Hi everyone -- (digging for basics)

[Frank Wappler]

How did you find PF?

(I had noticed for a long time -- mostly by googling -- that topics of my interest were being discussed here. ...)

Having sailed to a Ph.D. (exp. particle physics) and beyond more two decades ago, I've become increasingly critical about presentations and didactics of the theory of relativity, especially as far as it relates to experimental physics ...

This forum seems to be more inviting to exchanges of thorough arguments than some others to which I tried to contribute over the years. What made me finally join here was foremost the opportunity to issue a reply to:
https://www.physicsforums.com/insights/when-discussing-the-twin-paradox-read-this-first/

 

[berkeman]

Welcome to PF, Frank; it's good to have you here.

 

[Frank Wappler]

Well -- it says there:
https://www.physicsforums.com/insights/when-discussing-the-twin-paradox-read-this-first/

Leave a Reply
Want to join the discussion? Feel free to contribute!

-- but, unfortunately, there doesn't seem to be a link provided (for me, yet, having registered just minutes ago). So, meanwhile, here's the reply I had prepared for submission to this PhysicsForums - Insights article

When Discussing the Twin Paradox: Read This First
January 2, 2023/0 Comments/in Physics Articles/by PeterDonis

Reference: https://www.physicsforums.com/insights/when-discussing-the-twin-paradox-read-this-first/

PeterDonis wrote (January 2, 2023):
> When Discussing the Twin Paradox: [...] the basic point [...] you have two twins who take different paths through spacetime

Agreed.

> and those paths have different lengths

That's certainly the typical and intended setup constraint when "twin senarios" are put forward. (The more general, underlying task is to define, by means of relativistic spacetime geometry, how to compare the "lengths" of -- mostly disjoint -- timelike paths in the first place.)

> Those path lengths are invariants; they don’t depend on which frame you adopt. So both twins will agree on them.

The principles and methods of measurement ought to guarantee that everyone will agree on the (ratio of) lengths of specific (non-zero, timelike) paths.

> [...] if the scenario is well enough specified at all, it must contain enough information to calculate the paths of the twins through spacetime.

Alright. However, I still have an objection:
> [...] the lengths of the paths are the amounts that each twin ages during the trip.

> [...] If the twins have aged differently when they come back together, [...]

But "aging" is often used as "material or biological aging", in the sense of "wear and tear", "ripening", "development" or "decay", referring not to the "length" (duration) of specific a timelike path segment as such, but instead to individual manifestations of "change, in the course of having taken its (specific) path through existance/life/use".

We sensibly define and determine individual "rates of aging", as ratio of quantified change (or: age) and corresponding duration; accordingly we say that certain (mis)use or (bad) habits are corralated with "faster aging" of tools, machines, humans, for instance.

In contrast, the word "duration" corresponds to "spacetime path length" itself; and this use is certainly known and appropriate for referring to individual participants. So I prefer and suggest to say that "the lengths of the paths are the respective trip durations of each twin", etc.

 

[berkeman]

-- but, unfortunately, there doesn't seem to be a link provided (for me, yet, having registered just minutes ago).

I think you may have to be an Insights author to comment on Insights articles, but I'm not sure.

Feel free to start a new thread in the Relativity forum with a link to that Insights article to give your thread start some context. This thread here in the Introductions forum is for brief introductions only, so I'll go ahead and tie it off now.