Re: [Phys-l] three central misconceptions about relativity

[John Denker <jsd@av8n.com>]

On 10/14/2011 10:34 PM, Hugh Haskell wrote:
> Teaching students who are only barely comfortable 
> with two-dimensional geometry and trigonometry 
> about Minkowski diagrams and 4-space is really 
> not too practical. They already have problems 
> with abstractions, so those of us who teach 
> students t that level are pretty much required to 
> look at the differences between Newtonian and 
> Relativistic mechanics, and more or less leave it 
> there. In my classes I teach relativity by 
> starting with Einstein's postulates, and "derive" 
> the space and time dilation formulas 

One could easily turn that argument on its head:

a) If the students are not sufficiently comfortable
with geometry and trigonometry in the xy plane, then 
telling them about rulers that can't be trusted and 
clocks that can't be trusted will only make things 
worse.  Much worse.

b) There is at least a chance that talking about 
the geometry and trigonometry of the xt plane will
build on /and reinforce/ stuff they already know.

c) If they are really so clueless that they can't
handle item (b), then you shouldn't mention relativity
to them at all.

I'm serious:  Modern (post-1908) relativity is *less*
complicated than the other kind.  It's more pictorial,
more intuitive, and more connected to stuff students 
already know.  If they can't handle the geometric /
trigonometric approach, they have no chance of handling 
the other approach.

==============

> weird is clearly in the eye 
> of the beholder. 

Agreed.

> When something the student has 
> "known" for a long time is shown to be not true, 
> the "weird" label will almost always be applied 
> to the new material rather than the old.

True ... but not relevant to relativity, if it is done right.

Special relativity is just not very weird.  Little if any
conflicts with what they already know.  For example, it
firmly predicts that kinetic energy goes like half p^2/m
when the momentum is not overly huge.  That's not weird.
That's not even new.

Special relativity says that if you want to calculate the
length of a vector, you need to account for /all/ of the
components.  That's not weird.  That's not even new.

I still cannot imagine any good reason for taking a simple,
elegant subject and making it complicated and weird by thinking 
about it the wrong way.

================

Students don't have misconceptions about spacetime.  The 
teachers are the ones you need to worry about.  That's because
they have been exposed over and over again to clocks that 
(allegedly) can't be trusted, rulers that (allegedly) can't 
be trusted, mass that (allegedly) depends on velocity, and 
various other wacky ideas that are incompatible with any
real understanding of the subject.

=======================================================

On 10/13/2011 01:53 PM, Philip Keller wrote:

> > Like many high school teachers, I learned the physics I teach
> > mostly from textbooks --someone has to read them! As you know,
> > Halliday and Resnick, Serway, Sears Zemansky @ Young all introduce
> > relativity the historical way.

I am sympathetic to that point.  It's hard to teach something 
properly when there is no support for it in The Books or anywhere
else.  The Books are 100 years behind the state of the art.
Maybe in another 100 years or so they will get with the program.

I've been trying to speed up the process.  I've done some more 
work on
  http://www.av8n.com/physics/spacetime-welcome.htm
There's still a ways to go, but I think it has potential.

> > I think that when we teach it that way, it's because true or not, we
> > like the narrative.  And we like the dali-esque idea of shrinking
> > rulers and slow clocks.

For many years I had a cat named Salvador ... so named because
he liked to sleep draped over the back of the couch, crosswise,
oozing down on both sides.  (There are many other surreal stories
about that cat, but you wouldn't believe them.....)

So, I enjoy surrealism as much as the next guy, but I keep it 
separate from my physics.  I keep my physics real.  I don't brag 
on how abstruse and weird my physics is;  I brag on how sensible, 
useful, and elegant it is.

Remember: Dali was an artist.  There is artistry in physics, too.
Elegance.  Panache.

Dilated clocks and contracted rulers are not elegant.  They are
a disgusting kludge.  Spacetime is astonishingly elegant.

Please give it another look.  I think you will find it better in
every way:  Easier to teach, easier to learn, more powerful ...
and more artistic.