# New test of GR - verified to 1:20,000

#### Nereid

Staff Emeritus
Gold Member
In 2002, the Cassini spaceprobe, on its way to Saturn, passed close to the Sun as seen from the Earth (it was actually a long way from the Sun). Using the radio communications with Cassini, a stringent test of General Relativity was performed (a similar test had been done using the Viking lander on Mars, in 1979).

GR passed with flying colours.
This experiment shows GR to be valid to +/- ~5 x 10-5.

A popular account:
http://skyandtelescope.com/news/article_1066_1.asp

A preprint:
http://arxiv.org/PS_cache/gr-qc/pdf/0308/0308010.pdf [Broken]

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#### marcus

Gold Member
Dearly Missed
Yay Albert! This is great.
It is really close in spirit to the 1919 test by Eddington four years after Einstein proposed the theory----light bending as it passes sun (1919) and radiosignal slowed down as it passes close to sun (2002) are both showing the coefficient of refraction.

n = 1 + RSchw/r

Nereid you could probably expand on this a bit and give anyone who is interested some extra intuition about it. Wish you would. Too good an example and illustration of GR not to discuss further. How about it?

I always like to use the formula that the angle (in radians) that a ray is bent is equal to

2RSchw/r

where r is the distance of closest approach to the sun and the Schw radius is the familiar 2GM/c2

if you can relate this bending angle to the index of refraction and the timedelay effect on the radio signal from Cassini spacecraft as it passed the sun , or say anything to put a little meat on the bones, please do

Originally posted by Nereid
In 2002, the Cassini spaceprobe, on its way to Saturn, passed close to the Sun as seen from the Earth (it was actually a long way from the Sun). Using the radio communications with Cassini, a stringent test of General Relativity was performed (a similar test had been done using the Viking lander on Mars, in 1979).

GR passed with flying colours.
This experiment shows GR to be valid to +/- ~5 x 10-5.

A popular account:
http://skyandtelescope.com/news/article_1066_1.asp

A preprint:
http://arxiv.org/PS_cache/gr-qc/pdf/0308/0308010.pdf [Broken]

Last edited by a moderator:

#### Phobos

Staff Emeritus
Gold Member
I love it when they piggyback these kinds of experiments.

#### LURCH

Originally posted by Phobos
I love it when they piggyback these kinds of experiments.
Yes, with the expense of space missions, it is nice to see organisations like NASA and the ESO squeezing every bit of value out of their dime.

Nereid, thanks for reminding me, there's a sattelite-borne test of another of GR's predictions (frame-dragging) that I was going to make sure to follow. Haven't checked it in so long, I can't even remember the name. I'll get back with some data within 24.

#### wolram

Gold Member
http://www.phys.lsu.edu/mog/mog10/node9.html

Gravitomagnetism has a history that is at least as long as that of general relativity itself. The idea that mass currents might generate the gravitational analogue of magnetic fields, and crude experiments to look for such effects predated Einstein. Soon after the publication of general relativity (GR), Lense and Thirring calculated the advance of the pericenter and line of nodes of a particle orbiting a rotating mass.
______________________________________________________________________

for anyone that is unfamiliar with the term "frame dragging",

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#### LURCH

For those interested in following along, the new experiment in GR' frame-dragging prediction is called Gravity Probe B, or just "GP-B", and is scheduled for launch from Vandenburgh on in early December. Should be about two years in orbit to collect data.

#### marcus

Gold Member
Dearly Missed
Originally posted by LURCH
For those interested in following along, the new experiment in GR' frame-dragging prediction is called Gravity Probe B, or just "GP-B", and is scheduled for launch from Vandenburgh on in early December. Should be about two years in orbit to collect data.
Leonard Schiff thought that experiment up while taking a dip in someone's swimming pool back in 1960s. It has taken a long time to get it ready to fly. This is a postumous triumph for Schiff

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