#relativity — Public Fediverse posts

Frank Wappler @[email protected] · 2026-07-07 · 21:20 UTC

@level98 wrote (Jul 06, 2026, 21:46 UTC):

> In SR we are NOT suggesting one clock runs slower than one other clock.

This can be claimed MORE SINCERELY by you and me (who reject and avoid the *terrible* phrase) than by anyone using it uncritically.

p.s.
Other than this agreement (the main point) I continue to disapprove of your terminology; e.g.: What do you mean by a "location" ?

Note W. Rindler's description
_»An inertial frame is simply an infinite set of point particles sitting still in space relative to each other.«_

(The #relativity problem of first of all finding out whether any abstract-hypothetical reference frame is an inertial system, or not, you may delegate to Nenashev, Baranovskii https://arxiv.org/abs/2302.12209
and Your's truly.)

Rindler's description see at http://www.scholarpedia.org/article/Special_relativity:_kinematics

Frank Wappler @[email protected] · 2026-07-06 · 17:20 UTC

> [_» ... that there is no fact of the matter about who is moving and if I think you are moving then you think I am moving.«_ ]

> Given that was the whole point of the OP [...]

By "the OP" surely you mean what Level 98 wrote (22. Juni 2026, 02:45): https://mastodon.social/@level98/116790991658848783

>>> the words "moving clocks run slow" is a *terrible* phrase to use in talking about relativity. [...]

In my reading, quoting and responding to that/your OP, the sophomorics about "Who's moving?" were far from making up its *whole point*.

> The OP was not about what *you're* bothered with.

Out of an abundance of caution I repeat here what bothers me mainly about the *terrible* OP phrase: it refers to comparison of *rates* instead of comparison of *durations*.

If your OP was not about this concern *at all* then I hereby stand corrected.
(Please note my question at the end, though ...)

> [...] It's odd you commented [at all].

At the very least I commented to express my whole-hearted agreement with calling the phrase "moving clocks run slow" *terrible* for teaching #relativity.
(Thanks to mathstodon for enabling me to do so, publicly and archived.)

> i.e. what people in general struggle with in learning SR

I certainly don't mean to defend referring to _»the inertial system "at rest"«_
vs. _»the inertial system "in motion"«_;
such terminology surely contributes to the OP phrase being *terible*.

But any imaginable confusion due to this didactic blunder from 1905 vanishes, in my experience, as soon as the lectures turn to _»the platform system«_ vs.
_»the train system«_, for instance;
both (close enough to) being inertial systems, moving *wrt. each other*, mutually, interchangeably --
resulting in [contd.]

#relativity

Frank Wappler @[email protected] · 2026-07-06 · 17:20 UTC

> [_» ... that there is no fact of the matter about who is moving and if I think you are moving then you think I am moving.«_ ]

> Given that was the whole point of the OP [...]

By "the OP" surely you mean what Level 98 wrote (22. Juni 2026, 02:45): https://mastodon.social/@level98/116790991658848783

>>> the words "moving clocks run slow" is a *terrible* phrase to use in talking about relativity. [...]

In my reading, quoting and responding to that/your OP, the sophomorics about "Who's moving?" were far from making up its *whole point*.

> The OP was not about what *you're* bothered with.

Out of an abundance of caution I repeat here what bothers me mainly about the *terrible* OP phrase: it refers to comparison of *rates* instead of comparison of *durations*.

If your OP was not about this concern *at all* then I hereby stand corrected.
(Please note my question at the end, though ...)

> [...] It's odd you commented [at all].

At the very least I commented to express my whole-hearted agreement with calling the phrase "moving clocks run slow" *terrible* for teaching #relativity.
(Thanks to mathstodon for enabling me to do so, publicly and archived.)

> i.e. what people in general struggle with in learning SR

I certainly don't mean to defend referring to _»the inertial system "at rest"«_
vs. _»the inertial system "in motion"«_;
such terminology surely contributes to the OP phrase being *terible*.

But any imaginable confusion due to this didactic blunder from 1905 vanishes, in my experience, as soon as the lectures turn to _»the platform system«_ vs.
_»the train system«_, for instance;
both (close enough to) being inertial systems, moving *wrt. each other*, mutually, interchangeably --
resulting in [contd.]

#relativity

Frank Wappler @[email protected] · 2026-07-05 · 20:23 UTC

@level98
You wrote (Jul 05 2026, 2:34 AM):
> The intro's nice. Particularly [...]

I.e. concerning the recent arxiv article by D. Garfinkle, on which I commented via SciRate a few days ago.
(I rather raise interest in https://arxiv.org/abs/2302.12209 -- hoping for help on projects which exemplify _"well-stitchedness"_.)

> _»Time dilation is 'fast clocks run slow' while length contraction is 'fast rulers get short.' [...]«_

Cmp. p. 4 of https://arxiv.org/abs/2605.21660
In the very next paragraph he suggests

_»Thus there may be room for improvement in the way that special #relativity is often presented [...]«_

I strongly agree and I solve thus:

- Time dilation is: 'the arc-length of a straight/inertial timelike worldline section is shorter, by γ, than the arc-length of any straight/inertial timelike worldline section on which the former is (invertably) (Einstein-) projected'.

and

- length contraction is: 'a train is longer, by factor γ, than the platform on which the constituents of the train are (simultaneously) (Einstein-) projected'.

Curiously, Garfinkle doesn't arrive there ...

> _»On the other hand we are told that there is no fact of the matter about who is moving and if I think you are moving then you think I am moving.«_

I didn't even bother with such sophomorics; they're not involed in expressing what time dilation is, and what length contraction is.

What may be measured is:

- whether any one timelike worldline is straight (inertial),

- whether any two participants find constant ping durations wrt. each other.

p.s.
> You're trying to teach your grandma how to suck eggs.

Physicists do indeed make their points by considering grandmas, based on R.P. Feynman's appreciation.

#relativity

Frank Wappler @[email protected] · 2026-07-05 · 20:23 UTC

@level98
You wrote (Jul 05 2026, 2:34 AM):
> The intro's nice. Particularly [...]

I.e. concerning the recent arxiv article by D. Garfinkle, on which I commented via SciRate a few days ago.
(I rather raise interest in https://arxiv.org/abs/2302.12209 -- hoping for help on projects which exemplify _"well-stitchedness"_.)

> _»Time dilation is 'fast clocks run slow' while length contraction is 'fast rulers get short.' [...]«_

Cmp. p. 4 of https://arxiv.org/abs/2605.21660
In the very next paragraph he suggests

_»Thus there may be room for improvement in the way that special #relativity is often presented [...]«_

I strongly agree and I solve thus:

- Time dilation is: 'the arc-length of a straight/inertial timelike worldline section is shorter, by γ, than the arc-length of any straight/inertial timelike worldline section on which the former is (invertably) (Einstein-) projected'.

and

- length contraction is: 'a train is longer, by factor γ, than the platform on which the constituents of the train are (simultaneously) (Einstein-) projected'.

Curiously, Garfinkle doesn't arrive there ...

> _»On the other hand we are told that there is no fact of the matter about who is moving and if I think you are moving then you think I am moving.«_

I didn't even bother with such sophomorics; they're not involed in expressing what time dilation is, and what length contraction is.

What may be measured is:

- whether any one timelike worldline is straight (inertial),

- whether any two participants find constant ping durations wrt. each other.

p.s.
> You're trying to teach your grandma how to suck eggs.

Physicists do indeed make their points by considering grandmas, based on R.P. Feynman's appreciation.

#relativity

Frank Wappler @[email protected] · 2026-07-04 · 22:10 UTC

> [...] "time" vs "duration" [...] makes no difference. Except...

> t [...]

... i.e. individually: "a timestamp" also: "a reading"; (either way some real number value),
and collectively typically (thought of as) parametrizing a time-like worldline.

> [ vs. ] Δt

... i.e. individually (certainly "on first sight"):
"a numerical difference (of two timestamps)".

> I'd prefer Δ because it emphasises that it's a time

... time STAMP DIFFERENCE (btw., right ?) ...

> between two Events,

You're right in that, in order to specify and determine a duration, there are (typically) two arguments to be considered -- that is, besides considering exactly WHOSE worldline-section (duration) is to be determined (which can show up as third argument) -- namely the declarations BEGINNING FROM, and ENDING AT (a.k.a. UNTIL) to delimit the applicable worldline section.

Now, if you recall Fig. 1 of your article, there were events shown which you (and/or co-author?) had distinctly labelled with different integers ("1", "2"), while in my preceding post I had referred to them by explicitly listing all (relevant) participants; viz. "e_RY" for the event in which the two participants "R" and "Y" had met each other (once, in passing), etc.

So: just identifying an event (as the argument FROM) and another event (as UNTIL) wouldn't quite suffice to unambiguously declare WHOSE reading(s) are meant to be taken for calculating the sought difference, Δt.

That's why I prefer and suggest to consider instead the distinctive "indications" of individual participants, in the sense of their distinctive individual shares of the relevant event in which they took part. [contd.]

#relativity

Frank Wappler @[email protected] · 2026-07-04 · 22:10 UTC

> [...] "time" vs "duration" [...] makes no difference. Except...

> t [...]

... i.e. individually: "a timestamp" also: "a reading"; (either way some real number value),
and collectively typically (thought of as) parametrizing a time-like worldline.

> [ vs. ] Δt

... i.e. individually (certainly "on first sight"):
"a numerical difference (of two timestamps)".

> I'd prefer Δ because it emphasises that it's a time

... time STAMP DIFFERENCE (btw., right ?) ...

> between two Events,

You're right in that, in order to specify and determine a duration, there are (typically) two arguments to be considered -- that is, besides considering exactly WHOSE worldline-section (duration) is to be determined (which can show up as third argument) -- namely the declarations BEGINNING FROM, and ENDING AT (a.k.a. UNTIL) to delimit the applicable worldline section.

Now, if you recall Fig. 1 of your article, there were events shown which you (and/or co-author?) had distinctly labelled with different integers ("1", "2"), while in my preceding post I had referred to them by explicitly listing all (relevant) participants; viz. "e_RY" for the event in which the two participants "R" and "Y" had met each other (once, in passing), etc.

So: just identifying an event (as the argument FROM) and another event (as UNTIL) wouldn't quite suffice to unambiguously declare WHOSE reading(s) are meant to be taken for calculating the sought difference, Δt.

That's why I prefer and suggest to consider instead the distinctive "indications" of individual participants, in the sense of their distinctive individual shares of the relevant event in which they took part. [contd.]

#relativity

Frank Wappler @[email protected] · 2026-07-03 · 17:39 UTC

2/2:

> A key relationship to the basics of SR being that the two IRFs disagree on the synchronisation of the two clocks etc.

Important of course; but **not basic**.

"Synchronism" (Einstein, 1905) **presumes/requires** readings, i.e. t-values;

but the analysis linked above, and Einstein's simultaneity definition of 1916/17 https://archive.org/details/in.ernet.dli.2015.162673/page/21/mode/2up?q=mid-point
**do not**.

Consequence: t-values are **derived** from more basic
given observational circumstances:
coincidence determinations, causal relations.

Frank Wappler @[email protected] · 2026-07-03 · 17:39 UTC

2/2:

> A key relationship to the basics of SR being that the two IRFs disagree on the synchronisation of the two clocks etc.

Important of course; but **not basic**.

"Synchronism" (Einstein, 1905) **presumes/requires** readings, i.e. t-values;

but the analysis linked above, and Einstein's simultaneity definition of 1916/17 https://archive.org/details/in.ernet.dli.2015.162673/page/21/mode/2up?q=mid-point
**do not**.

Consequence: t-values are **derived** from more basic
given observational circumstances:
coincidence determinations, causal relations.

Frank Wappler @[email protected] · 2026-07-03 · 17:31 UTC

1/2:

> This simple scenario

... To be sure, that's Fig. 1 of your Austr. Phys. 56 (6), 2019,
and as analyzed (to my ability & satsfaction) in section 2. of https://physics.stackexchange.com/a/686765/12262 ...

> compares a measurement of time between the two Events where we're comparing a single clock along its (inertial) worldline, with that of two (synchronised) clocks in another inertial reference frame.

I recognize this terminology from your article; there are reasons the terminology I use instead. Briefly:

- The very old word "time" has very many (more or less related but) different meanings.
Use "duration", i.e. the **measure of** temporal sepratation, when we **mean duration**. Last but not least, see the word/notion "duration" used here: https://www.bipm.org/en/si-base-units/second

- "between two (not necessarily distinct) events", say e_{RY} and e_(LY) of your Fig. 1 are

(1) in generality: "Lorentzian distance" &ell;[ e_{RY}, e_(LY) ], cmp https://en.wikipedia.org/wiki/Globally_hyperbolic_spacetime#Definitions also "Beem-Ehrlich-Easley",

(2) "under certain circumstances": Synge's world function &ell;[ e_{RY}, e_(LY) ] , https://en.wikipedia.org/wiki/Synge%27s_world_function

(3) and (surely only) in flat spacetime regions: the interval s^2[ e_{RY}, e_(LY) ]

> comparing a single clock along its (inertial) worldline

That's in fact pretty much the duration (1).

> with that of two (synchronised) clocks in another inertial reference frame.

And to repeat, that is (in Fig. 1):

"The duration of R, from having met/passed Y
until R's indication simultaneous to L's indication of having met/passed Y."

which (by definition) is equal to

"The duration of L, from L's indication simultaneous to R's indication of having met/passed Y, until L having met/passed Y."

#relativity

Frank Wappler @[email protected] · 2026-07-03 · 17:31 UTC

1/2:

> This simple scenario

... To be sure, that's Fig. 1 of your Austr. Phys. 56 (6), 2019,
and as analyzed (to my ability & satsfaction) in section 2. of https://physics.stackexchange.com/a/686765/12262 ...

> compares a measurement of time between the two Events where we're comparing a single clock along its (inertial) worldline, with that of two (synchronised) clocks in another inertial reference frame.

I recognize this terminology from your article; there are reasons the terminology I use instead. Briefly:

- The very old word "time" has very many (more or less related but) different meanings.
Use "duration", i.e. the **measure of** temporal sepratation, when we **mean duration**. Last but not least, see the word/notion "duration" used here: https://www.bipm.org/en/si-base-units/second

- "between two (not necessarily distinct) events", say e_{RY} and e_(LY) of your Fig. 1 are

(1) in generality: "Lorentzian distance" &ell;[ e_{RY}, e_(LY) ], cmp https://en.wikipedia.org/wiki/Globally_hyperbolic_spacetime#Definitions also "Beem-Ehrlich-Easley",

(2) "under certain circumstances": Synge's world function &ell;[ e_{RY}, e_(LY) ] , https://en.wikipedia.org/wiki/Synge%27s_world_function

(3) and (surely only) in flat spacetime regions: the interval s^2[ e_{RY}, e_(LY) ]

> comparing a single clock along its (inertial) worldline

That's in fact pretty much the duration (1).

> with that of two (synchronised) clocks in another inertial reference frame.

And to repeat, that is (in Fig. 1):

"The duration of R, from having met/passed Y
until R's indication simultaneous to L's indication of having met/passed Y."

which (by definition) is equal to

"The duration of L, from L's indication simultaneous to R's indication of having met/passed Y, until L having met/passed Y."

🔗 https://doi.org/10.7551/mitpress/15746.001.0001

#relativity

UniversityofGroningenLibrary @[email protected] · 2026-07-03 · 09:16 UTC

New in our electronic collection:

➡️ Testing #Einstein: One Hundred Years of #Experimental #Relativity

#OpenScience #OpenAccess #books #bookstodon #physics #science #history

#einstein #experimental #relativity #openscience #openaccess #books

UniversityofGroningenLibrary @[email protected] · 2026-07-03 · 09:16 UTC

New in our electronic collection:

➡️ Testing #Einstein: One Hundred Years of #Experimental #Relativity

🔗 https://doi.org/10.7551/mitpress/15746.001.0001

#OpenScience #OpenAccess #books #bookstodon #physics #science #history

#einstein #experimental #relativity #openscience #openaccess #books

Frank Wappler @[email protected] · 2026-07-02 · 17:47 UTC

Hi Theo,

thanks for your replies.
First of all, however, I'd like you to know that I managed to get a copy of your Austr. Phys. 56 (6), 2019, thanks to your website and the excellent archive of the AIP.

Interesting article! While your repeated attempts to "save [parts of] that oft-used [abominable] phrase" didn't succeed, I was very glad that you -- almost in passing -- struck upon **the** **appropriate** phrase:

> _»We know that it is you [circled Y in Fig. 1] who measures the proper time, and hence the shorter time interval.«_

So the key/solution is: to refer to **duration**, certain clocks running **longer** (or **shorter**) than certain other clocks;
instead of referring to clock rate (frequency), i.e. certain clocks running faster (or slower) than certain others.

This holds for #GeneralRelativity just as well.

All that inappropriate referring to clock rate has been nothing but a "red hering" from the outset (Einstein Ann. Phys. 17, 891 (1905))!

In terminology I prefer, I teach the Fig.1-applicable appropriate #SpecialRelativity phrase as

"The duration of Y, from having met/passed R until having met/passed L,
is shorter, by factor √{ 1 - β^2 }, than
the duration of R, from having met/passed Y
until R's indication simultaneous to L's indication of having met/passed Y."

p.s. For your kind consideration, here's "putting my money where my mouth is": https://scirate.com/arxiv/2605.21660

p.p.s. The notion #Duration may have been depreciated in #Relativity due to nonsense by the French philosopher H. Bergson. Instead, "duration" plainly is in the SI-second-def

p.p.p.s. I can mostly recommend tooting (!) by mathstodon: 1729 chars. + \(\LaTeX\) -- only "Markdown" still doesn't work!

#relativity #duration #specialrelativity #generalrelativity

Frank Wappler @[email protected] · 2026-07-02 · 17:47 UTC

https://mastodon.social/@level98/116790991658848783

Hi Theo,

thanks for your replies.
First of all, however, I'd like you to know that I managed to get a copy of your Austr. Phys. 56 (6), 2019, thanks to your website and the excellent archive of the AIP.

Interesting article! While your repeated attempts to "save [parts of] that oft-used [abominable] phrase" didn't succeed, I was very glad that you -- almost in passing -- struck upon **the** **appropriate** phrase:

> _»We know that it is you [circled Y in Fig. 1] who measures the proper time, and hence the shorter time interval.«_

So the key/solution is: to refer to **duration**, certain clocks running **longer** (or **shorter**) than certain other clocks;
instead of referring to clock rate (frequency), i.e. certain clocks running faster (or slower) than certain others.

This holds for #GeneralRelativity just as well.

All that inappropriate referring to clock rate has been nothing but a "red hering" from the outset (Einstein Ann. Phys. 17, 891 (1905))!

In terminology I prefer, I teach the Fig.1-applicable appropriate #SpecialRelativity phrase as

"The duration of Y, from having met/passed R until having met/passed L,
is shorter, by factor √{ 1 - β^2 }, than
the duration of R, from having met/passed Y
until R's indication simultaneous to L's indication of having met/passed Y."

p.s. For your kind consideration, here's "putting my money where my mouth is": https://scirate.com/arxiv/2605.21660

p.p.s. The notion #Duration may have been depreciated in #Relativity due to nonsense by the French philosopher H. Bergson. Instead, "duration" plainly is in the SI-second-def

p.p.p.s. I can mostly recommend tooting (!) by mathstodon: 1729 chars. + \(\LaTeX\) -- only "Markdown" still doesn't work!

#relativity #duration #specialrelativity #generalrelativity

Frank Wappler @[email protected] · 2026-07-02 · 00:49 UTC

https://mastodon.social/@level98/116790991658848783

Level 98 wrote (22. Juni 2026, 02:45):
> the words "moving clocks run slow" is a *terrible* phrase talking about #relativity.

Agreed.

Who would ever distort into such an abomination a perfectly legitimate standard phrase such as, say

"clock \(A\) has run _slower_, i.e. at lower clock rate \(\nu_A\), on average, on its (time-like) worldline segment starting from event \(\varepsilon_{ABJ}\) and ending with event \(\varepsilon_{ABK}\)
than
\(\nu_B\), the rate clock \(B\) has run, on average, on _its_ (time-like) worldline segment starting (likewise) from event \(\varepsilon_{ABJ}\) and ending (likewise) with event \(\varepsilon_{ABK}\)"
?

> The reality of SR is far cooler and (according to a teacher I was helping) easier to understand, if one doesn't use such pop sci phrases.

How else could you even begin to communicate (e.g. to students) that, accordingly, a comparison is to be made between

- the ratio \(\nu_A := \frac{(t_A[ ~ \text{_BK} ~ ] - t_A[ ~ \text{_BJ} ~ ] )}{ \tau A[ ~ \text{_BK}, \text{_BJ} ~ ]}\)

and

- the ratio \(\nu_B := \frac{(t_B[ ~ \text{_AK} ~ ] - t_B[ ~ \text{_AJ} ~ ] )}{ \tau B[ ~ \text{_AK}, \text{_AJ} ~ ]}\)

?

p.s.
Did you (or that fine teacher you mentioned) come up with a more efficient notation for identifying

- distinctive events (by the names of their distinguishable coincident participants),

- distinctive indications of any one participant (by the names of all other distinguishable participants coincident in the resp. event),

- the (not necessarily distinct) readings "t" assigned to indications,

- the durations (a.k.a. arc-lenghts) \("\tau"\) of specific worldline segments
?

For #SpecialRelativity and in general.

#specialrelativity #relativity

Frank Wappler @[email protected] · 2026-07-02 · 00:49 UTC