Re: SR cannot determine Contraction

On Feb 24, 8:21 pm, "Artful" <art...@xxxxxxxxxx> wrote:
"N:dlzc D:aol T:com (dlzc)" <dl...@xxxxxxx> wrote in messagenews:mrpwj.3400$kp6.3095@xxxxxxxxxxxxxxx

Dear Artful:

"Artful" <art...@xxxxxxxxxx> wrote in message
news:13s41e3pgqbgic8@xxxxxxxxxxxxxxxxxxxxx
"Dono" <sa...@xxxxxxxxxxx> wrote in message
news:1a9d4300-fd43-45a6-b051-31d97de34a52@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I know the paradox, this simple exercise doesn't
support your view.

Yes .. it does.  If the pole was not taking up less
space in the barn's frame of reference, it would
not fit inside the barn.

There is no "barn's" frame of reference.  In the rest frame of the barn,

You just said there wasn't one.

an observer in the front of the barn and an observer in the back of the
barn will not agree that both doors were closed "simultaneously".

They certainly can.  Simulaneity is well defined in SR

It is very important that you not imagine you can have instantaneous
signalling from the front to the back of the barn.

You don't need it.  You can have a signal sent from the centre of the barn
to the two equidistant observers.  Anyawy .. this is not about the physical
ability to construct a real and literal pole-in-the-barn experiment.  It is
about the fact that, in SR, a moving object occupies less 'space' in a given
frame of reference at a given time than it does if it were at rest.

The pole is "really" contracted,

Yes .. it is.  In the pole-and-barn for a ceratin time, the pole is entirely
within the barn (whether or not you close the doors), even though the barn
has a shorter rest length than the pole.

AND it is entirely a matter of perspective.

Yes, it is.  Reality is what we measure.

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.

PD
.

Relevant Pages

  • Re: TOM ROBERTS - Dono is confused, please help him out (was SR cannot determine Contraction)
    ... as the long axis of the rod) in some inertial frame of reference, its length taken at some given time within the frame of reference will be shorter than its length when at rest .. ... He says the pole and barn thought experiment doesn't support my point of view, and that the pole does not at any time fit within the barn, because length contraction is not physical. ...
    (sci.physics.relativity)
  • Re: TOM ROBERTS - Dono is confused, please help him out (was SR cannot determine Contraction)
    ... the long axis of the rod) in some inertial frame of reference, its length taken at some given time within the frame of reference will be shorter than its length when at rest .. ... pole-and-barn thought experiment where as some given time in the FoR of the barn, the pole in motion is fully within the barn, even though the rest length of the pole is longer than that of the barn. ...
    (sci.physics.relativity)
  • Re: SR cannot determine Contraction
    ... There is no "barn's" frame of reference. ... In the rest frame of the barn, ... In the pole-and-barn for a ceratin time, ...
    (sci.physics.relativity)
  • Re: SR cannot determine Contraction
    ... What I asked is can the pole fit in the ... > barn with both doors closed simultaneously in the barn frame? ... but observations made from another frame cannot possibly affect ... The issue was that you asserted VERY clearly that in the thought experiment, SR would predict that the rod would NOT fit within the barn. ...
    (sci.physics.relativity)
  • Re: SR cannot determine Contraction
    ... If the pole was not taking up less ... not fit inside the barn. ... There is no "barn's" frame of reference. ...
    (sci.physics.relativity)
Loading