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why would it look like an oval if it is shrinking along the rotating line of motion?  I don't have the vocabulary for this.

I will dig up the section from Einstein's book.

eric

On Mar 15, 2013, at 11:16 AM, Marc Taylor <[log in to unmask]> wrote:

> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related institutions.
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> 
>> 2) even in planar geometry, I seem to remember reading that
>> pi is variable depending on frames of reference.  Imagine a
>> disk spinning at a significant fraction of the speed of
>> light.  It gets shorter along the line of direction, in other
>> words the circumference shrinks.  But a radius, which is not
>> moving along the line of direction approaching the speed of
>> light, doesn't shrink.  I think in Einstein's little book
>> called Relativity this is an example he gives of how the
>> "laws of nature" appear to change as you approach the speed of light.
>> 
>> Again, am I missing something?
> 
> I don't see how pi would change. No matter how the disk is moving, it will either look like a circle or an oval... If you are seeing it under conditions when "pure" Lorentz contraction is visible, it will look like an oval, otherwise, it will look like a disc. Neither of these involve any magical geometry, like Terry Prachett's five-sided squares. Take a picture of the disk and it will either look like a circle or an oval. Become comoving with the disk, and you measure it as a circle. In either case, pi pops out as per normal.
> 
>> Curiouser and Curiouser.
> 
> Indeed.
> 
> Marc Taylor
> Manager, Planetarium and Science Programs
> Hudson River Museum
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