B Understanding twin paradox without math

[robphy]

Exactly, this is Brian Greene's explanation of the twin paradox the only measurements you can trust are from an observer in an inertial frame. No time skips. Which is why I drew those diagrams, from three inertial frames.

I'd be interested in a reference to where Greene says this. Either there's some context you're missing or Greene is saying wrong things in his popularisations (again). There are plenty of non-inertial frames one can work with where there are no time skips (and Greene knows this - the study of non-inertial frames was a step on the road to the discovery of general relativity). They are just mathematically harder to use than inertial frames.

WSU: Special Relativity with Brian Greene (The Twin Paradox, Explanation #1 &t=09h26m30s )


Here is how I would paraphrase his statements:

You must be an "inertial observer for the whole trip"​

in order to claim that you (tautologically) are an "inertial observer for the whole trip".​

Otherwise, the full analysis from a non-inertial observer (Gracie)​

is not valid as if Gracie were an "inertial observer".​

​

I think Greene's presentation of this point is fine (**in this format [of a recorded non-interactive lecture]**)...
although sometimes soundbites with missing context can lead to misinterpretations of the full message.
(More succinctly, as I often say, "𝐁𝐞𝐢𝐧𝐠 𝐚𝐛𝐥𝐞-𝐭𝐨-𝐛𝐞-𝐚𝐭-𝐫𝐞𝐬𝐭"≠"𝐁𝐞𝐢𝐧𝐠 𝐢𝐧𝐞𝐫𝐭𝐢𝐚𝐥". )
**I think that if subtleties were raised by a live questioner, Greene would clarify his statements.**

In my interpretation, it's too strong to draw a conclusion that any analysis (any measurement) from Gracie is invalid.
As @Ibix suggests, one needs more care to handle non-inertial Gracie's measurements compared to inertial George's measurements.

 

[Ibix]

In my interpretation, it's too strong to draw a conclusion that any analysis (any measurement) from Gracie is invalid.

Thanks. I wondered if it was something like that. 'If you want to use a naive SR analysis you have to use a single inertial frame for the whole thing' is true and is at least half an explanation for why the twin paradox isn't a paradox, but it is not as general a claim as @obtronix is making.

 

[Orodruin]

Telling a student that the thing that confuses them is a “pretty simple no brainer” may not be as helpful to the student as you think. Usually they require a teacher to address their actual misunderstanding, not insult them.

Making a set of video lectures about a decade ago made me realize how often I would use ”simply” as a filler word when describing some result. Most of the time it is not needed for the description and only has a negative effect. For students who actually think it is simple it adds nothing and for students who don’t it comes off as somewhat insulting or demoralizing. I had to spend a lot of time consciously thinking about it to work it out of my typical lecture jargon. When writing my first book one of the last things I did was to search through the source LaTeX for the word ”simple” and eliminated it unless I felt it actually added substance.