Re: relatività ed altro

From: cometa luminosa <a.rasa_at_usl8.toscana.it>
Date: 1 Apr 2007 12:16:40 -0700

On 1 Apr, 15:15, lulla <l..._at_nospam.it> wrote:
> Sono una ignorante scusate ....
> ma vorrei fare una domanda.
>
> Se la relativit� ristretta predice che un corpo accelerato a velocit�
> prossime a quella della luce aumenta la propria massa fino ad impedire
> ogni ulteriore aumento della velocita' limitando la massima velocita' a
> *c*, tale aumento di massa non comporta un aumento della forza di gravit�
> di tale corpo?


In relativit� non c'� un solo concetto di massa, ma parecchi. Di
solito ci si riferisce alla "massa propria" o "invariante" o "a
riposo", in quanto appunto non varia da un sistema di riferimento
inerziale all'altro. Questa massa perci� non varia con la velocit�. 1
grammo rimane tale sia a v = 0 che a v = 299.792.457 m/s (per fare un
esempio).

Per quanto riguarda il campo gravitazionale generato da un oggetto
massivo in moto, pare che la risposta non sia cos� semplice:
http://en.wikipedia.org/wiki/Mass_in_general_relativity

<<Doesn't this imply that a moving particle has "more gravity" than a
stationary particle?

This remark is probably true in essence, but it is difficult to
quantify.
Unfortunately, it is not clear how to measure the "gravitational
field" of a single relativistically moving object. It is clear that it
is possible to view gravity as a force when one has a stationary
metric - but the metric associated with a moving mass is not
stationary.
While definitional and measurement issues constrain our ability to
quantify the gravitational field of a moving mass, one can measure and
quantify the effect of motion on tidal gravitational forces. When one
does so, one finds that the tidal gravity of a moving mass is not
spherically symmetrical - it is stronger in some directions than
others. One can also say that, averaged over all directions, the tidal
gravity increases when an object moves.
Some authors have used the total velocity imparted by a "flyby" rather
than tidal forces to gain an indirect measure of the increase in
gravitational "effective mass" of relativistically moving objects
(Olson & Guarino 1985)
While there is unfortunately no single definitive way to interpret the
space-time curvature caused by a moving mass as a Newtonian force, one
can definitely say that the motion of the molecules in a hot object
increases the mass of that object.
Note that in General Relativity, gravity is caused not by mass, but by
the stress-energy tensor. Thus, saying that a moving particle has
"more gravity" does not imply that the particle has "more mass". It
only implies that the moving particle has "more energy">>.
Received on Sun Apr 01 2007 - 21:16:40 CEST

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