Re: SR cannot determine Contraction

"PD" <TheDraperFamily@xxxxxxxxx> wrote in message
news:bb460ef8-1117-41cb-8c3e-9bc7dfe053ea@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have to agree with Artful on a simple but important point.
Length is a purely spatial quantity by definition. The invariant
interval between two events that occur at the ends of the rod may
indeed be frame-independent, but it is in no way a length, because in
any other frame, it involves both space and time components. What is
true is that in a particular frame, the invariant interval and the
length happen to numerically coincide. This is the frame where the
time difference between the two rod-end events happens to vanish. This
coincidence in this frame in no way turns the interval into a length.
The length is an inherently 3D quantity. The interval is inherently a
4D quantity.

Another example can be taken from simple Euclidean 2-D geometry. Draw
two orthogonal axes and a vector from the origin, so that there is a
component of that vector along one of the axes. By choice of axes, you
can make it so the component numerically coincides with the vector,
but even in this case the component is NOT the vector. The vector is a
2D object, and the component is a 1D object by definition.

Nicely put. Now we've just got to convince dono that an object *does*
'physically' have a shorter spatial length (ie take up less physical space;
have a shorter spatial distance between its endpoint) at a given instant
within a frame of reference in which the object is moving, than in a frame
of reference in which it is at rest.


.

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