Talk:Proper time

Corrected some spelling
I have corrected some spelling, and I have changed some headings. The wikipedia Manual of style recommends: Avoid links within headings. --Cleon Teunissen | Talk 14:57, 16 May 2005 (UTC)

"Path integral" vs. "line integral"
"Joke" - I decided to change "line integral" back to "path integral" primarily becuase your change left the article inconsistent both internally and with Wikipedia itself. Internally, there are several references to the path integral in this article, but you only changed one. Also, in Wikipedia, the title of the article is "path integral", not "line integral" (although it does acknowledge line integral as being synonomous with path integral).

If you feel strongly enough on this matter you may change the wording again. All that I ask is that this time you change all occurances of the term "path integral" instead of just one.

--EMS | Talk 17:53, 29 May 2005 (UTC)


 * Oh, I didn't notice that it was in multiple places in the article. It's not a big deal, it's just that physicists call functional integrals path integrals. --Joke137 21:06, 1 Jun 2005 (UTC)


 * I am aware of Feynman diagrams but not of the intergration being called a "path integral" also. So I now see what my usage is jarring to you.  As I wrote above, my big complaint is the loss of consistency instead of the change of terminology.  I do not revert edits without a good reason to do so, and would not have reverted yours if the change had been done throughout the article. --EMS | Talk 22:11, 1 Jun 2005 (UTC)

I've chanced instances of path integral to line integral because of Joke137's comments. HissingFauna (talk) 04:56, 11 January 2009 (UTC)

Minor typo in equation?
The last equation seems to be missing a 'dt' at the end; could someone who knows how to edit the equations fix it? Thanks! Fasrad 13:57, 16 October 2006 (UTC)


 * Done. Thanks for noticing it. --EMS | Talk 14:30, 16 October 2006 (UTC)

Restructuring
I've rewritten the verbal definition in the introduction to stress how proper time differs from coordinate time. I've also brought the mathematical definition for SR ahead of the GR definition. Most readers of this article are likely to be learning SR and won't be familiar with GR, index notation, summation conventions and metric tensors.

I think a little further copy-editing may be required to make the whole article flow consistently. I plan to modify the article on coordinate time later.--Dr Greg 13:20, 12 December 2006 (UTC)

Your definition of proper time Is meaningless
In the special relativity section you define proper time as tau a function of t called coordinate time. Going to the definition of coordinate time it tells me that coordinate time is proper time in special relativity. Your definition is therefore wrong, confusing and just plain stupid. Wikipedia is again demonstrated to have poor editors who dont know what they are doing.72.84.64.212 (talk) 13:31, 28 March 2008 (UTC)


 * The definition of coordinate time does not say "coordinate time is proper time". It says coordinate time (relative to an inertial observer) is the proper time measured by a clock


 * (a) that is at the same location as the event,


 * (b) that is stationary relative to the observer and


 * (c) that has been synchronised to the observer's clock using the Einstein synchronisation convention.


 * All three conditions (a) (b) and (c) are necessary: in situations where any of them is false, coordinate time is not the same as proper time.


 * When (a) (b) and (c) are all true then, in the mathematical definition of proper time v(t) = 0 (gamma = 1), so tau = t.


 * I can assure you the definition given is correct. I have added some extra words of clarification. If you still think it is confusing, can you think of a better way of phrasing it so it isn't confusing? Or can you put your finger on the confusing part? --Dr Greg (talk) 17:18, 1 April 2008 (UTC)

The definition of proper time as tau, indicates that it is less than the coordinate time t. Therefore it indicates that the moving clock tau time is running fast, since the time intervals on this clock are shorter than the time intervals on the rest clock t. This is not time dilation, so either the theory is wrong, or your presentation is wrong. Please fix this mistake.72.84.64.212 (talk) 13:45, 28 March 2008 (UTC)


 * If tau is less than t, the tau clock is running slow relative to the t clock. If the clocks are synced at 12 o'clock, then when t reads 2 o'clock, tau might read 1 o'clock, for example (1 < 2). --Dr Greg (talk) 17:18, 1 April 2008 (UTC)

Time corrected by gamma
Would it be accurate to say that proper time is time over gamma?

206.109.195.126 (talk) 15:51, 22 August 2009 (UTC)


 * In the special case of an inertial observer measuring an inertial object, yes, the proper time of the object equals the coordinate time of the observer divided by the Lorentz factor γ. This follows from the very first equation in the "In special relativity" subsection of the "Mathematical formalism" section. But it wouldn't be true in other circumstances.


 * By the way, new sections go at the bottom of the page, not the top. --  Dr Greg   talk  19:30, 22 August 2009 (UTC)

Please review Proper Time twin paradox

 * The following is copied from 

Dear Dr. Greg:

I am new to this and this is the only way I see how to talk to you.

I ask you to review your application of proper time to the twin paradox.

Your account shows the rocket ship aging more than on earth.

Proper Time is an invariant in relativity and is the same for all observers. Using local measurements different observers get the same value for proper time.

If you think I am nuts you are in good company. I do think I am right.

Donald Lem Donald Lem (talk) 18:13, 22 January 2015 (UTC)


 * Proper time depends not only on the start event and the end event, it also depends on the route taken between those events. Once the two events and the route has been decided, then it is true that all observers will calculate the same proper time from their own coordinate system using the formulas in this article.


 * However, none of this is actually relevant to the twin paradox example that you edited, which uses only one coordinate system throughout. The twin paradox concerns two twins who take different routes between the same pair of events, $$(0,0,0,0)$$ and $$(10\mathrm{ y},0,0,0)$$.


 * Your example is completely different and is not the "twin paradox" scenario.


 * Please read the Wikipedia's twin paradox article and http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html, which both explain this in detail.


 * Wikipedia is an encylopaedia for reporting what is established in reliable published sources, not a place for you to publish your own original research. --  Dr Greg   talk  20:17, 23 January 2015 (UTC)


 * And this is not really the place discuss the subject—see wp:Talk page guidelines. User Donald Lem could (and perhaps should) try at our wp:Reference desk/Science. This article talk page is definitely not the place to discuss this. Cheers all. - DVdm (talk) 21:28, 23 January 2015 (UTC)
 * Can you, please , provide a source citation where the proper time is applied to the twin paradox scenario ? 82.54.63.226 (talk) 06:56, 1 May 2023 (UTC)

Proper Time for Lightlike Interval
I removed the part of the sentence suggesting that light beams experience no passage of proper time. It is problematic to claim this for a few reasons, of which I will go into one unless someone feels like more explanation is needed. Special Relativity is all about relating what is happening in one reference frame to what is happening in another. There is a conversion between the time measured between events in one reference frame to the time measured between events in another frame. Indeed, if you attempt to calculate the proper time for a light-like interval, you get zero. The problem is that if you invert that equation to get the time measured by an observer who sees the light beam traveling from one point to another, you find that the time measured by an outside observer is undefined. In other words, you end up dividing by zero. Why does this happen? The reason is that the rules of Special Relativity, that is, measuring the time and space between events, are not defined for reference frames moving at the speed of light. It doesn't make sense to ask the question "What does the photon see?" in terms of measuring the time and space between events. I welcome any questions or corrections. Cougar2013 (talk) 16:47, 2 February 2015 (UTC)
 * One should take care not to get snagged on a particular description used to make something interpretable in everyday terms. A concept such as proper time is best defined mathematically in terms of an arbitrary (nonsingular) set of coordinates, and one does derive that the proper time along a lightlike world line remains constant. It is true that one cannot coordinatize spacetime from the perspective of "an observer travelling at the speed of light" (with proper time as one of the coordinates), but this does not present an obstacle to the definition of proper time. "The passage of proper time along a world line between two events" can thus be defined consistently for a light-like worldline and is zero, and is invariant for all observers.  The problem arises in trying to interpret proper time as a coordinate for this case. —Quondum 17:11, 2 February 2015 (UTC)


 * You cannot invert the proper time of a light like interval to give the time measured by an observer watching the light beam. It implies that "photons don't experience time", which is an entirely false notion. This means that it doesn't make sense to set v=c in Special Relativity equations. The issue isn't the definition of proper time, the issue is taking an unphysical limit of an equation. Furthermore, all observers will notice the state of a photon change as it propagates along. This is not consistent with the idea that no proper time elapses for a photon. It doesn't make sense to talk about the proper time for events separated by a light like interval. Source: PhD in particle physics. Cougar2013 (talk) 17:41, 2 February 2015 (UTC)


 * I have added a sourced statement. That should settle it. - DVdm (talk) 17:52, 2 February 2015 (UTC)


 * Thanks for the reference. That works for me as long as nobody is claiming that "photons don't experience time" which is the common confusion that always seems to spring up from this topic.Cougar2013 (talk) 18:14, 2 February 2015 (UTC)


 * Quite. Besides, talking about photons in the context of classical—i.o.w. non-quantum—relativity should probably be avoided. Light signals is a much safer choice. Cheers - DVdm (talk) 19:46, 2 February 2015 (UTC)
 * I was surprised to find a source defining the proper time along a lightlike worldline as zero. In my experience, everyone leaves proper time along a lightlike worldline as an undefined concept; it makes things easier because you don't need to make exceptions for light. As well as the avoiding the potential confusion noted above by Cougar2013, it's also then compatible with the operational definition that proper time "is what clocks measure". (Of course, clocks can't travel at the speed of light.) For example, Woodhouse (Special Relativity, p.104) uses this operational definition, and Rindler (Relativity: Special, General and Cosmological) gives both operational (p.65) and mathematical (p.98) definitions, both of which apply only to massive particles. --  Dr Greg   talk  20:23, 2 February 2015 (UTC)


 * This sounds like a war between a clean (mathematical) definition and a physicists insistence on a term being intuitive throughout its domain of applicability. No exception needs to be made for light; the proper time is simply defined as the integral of a square root.  The only problem arises when the square root goes imaginary (spacelike world line tangent).  I'd have to review the sources to see whether they are quite as unyielding as you present them.  —Quondum 20:44, 2 February 2015 (UTC)


 * Tricky indeed. How about this?
 * "... for which the proper time interval between transmission and reception is sometimes characterised as zero, and sometimes as undefined.
 * - DVdm (talk) 21:44, 2 February 2015 (UTC)
 * That would work for me. (Note: you've omitted the final "5" character from the "page=" parameter of the 2nd ref.) Perhaps it would be slightly clearer to say "...sometimes characterised as zero, by some authors, or as undefined, by others." --  Dr Greg   talk  22:09, 2 February 2015 (UTC)
 * Ok, I've put that it in the article. Also removed the faulty (and superfluous) page qualifier  from the page param—thanks for spotting that. - DVdm (talk) 22:50, 2 February 2015 (UTC)
 * Ok, I've put that it in the article. Also removed the faulty (and superfluous) page qualifier  from the page param—thanks for spotting that. - DVdm (talk) 22:50, 2 February 2015 (UTC)

Special Relativity/Proper Time for Light Like Interval
Again, we are seeing edits giving the proper time of a light like interval as zero. This leads to the incorrect idea that photons somehow "don't experience time". If proper time is to be defined as the time measured by a clock following a certain world line, then a light like world line will have an undefined proper time since no clock can travel at the speed of light. In addition, the proper time for a light like interval cannot be inverted to compute the coordinate time between two events.Cougar2013 (talk) 14:54, 14 April 2015 (UTC)


 * What about re-adding something like this? - DVdm (talk) 15:44, 14 April 2015 (UTC)


 * I don't mind, it's just that the page keeps getting edited, and the editor is leaving no comments in the talk section.Cougar2013 (talk) 15:57, 14 April 2015 (UTC)


 * Perhaps we can wait until is done, and the article becomes stable again. - DVdm (talk) 15:58, 14 April 2015 (UTC)


 * Sounds like a plan!Cougar2013 (talk) 16:08, 14 April 2015 (UTC)


 * I planned to re-add something like that. In the meanwhile, I removed all references to "lightlike". I'll leave the article in peace for the day for you. YohanN7 (talk) 16:21, 14 April 2015 (UTC)
 * And, oh Cougar2013, don't write on my user page. Use my talk page if you must, but this talk page is sufficient. And don't be in such a god damned hurry. YohanN7 (talk) 16:41, 14 April 2015 (UTC)
 * You're the one who made edits with no comment in the talk section. That is just common courtesy.Cougar2013 (talk) 16:59, 14 April 2015 (UTC)
 * No, you don't understand. Both you and I can edit without rambling on the talk page, see WP:BOLD. I do reply on the talk page (as you can see), just not within the minute each time.
 * Since the references give versions with or without lighlike worldlines, we should reflect that, not Cougar's personal theories. YohanN7 (talk) 16:45, 14 April 2015 (UTC)
 * I have put forth no personal theories. I'm trying to avoid passages that lead people to think that photons "don't experience time". It's a bothersome idea and it makes no sense.Cougar2013 (talk) 16:59, 14 April 2015 (UTC)
 * Which sounds exactly like your personal theory. We can't have that. We need references. YohanN7 (talk) 17:03, 14 April 2015 (UTC)
 * Are you going to tell me that photons don't experience time? We had references to what was in the article before you removed it with no note or explanation in the talk section. Nobody mentioned photons experiencing time in the article. Why don't you try fully reading the comments here before reacting to them.Cougar2013 (talk) 17:06, 14 April 2015 (UTC)


 * I'm not going to tell you anything, but for fun of it, and to see what the "public opinion" is I Googled the question: Does photons experience time? Judge for yourself, but spare us your OR. I said I planned to include (or reinstate) sample references which do, respectively do not include "lightlike" in the definition. You are the one having problems reading comments in full. You are obviously new to Wikipedia. What you (or I) believe doesn't matter. What matters is what the references say. YohanN7 (talk) 17:30, 14 April 2015 (UTC)
 * We had perfectly good references before you changed the article with no explanation. Public opinion on this particular issue doesn't matter, and actually is the problem. Being newer than you to wikipedia fortunately has no impact on what is factually correct about physics. As someone who has a PhD in physics, specifically particle physics, I have an interest in people being taught things that make sense in regards to particle physics. My initial comment in this talk section was "If proper time is to be defined as the time measured by a clock following a certain world line, then a light like world line will have an undefined proper time since no clock can travel at the speed of light." I'd like you to point out what is wrong with that, if you don't mind.Cougar2013 (talk) 17:52, 14 April 2015 (UTC)
 * Well, do that but supply references, and don't remove the opposing (referenced) viewpoint. Again, it is totally uninteresting that you think that it doesn't make sense for photons to "not experience time". Besides, neither you nor I need to ask permission on the talk page before editing. I am not going to discuss your personal POV. Waving your credentials also does not impress anyone around here. YohanN7 (talk) 18:45, 14 April 2015 (UTC)
 * We had references. The question is why you felt the need to edit the article. It was fine the way it was. I don't care if anyone is impressed with my credentials, I gave them to justify my concern for the subject and how the material is presented. I never edited the article by inserting any of my own information into the actual page. I removed a line which, as we decided upon before you did your editing, was unclear and not universally agreed on by the sources that we listed. I know you're mad that I wrote on your page, but it was about time that you justified your edit since others are concerned about the content on this page. This has nothing to do with my POV, and for you to suggest so is missing the point.Cougar2013 (talk) 19:07, 14 April 2015 (UTC)
 * It has more references now, including what it had before. I edited it because of its poor quality. Do you really think the article was better this morning than now? Which (edited) parts are not better now? I'm not claiming it is perfect, but I do think it is better now.
 * I'm not mad at you for writing on my user page. Everybody makes mistakes. YohanN7 (talk) 19:31, 14 April 2015 (UTC)

F.w.i.w. I think that the artcile is much better than it was this morning. The addition about undefined/zero lightlikeness sounds perfect to me. Thanks YohanN7, good job. Hey... let's work together in peace here. - DVdm (talk) 20:13, 14 April 2015 (UTC)
 * Right, yes, in peace. Thank you for the kind words. YohanN7 (talk) 22:34, 14 April 2015 (UTC)

Concerning proper time for lightlike intervals
I see there are two past discussions of this topic, but I'd like to open a new one since the prior ones seem to be complete conversations. I think it is highly inappropriate to consider proper time for lightlike intervals. The first sentence of the article,

"In relativity, proper time along a timelike (or lightlike) world line is defined as the time as measured by a clock following that line."

is 100% incorrect. In relativity, it is impossible that such a clock can exist on a lightlike trajectory. The spacetime interval which can be linked to proper time for timelike intervals, but not necessarily so, is identically zero for lightlike trajectories. In addition, the footnote on Landau and Lifshitz's text "Yet others aren't clear on whether lightlike intervals are included" is also incorrect as L&L make abundantly clear in the beginning of the section that they are considering a reference frame which travels on the trajectory. Light cannot, I repeat cannot have such a trajectory. Further still, on page 245 of the same text, L&L correctly point out that,

"The equation of a geodesic in the form [involving the spacetime interval] is not applicable to the propagation of a light signal, since along the world like of the propagation of a light ray the interval ds, as we know, is zero, so that all the terms in [the geodesic equation] become infinite."

L&L go on further that to define null trajectories in a purely geometrical context which while resulting in the same form as the original geodesic equation does not invoke the notion of time in any way shape of form. Other references can be supplied to support this, this is not controversial in the physics community except among those who do not fully understand the theory. Simply put, lightlike intervals are not included. The other reference, Lawden on page 116 make the same statements as L&L,

"The definition of a geodesic which has been given at the beginning of this section cannot be applied to the class of curves for which the interval ds between adjacent points vanishes."

Thus the other footnote, "Some authors include lightlike intervals in the definition" is also completely incorrect and results from a incomplete reading of Lawden's page 150 who relates the trajectory to the observer's coordinates. To make this as absolutely clear as possible: The lightlike/null trajectory is not in any way associated with a concept of proper time and must be parameterized in a completely separate context. I am going to remove the parenthetical inserts involving lightlike trajectories in this article's discussion of proper time and correct the footnotes as I've explained above. Those who disagree, please reply on this talk page. 150.135.210.29 (talk) 07:29, 5 December 2015 (UTC)


 * I've made the edits and added several additional references and footnotes, as well as a short statement on when and when not the mathematical procedure of the article is mathematically and physically valid. 150.135.210.29 (talk) 09:15, 5 December 2015 (UTC)


 * Edit, I did a rereading of Lawden. (Specifically section 7 p.17, https://archive.org/details/AnIntroductionToTensorCalculusRelativity) The original footnote is not incorrect as I first accessed, this author generalizes proper time to be synonymous with space time interval even goings as far as to label even proper distances as 'imaginary proper times'. This generalized definition of perfectly consistent, but would imply that the proper distance and proper time articles should be merged. I believe the reader is better served if the two notions are kept separate and proper time left to its restricted form involving a measurable quantity constructed from a reference frame as seen in the derivation of this article. 150.135.210.29 (talk) 10:19, 5 December 2015 (UTC)


 * I agree with your entire analysis and edits. Good job. However, your most recent edits include some slight non-neutrality as wp:OR: eventhough it is just in a footnote, the phrase "... but this comes at the cost of also including the spacelike proper distances" is not in the source. I have rephrased that into "... and also include the spacelike proper distances as imaginary proper times". - DVdm (talk) 11:19, 5 December 2015 (UTC)


 * Might add that L&L also use imaginary intervals. They define the interval as $ds$ rather than $ds^{2}$ and on occasion mention imaginary intervals. YohanN7 (talk) 12:06, 5 December 2015 (UTC)


 * Thanks for the fix DVdm. To YohanN7, this is the sign convention issue raising its ugly head. L&L uses (+---), so imaginary intervals are spacelike and real intervals are timelike. In the (---+) convention such as used by Lawden and others, the vice versa is the case. 150.135.210.29 (talk) 00:34, 6 December 2015 (UTC)


 * Yes, of course. By the way, it was originally my personal blooper to include the L&L ref as being "undecided" when it came to light-like intervals. I took it from memory that turned out to be faulty, and then I never returned to the issue. The other references weren't available to me. Thanks for fixing this. YohanN7 (talk) 10:38, 7 December 2015 (UTC)

Effect Of Extreme Acceleration
There doesn't appear to be any mention of difficulties or impossibility of measuring proper time in a frame of reference undergoing extreme acceleration. Ref: http://iopscience.iop.org/article/10.1088/0264-9381/32/17/175003/meta;jsessionid=D5CC1EA3EE4769553387910B5BD41133.c1. Anyone up for the task? RegulatorRectifier (talk) 18:29, 13 December 2015 (UTC)


 * "Difficulties" not worth mentioning since proper time measure in 10^16 m/s² frame was done in the 70's, to a couple ppm. 2601:542:103:A1F0:8DAE:9D41:350A:DD10 (talk) 21:47, 1 August 2023 (UTC)

Requesting clarification of ambiguous sentence in introductory paragraph
The introductory paragraph contains the sentence:

> It is thus independent of coordinates, and a Lorentz scalar.[1]

To me, this sentence is elliptical and has (at least) two readings:

> It is thus independent of coordinates and [it is] a Lorentz scalar.

or

> It is thus independent of coordinates and [it is also independent of] a Lorentz scalar.

I think the comma splice implies the latter. As a layman reader, I'm not sure which interpretation is correct, so I don't know how to help. Therefore, I report the issue on this talk page for the editors to consider. Cheers and thanks for your work! — Preceding unsigned comment added by 63.150.103.146 (talk) 16:03, 19 July 2019 (UTC)
 * A Lorentz scalar is, by definition, independent of coordinates. I've edited to remove the ambiguity. --  Dr Greg  talk 16:38, 19 July 2019 (UTC)
 * Quite dismaying for a lede's 2nd sentence to begin with a logical conclusion "It is thus ..." when it doesn't seem true.
 * Any clock's time must depend on its choice of t=0, so a clock with world line thru Greenwich and set there would show its proper time as different than a clock with coordinates in NYC. So I'm pretty sure proper time is NOT independent of coordinate(s).
 * Paragraph 3 tells us that the proper time INTERVALS from event A to event B depend on which paths the clocks follow from A to B ... so the process integrals differ even if the end points are stated as the same - but certainly the process depends on those end points (so is not independent of them).
 * If the *interval* "transforms as a Lorentz scalar" that might be useful, but there seems to be only 1 inertial frame that has a proper time interval from A to B. So if you transform it to a different frame, your different interval value doesn't even end at B! (so is pretty useless imo)
 * Can somebody clear this up? 2601:542:103:A1F0:8DAE:9D41:350A:DD10 (talk) 22:53, 1 August 2023 (UTC)