Re: sources of gravity
From: Jonathan Thornburg (jthorn_at_xeon44.aei.mpg.de)
Date: 11/30/04
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Date: Tue, 30 Nov 2004 18:48:16 +0000 (UTC)
<carlip-nospam@physics.ucdavis.edu> wrote in message
[[there are]]
# easily a dozen groups investigating the question of whether
# composition affects gravitational fields.
#
# So far, though, not one speck of evidence has been found to
# suggest that ``some forms of energy do not curve space-time.''
# There are strong limits for electrostatic and magnetstatic
# energy,
Someone who (if I've unwrapped the nested quoting correctly) might well
be greywolf42 <mingstb@marssim-ss.com> asked for references for this claim.
I offered
> (The absence of) the lunar Nordtvedt effect.
>
> http://scholar.google.com/scholar?hl=en&lr=&q=Nordtvedt+effect&btnG=Search
> found about 540 hits, of which numbers 1, 2, 4, and 5 look like
> good answers to your question.
>
> Notably, the top google hit there, a pdf version of
> http://relativity.livingreviews.org/Articles/lrr-2001-4/
> contains a section 3.6 "Tests of the strong equivalence principle",
> devoted to this topic (which includes references to the original
> literature). I believe this is freely available online; if not, try
> http://arxiv.org/abs/gr-qc/0103036
greywolf42 responded
> In the specific case of Will's link, above, there is simply no experimental
> substance behind his claims. [[...]]
I'm sorry, but this is simply not true. Quoting from section 3.6 of
http://relativity.livingreviews.org/Articles/lrr-2001-4/
# Since August 1969, when the first successful acquisition was made of a
# laser signal reflected from the Apollo 11 retroreflector on the Moon,
# the lunar laser-ranging experiment (LURE) has made regular measurements
# of the round-trip travel times of laser pulses between a network of
# observatories and the lunar retroreflectors, with accuracies that are
# approaching 50 ps (1 cm).
These are direct *experimental* measurements: you fire a (intense)
laser pulse at the moon, and time how long it takes for (a very small
fraction of) it to bounce back to you. That is, you *measure* the
round-trip light-travel time from your apparatus on the Earth, to a
set of corner cubes sitting on the surface of the moon.
Two good references for further information on this are:
@article{
Nordtvedt-1999-lunar-laser-ranging-vs-GR,
author = "Kenneth Nordtvedt",
title = "30 years of Lunar Laser Ranging
and the Gravitational Interaction",
journal = "Classical and Quantum Gravity",
volume = 16, number = 12A",
pages = "A101--A112",
year = 1999, month = "December",
doi = "10.1088/0264-9381/16/12A/305",
url = "http://stacks.iop.org/0264-9381/16/A101",
}
@article{
Turyshev-etal-lunar-laser-ranging-2004,
author = "Slava G. Turyshev and James G. Williams and Michael Shao
and John D. Anderson and Kenneth L. Nordtvedt Jr
and Thomas W. Murphy Jr",
title = "Laser Ranging to the Moon, Mars and Beyond",
eprint = "gr-qc/0411082",
note = "Invited talk given at The 2004 NASA/JPL Workshop on
Physics for Planetary Exploration, April 20-22, 2004,
Solvang, CA, USA",
}
> This is not terribly surprising for a website.
The top-level web page of the website in question,
http://relativity.livingreviews.org/
describes itself as
| Living Reviews in Relativity
| A refereed solely electronic journal offering reviews in
| all areas of relativity, including an extensive reference database
The word "refereed" here has the usual meaning in scientific discourse.
This is *not* the author's personal web site, rather this is a
refereed scientic journal's web site.
> However, if one looks at Will's book (which gives more detail than Will's
> webpage), one can see that Will simply assumes his conclusion. Will simply
> assumes that GR is correct, and any variations are simply due to "ocean
> tides".
Again, this is simply not true. Will's book,
@book {
Will,
author = "Clifford M. Will",
title = "Theory and Experiment in Gravitational Physics",
edition = "Revised",
publisher = "Cambridge University Press",
address = "Cambridge (UK)",
year = "1993",
isbn = "0-521-43973-6 (paperback)",
snote = "Only adds one new ``update'' chapter to 1st edition",
}
contains (section 8.1) a fairly detailed calculation of various
effects relevant to these measurements.
Even a brief perusal of this calculation reveals that it does *not*
assume GR. On the contrary, it works within a larger "meta-theory",
the "PPN formalism", which includes GR, Whitehead's theory, Brans-Dicke
theory, and (infinitely) many other possibilities.
[Crudely speaking, the PPN (parameterized post-Newtonian)
formalism allows arbitrary Taylor-series expansions of
various formulas about their Newtonian backgrounds.
Any given gravity theory (GR, Whitehead's theory, Brans-Dicke,
etc) should make specific predictions for the Taylor-series
coefficients; we can then test the theory by experimentally
measuring these coefficients. Section 3.2 of the article
I cited above,
http://relativity.livingreviews.org/Articles/lrr-2001-4/
contains a brief introduction to the PPN formalism. Chapter
4 of Will's book (cited above) is a more extensive presentation.]
[[potential conflict-of-interest disclaimer: I am an employee of the
same institution that publishes Living Reviews in Relativity.]]
ciao,
-- -- Jonathan Thornburg <jthorn@aei.mpg.de> Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut), Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html "Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral." -- quote by Freire / poster by Oxfam
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