Re: Doubts about relativity 'dogmas'


On Jan 28, 4:58 am, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
Juan R. wrote:
On Jan 21, 6:50 pm, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote [in
reply to TvF]:
You insist on using "force" where I would use "connection". Due to the
presence of the massive earth, there is a nonzero connection where the
moon is initially located, and once you cease holding the moon on a
worldline parallel to the earth, its geodesic path will converge with
the earth's -- what you would call a 3-space acceleration toward the
earth. If you insist, you can call that a "force in 3-space", but in GR
we call this geodesic deviation (considering the moon to be of small
enough mass that its effect on the geometry can be neglected).

You sound completely inappropriate trying to find errors in tvf where
he did *none*.

Then you apparently share his lack of knowledge. TVF has proved that he
simply does not understand the basics of GR. <shrug>

If you read with care my posting you can see i disagree with him in
his interpretation of inertiality in geodesic motion; however he is
right in other points. Once remarked this, it may be also stated that
you are wrong in many of your claims regarding GR forces and
'potentials'.

1) The geodesic equation of motion is valid only as _approximation_;
the geodesic equation is often not used by astronomers for computing
orbits of objects.
Sure it is an approximation that small objects follow geodesics. Do you
seriously think that a baseball is of significance compared to earth and
sun????


I did not said that approximation was not good for that example (it
appears to me that you interpret my posting in that way).

I said is that geodesic equation of motion is approximate. Many many
people (specially physicists with a formal training in GR) think that
gravity is geodesic motion, which is not true except as approximation.
However, the coordinate equations of motion are exact. The forces tvf
talks are called coordinate or gravitational forces in GR and are
exact in GR, no the result of approximations. Forces are related to
rate of change of four momentum by unit of time and that is also
exact.

Yes, astronomers use a different approximation.

In fact, astronomers compute the perihelion for Mercury using the
gravitational force in GR. The geodesic equation is more 'academic'
than practical.

Gravitational force is _not_ zero for the change of 4-momentum on
laboratory time. In standard GR, forces associated to the _coordinate_
form of equations of motion are called _ coordinate forces_.
One can make up new language to one's heart's content. But it does not
change anything.

It is not new language, it is at least 50 years old in relativistic
literature. Simply you do not know.

In GR it simply is not possible to model gravitation as
a "force". In weak-field situations, such as the solar system, it can be
APPROXIMATED as a "force".

That is completely wrong. In the coordinate form of GR (that tvf call
the Einstein original force view), forces are also valid for STRONG
regimes. There is *none* approximation, just your misunderstanding of
a different formulation of GR.

Very interesting thought "In making the approximation some of the
concepts of GR were violated". If true, logical conclusion would be
"we are testing something is not GR" or "we are testing in laboratory
models violating GR" or similar.

An APPROXIMATION to GR is tested. And it is possible to show that the
assumptions used in the approximation are valid, and the values computed
using the approximation do not differ significantly from values computed
using the full theory (if that were possible).

This is no different from any other experiments -- _ALL_ experiments and
measurements make various approximations.

I agree on that, but you said "In making the approximation some of the
concepts of GR were violated". Therefore, would be emphasized that
experimental test was more empirical (values) that conceptual. And
would be remarked in popular literature that in tests are violated
some GR concepts.

Difficult to believe tvf was claming that because the dimensions of
the coefficients are not even those of force per unit of mass.
Sure they are, for the right set of coefficients and a reasonable choice
of units and coordinates.

Some example please?

But he thinks they are "real", and he thinks they are
"representative" of GR. That is false.

No, forces are physical in the force formulation of GR. In fact,
whereas the coordinate form of GR is exact and forces are exact, the
geodesic approach is not exact because only valid for 'small' test
bodies.

In fact, from a practical point of view the geodesic motion is not
very practical and reason that tvf, as astronomer, was thought the
force version of GR. Equations are four dimensional and forces are
four dimensional, but the temporal component of the 4-momentum has
zero interest for an astronmers computing the orbit of a celestial
object, therefore you take the 3 'projection' of equations of motion
and you obtain an expression where 3-forces are related to changes on
3-momentum. Equations are exact, valid for strong and weak limits.

I already explained why the force is zero in the geodesic approach,
but forces are non-zero in the coordinate form of GR. See equations i
wrote and try to understand GR before writting completely wrong
comments.

.

Relevant Pages

  • Re: Doubts about relativity dogmas
    ... Sure it is an approximation that small objects follow geodesics. ... I said is that geodesic equation of motion is approximate. ... the coordinate equations of motion are exact. ...
    (sci.physics.relativity)
  • Re: Doubts about relativity dogmas
    ... When I wrote “the own equation of motion in GR is NOT ... geodesics. ... In the force formulation of GR, ... No approximation has been introduced. ...
    (sci.physics.relativity)
  • Re: Doubts about relativity dogmas
    ... You can work with relative geodesics ... It would be tricky using the geodesic equation of motion for computing ... Mercuri orbit, because you would translate magnitudes to laboratory ... approximation valid under some but not all circumstances; ...
    (sci.physics.relativity)
  • Re: Doubts about relativity dogmas
    ... That would be the force of gravity acting in 3-space. ... you do not understand tvf is writting. ... the motion of the object is indeed along ITS OWN proper ... No any body follows geodesics in GR. ...
    (sci.physics.relativity)
  • Re: On writing negative zero - with or without sign
    ... of representing an "exact" zero... ... My definition of an "exact" zero is whatever internal representation ... *only* value associated with that approximation. ... A subset of floating point numbers can have an exact representation. ...
    (comp.lang.fortran)
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