When I first posted on the PSX, I kept using the term "age difference" as meaning something other than the term "reciprocal time dilation". It turns out that age difference here means the coordinate time difference between where a perspective's line of simultaneity intersects the two velocity lines. Basically, it means the same as reciprocal time dilation.

So at .6c if Bob's line of stationary simultaneity intersects his velocity at t=4 and Alice's at t'=3.2, the age difference is 4-3.2=.8.

But from the Loedel reference frame, which is defined on wiki as the frame of reference in which two collinear velocities become equal speeds in opposite directions, the Loedel velocity of 1/3c lends a perspective and Loedel lines of simultaneity where the Loedel age difference is 4-4=0 according to this Md:

Loedel age difference

The Loedel reference frame's lines of simultaneity join coordinate times that match the time labels of proper time. In conjunction with this question here, the Loedel age difference gives a picture of how age difference progresses for proper time coordinate labels (but not proper time itself as that is invariant of perspective more info why here).

So my question is whether "Loedel age difference" can be used as a valid term distinct from the term "age difference" used here?

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    $\begingroup$ Hi, we've noticed that you have made a large number of minor edits to this post. Please be mindful that every edit bumps the post in the "active" tab of the site and try to make your edits substantial. If you foresee improving this post repeatedly, maybe collect several edits and make them in one go instead of submitting them individually. $\endgroup$
    – Chris
    Jan 1, 2020 at 8:01
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    $\begingroup$ The terms "Loedel age difference", "Loedel reference frame" do not appear anywhere on the internet outside of this question or other posts by you on our site. It does not seem as if your edits make this question clearer or more understandable to others. You've edited this question 11 times after Chris asked you not to do so many edits, we are now at revision 38. At this point, it is impossible to tell whether this is in any meaningful sense an "edit" of the first version or an entirely different question. Please do not edit this post any further; ask a new question if you have a new question. $\endgroup$
    – ACuriousMind
    Jan 31, 2020 at 21:17
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    $\begingroup$ Why were all my comments deleted by a person unknown and no record of them having existed? @Chris $\endgroup$
    – ralfcis
    Feb 12, 2020 at 0:48
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    $\begingroup$ @ralfcis They were not deleted by me, but most likely it's because you continued to post comments here instead of using the chat room intended for that purpose and/or they were deleted as "no longer needed" as they were not suggestions to improve the post or responses to said suggestions. $\endgroup$
    – Chris
    Feb 12, 2020 at 0:54
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    $\begingroup$ @ralfcis In general, comments are subject to be deleted at any time- you shouldn't assume they will hang around. This holds especially when a chat room has already been created expressly for that purpose. $\endgroup$
    – Chris
    Feb 12, 2020 at 1:04

1 Answer 1


You are playing fast and loose with notations like $t$, $t'$, and $t''$, assigning them different meanings at different stages in your analysis. So let's fix some meanings:

$t$ is Bob's time coordinate.

$t'$ is outbound-Alice's time coordinate.

$t''$ is inbound-Alice's time coordinate.

$E$ is the event where Alice starts her journey. $F$ is the event where she reaches her destination and turns around. $G$ is the event where she arrives back home.


$t(E)=t'(E)=0\quad t''(E)=-4.5$.

$t(F)=5\quad t'(F)=t''(F)=4$.

$t(G)=10\quad t'(G)=12.5\quad t''(G)= 8$.

It makes no sense to write things like $t=Yt'$. You must first specify an event where you're measuring. [You will in fact sometimes see equations like $t=Yt'$ in print, but only when it's already clear which event we're evaluating at.]

It makes no sense to ask whether an equation is true "from Alice's perspective". An equation is either true or false, period. $t'(F)=(4/5)t(F)$, from any "perspective". And $t(G)=(4/5)t'(G)$, from any "perspective".

In fact, every single place where you use the undefined word "perspective", you are creating amibguity and setting yourself up for sloppy reasoning. I strongly suggest that you review your writings and eliminate this and other undefined terms wherever they occur.

This should fully answer all of your conceptual questions. The answer to most of your vocabulary questions is "No, there are no standard names for these things, in some cases because you're asking about the names of things you've never clearly defined."

And the main lesson is: It's not enough to just string a bunch of words and symbols together. Words need meanings, and if you don't stick to those meanings, you are only going to confuse yourself and frustrate everyone who tries to help you.

  • $\begingroup$ 1. She travels 4 yrs each way. 2. All perspectives including Alice's agree at co-location at t'=8 that Alice is 8 and Bob is 10 but at 6ly separation, Alice's perspective is she is she is t'=8 and Bob is t''=6.4. 3. What is your answer because I've seen many. 4. t=Yt' from Bob's perspective and t'=Yt'' from Alice's. 5. I'm just interested in the names of what I've described not in the million others. I assume you're saying there are no proper names for the proper time perspective. $\endgroup$
    – ralfcis
    Dec 23, 2019 at 13:48
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    $\begingroup$ @ralfcis: If you insist on using notation like $t$ and $t'$ in some nonstandard way known only to you, then only you can understand or answer your questions. If you insist on using phrases like "Alice's perspective" to mean two different things in the same sentence, then your questions will never make sense in the first place. And if you choose to just keep ignoring these issues, people are very quickly going to give up on even trying to help you. $\endgroup$
    – WillO
    Dec 23, 2019 at 15:10
  • $\begingroup$ I realise my questions don't make sense because I don't know the terms.I want to learn the terms so I'm describing them. t=Yt' where t is from Bob's perspective blue line of simultaneity and t'=Yt'' from Alice's perspective red line of simultaneity. For Alice's time of t'=8, Bob's is t=10 and t''=6.4. What are your terms. All I have is Brian Greene's terms from his on-line course. $\endgroup$
    – ralfcis
    Dec 23, 2019 at 15:51
  • $\begingroup$ PS. and I realise Alice's lines of simultaneity point upwards on the inbound leg but at co-location the red line is a point, not a line. At the turnaround point I've heard so many interpretations of how that 2yrs disappears at the co-location point (correct term?) that any answer I give will be wrong to someone so I don't want to give my answer to your question. $\endgroup$
    – ralfcis
    Dec 23, 2019 at 16:06
  • $\begingroup$ Thank you, I understand your notation but I've never seen that before. $\endgroup$
    – ralfcis
    Dec 23, 2019 at 17:11

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