Talk:Spacetime diagram

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Complete new Version

It's a translation of the german article I wrote in feb. 2005. Minkowski diagrams offer a great possibility to understand relativity graphically and without mathematics. I hope the article remains free from further formulas ;-). --Wolfgangbeyer 14:38, 7 June 2007 (UTC)[reply]

I have reworked the section on "The speed of light as a limit", to smooth out wording, clarify and amplify. I have no doubt that the essential proposition is true if our usual notions of time and causality are correct, but I hope this is stated explicitly in Rindler (the only ref cited); otherwise, we are in danger of WP:OR charges, and need to find some further sources, not within the Wiki project. Cheers, Wwheaton (talk) 17:03, 16 March 2008 (UTC)[reply]

The reference to Rindler's book is a holdover from the previous version of this article. If you look at the German version, of which this new English version is a translation, Rindler's book isn't referenced, so you may not find it. DonQuixote (talk) 16:02, 19 March 2008 (UTC)[reply]

Oh dear, that means we really must have a better reference asap. I think it is widely understood among physicists, but I do not immediately know where it is explicitly stated and explained. I suppose Taylor & Wheeler's old book Spacetime Physics for undergraduates likely has it, but I do not have a copy handy. Does anyone who has a copy know if it is in there, or if not where a good reference can be found? I have put a request on Wolfgangbeyer's talk page, maybe he can supply something. Bill Wwheaton (talk) 18:08, 19 March 2008 (UTC)[reply]

Minkowski diagram vs. Loedel diagram

The main page seems to mix the Minkowski spacetime diagram and the so-called Loedel spacetime diagram (by Enrique Loedel Palumbo) freely. The very first figure is actually a Loedel diagram, with two non-orthogonal axes systems. Technically and historically, the Minkowski and Loedel diagrams are not the same.

As far as I could establish [Shadowitz A. (1988) Special Relativity, Dover], the Loedel diagram was proposed in 1948 as an aid to teaching special relativity. I recommend that the author of the main Minkowski diagram article edit the page to reflect these facts.

Jorrie 03:53, 1 December 2007 (UTC)[reply]

You are right. I have added a remark to the article so as to make reference to the term Loedel diagram. Perhaps the article needs some further modifications on this point. On the other hand, a Loedel diagram for the mutually moving reference systems A and B is nothing more than a Minkowski diagram for a "symmetrical observer" C (i.e. an observer for which A and B are moving in opposite directions with symmetrical speeds). Isn't it? -- JocK (talk) 13:01, 2 March 2008 (UTC)[reply]

Given that Loedel diagrams (proposed in 1948), are a subset of Minkowski diagrams, and that Minkowski obviously originated the larger idea (he died in 1909), it seems to me that this distinction is a pedagogical nit that deserves a footnote, but is not worth encumbering the article. A Loedel diagram is a Minkowski diagram, of a special type, right? Wwheaton (talk) 20:33, 10 April 2008 (UTC)[reply]

The Loedel diagram IS just a special type of Minkowski diagram, where the other two observers move with equal speeds in opposite directions relative to some third frame (which we can always find, like an average). This means that the factor commonly called "gamma" is the same in both these observers frames, since it depends only on relativistic speed, and not direction. This "gamma" factor is called this since it is the ratio between measued time differences OR lengths between two different frames, and is used all the time in relativistic physics. It also happens to be factor that the scale mark is increased by, as some simple invarient interval calulations/hyperbolas will show. My physics 133 class used "Six Ideas that shaped Physics: The Laws of Physics are frame independant" by T.A. Moore to cover just about anything you can do with Lorentz transformations and Minkowski diagrams, they're introduced on page 30 and used to the end, as they're useful with deriving "four momentum" hyperbolas and the fact that mass is invariant (mass squared, technically) for a given object. The Loedel diagram is just a special minkowski diagram as it is explained on the page, but this is a pretty sorry page... as a note: (gamma)=y=(1+(v/c)^2)^(-1/2), dt'=y*dt, dx'=y*dx where primes denote tilted axes, and d_ is a measured difference on the diagram axes. also c=c not because of this diagram, but because a simple differential equation (wave eqn.) says c=c for a photon, since it's a wave. If not, we could actually have any limiting speed, even an infinite one (where Galilean transformations physically work). 172.130.75.2 (talk) 09:06, 9 May 2009 (UTC)[reply]

Time Dilation

Under the heading "time dilation" there appears the sentence, "Due to OB<OA he concludes that the time passed on the clock moving relative to him is smaller than that passed on his own clock since they were together at O." This implies that change in time is equal to the length of the the line. This is false; it should be corrected. 03:04, 31 January 2008 (UTC)

Quantitative understanding without mathematical equations

The Minkowski diagram is an important tool for explicating relativity, and thus we do well to improve it. The phrase currently in the lede about quantitative understanding without mathematical equations signals the disappointing state of the current article. Relativity marks the first real mathematical physics, a study without manipulables. Popular books frequently seduce readers by promising insight without tears. With our wiki-links we can build understanding from fundamentals in quantitative study. So far this article has not included hyperbolas in the Minkowski diagram. Most texts consider them as much a part of a Minkowski diagram as the various time and space axes. The configuration now called a Minkowski diagram preceeded Minkowski's 1908 paper. For instance, in 1900 Alexander Macfarlane (mathematician) included such a diagram in his paper on hyperbolic quaternions. The importance of this article, and its close relation with split-complex numbers, give me reason to consider the task of revision. First, however, I'd like to hear from other editors.

Rgdboer (talk) 01:01, 13 January 2009 (UTC)[reply]

First time I hear about split-complex numbers. Interesting stuff. Are you suggesting that utilising the formalism of split-complex numbers will give us an opportunity to improve the didactical quality of Minkowski diagram? JocK (talk) 03:17, 13 January 2009 (UTC)[reply]

For reference see [[1]] for the original Minkowski diagram. I have been working to improve basic classical linear algebra articles that support spacetime study, such as versor#Hyperbolic versor which is an equivalent concept to Lorentz boost but arose earlier.Rgdboer (talk) 20:56, 29 May 2009 (UTC)[reply]

Thanks to 84user we have diagrams in Commons to draw on. Note that WK Clifford has a diagram on page 90 of his Elements of Dynamic (1878) that contains information usually associated with the Minkowski diagram. We can make relativity more easily understood by improving this page to provide the view of spacetime that these Clifford-Macfarlane-Minkowski diagrams afford.Rgdboer (talk) 03:31, 8 December 2010 (UTC)[reply]

Upon reflection, now it seems indeed the Minkowski diagram can bring understanding without equations. In fact, the diagram is a tool in synthetic geometry. However, contrary to the current state of the article, the literature uses a "calibration hyperbola" to illustrate the effect of a Lorentz transformation. See unit hyperbola for the geometric context of the Minkowski diagram.Rgdboer (talk) 03:06, 20 March 2011 (UTC)[reply]

bisector

Sorry to be picky, but should not the bisector referred to in para 3 have the equation ct = x'? Chumod (talk) 16:05, 10 June 2011 (UTC)[reply]

It doesn't if c is normalized to 1, which probably should be mentioned. DonQuixote (talk) 20:44, 10 June 2011 (UTC)[reply]

There is a continuing tradition to use ct instead of t alone in spacetime diagrams so that all coordinates refer to spatial measure. Since relativity refers to blend of space and time, this tradition inhibits the modern view of spacetime. The second paragraph says the temporal axis runs vertically and that units are chosen so light lives on the lines of slope plus or minus one.Rgdboer (talk) 20:58, 10 June 2011 (UTC)[reply]

Galilean space-time diagram

In the section titled "Path-time diagram in Newtonian physics", the article claims to provide a picture of a Galilean transformation; however, the point A is farther along the blue (ct') axis than the black (ct) one (indicating a sort of time dilation). This doesn't seem correct to me, since time dilation is a concept encapsulated by a Lorentz transformation (as opposed to a Galilean one). Am I misinterpreting the diagram, or is the diagram wrong? Luolimao (talk) 22:22, 22 May 2013 (UTC)[reply]

You are right that time dilation is a topic peculiar to relativity theory. The section is about pre-relativity where absolute time and space are presumed. The problem enters with associating the diagonal length to event A as a measure of time. In the Galileo-Newton point of view time is marked by lines parallel to the x-axis, so the time to A along the blue diagonal is the same as along the black vertical. Thank you for a thoughtful question that led to some edits to improve this project.Rgdboer (talk) 23:54, 22 May 2013 (UTC)[reply]

Ah, so the time elapsed is represented by the (perpendicular) distance to the space axis, not some length along the time axis? Luolimao (talk) 02:24, 23 May 2013 (UTC)[reply]

Yes, that is correct for absolute time and space. In the Minkowski diagram, where hyperbolas mark out proper time, the diagonal lengths become time measures.Rgdboer (talk) 19:12, 23 May 2013 (UTC)[reply]

When the stsndard (Galilean) spacetime diagram article was merged into the Minkowski diagram article, the general logic of the ST diagram was lost by the merging of the general into the specific. So now it is no longer possible to separate 2 particles in space (at the same time) by a horizontal distance in the diagram. And it would be interesting to know how the standard (abscissa = distance and ordinate = time) coordinate system got corrupted by Feynman and others into the alternative. And as the result we no longer have a logical set of ideas about a logical sequence of events in the diagrams. And so when we have a line showing the backwards movement of an antiparticle during which time period it emits a particle, we wind up with the emission point in time being before the time of creation of the particle. Shouldn't there somewhere be an explanation of the standard set of logical associations that can be made about the details of the standard ST diagram?WFPM (talk) 00:26, 18 December 2013 (UTC)[reply]

Error in definition of Loedel Diagram?

Could someone clarify this for me? The article says:

"To avoid this problem it is recommended that the whole diagram be deformed in such a way that the scales become identical for all axes, eliminating any need to stretch or compress either axis. This can be done by a compression in the direction of 45° or an expansion in the direction of 135° until the angle between the time axes becomes equal to the angle between the path axes."

But the equality of angle between time axes and path axes is already assumed for a Minkowski Diagram:

"If ct instead of t is assigned on the time axes, the angle α between both path axes will be identical with that between both time axes."

Shouldn't it read, for the Loedel Diagram:

"To avoid this problem it is recommended that the whole diagram be deformed in such a way that the scales become identical for all axes, eliminating any need to stretch or compress either axis. This can be done by a compression in the direction of 45° or an expansion in the direction of 135° until the angle between ct and x becomes equal to the angle between ct' and x'." — Preceding unsigned comment added by Aendolin (talk • contribs) 14:49, 23 June 2013 (UTC)[reply]

Thank you for your attention to this section. Since it refers to stretch and compress it presumes technical knowledge. The Loedel diagram material has been replaced with a pair of links to the article where the idea is expanded. After all, it is but a particular type of Minkowski diagram, and this article provides the basic details appropriate to Minkowski diagrams.Rgdboer (talk) 22:51, 23 June 2013 (UTC)[reply]

Lede needs a lot of work.

First, I need to qualify my remarks by stating that I am assuming that a Minkowski Diagram is the graph that I used in my Special Relativity course I took decades ago, as well as what is generally used in SR texts and courses today (for instance: Susskind's 2007 video course (YouTube or iTunes)). It's far from clear to me that this is what is being described here. That should be unacceptable to the authors: they've failed to communicate what it is they are talking about to someone with a passing knowledge of the subject. I will confine my comments to the 3rd paragraph of the lede, which badly needs to be rewritten. Here it is: "A particular Minkowski diagram illustrates the result of a Lorentz transformation. The origin corresponds to an event where a change of velocity takes place. The new worldline forms an angle α with the vertical, with α < π/4. The Lorentz transformation that moves the vertical to α also moves the horizontal by α. The horizontal corresponds to the usual notion of simultaneous events, for a stationary observer at the origin. After the Lorentz transformation the new simultaneous events lie on the α-inclined line. Whatever the magnitude of α, the line t = x forms the universal[2] bisector."

1. A diagram "illustrates the result" ? What this sentence is trying to say isn't easy. Isn't the USE of a diagram two-fold? It allows users to DETERMINE the results of a Lorentz transformation, AND it EXPLAINS those results in a graph in a way equivalent to solving the Lorentz transformations.
2. The origin corresponds to an event where a change... This is why I am writing this. This is simply wrong. I have seen dozens (at least) of these diagrams and I've never seen them used for a change in velocity (of WHAT??) at the origin. Of course, they can be, my point is that they are NOT what the novice will be seeing. So, where is the idea of an observer and his/her (inertial) frame of reference?? It SEEMS to be implying that the observer (and his reference frame) will suddenly move off at an angle upon a change in his motion! WRONG! (as you all know).
3. The new worldline will... ?????? Don't we need a 11-dimensional worm hole to "create" a new worldline? (Sarcasm). A change in the curvature of the worldline does NOT create a "new" one. This is really wrong. The origin is selected arbitrarily at a certain time and location in the observer's F.O.R., NO "change" is necessary. Observations of anything in motion (along the selected x axis) will necessarily be at an angle α --as measured from the POSITIVE t axis (not simply "the vertical axis").
4. The Lorentz transformation that moves the vertical to α also moves the horizontal by α. This is absolutely horrible! The vertical direction can not be "moved", sorry. What I expect you mean is that the L.T. that rotates t into t' by an angle α from the vertical, also rotates x into x' by an angle of α from the horizontal, with the effect that both t' and x' are symmetrical about the diagonal line t=x, but no longer at a 90° angle from each other as depicted on the graph. As the velocity increases, both lines approach the diagonal (the speed of light, c, chosen to equal 1 in these diagrams).
5. The horizontal corresponds to the usual notion of simultaneous events for a stationary observer at the origin. WOW! more obfuscation. The horizontal WHAT??? For any observer in the F.O.R. of "the" observer, horizontal lines are lines of constant time. Any two events on the same horizontal line are simultaneous to ANY observer in that F.O.R., not just one at the origin. In the reference frame of the object in motion, simultaneous events are any events on the same x'= k line, at an angle of α above the horizontal.
6. Whatever the magnitude of α.... What? more obfuscation. The Minkowski diagram strength is from the fact that light always travels with a slope of ±1, regardless of the x' and t' angle α, regardless of the velocity of the object. Lines of negative slope depict motion to the left (in the negative x direction).

If I were sure that this correctly captures the basic ideas (note I didn't go into worldlines, a more advanced topic imho). I'd make the changes myself. As it is, I defer to the experts.

P.S. is there really ANY reason to use the terms abscissa and ordinate here? (see next section) They are synonyms (usually) for horizontal and vertical axis. I suggest you pick a term and stick with it, and the terms need to be vertical and horizontal. 173.189.73.122 (talk) 03:16, 8 February 2014 (UTC)[reply]

Thank you in Ohio for a detailed criticism of an important paragraph. Two relevant hyperlinks have been added today. Some of your concerns have been addressed by edits also, but your insight may continue to be useful as this article serves many trying to get an understanding of modern physics. Getting rid of obfuscation is our common purpose.Rgdboer (talk) 21:30, 8 February 2014 (UTC)[reply]

World lines?

The use of "world line" vs "worldline" is inconsistent in the article. I recommend the former as the latter redirects to financial transaction processing company, as can be seen via the faulty link in the description: http://wiki.riteme.site/wiki/Worldline BoltNinja (talk) 11:07, 1 May 2015 (UTC)[reply]

I have reverted worldline back to the redirect to world line that it was until 14 April, on the grounds that the new article was unsourced, with no notability asserted and was overriding an established redirect with over 30 incoming links with no disambiguation hatnote. -- Dr Greg talk 18:17, 1 May 2015 (UTC)[reply]

Could somebody tell me if these images are related to the Minkowski diagram?

The first image was used to explain the impossibility of superluminal information speed (simplified to the special case of instantaneious flow of information). See also Wikiversity:Minkowski diagram and Superluminal communication. I would greatly appreciate help from an expert.--Guy vandegrift (talk) 21:23, 17 May 2016 (UTC)[reply]

I have removed the image and the analysis per wp:unsourced and wp:NOR: [2]. Do you have a reliable source that supports the image and the content in the caption? Do you realise that if a train in such a diagram is supposed to be moving w.r.t. the (x,t) frame, it cannot be drawn parallel with the x-axis? - DVdm (talk) 06:43, 18 May 2016 (UTC)[reply]

I realize that it is misleading to draw the train horizontally if it is moving, but what if the train is not "part of the picture",so to speak? What if the space-time drawing contains only the three portions of the train (F,C,R)=(front, center, rear) these may be drawn. The image of the train just shows where the stationary observer defines the train to be at a given point. In the subsequent diagrams I acknowledge this by drawing the train with different skew. I am confident that the diagram is basically correct and will revert on the grounds that it is a space-time diagram with the train as simply an illustration of where the entire thing is from one perspective. Maybe the dispute will bring in somebody who has seen the diagram in a book.--Guy vandegrift (talk) 07:56, 18 May 2016 (UTC)[reply]

I have removed the image and the analysis again per wp:unsourced and wp:NOR ([3]) and left a formal 2nd-level warning on your talk page ([4]) - see wp:BRD and wp:BURDEN, which is yours. - DVdm (talk) 08:22, 18 May 2016 (UTC)[reply]

Do you know how to bring in more experts? If a figure states the obvious, you don't need a reference. I do see your point, and hope someone else steps into this discussion soon. I will post a notice at Special relativity because not everybody posts the pages this is on. If two or three other people with knowledge get involved, one of them might have seen a similar diagram. Otherwise, if it truly is original research, then it needs to be published first. Keep in mind, that if someone puts uncited material that is likely to be true, it's often allowed to stay until someone finds a source. --Guy vandegrift (talk) 08:32, 18 May 2016 (UTC)[reply]

Not only does it need to be published first. It also needs to be quoted and mentioned in the relevant literature, otherwise wp:UNDUE. Count on at least 5 years before that happens. - DVdm (talk) 08:41, 18 May 2016 (UTC)[reply]

After reading the literature, I am convinced that these can be called Minkowski diagrams, even if they are not exactly what Minkowski drew. (BTW I am pretty sure that this is what he drew, and also don't think I will be posting any of this on Wikipedia in the near future. Even if I manage to publish this stuff (its doubtful), I never cite my own work on WP--Guy vandegrift (talk) 04:46, 19 May 2016 (UTC)[reply]

Reference for so-called “path axis“ (labeled x, usually)

I can’t say I have ever heard the term “path axis”. So, when someone used the term (which, I found out later, they got from this WkiPedia page), I thought they were referring to the spacetime path (“worldline” or “world line”).

I have performed an Internet search for “path axis” used in conjunction with Minkowski Diagrams, but the only hits I get are all quotes from this WikiPedia page.

Where is any primary reference to the use of this term in conjunction with Minkowski Diagrams?

DWHalliday (talk) 23:14, 11 November 2018 (UTC)[reply]

@DWHalliday: The term "path axis" was introduced to this article in this 7 June 2007 edit by User:Wolfgangbeyer who was mainly active on the German Wikipedia. See his German user page. I suspect that "path axis" is some literal translation of the German "Wegachse", which I can trace in the German literature as far back as 1997 using Google scholar. Like you, I cannot find any English use of the translated form that is not traceable to this article.

Wikipedia is the source of several highly unfortunate English neologisms. Consider the term "Fizeau–Foucault apparatus", the original posting of which was by a currently banned user who didn't know the difference between two extremely different measurements of the speed of light, and who which, so far as I can see, was a rather argumentative editor.

The term "path axis" should, in my opinion, be edited out of this article. Prokaryotic Caspase Homolog (talk) 08:07, 12 November 2018 (UTC)[reply]

@Prokaryotic Caspase Homolog: I quite agree with you. In fact, I had suspected a transliteration from the German as you express. DWHalliday (talk) 02:04, 13 November 2018 (UTC)[reply]

@DWHalliday: Would you be able to take on the task of removing these Germanic phrasings? I'm very busy right now with extensive changes to Special relativity. The version that I started with was pretty dreadful. After 2 1/2 weeks of edits, I believe that I've improved the article from "dreadful" to "mediocre".

I've extensively reorganized it, pushing some unsourced and much-too-technical-for-the-target-audience material to Technical discussion of spacetime at the end. I have added new sections Graphical representation of the Lorentz transformation, Relativistic Doppler effect, and Invariant interval. I also greatly expanded Thomas rotation and Causality and prohibition of motion faster than light. Right now I'm working to address various issues with my treatment of Invariant interval that Purgy noted. If you could review my work and offer your criticism, I would much appreciate it! Prokaryotic Caspase Homolog (talk) 03:50, 13 November 2018 (UTC)[reply]

I understand this axis as representing a one-dimensional subspace of the common (flat) 3-d Euclidean space. I think one might call this a straight path and a general path might certainly be something that is "curved" (in one dimension?). Nevertheless, I do not experience "path" as that bad a term in this context here. I think its nearness to a spatial analogue in a pasture contrasts nicely to the other, assumed one-dimensional, axis that fixes temporal coordinate values. Worldlines are then no paths anymore. I am quite unsure about a rigorous interpretation of oblique coordinate pairs within an -at the root- orthogonal diagram. Nevertheless, spacetime diagrams for ever! Purgy (talk) 10:20, 12 November 2018 (UTC)[reply]

Wikipedia is not supposed to be a source of innovation in terminology. Despite the (apparently) established history of "Wegachse" in German, the translated term is not established in the English literature. I have just checked Rindler and do not see the term "path axis" used in his textbook. Prokaryotic Caspase Homolog (talk) 13:11, 12 November 2018 (UTC)[reply]

I never intended to make WP a source of innovation in terminology, I just wanted to express my opinion that the word "path", especially as "linear path", in its everyday meaning, bound to traversing spatial distances, might be very apt to lead uninitiated readers to the notion that is targeted in this context, without creating "terminology", not even jargon. Afaik, there are position/distance/displacement vs. time graphs around, all with a specific, "determined" meaning, but not bound to LT. I would not move even a brow, if "path" were edited out, but I do not experience its use as innovating terminology, or otherwise tumbling any pillars, but rather as harmlessly using an existing and reasonable association in explaining (1+1)-dimensional spacetime. Purgy (talk) 08:21, 13 November 2018 (UTC)[reply]

@Purgy Purgatorio: Unlike the spacetime path, often called a “worldline” or “world line”, which is a path, there are no “paths” for the orthogonal axes: such are sets of simultaneous events. Nothing travels or traverses such lines. DWHalliday (talk) 02:04, 13 November 2018 (UTC)[reply]

@DWHalliday, I perceive a certain amount of condescending briefing in the above I consider as superfluous. I will continue to avoid calling worldlines in the context here "paths" (to me they are preferably curves in a (1+1)dim spacetime) and their events, projected to the spatial dimension will still be confined to this "axis" (as you call it), either "at rest" or "travelling" or "traversing", even when -better avoided- "not simultaneous". Purgy (talk) 08:21, 13 November 2018 (UTC)[reply]

Wtf is STR???

Using abbreviations which are not explained in the text is the ultimate sign of a writer who is not knowing for whom he/she is writing!!! — Preceding unsigned comment added by Koitus~nlwiki (talk • contribs) 19:21, 27 March 2019 (UTC)[reply]

"STR" is a an abbreviation for the special theory of relativity, and nowadays is used much less frequently than "SR". I have changed the text to "special relativity (SR)". Prokaryotic Caspase Homolog (talk) 21:25, 27 March 2019 (UTC)[reply]

Accelerating observers

Why are all the points on the diagram persistently moving downwards (in conjunction with the Lorentz transformation that appears to be "cycling back and forth"). What kind of acceleration is the observer experiencing? I would assume the acceleration matches the back and forth dilation of the diagram, so I'm not sure why all the spacetime events (which are points that occur at a particular time) are moving downwards. I feel this section needs a little more explanation as to what it's depicting.

I'm not a physicist nor an expert in relativity but I'm sure I would be able to understand a sufficiently good explanation. The rest of the article makes sense to me. Hddharvey (talk) 00:09, 4 November 2021 (UTC)[reply]

@Hddharvey: I was never too happy with this article's handling of that diagram, so I expanded the section somewhat. I incorporated text that I wrote for other Wikipedia articles (self-plagiarism) as well as text that other people have written for other articles (in particular, Theoretical motivation for general relativity). Hope this works better for you! Prokaryotic Caspase Homolog (talk) 02:43, 6 November 2021 (UTC)[reply]

@Prokaryotic Caspase Homolog: Thanks. The extra figure (Fig 5-1) added to that section makes it make perfect sense.

Should "within the past light cone" be "on past light cone boundary"?

The article states "If one imagines each event to be the flashing of a light, then the events that are *within* the past light cone of the observer are the events visible to the observer". This doesn't seem to be quite right. If the event is a flash of light (in a vacuum), I think it will arrive at (be visible to) the observer at the point where the event *crosses* the past light cone boundary. I think that it will not be visible after that, as a flash is by nature instantaneous. If on the other hand the event is a light beginning to shine then the *light* will continue to be visible as the event passes further inside the past light cone, although the event itself (the beginning of the light shining) is still instantaneous and only visible once i.e at the point where it crosses an observer's past light cone boundary. However that is my reading of it... perhaps that is not the intended meaning of the wording. I think it might be helpful if it was reworded to clarify? The section which it concludes is a particularly excellent part of the article, it would be good to maintain the same high standard here too. 2A02:C7E:3525:0:9818:5454:1986:C0A0 (talk) 12:13, 30 September 2022 (UTC)[reply]