Re: Some questions about gravity

thomas.hergenhahn@xxxxxx (Thomas Hergenhahn) wrote in message news:<8537e997.0504181543.25fe0913@xxxxxxxxxxxxxxxxxx>...
> Hello to all.
> I'm just a layman in physics. Recently I reread some popular books on
> modern physics and I have nobody to discuss the subjects with. So I
> try it here and beg your pardon if my questions are stupid, trivial or
> far below the level of this group. Nevertheless, I should be glad if
> someone would be willing to answer them.
>
> 1. I read somewhere that observations of the light from distant
> star(s) during an eclipsis of the sun provided first evidence that
> confirmed Einstein's general ralativity. Is this right?

Yes, in 1919. You can look up the name Eddington. He led one of
the two teams who did the observations.

> 1a. Wouldn't special relativity alone predict the deflection of a
> lightbeam, as light has mass according to e=mc2?
> 1b. Is the amount of deflection predicted by general relativity
> different from what special relativity predicts?
> 1c. The motion of planets is still in good accordance with the
> Newtonian picture, AFAIK? If so, a deflection of light in accordance
> with it's mass would fit into the Newtonian picture of gravity.

It would if photons (light) had mass, but they don't and so dont
directly obey the newtonian calculations that are valid for mass.

As it stands, photon energy behaves as if it amounted to only half
the mass that their energy would account for in massive particles.

> what are the conditions, where the extra contribution from general
> relativity leads to nonnelectible effects?

The difference is accounted for in GR by the spacetime curvature
caused by the mass of the Sun.

> 2. If I understood right, according to general relativity, a mass
> distorts space time, while another body continues to follow a geodesic
> path, which is not a straight line any more.
> 2a. It is still possible to measure space alone in euclidian
> coordinates. Otherwise, how could one "measure" the curvature? So
> space is not distorted, but space-time?
> 2b. Bricklayers use a chord or thread to determine the straight line.
> Nowadays they tend to use laser gauges. Could I still use such a
> (infinitely lightweight) thread between to points of the light beam
> deflected near a massive body and measure the curvature with reference
> to it?
> 2c. But the new laser gauges would follow the path of the lightbeam,
> as in their case there is light moving with time while the thread
> consists of particles "at rest"?
> 2d. In particle accelerators, electrons emit the "synchrotron
> radiation" when forced to go round a bend by magnetic fields.

Not exactly. They emit the synchrotron radiations only if the
beam is "wiggled" (the beam is made to oscilate axially which
constantly changes the translation diameter of the "orbit") a burst
of radiation is emitted each time the beam moves to the tighter
radius.

> Would they do the same when they were forced around the same
> curve by a very massive body?
> 3. Physicists are seeking a unified theory and they expect that it
> should"explain" or "treat" all forces likewise/from a common origin/in
> a common manner. Gravitation seems to be a very special thing if it
> modifies spacetime.
> Why does nobody say that electromagnetism distorts spacetime, but only
> for particles susceptible to electromagnetic forces? Or likewise with
> weak and strong nuclear forces? Would that make sense? If not, why
> should one expect a mechanism of particle (graviton) exchange, if the
> working of gravity is already explained by spacetime curvature?
> Wouldn't that mean to attribute TWO underlying mechanisms to gravity?

You may be interested to know that GR does not explain all gravitational
phenomena. Look up info on the Pioneer 10/11 anomaly. GR is unable
to account for the trajectories of these two crafts that are on
escape trajectories from the sun.

André Michaud
.

Relevant Pages

  • Ref: www.agora.demon.co.uk/dynamics.txt
    ... >presently occupied by Special and General Relativity ... and matter has mass (and thus ... particles even if they have individual spin, ... particles do not have a 'one value' velocity. ...
    (sci.physics.relativity)
  • Re: About Gravity, Inertia and Mass
    ... applicable when describing matter. ... Relativity can only refer to relative time or length because it is the ... CMBR is a mix of the particles that make up material space. ... Gravity is caused by this space flow. ...
    (sci.physics)
  • OBarr: physical gravity.
    ... What is gravity? ... exchanges in mass can occur is by spalls. ... identical mass as any of the original particles. ...
    (sci.physics.relativity)
  • About Gravity, Inertia and Mass
    ... applicable when describing matter. ... Relativity can only refer to relative time or length because it is the ... CMBR is a mix of the particles that make up material space. ... Gravity is caused by this space flow. ...
    (sci.physics)
  • About Gravity, Inertia and Mass
    ... applicable when describing matter. ... Relativity can only refer to relative time or length because it is the ... CMBR is a mix of the particles that make up material space. ... Gravity is caused by this space flow. ...
    (sci.physics)