Re: Q question about relativity

From: N:dlzc D:aol T:com \(dlzc\) (net_at_nospam.com)
Date: 09/02/04 

Date: Thu, 2 Sep 2004 06:50:43 -0700

Dear Ohad:

"Ohad" <ohadasor@netvision.net.il> wrote in message
news:41370586$1@news.barak.net.il...
> Hello,
>
> About the General Theory of Relativity. Newton said that gravitation is
> according to the mass of the two bodies, and according to the distance
> between them. Einstein said that a proof of his theory, about changing
> the
> space, is seeing that even light is bent according to gravitation.

Actually, no. Light travels the straightest possible line, in space that
is curved by the presence of mass.

> My question is: the mass of the photon is zero, so the light beam cannot
> be
> bent according to Newton, only according to Einstein. But notice that
> only
> the rest mass of the photon is zero! The photon is moving in the speed of
> light, and its mass in velocity isn't zero, is a positive number!

Actually, no. Relativistic mass is not the same as gravitational mass. If
you imagine what you might see as you approach or recede from the mutual
orbit of two bodies, factoring out relativistic Doppler shift. You will
see that the bodies orbit normally. You see that they do not become black
holes. You will see that your own path is not deflected unusually.

It is common to add the adjective "relativistic" in front of mass, to let
people know that you cannot remember the correct formula for the total
energy of a partcle. This mistake comes about when people use E=mc^2 for
the total energy, when this is really only the rest energy. The total
energy is
E^2 = (pc)^2 + (mc^2)^2. You can see that the momentum term, based on a
vector quantity, has its own existence, and this existence is all there is
for the photon.

> So even to
> Newton the beam of light should be bent. So what's Einstein great
> discovery
> in the General Relativity, and why light bending is a proof for it? The
> light should be bent even according to Newton!

I hope you have seen now, one problem (of many) with "relativistic mass".
The rest mass of photons has been measured, it is far less than the total
energy of the photons involved would indicate, and its determined value is
consistent with zero rest mass.

David A. Smith

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