Gravity from the Ground up -- Schutz

Matt Giwer
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Topic 195696

Cambridge U Press 2003

Everything you ever wanted to know about gravity starting from, as it says, from the ground up and builds up to the current issues carefully.

Included are dark matter and energy and gravity waves. It has enough math to satisfy satisfy people with degrees in physics but not in the field and enough words to explain things to the proverbial English major.

In the word explanations are more than enough "what it means" as well and "what it does not mean" to give a good feel for the subject. It also includes enough usage and explanation of scientific concepts such as the difference between a description and a theory so the English major is not left behind when going beyond observation.

It is applicable to both Einstein@h and Milkyway@h and explains what can come from them beyond the academic accomplishment. For example, not matter how good the telescope it cannot see back in time before light was able escape long after bang time -- but gravity could. We can see the cosmic background radiation so there should be observable gravity waves going back to the bang itself. Not next year or next decade but eventually.

I found an explanation as to why brown dwarves in galaxies are not a candidate. There is observational evidence of concentrations of dark matter in the empty space between clusters of galaxies. There is no way to explain only dwarves would form between galaxies in clusters.

Associated with the book is
http://www.gravityfromthegroundup.org/

From the text

Quote:

http://www.gravityfromthegroundup.org
It contains
• the Java programs for you to download;

• a free version of the Triana software environment for running the programs and displaying their results graphically;

• solutions of all the exercises;

• links to allow you to download and install Java and other programs needed for your computer;

• additional illustrations for some of the chapters;

• a way of submitting comments, misprints you have found, or suggestions that could be incorporated in future editions; and xviii Preface

• links to useful websites where you can follow up some of the material covered in the book.

Visit the website: it is a valuable addition to this book, and it is completely free.


Matt Giwer
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Gravity from the Ground up -- Schutz

Also New Scientist 11/02/05 with a modest non-technical discussion of dark matter with lots of colorful illustrations -- :( in spades. However it is eight years after the book and nothing significant is different.

Matt Giwer
Matt Giwer
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A high math content source

A high math content source

Data Analysis of Gravitational Waves, Sanjay Kumar Sahay, ISBN 978-91-85917-05-1

Extensive math presentation but with sufficient verbiage to describe what the math is about.

mikey
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Saw a show on tv last night

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.

tullio
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I think theories can be

I think theories can be changed only in front of some experimental evidence. See a recent "Nature" article about search of dark matter in the underground Gran Sasso Laboratory in Italy:
Dark Matter
Tullio

Mike Hewson
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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.


None of us know everything, Newton included .... :-)

This the area of MOND : MOdification to Newtonian Dynamics ( many variants ). Essentially the idea is a fair one, but to date it hasn't yielded much fruit. The missing mass is the difference ( or placeholder for the label ) between what matter we see and what matter ought be there to explain the rotation curves of galaxies. As you recede from a galactic centre the assumption is Keplerian orbits, or at least approximately so, by allowing the galaxy's mass ( within a given radius ) to act in effect from it's centre point. That means periods should diminish with distance from the centre according to Kepler's Third Law ( radius cubed goes like period squared ) after allowing for known mass distributions. They don't. They have rather higher rotation rates ( shorter periods ) in outlying galactic areas.

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 ). Interestingly globular star clusters are well understood without varying his laws one whit, so 'some circumstances' might refer to the contemporary theorist's preference towards his objects of especial interest! :-)

One can always introduce a variant force profile to answer the given case of a certain galaxy, but you have to change that in detail when you look at another galaxy. You see we seek universal laws, so the question is begged as to why 'law' on a per galaxy basis ..... either you make the rules uniform and/or explain why 'local' factors produce variation. The current 'status quo' is to accept Newton's Laws en face but suspect extra local mass. Either way the onus still resides upon the theorist to resolve toward the observations ..... if it was described as a 'competing theory' that's probably rather cheeky, and overstates the merit or generality I'd say. Bit like saying that I could 'compete' with an Olympic sprinter simply by arriving at the starting blocks. Probably kinder to say that investigations are continuing to find alternate fits to data. No one has yet found the Dark Matter either .....

Cheers, Mike.

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

tullio
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Sorry Mike, periods should

Sorry Mike, periods should not increase with distance?
Tullio

Mike Hewson
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RE: Sorry Mike, periods

Quote:
Sorry Mike, periods should not increase with distance?
Tullio


Whoops, yes my bad. Radius and periods trend together. I was thinking of the speed curves, as they measure that across the face of the galaxy's disk and that tells them that the speed doesn't go down as expected. It's supposed to increased to a certain radius and then dip and die away, but it stays well up toward the visible edge of the galaxy.

I should have also mentioned that while GR is a modification to Newton's Laws ( but isn't called MOND ), it isn't thought to apply to the detail here as neither accelerations/velocities/mass-densities are high enough to bring sufficient correction to align to observations.

There's a group that have been laser range finding to the Moon on the corner-cube arrays left by Apollo. Bit of a hit-or-miss affair apparently but they kept at it! In their modelling they've been allowing free variation of all sorts of parameters - including ones which would say, breach the big conservation laws - to at least locate in parameter space where any MONDs should go or apply. But that's only over Earth_to_Moon scale so wouldn't necessarily apply galactically ie. if it shows no variations ( or places upper bounds too small upon variations ) from Newton then what of it?

Also the further away into the universe that we look is also older light from earlier times, but that's more an issue for ( or a trigger indeed to invent ) that other Darkness - Dark Energy. Interestingly that is yet another placeholder for our ignorance of scales well above our local neighborhood. As we could well require another data collection point somewhere well far away ( not in my lifetime at least! ) to sort this out, then we may always have that level of ignorance. Meanwhile we use all manner of proxies.

Cheers, Mike.

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

ML1
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RE: ... Radius and periods

Quote:
... Radius and periods trend together. I was thinking of the speed curves, as they measure that across the face of the galaxy's disk and that tells them that the speed doesn't go down as expected. It's supposed to increased to a certain radius and then dip and die away, but it stays well up toward the visible edge of the galaxy. ...

OK...

So to gather the light to determine the speed curves, the measurements must have to in effect average the light from many individual stars...

You get the expected results for globular clusters, but not for the older disk galaxies...

Hence:

Has allowance been made for the stars following elliptical orbits about their galactic centre rather than approximating to circular orbits?

Also, might the results be getting skewed by the measurements in effect thresholding out a great number of stars that form a lower visible density of slow far-flung stars versus the higher density of (more visible) stars 'seen' nearer the galactic centre?

And/or could a cloud of far-flung stars around what we see as galaxies add up to the 'dark matter' effect?

What is different between globular clusters and disk galaxies? Size? Mass? Age?

Keep searchin',
Martin

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ML1
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RE: ... Dark Energy.

Quote:
... Dark Energy. Interestingly that is yet another placeholder for our ignorance of scales well above our local neighborhood. As we could well require another data collection point somewhere well far away ( not in my lifetime at least! ) to sort this out, then we may always have that level of ignorance. Meanwhile we use all manner of proxies.

A small step beyond the moon might be Pioneer. The Planetary Society rescued the Pioneer anomaly data. Has anything further become of that?

Keep searchin',
Martin

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Mike Hewson
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RE: So to gather the light

Quote:

So to gather the light to determine the speed curves, the measurements must have to in effect average the light from many individual stars...

You get the expected results for globular clusters, but not for the older disk galaxies...


Yep. The arguments are statistical in both instances. It's easier for globular clusters as one can view them like 'boiling atoms' - one star is a particle - and apply what's called the virial theorem to get a stochastic distribution of measurable quantities. If a Newtonian potential ( 1/r ) is used then the fit is pretty OK. So for that type of star grouping at that scale nothing is 'missing'.

Quote:

Has allowance been made for the stars following elliptical orbits about their galactic centre rather than approximating to circular orbits?

Also, might the results be getting skewed by the measurements in effect thresholding out a great number of stars that form a lower visible density of slow far-flung stars versus the higher density of (more visible) stars 'seen' nearer the galactic centre?

And/or could a cloud of far-flung stars around what we see as galaxies add up to the 'dark matter' effect?


That I think is the rub, you have to make more assumptions for something as complex as a galaxy. Certainly halos have been considered.

Quote:
What is different between globular clusters and disk galaxies? Size? Mass? Age?


Yes .... :-)

Quote:
A small step beyond the moon might be Pioneer. The Planetary Society rescued the Pioneer anomaly data. Has anything further become of that?


From memory that was resolved adequately by including radiation pressure from the quite knackered onboard nuclear thermopile. It still radiates a squidgen of heat ( into a 4K background ) and after a while the momentum of the emitted photons will accumulate to cause a tiny change in the craft's dynamics.

Cheers, Mike.

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

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