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
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.
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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.
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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.
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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 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
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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.
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"I have made this letter longer than usual, because I lack the time to make it short." - Blaise Pascal
Sorry Mike, periods should not increase with distance?
Tullio
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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.
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"I have made this letter longer than usual, because I lack the time to make it short." - Blaise Pascal
... 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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... 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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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'.
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.
What is different between globular clusters and disk galaxies? Size? Mass? Age?
Yes .... :-)
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.
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"I have made this letter longer than usual, because I lack the time to make it short." - Blaise Pascal
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.
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...
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.
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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
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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.
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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
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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.
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A rather critical "Nature" article:
Gravity Probe B
Tullio
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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.
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"I have made this letter longer than usual, because I lack the time to make it short." - Blaise Pascal
An article on a possible explanation for dark matter:
http://news.yahoo.com/s/afp/20110527/sc_afp/australiaastrophysicsscience
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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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!
Thanks for that input! I couldn't recall whom to attribute that work to. :-)
Cheers, Mike.
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"I have made this letter longer than usual, because I lack the time to make it short." - Blaise Pascal
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!
Thanks for that input! I couldn't recall whom to attribute that work to. :-)
Cheers, Mike.
There's been made an analysis with similar results at Zentrum für angewandte Raumfahrttechnologie und Mikrogravitation (ZARM) in Bremen. Other than Frederico Francisco, they used the finite elements method to calculate the thermal repulsive forces. (Annalen der Physik, June 2011 / Spektrum der Wissenschaft, July 2011)
Gruß,
Gundolf
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!
http://arxiv.org/abs/1103.5222
Modelling the reflective thermal contribution to the acceleration of the Pioneer spacecraft
Authors: F. Francisco, O. Bertolami, P. J. S. Gil, J. Páramos
(Submitted on 27 Mar 2011)
Abstract: We present an improved method to compute the radiative momentum transfer in the Pioneer 10 & 11 spacecraft that takes into account both diffusive and specular reflection. The method allows for more reliable results regarding the thermal acceleration of the deep-space probes, confirming previous findings. A parametric analysis is performed in order to set an upper and lower-bound for the thermal acceleration and its evolution with time.
Comments: 11 pages, 7 figures, 3 tables
Subjects: Space Physics (physics.space-ph); Instrumentation and Methods for Astrophysics (astro-ph.IM); General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1103.5222v1 [physics.space-ph]
Submission history
From: Frederico Francisco [view email]
[v1] Sun, 27 Mar 2011 15:07:48 GMT (275kb)
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Thank you Matt. I will closely read that one. I immediately see that the modeling depends not only upon the decay of the onboard power generator but also the thermodynamic geometry of the craft and degradation of what putatively remains of it's functioning systems. Even when barely/not operating to specification it still speaks lessons to us from such an enormous distance. They are remote human artifacts and we can but hope that they reach the scrutiny, maybe even with insight and benefit, of sentient beings!
I visited Tidbinbilla near Canberra about 5 years ago and I believe they still are following the Voyager craft too.
Cheers, Mike.
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"I have made this letter longer than usual, because I lack the time to make it short." - Blaise Pascal
Thank you Matt.
You want to while away an afternoon just scan what is available at arvix.org and collect more papers than you will ever find time to read. It is a real exercise in learning to prioritize.
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