Gravity: real force or artefact of acceleration?

[robphy]

Indeed... It's best to learn modern general relativity from a modern expert in general relativity.
A Nobel or Fields Medal prize winner is not necessarily such an expert in modern GR.
A Caltech or Harvard professor is not necessarily such an expert in modern GR.
An early researcher in relativity is not necessarily such an expert in modern GR.

 

[pmb_phy]

The cornerstone of Einstein's theory, however, is the proposition that gravity is itself a fictitious force (or, rather, that it is indistinguishable from a fictitious force).

I disagree. What Einstein said was that the force of gravity cannot be distinguished from inertial forces (aka "fictitious" forces) and as such what used to be referred to as "fictitious" forces can just as easily be taken as "real".

I got into this much deeper in a new web page I created which is located at

http://www.geocities.com/physics_world/gr/inertial_force.htm

Notice Einstein's comments in the February 17, 1921 issue of Nature [28]

Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the Earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.

Pete

 

[mendocino]

Hi, mendocino,

I hope you noticed that Politzer was writing for a popular science organ (Scientific American). His statement is an oversimplified reference to the Equivalence Principle (c.f. "elevator experiment"), which is indeed one of the insights which guided AE on the path towards gtr, but gtr in fact treats gravitation rather differently from the impression which might be left by his statement read in isolation.

See for example the popular book by Robert M. Wald, Space, time, and gravity : the theory of the big bang and black holes, University of Chicago Press, 1977. Wald is a specialist in gtr and IMO this little book paints a much more faithful picture for general audiences of how gtr treats gravitational phenomena and how its predictions differ from Newtonian gravitation.

You might also try various recent posts by myself in which I tried to clear up a common confusion concerning how "acceleration" is treated in gtr, as compared to how the effects of a "gravitational field" on the motion of small objects is treated. See [post=1494972]this[/post], [post=1498288]this[/post], and [post=1482542]this[/post], plus older posts collected in my current sig.

What's so different?
Could you elaborate the following statement?

>>> gtr in fact treats gravitation rather differently from the impression which might be left by his statement read in isolation.

 

[Chris Hillman]

Ftfl!

Could you elaborate the following statement?

I have done so in the past few days; please see the posts I cited.

 

[mendocino]

Can you tell me the following link is what you referred to?
https://www.physicsforums.com/showthread.php?t=196359
If not, where can I find it?
Thanks...

 

[Chris Hillman]

:uhh: The links I mentioned were the links I mentioned.

 

[mendocino]

What's "modern General Relativity"?

Indeed... It's best to learn modern general relativity from a modern expert in general relativity.
A Nobel or Fields Medal prize winner is not necessarily such an expert in modern GR.
A Caltech or Harvard professor is not necessarily such an expert in modern GR.
An early researcher in relativity is not necessarily such an expert in modern GR.

Can you tell me what's this "modern general relativity"?
What's the difference between Einstein's GR and so called "modern GR"?

 

[robphy]

In my opinion,
I would describe "modern general relativity" as the geometrical formulations of relativity as found in modern textbooks like Wald and Hawking&Ellis. These texts [based on modern research in relativity, starting from, say, from 1956 (Synge's relativity text)] represent the attempt to capture physical ideas with precise mathematical structures that model them. With such precision, it becomes easier to discuss, analyze, and make predictions of the physics.

I'm not sure how Einstein would react to the formalism...
...but one might look to his initial reaction to Minkowski's reformulation
...then his eventual acceptance and extension of it.

 

[Chris Hillman]

I Hope You Aren't Just Playing Word Games...

What's the difference between Einstein's GR and so called "modern GR"?

For quite a few years after Einstein introduced gtr, physicists (including AE) had great difficulty disentangling geometric phenomena from artifacts of a particular coordinate chart. It took many decades before most physicists working on gravitation finally understood such a basic point as the fact that according to gtr, a gravitational wave consists of ripples in curvature. During these difficult years, mathematicians--- working in part to fill the need for simple computational tools--- introduced and popularized methods which focused attention on geometrical phenomena. During the Golden Age of Relativity, c. 1960-1970, many major advances where made due to adoption by the leading researchers of the new methods. A well known example is the kinematic decomposition of a timelike congruence in a Lorentzian manifold into acceleration and vorticity vectors and expansion tensor. One might also mention the optical scalars and various decompositions of the Riemann tensor.

These techniques were not discussed in the earliest textbooks on gtr because they were not yet available when those books were written. In 1973, two landmark books appeared: the monograph by Hawking and Ellis and the textbook by Misner, Thorne, and Wheeler. These finally made available the geometric viewpoint widely avaible to students and nonspecialists.

So roughly speaking, gtr textbooks published after 1973 are modern; those published before are premodern.