Gravity from the Ground up -- Schutz

Matt Giwer
Matt Giwer
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RE: Saw a show on tv last

Quote:
Saw a show on tv last night about Dark Matter and computer simulations and all that and one competing theory is apparently about 'variable gravity' where gravity is not a constant but can change in some circumstances. Has anyone else ever heard of this? This is contrary to Newton's theories but the proponents say Newton knew what he knew but didn't know everything, and as new stuff comes in theories must be changed.

I don't know about that.

I do know that the solutions to the tensor equations in GR are so complex only a few of the simplest have been solved. I also know we have not solved the three body problem even with Newtonian gravity much less any 3+n problem. I also know that while we can make certain Newtonian simplifications using the center of mass, we do not know enough about the distribution of dark matter to estimate the center of dark mass -- assuming the center of dark mass can be treated the same as the center of mass which is nothing but an unproven assumption.

All that said and certainly more if I think about it more and hugely more that I don't know enough to add more, I think it is a bit premature to run with a journalism major's description of the problems.

With all the simplifications that ignore all the interesting issues and in fact using the Newtonian gravity model the idea of "variable" gravity is just something to try first and hope it is an elegant solution. If it works, great. If it fails and it appears to have failed, the really hard work has to begin.

Occam warns against superfluous complexity. In this matter it is also a justification for the lazy man. Exhaust the easiest possibilities before looking further.

Matt Giwer
Matt Giwer
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RE: ... So I'd assume that

Quote:

...
So I'd assume that 'variable gravity' is a phrase denoting a modified inverse square force law at rather larger distances than it was originally hypothecated from. Strictly speaking Newton himself could only verify his famous inverse square gravity law within the confines of the solar system ( apple -> Moon -> planets ).
...
Cheers, Mike.

If I might add here the correct expression of Newton's contribution was UNIVERSAL gravitation. As it is on earth so also in the heavens. It was the extension of the gravity on earth as defined by Galileo to the moving heavens was the great leap.

It took me a lot of years to realize this as it was the basis for a total rethinking of the known universe. No longer were the heavens governed by different rules, celestial spheres and such. Rather the abode of THE God or the gods for millennia were over night changed to no different from Earth.

It was a huge change in thinking -- which was totally confused by philosophers who are an abysmally foolish lot. The heavens lost their special standing. All the speculation about Platonic solids as the form of the celestial spheres and a host of other speculations were replaced by a simple equation that was no different from the legendary tower of Pisa experiment.

The most obvious change was with Newton's equation it was obvious that the equation would permit the determination of the masses of the planets and the sun -- after a lot of hard work but a doable thing. This was not just the exchange of earth centered to sun centered which leads no where by itself. This was a change that openly an entirely new field of study and guaranteed job security for generations of physicists as yet unborn.

tullio
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Speaking of gravity, the ESA

Speaking of gravity, the ESA GOCE satellite has mapped the Earth's gravitational field with great accuracy. If you go to the ESA portal you can read about it and also see a video of a press conference.
Tullio

Matt Giwer
Matt Giwer
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RE: Speaking of gravity,

Quote:
Speaking of gravity, the ESA GOCE satellite has mapped the Earth's gravitational field with great accuracy. If you go to the ESA portal you can read about it and also see a video of a press conference.
Tullio

Got it. Odd things happen.

Reminds me of the melters talking about sea level rise if Greenland melts. The sea level around Greenland falls while the rest rises because the gravitational attraction of the glaciers is gone.

Greenland gains land and a better climate so it should be in favor of global melting.

tullio
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Next Wednesday at 1 pm EDT

Next Wednesday at 1 pm EDT NASA will host a press conference, also streamed in NASA TV, on the results of Gravity Probe B.
Tullio

tullio
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Francis Everett and Clifford

Francis Everett and Clifford Will said that an article has just been accepted by Physical Review Letters.
Tullio
To summarize, the geodetic effect has been measured with great accuracy, while frame dragging has still a 20% margin of error.

tullio
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A rather critical "Nature"

A rather critical "Nature" article:
Gravity Probe B
Tullio

Mike Hewson
Mike Hewson
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RE: A rather critical

Quote:
A rather critical "Nature" article:
Gravity Probe B
Tullio


Certainly in the reader's comments below the article there's a spot of face slapping going on, and accusations of sock puppetting. Pistols at dawn might sort that perhaps. I gather the criticism has several thrusts :

- total cost. Bear in mind this project was not-quite-throttled-to-death for funding for several decades : undoubtedly a good fraction of the $750M USD was stuffing about with this prolongation. This is an issue of science funding administration, not poor science.

- a suggestion of 'targeted analysis' or not-quite-so-subtle hints of rigging the numbers toward expectations. Ever so politely phrased : "It may be that people repeating this analysis with another working hypothesis on the nature of the systematic errors would get another result". Phew! This is the equivalent of saying to me that I have no diagnostic expertise : an allegation of basic incompetence, no less. Wow. Guess who's not on the X-mas prezzie list then? :-)

- same thing already done other ways. And cheaper. Bit of an apples vs oranges argument I think, as by definition what's measured on the ground vs. low earth orbit vs. higher orbit and polar are going to vary for lots of reasons. Both signal and noise. So while one might criticise the polhode analysis of GP-B then what of the atmospheric drag modelling for other satellites ( in lower orbit they'll be hitting alot more atmosphere ) ? So you swap one set of imperfect analytic constraints for another set of imperfect analytic constraints.

- targets not met. I reckon it's a bit disingenuous to expect the 1960-something initial predictions to be held to account several decades later after so many design and funding changes - externally imposed to boot.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

robertmiles
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An article on a possible

An article on a possible explanation for dark matter:

http://news.yahoo.com/s/afp/20110527/sc_afp/australiaastrophysicsscience

PhysicsGroupie
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Yes, I believe the Pioneer

Yes, I believe the Pioneer Anomaly has been solved. It wasn't due to low-frequency gravitational waves, or any one of a number of other theories.

The final resolution came from the recent work by Portuguese scientist Federico Francisco and colleagues at the Instituto de Plasmas e Fusao Nuclear in Lisbon, claiming the original thermal calculations -- the ones that ruled out heat as a possible explanation for the anomaly -- were wrong!

Specifically, Francisco et al. reworked the calculations taking into account not just how heat is emitted, but also how it gets reflected off various parts of the spacecraft. As a result, the anomaly disappears.

Whew!

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