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Re: [Phys-l] Teaching Special Relativity



On Sat, 4 Jul 2009, Moses Fayngold wrote:

Hi all-
See
http://en.wikipedia.org/wiki/Proper_length
as an example of the use of the word "proper" in ordinary relativistic parlance. "Proper" and "invariant" are, for all practical purposes
the same.
Regards,
Jack
________________________________
On Saturday, July 4, 2009 5:17:47 PM, John Denker wrote:

On 07/04/2009 01:31 PM, Moses Fayngold wrote in part:

"While we're on the subject, it strikes me as beyond bizarre that
anyone would ask students to give up the idea that the length of a
ruler is /the/ length of the ruler ... and that the length is
invariant with respect to rotations. The spacetime approach says the
length is invariant under rotations in the XY plane and also
rotations in the XT plane, i.e. boosts, i.e. changes of velocity."

How interesting! And I thought this was quotation from your own previous message!

">> As to the XT rotations, the proper length is NOT invariant under these rotations! (Now I recognize my words! - MF)

">Baloney"

A strong word, but not compelling argument.

"">> At least not in the sense usually associated with the word "invariant" ""

"> In this context, invariant means Lorentz invariant. Nothing more,nothing less. Proper length is Lorentz invariant."

True for proper distance. False for proper length. I showed this in my previous message. Please, show my error.

"">> And it is not necessary that you mark them simultaneously in K0.""

"> Actually it is necessary. Otherwise you are measuring something else, not the _proper_ length."

Please, read again my argument and show where and why it is wrong.

"">> .... We can even consider such measurement as a possible operational definition of proper length"".

">What do you mean by "we", Kemosabe?"

You are totally free not to join the company.

">Proper length has a well-established definition. It is defined that way for good reasons. The redefinition given above is a mockery of the definition, and a travesty of the rationale behind the definition".

It would be really instructive if you just formulate this well-established definition and explain what is wrong in my definition.

"> When people criticize special relativity by imposing heretical definitions, and then criticizing the impracticality of their own figments, it's the worst sort of straw-man fallacy. This is Pentcho Valev territory.
People who pointedly refuse to use spacetime ideas in their own work should refrain from imposing their wacky definitions on people who do use spacetime ideas. As Harry Emerson Fosdick put it, person saying it cannot be done is liable to be interrupted by persons doing it. "

Good rhetoric for a political forum, not for Physics Forum.

Moses Fayngold,
NJIT

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Forum for Physics Educators
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


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