Re: [Phys-l] velocity-dependent mass (or not)

[John Denker <jsd@av8n.com>]

On 06/28/2009 05:25 PM, I wrote:

> .... any class time spent on
> the spacetime approach to relativity reinforces and deepens
> the students' understanding of the geometry and trigonometry
> of ordinary 3-space, their understanding of vectors, et cetera.
> This is important, since many of them will not have much direct 
> use for relativity in their future lives.  Therefore the "street
> value" of the coursework comes down to gee-whiz value and side
> effects.  The gee-whiz value of spacetime (elegant) is *more* 
> than the value of a bucket of paradoxes (ugly) ... and the side
> effects (geometry, trigonometry, and vectors) are incomparably
> more valuable.

That's all true, but I left off the other half of the argument,
the half that applies if/when some of your students are going to
grow up to be physics majors:

The issue here is unlearning.  Unlearning is always hard and
unpleasant for the student and for the teacher.  I guarantee you
that if you teach the "contraction / dilation / velocity dependent
mass" approach to special relativity, your students will have to 
unlearn every bit of that when they start their first course in 
_general_ relativity.  To say the same thing the other way, general
relativity is all about spacetime, so there is a big advantage to
taking the spacetime approach to special relativity, starting from
Day One.

So, please please please, as a favor to your students, and to the
teachers who will come after you, please teach special relativity
using the modern (post 1908) spacetime approach.

In my experience, virtually all of the people who argue that moving
clocks can't be trusted and moving rulers can't be trusted and moving
particles have a velocity-dependent mass (or three different velocity-
dependent masses) are people who have never cracked open a copy of 
MTW or Weinberg or any other general relativity book.

Special relativity is not weird or paradoxical.  It is just the
geometry and trigonometry of spacetime.

> The idea that time is the fourth dimension has been around since
> 1908, i.e. for 101 of the 104 years that relativity has existed.
> It is so ingrained in our culture that it is taken for granted in
> TV shows, even non-science-oriented shows like /Buffy/.  Why is
> it not taken seriously in physics class?