Register to reply 
The accurate law of gravity 
Share this thread: 
#1
Feb1612, 08:05 PM

P: 24

hi
I head that the laser moon mirror experiment showed that the following equation is not so accurate. is there in general relativity a precise equation that links the mass of the moon and the mass of the earth and the force and the distance between them ? thx 


#2
Feb1712, 01:59 PM

Sci Advisor
HW Helper
Thanks
P: 26,160

hi avery!
that's the equation for two uniform spheres … it doesn't apply if your moon is lumpy! 


#3
Feb1712, 03:35 PM

Sci Advisor
PF Gold
P: 5,027

The simplest approximate corrections to the Newtonian formula is the EinsteinInfeldHoffman equations of motion. The first term, below, is the Newtonian formula: http://en.wikipedia.org/wiki/Einstei...mann_equations 


#4
Feb1912, 05:19 AM

P: 24

The accurate law of gravity
well I guess they take that in consideration when they measure the distance between the mirror on the moon and their location on earth. 


#5
Feb1912, 08:53 AM

Mentor
P: 15,055

What the lunar laser ranging experiments did do was to test one of the key tenets of general relativity, the equivalence principle. The Moon has a rather weird geometry. The far side of the Moon has a much thicker crust than the near side. This results in about a two kilometer offset between the Moon's center of mass and its geometric center. It is this offset that gives the laser ranging experiments the ability to test the equivalence principle to an amazing degree of precision. The best place in the solar system to look to to see the difference between Newtonian mechanics and general relativity is not the Moon. It is the orbit of Mercury. Suppose Mercury was the only planet in the solar system. Newtonian mechanics would indicate that Mercury would return to exactly the same spot in space after one orbit. General relativity says otherwise. It says that Mercury's orbit would like something like this: The orbit is still more or less elliptical, but the perihelion (the point of the closest approach to the Sun) advances with each orbit. The portrayal in the above image is greatly exaggerated. Imagine a slightly larger scale model of Mercury's orbit such that the perihelion distance is one meter. Instead of four orbits as depicted above, imagine the precession over 415 or so orbits (100 years). Imagine placing a meter stick that goes from the center of the Sun to Mercury's perihelion at the start of this 100 year period, another meter stick that goes from the center of the Sun to Mercury's perihelion at the end of this 100 year period. Per general relativity, the angle subtended by those two meter sticks will be about 43 arcseconds (and this agrees with observations). That 43 seconds of arc: That is the thickness of a postcard between the tips those two meter sticks. Tiny! Bottom line: Even for the orbit of Mercury, Newtonian mechanics misses the mark by a tiny, tiny bit over the course of a century. One has to look at the solar system for a very long time, or look outside the solar system at huge masses orbiting very closely to one another, before you can say that Newtonian mechanics "is not so accurate". Those who claim that general relativity falsifies Newtonian mechanics have missed the mark. It doesn't. What general relativity did do was to show that Newtonian mechanics is not universally true. Newtonian mechanics is instead merely an approximation of reality. Then again, this is most likely the case for general relativity as well. Physicists have yet to reconcile general relativity with quantum mechanics, and when they do, the combined theory will have to differ from both in some subtle regard. To me, the most profound difference between Newtonian mechanics and relativity is how the two look at space and time. Newtonian mechanics postulates that space and time are rigid, absolute, universal, and disconnected from one another. Relativity theory says otherwise. It is this "otherwise" that forces us to look at the universe in a very different regard. The accuracy (or lack thereof) of Newtonian mechanics with regard to lunar laser ranging experiments, or even the orbit of Mercury, is very much secondary to this huge mental shift in the nature of space nd time. 


#6
Feb1912, 09:17 AM

PF Gold
P: 4,087




#7
Feb1912, 10:36 AM

Sci Advisor
PF Gold
P: 5,027

While I completely agree with the last two points, in the process of looking for a place to link to the EIH equations of motion, I was surprised to discover that In the most recent editions of Astronomic Almanac, all planetary calculations now use EIH, and this proved necessary with increasingly precise observational capabilities. All those terms after the primary Newtonian term are very small, but they are now observable for the solar system.
See section 8.3 of: http://iaucomm4.jpl.nasa.gov/XSChap8.pdf 


#8
Feb1912, 11:29 AM

Mentor
P: 15,055

Some of the observations used to generate those ephemerides are very, very old. Thousands of years old. The ancients recorded eclipses and transits, and those old records, while scanty, are a part of what give the ephemeris models a long term basis. 


Register to reply 
Related Discussions  
Accurate RA and DEC  Astronomy & Astrophysics  0  
How accurate are Newton's formulas concerning Gravity, and other questions  General Physics  4  
Does this look accurate to you?  Astronomy & Astrophysics  7  
Newton's gravity and Coulomb's law are not quite accurate>  General Physics  16  
More Accurate Value for G  Classical Physics  1 