Re: I got stuck on Part II of Special Theory of Relativity (by Albert Einstein) textbook, please help

"Albertito" <albertito1992@xxxxxxxxx> wrote in message
news:9f3df407-4417-4a08-a3a6-493054f2ac1f@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi experts,

My aim is to understand General Relativity Theory, but somebody
advised me to study Special Relativity before that GR attempt.

Yeup .. its much simpler, and is a limiting case of GR

So,
Here I am, stuck on Part II of Special Theory of Relativity by
Albert Einstein. It seems a blind alley to me. This Part II refers to
"The system of Co-ordinates". I have some 'naive' questions:

1. What is a rigid body?.

It is something that anyone with any knowledge of physics, or eve of hte
English language (or the equivalent in German) would undersatnd. It is one
that doesn't distort (doesn't expand or contract in its own frame of
refernnce). There is no such think in reality .. everything can distort in
some sense .. but one can do some physics on the assumption that a body is
rigid (theories tend to be done in terms of ideals and then allowances made
for things like friction, no-rigidity etc)

2. How can a system of co-ordinates be based on distances
through a rigid body, if you have not previously defined
rigorously what a rigid body is?.

It was taken that any reasonable person would now what the word 'rigid'
means

3. Do rigid bodies actually exist in Nature?.

No .. not ones with any length at least :)

4. What would happen if you define a system of co-ordinates
based on the concept of non-rigid body?.

A system of coordinates is always rigid, otherwise they aren't any use. And
as a system of coordinates is a concept only, it can indeed be truly rigid.

5. Is distance that what a ruler can measure?.

You can measure distance with a ruler (the absolute difference in spatial
coordinates between two events with the same time coordinate in a give
nframe of reference)

6. What would happen to a ruler's measurement if the ruler's
rigidity is slightly altered in some parts of it, in a manner
that
we cannot detect by means of any other ruler?.

All rulers (like any other material object) are non-rigid .. which is why
when you want to measure things accurately, a ruler is not a good choice.
In general they are rigid enough for doing things like seeing how long a
piece of wood is (say).


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