What is the role of quantum gravity in understanding our universe?

[wolram]

http://arxiv.org/list/gr-qc/0311

http://arxiv.org/PS_cache/gr-qc/pdf/0311/0311021.pdf

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=30467

http://www.cerncourier.com/main/article/42/7/18.
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some interesting papers on gravity research
the top address is the main page for second.
the second one is about how observations of the moons
orbit at mm scale can help unravel the mysteries
of gravity.

 

[Creator]

Originally posted by wolram
http://arxiv.org/PS_cache/gr-qc/pdf/0311/0311021.pdf


the second one is about how observations of the moons
orbit at mm scale can help unravel the mysteries
of gravity. [/B]

Interesting, Wolram, that is, the second one:
http://arxiv.org/PS_cache/gr-qc/pdf/0311/0311021.pdf

However, are you farmiliar with the de Sitter effect? I can't quite see how better lunar ranging (alluded to in the article) can verify this particular relativistic effect. This is a geodetic effect. De Sitter calculated that the earth-moon system should rotate (precess) in the plane of the ecliptic about 19 milli arcseconds/year due to its motion through the space-time curvature of the sun. How could lunar ranging verify precession of the earth-moon system? [?]

Creator

 

[wolram]

posted by creator.
However, are you farmiliar with the de Sitter effect? I can't quite see how better lunar ranging (alluded to in the article) can verify this particular relativistic effect. This is a geodetic effect. De Sitter calculated that the earth-moon system should rotate (precess) in the plane of the ecliptic about 19 milli arcseconds/year due to its motion through the space-time curvature of the sun. How could lunar ranging verify precession of the earth-moon system?
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http://www.iop.org/EJ/article/0264-9381/16/4/016/cq16004l2.html

As applied to the Moon's orbit, this radial perturbation was tabulated some decades ago as part of the collection of possible relativistic gravity effects which could be sought using lunar laser ranging (LLR) techniques [4]. But because of the relatively large distance to the Moon which renders LLR primarily sensitive to the radial orbital perturbations, tangential perturbations of the lunar orbit were not included in that cited study. Ranging to near-Earth satellites (SLR) is a different situation, however; in such configurations the range measurements are comparably sensitive to tangential and radial perturbations.
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this is all getting to deep for me, i think i understand,
but would be much happier if someone explianed it to me.

 

[Creator]

Ah...Its the Nordtvedt effect! Apparently the other article mistakenly called it the de Sitter effect, which in its original form includes no radial variation in earth-moon range. The Nordtvedt effect does clearly point to a radial oscillation in the earth-moon acceleration...as also evidenced by his letter to the editor (which you included):

"The time-varying acceleration in equation (2) produces an oscillatory radial perturbation of a near-circular satellite orbit"..

Thanks for including the (Nordtvedt) reference.

Creator

 

[marcus]

Originally posted by wolram


http://arxiv.org/gr-qc/0311021

LLR, Lunar Laser Ranging
the shiny muffin tin has not been explained in this thread yet?

the idea is to send beam back in exactly the same direction?

I thought the corner of a cube (from inside, the concave version of the corner of a cube) would do that.

why this thing that looks like a muffin tin?

 

[marcus]

ah so

had another look
each one of those round dimples in the "retroreflector"
is, if you look real close, the inside of the corner of a cube.
Or so it looks to me.

no matter what direction the beam comes in from
the reflector is designed to send it back in an exactly parallel
direction

a simple flat or curved mirror will not do that, but
the corner of a cube will IIRC
someone who knows please confirm

in two dimensions (in a flat world) a simple picture shows it works
the inside of the corner of a square, if made of mirror, sends
the beam back in the same direction it came from

intuitive that it would work in 3D

 

[wolram]

http://www.jpl.nasa.gov/releases/99/lunarlaser.html

The reflector consists of a checkerboard mosaic of 100 fused silica half cubes (roughly the size of the average computer monitor screen), called corner cubes, mounted in a 46-centimeter (18-inch) square aluminum panel. Each corner cube is 3.8 centimeters (1.5 inches) in diameter. Corner cubes reflect a beam of light directly back toward the point of origin; it is this fact that makes them so useful in Earth surveying
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propper nuts and bolts science, i am still struggling with
the theory behind this experiment but i learn a little every
day, and find it encouraging that scientists are doing this
type of experiment.

 

[wolram]

http://relativity.livingreviews.org/Articles/lrr-2001-4/node12.html

The next class of solar-system experiments that test relativistic gravitational effects may be called tests of the strong equivalence principle (SEP). In Sec. 3.1.2 we pointed out that many metric theories of gravity (perhaps all except GR) can be expected to violate one or more aspects of SEP. Among the testable violations of SEP are a violation of the weak equivalence principle for gravitating bodies that leads to perturbations in the Earth-Moon orbit; preferred-location and preferred-frame effects in the locally measured gravitational constant that could produce observable geophysical effects; and possible variations in the gravitational constant over cosmological timescales.
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i found this site, i think it gives a better background to
the experiment.