by 75.185.132.42 (talk) 03:51, 8 February 2008 (UTC)

[edit] "Stationary"?

The reference to "stationary" in this article when discussing special relativity is inadvisable. Among other things, it is extremely misleading to the naive reader as it conjures up the idea that there is some special frame, as in aether theory, against which (absolute) velocity is measured - even if that was not what was meant or explicitly stated. Einstein and special relativity are quite clear that all inertial frames are on equal footing and to claim that one frame, even in the context of one example, is a special ("stationary") frame is, at the very least, misleading.

I see that some here understand this: "That animation is designed to hammer in the point that a distinction between "being stationary" and "having a velocity" does not enter special relativity, the symmetry is unconditional" - Cleonis

In particular, there is absolutely no justification to claim, "The Einsteinian takes seriously the thesis that all motion is indeed relative to some actual (if specified only by implication) "benchmark" that is regarded as stationary, ...".

Hence, I would recommend changing such references and would plan to do so after, of course, waiting for persuasive feedback.

As it happens, "stationary" is also used in the article when discussing GRT and as such is not subject to the above comments. The quote is "Thus gravitational time dilation is agreed upon by all stationary observers, independent of their altitude". As a minor note, I think it would be clearer to delete "stationary" from that sentence. (It's usually dropped as the key for agreement is not the observer's motion relative to the observed, but the observed's motion relative to one another or rather their lack of or negligible relative motion.) TwPx 03:00, 14 November 2007 (UTC)

There is a responsibility at Wikipedia to keep things in layman's terms. Using the word "putative" in front of "stationary" is a clue that there is more to it for those who wish to read on further. This way the article increases in complexity for those who want to make the effort of reading on. Jok2000 21:47, 14 November 2007 (UTC)

[edit] Observers on the ground will NOT agree

"an observer at the top of a tower will observe that clocks at ground level tick slower, and observers on the ground will agree."

I see this has been brought up but I don't see an explanation and experiments cited in the article seem to dispute it completely. Shouldn't we remove that line?

If the clock is moving slower due to time dilation from gravitational effects the observer next to the clock will be in the same gravitational field and therefore the same realm of time as the clock. So no, the observers on the ground will NOT agree with the person on the tower. Observers on the ground will see their own clock as moving at the normal rate while the clock on the tower will appear to be moving fast. Observers on the tower will see their clock run at the normal rate while the clock on the ground will appear to be moving slow.

"Hafele and Keating, in 1971, flew cesium 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 US Naval Observatory. Two opposite effects came in to 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. Pound, Rebka). But also, contrastingly, the moving clocks were expected to age more slowly because of the speed of their travel. The gravitational effect was the larger, and the clocks suffered a net gain in elapsed time. To within experimental error, the net gain was consistent with the difference between the predicted gravitational gain and the predicted velocity time loss."

Cyberben3d 17:28, 14 November 2007 (UTC)

I did not write the quoted sentence in the article, but let me confirm that it is correct. I think the problem maybe that you, Cyberben3d, are interpreting it differently than it was intended to read and how I, as a first time reader of it, interpret it. It's correct in the limited assertion that both observers will agree that the clock on the ground ticks slower than the clock in the tower.
Your argument doesn't seem to debate that point. You are correct that the two observers will see things differently in that both will see their own clock as ticking at a "normal" rate, but that is not in contradiction to the quote's claim.
I see that you are already familar with the original citation, Pound, R. V.; Rebka Jr. G. A. (November 1, 1959). "Gravitational Red-Shift in Nuclear Resonance", Physical Review Letters 3 (9): 439-441, supporting the quote.
TwPx 19:44, 14 November 2007 (UTC)

[edit] Problem with the article that is misleading readers

There is a problem with the article that is misleading readers. It is NOT pointed out that the sign of the 'Delta T' is dependent upon direction of motion. Dilation is NOT the only choice.
The Lorentz-Einstein time transformation equation can be written t' = gamma (t - vx/c^2)
And this form makes it clear that the sign of the correction depends on the sign of v.
Normal convention has v as positive when the distance is growing and v as negative when the distance is shrinking.
x is, of course, the location on the x axis or the distance between the observer and the object being studied.
You will also notice that relativistic time correction can involve dilation or contraction, matching the relativistic Doppler effects.
The idea that 'ALL moving clocks run slower than our clock' is incorrect. Only those moving away run slower.
Those moving toward us will appear to be ticking faster than our clock to us.
This SOLVES the 'paradox' in the symmetric acceleration example by removing the apparent problem some see because they do not realize that relativity can compress the time (as WE see it) of an oncoming object.
N5bz (talk) 20:27, 27 November 2007 (UTC)

With the usual definition (see article) for time dilation to come into play, the x in the equation t' = gamma (t - vx/c^2) is supposed to be 0, meaning that the clock is at rest in the unprimed coordinate system (x,t). In that case the equation reduces to t' = gamma t, meaning that the time between the 0-tick and the t-tick on that clock is measured to be t' in the primed coordinate system (x',t') in which the clock is moving, t' being larger than t by the factor gamma, hence the term time dilation. DVdm (talk) 22:34, 27 November 2007 (UTC)


Perhaps I am wrong. I thought that x and v were measured in the unprimed frame and refered to the

distance to the OTHER clock, and the velocity of the OTHER clock as measured in the stationary frame.

[quote from AE 1905 paper]

Now to the origin of one of the two systems (k) let a constant velocity v be imparted in the direction of the increasing x of the other stationary system (K), and let this velocity be communicated to the axes of the co-ordinates, the relevant measuring-rod, and the clocks. To any time of the stationary system K there then will correspond a definite position of the axes of the moving system, and from reasons of symmetry we are entitled to assume that the motion of k may be such that the axes of the moving system are at the time t (this “t” always denotes a time of the stationary system) parallel to the axes of the stationary system.

[unquote]

so the velocity and distance traveled are as measured in the stationary system but they are of the moving object. And he is talking about something moving AWAY at v. "in the direction of increasing x".

[quote]

Further, we imagine one of the clocks which are qualified to mark the time t when at rest relatively to the stationary system, and the time tau when at rest relatively to the moving system, to be located at the origin of the co-ordinates of K, and so adjusted that it marks the time tau. What is the rate of this clock, when viewed from the stationary system? Between the quantities x, t, and tau, which refer to the position of the clock, we have, evidently, x = vt and tau = gamma (1-vx/c^2).

[end quote][note: I substituted gamma for 1/sqrt(1-v^2/c^2) in this quote]

[quote]

whence it follows that the time marked by the clock (viewed in the stationary system) is slowed....

[unquote]

So I read AE as speaking about a clock that is moving AWAY at v. So, unless I am misreading badly, the x means the distance (as measured in K) of THE MOVING CLOCK. And v is the velocity of same as measured in K.

The fact that AE has not bothered to explain the case where the clock is approaching the origin of K at -v and is located at x does not seem to me to imply that Tau will ALWAYS be less than t.

I think he just expected us to understand that tau could be greater or less than t, depending on sign of v.

I suspect that if we start his derivation using '-v in direction of decreasing x' we will derive exactly the same equation as he did using 'v in the direction of increasing x'.

But perhaps I am misreading. --N5bz (talk) 00:09, 28 November 2007 (UTC)


Perhaps you're just a bit confused by the inevitable multitude of choices of variables notation. For a clear understanding I'd refer to the section Time dilation and length contraction in the main Special relativity article, and then review the definition (using slightly different notation) in the overview of this article.
If you have a clear understanding of that, then keep in mind that above (with your choice of the equation t' = gamma (t - vx/c^2) ), we have implicit deltas, meaning that (x,t) = (x',t') = (0,0) is the event of coincidence of the two origins, such that in terms of the deltas, we have Delta(x) = x-0, Delta(t) = t-0, Delta(x') = x'-0, Delta(t') = t'-0.
If this does not help, since this article content/format talk page is not the place to explain such things, perhaps you could go to a Usenet forum like for instance sci.physics.relativity, where surely you will find someone who will help you. Good luck. - DVdm (talk) 09:22, 28 November 2007 (UTC)

[edit] Apparent contradiction

Under "Velocity and gravitational time dilation combined-effect tests" (specifically the bullet about GPS), the article states that "the in-orbit clocks are corrected for both special and general relativistic time-dilation effects so they run at the same (average) rate as clocks at the surface of the Earth."

A few paragraphs later, under "Time dilation and space flight", it says "at the velocities presently attained, however, time dilation is not a factor in space travel."

These two statements appear to contradict one another. If differences in clocks on moving GPS satellites are found, then certainly differences in clocks on our spaceships can be found as well. What does "not a factor" mean? Would a two-second difference on a moon mission qualify as "not a factor"? Certainly the astronauts wouldn't notice when they return to earth. But it's still a factor, I would argue, even if it's only a fraction of a second.

I cannot find any valid reference on the internet to the difference in time that was observed between the clock placed on Apollo 11 and the stationary clock on earth. However, I do recall reading, in the 1969 Worldbook, that a difference was observed. I will keep looking. Dbrashj (talk) 16:32, 28 November 2007 (UTC)

From Global Positioning System#Relativity:
"For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45.9 microseconds (μs) per day, because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7.2 μs per day. When combined, the discrepancy is about 38 microseconds per day; a difference of 4.465 parts in 1010.[18] To account for this, the frequency standard onboard each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.[19]"
The Apollo missions went somewhat farther out of Earth's gravity well of course, but I kind of doubt they carried a clock capable of measuring measuring the difference over the few days of the missions.
—wwoods (talk) 21:11, 28 November 2007 (UTC)


Of course it couldn't possibly be due to a doppler effect through a medium, to think so is heretical. 98.165.6.225 (talk) 20:23, 14 April 2008 (UTC)

[edit] Time dilation is symmetric...

Common sense would dictate that if time passage has slowed for a moving object, the moving object would observe the external world to be correspondingly "sped up". Counterintuitively, special relativity predicts the opposite.

A clock is a frequency, therefore doppler shift applies. Events happen faster in the forward direction and slower in the backward direction. Referring to the twin paradox example, the traveling twin records every event that happens for the homebound twin, but in a shorter time span by his clock. The same number of events happened in a shorter time interval, thus the rate of activity was greater for the moving twin. The moving twin (or a detection device) has altered perception due to time dilation, it's not magic. —Preceding unsigned comment added by Phyti (talk • contribs) 23:20, 6 March 2008 (UTC)

[edit] Formula for proper time is wrong.

The article lists the formula for proper time as:

t^*=\frac{c}{g} \cdot \ln \left( \left(\sqrt{c^2 + v_0^2} - \frac{v_0}{\sqrt{1-\frac{v_0^2}{c^2}}} \right) \cdot \frac{\sqrt{c^2 + (g \cdot t + v_0/\sqrt{1-\frac{v_0^2}{c^2}})^2} + g \cdot t + v_0/\sqrt{1-\frac{v_0^2}{c^2}}}{c^2} \right)

Any value of v0 greater than 235682253.9 m/s will cause sqrt(c2+v02)-v0/sqrt(1-v02/c2) to be negative, causing the value inside the ln() function to be negative, resulting in an invalid result. It appears to be correct for v0=0. I suspect the formula is wrong for any non-zero v0. Unfortunately, I don't understand the article referenced by the section to pull the proper formula out. 70.95.251.55 (talk) 10:01, 16 March 2008 (UTC)

In the spacetime slice of a single map frame used to define simultaneity, proper time τ elapsed during constant proper acceleration g from a co-linear initial velocity vo follows from the integrals g = Δw/Δt = cΔη/Δτ and the velocity conversions η=tanh-1[v/c]=sinh-1[w/c]. The result in terms of elapsed map time Δt is \Delta \tau = \frac{c \Delta \eta}{g} = \frac{\sinh^{-1}[\frac{g \Delta t}{c}+\sinh[\eta_o]]-\eta_o}{g/c}. I think the expression above does work for vo=0, as you suggest, since ln[sqrt[1+t2]+t]=sinh-1[t]. Thermochap (talk) 12:09, 17 March 2008 (UTC)
Note: The accelerated motion expressions for x (compared to Δx=c2Δγ/g) and v (considering Δη=gΔτ/c) in that same section are correct for any vo, while the expression for t (compared to Δt=Δw/g) also works only when vo=0. Thermochap (talk) 12:01, 18 March 2008 (UTC)

[edit] Overview

"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."

I believe the speed of light is only around 300km/sRMFan1 (talk) 12:53, 28 March 2008 (UTC)

You're misreading or misremembering. The speed of light it 3x10^8 m/s = 300,000 km/s = 300 Mm/s. Consider, the U.S. is ~5,000 km across; how many seconds does it take light to cross that distance?
—WWoods (talk) 20:27, 28 March 2008 (UTC)

[edit] Remove section Time dilation in popular culture

I propose we get rid of this section Time dilation#Time dilation in popular culture with its potentially endless and i.m.o. rather silly list of science fiction titles. I don't think that this belongs here. For those who really want to keep the list for easy reference, the entire section can be moved to Science fiction#Usage of time dilation or something. Comments? DVdm (talk) 19:10, 15 May 2008 (UTC)

Since there was no comment, I went ahead and removed the science fiction section. DVdm (talk) 11:32, 16 May 2008 (UTC)
congratulations! ErNa (talk) 20:37, 18 May 2008 (UTC)