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Re: [Phys-l] Teaching Special Relativity



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."

Nobody is asking the students to do it, as far as the XY rotations
are concerned.

OK so far.

As to the XT rotations, the proper length is NOT invariant under
these rotations!

Baloney.

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.


.... If you are in the rest frame K0 of the
rod, and you want to measure its length, you can do it by first
marking its end-points.

OK, if it's done right, as discussed below.

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.

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

What do you mean by "we", Kemosabe?

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.

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.