Re: Domanda sulla Relatività Ristretta

From: Pier Franco Nali <ampfn_at_tiscali.it>
Date: Tue, 4 Jul 2023 11:46:56 -0700 (PDT)

Il giorno lunedì 3 luglio 2023 alle 08:05:04 UTC+2 Dino Bruniera ha scritto:
> Il giorno domenica 2 luglio 2023 alle 17:25:04 UTC+2 Giorgio Pastore ha scritto:
> > Il 02/07/23 11:42, Dino Bruniera ha scritto:
> >
> >
> >
> > Sul reale/apparente non credo che F. ti seguirebbe. E neanch'io: vedo
> > una nozione "ingenua" di realtà. …….
>
> >
> > Astratto quindi, ma non apparente.
> Si, forse in questo caso non è molto corretto usare il termine apparente.
>

Ancora sul dualismo reale/apparente della contrazione forse può essere utile riportare il seguente brano, sempre dal libro di Born (p. 213):
<< The view of Einstein’s theory about the nature of the contraction is as follows:
















A material rod is physically not a spatial thing but a space time configuration. Every point of the rod exists at this moment, at the next, and still at the next, and so forth, at every moment of time. The adequate picture of the rod under consideration (one-dimensional in space) is thus not a section of the x-axis but rather a strip of the xt-plane (Fig.120). The same rod, when at rest in various moving systems S and S' is represented by various strips. There is no a priori rule as to how these two-dimensional configurations of the xt-plane are to be drawn so that they may represent the physical behaviour of one and the same rod at different velocities correctly. To achieve this a calibration curve in the xt-plane must first be fixed. Classical kinematics draws this differently from Einsteinian kinematics. It cannot be ascertained a priori which is correct.In the classical theory both strips have the same width as measured parallel to a fixed x-axis. In Einstein's theory they have the same width as measure
d in the various x-directions of the systems of reference in relative motion and with different but determinate units. The "contraction" does not affect the strip at all but rather a section cut out of the x-axis. It is, however, only the strip as a manifold of world-points, events, which has physical reality, and not the cross-section. Thus the contraction is only a consequence of our way of regarding things and is not a change of a physical reality. Hence it does not come within the scope of the conceptions of cause and effect. The view expounded in the preceding paragraph does away with the notorious controversy as to whether the contraction is "real" or only "apparent." If we slice a cucumber, the slices will be the larger the more obliquely we cut them. It is meaningless to call the sizes of the various oblique slices "apparent," and to call, say, the smallest which we get by slicing perpendicularly to the axis the "real" size. In exactly the same way a rod in Einstein's theory has various lengths accor
ding to the point of view of the observer. One of these lengths, the statical length, is the greatest, but this does not make it more real than the others. The application of the distinction between this naive sense is no more reasonable than asking what is the real x-co-ordinate of a point x, y when it is not known which xy-co-ordinate system is meant.>>

Saluti, PF
Received on Tue Jul 04 2023 - 20:46:56 CEST