Re: Length contraction is not an intrinsic property of an object

From: Tom Roberts (tjroberts_at_lucent.com)
Date: 12/20/04 

Date: Mon, 20 Dec 2004 10:36:36 -0600

Harry wrote:
> "Tom Roberts" <tjroberts@lucent.com> wrote in message
> news:cpafu3$oqh@netnews.proxy.lucent.com...
>>[Subject line]
>>"Length contraction" cannot possibly be intrinsic to any object -- it is
>>frame dependent. Anything that depends upon how one looks at an object
>>is not intrinsic, by definition.
>
>Did you read the recent paper in the AJP that concludes just the
> opposite?
> -> American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp.
> 1316-1324, Reflection of light from a uniformly moving mirror, Aleksandar
> Gjurchinovski

You are intermixing different concepts. The reflections Gjurchinovski
discusses are "real" in that they can be measured, as can the
consequences of length contraction. But they are not INTRINSIC to the
mirror, or to any other object in his discussion.

As I have said so often, length contraction is simply geometrical
projection. A Euclidean analogy: in one orientation a tall ladder can be
carried horizontally through a doorway, and in another orientation it
cannot -- what matters is the PROJECTION of the ladder's length onto the
width of the doorway. This obviously has physical consequences (getting
through the doorway or not), but nothing intrinsic to the ladder has
changed; what changed is the RELATIONSHIP between ladder and doorway
(their relative orientation).

Similarly in SR, what changes is the RELATIONSHIP between the (moving)
object and the measuring tools used to measure its length.

IOW: "real" != "intrinsic".

Tom Roberts tjroberts@lucent.com

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