Re: [Phys-l] starlight deflection

[carmelo@pacific.net.sg]

You could be surprised to realize that in the latest issue (Nov 07) of  
Ameican Journal of Physics, David Topper concluded that "Einstein  
stayed well clear of defining the relativistic mass parameter, mR,  
which many relativity experts today see as a nonfundamental  
construction", based on Einstein's 1934 two-blackboard derivation of  
energy-mass equivalence. Topper seems to suggest that Einstein was not  
in favour of relativistic mass even as early as 1934.

However, in Einstein's “Autobiographical notes” published in 1949, he  
wrote that "1. From general considerations of special relativity  
theory it was clear that the inert mass of a physical system increases  
with the total energy (therefore, e.g., with the kinetic energy). 2.  
From the very accurate experiments… it was empirically known with  
very high accuracy that the gravitational mass of a body is exactly  
equal to its inert mass."

In addition, Einstein also wrote that "A beam of light carries energy  
and energy has mass." (The Evolution of Physics, published in 1938)  
Hence, many physicists suggest that light has relativistic mass, or  
more commonly, effective mass.

The issue is fundamentally a matter of semantics.


Alphonsus

Quoting "Fayngold, Moses" <fayngold@ADM.NJIT.EDU>:

> David Ward wrote:
>
>  "Starlight deflects as it grazes the sun, as the sun's mass warps   
> space...so the vector momentum of a photon is altered by its   
> encounter with the sun. But an altered momentum means that a force   
> was exerted. So, does the starlight tug ever-so-slightly on the sun?  
>  Photons lack mass, so I suspect they don't warp spacetime..."
>
>   The last sentence in this question illustrates disadvantages of   
> the current fashion to consider ONLY invariant characteristics of an  
>  object as something worth considering. Such view applied   
> straightforwardly to the described situation might indeed tempt one   
> to think that a photon does not warp spacetime since its rest mass   
> is zero. Such conclusion would be wrong for two reasons. First, in   
> case of a photon, its rest mass does not store any information about  
>  this photon whatsoever (except for the trivial notion that it moves  
>  with the invariant speed.) It is its RELATIVISTIC MASS (its energy)  
>  that is relevant characteristic. Second, the spacetime is warped by  
>  RELATIVISTIC MASS (more accurately, by energy-momentum) of an  
> object  rather than by its rest mass alone. Neglecting this  
> distinction is  OK in case of a massive stationary object, but not  
> OK for a moving  object, especially such as a photon. A photon does  
> warp spacetime,  and the resulting spacetime curvature is entirely det
>  ermined by its relativistic mass.
>
>   "In any event, this boils down to "if the sun exerts a force   
> (momentum alteration) on a photon does the photon do the same to the  
>  sun?" My general relativity is so very poor- I suspect the answer   
> may lie there."
>
>  The answer to this question is yes, although its details may be   
> rather subtle. If you have two electric charges - one stationary and  
>  the other passing by, they exert forces on one another, but these   
> forces do not generally satisfy the requirement of Newton's third   
> law (equal in magnitude and opposite in direction); this may appear   
> to contradict conservation of the net momentum, but it does not,   
> because the net momentum includes also the momentum of the EM field   
> of the system, which was left out when we considered the charges only.
>  The same reasoning applies to masses and gravitational field  of a system.
>
>
> Moses Fayngold,
> NJIT
>
>
>
>
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