- #36
Chris Hillman
Science Advisor
- 2,355
- 10
The "Acceptable Sliver" in PPN Space
Hi, rbj,
Regarding "confirm the GTR quantitative prediction of the amount of de Sitter precession over the quantitative preditions of the other theories", this phraseology seems to miss one of the points I was trying to emphasize. See the review paper by Will on the webpage I cited and look for the diagrams showing regions of "allowed theories" parameterized by the PPN formalism. Observe that in the PPN picture, gtr sits smack dab in the center of a very thin "acceptable sliver" sitting inside a large (but finite dimensional) space of alternative gravitation theories. This sliver represents the theories which are in agreement with current observation/experiment. (Modulo inevitable quibbles over what observations to accept at face value.) However, there are still points in this sliver which are arbitrarily close to the point representing gtr.
The "acceptable sliver" has just been reduced in one dimension, if you like, because GP-B has measured the de Sitter precession to greater accuracy than ever before. Now, gtr still sits in the center of the newly reduced "sliver", but given the fact that limits of accuracy will always be with us, you can never hope to reduce this sliver to a single point! By Occam's razor, gtr is preferred theoretically because it happens to be the simplest of all these theories, which is one reason for paying so much more attention to it than to the other theories in the "acceptable sliver".
As for "hedging", the whole point of my post was to explain why there is no "hedging" from the Stanford team! Rather, they said that they are not yet ready to say anything about the more interesting Lense-Thirring precession. While this is certainly disappointing, they went on to carefully explain why that is the case. They carefully explained why they think they know what they have to do in order to "safely remove" the unanticipated errors in order to finally obtain the desired clean test, and they said they hope to complete this work by December. Science is hard, and sometimes you just have to be patient while people work out some wrinkles.
Hi, rbj,
rbj said:
Regarding "confirm the GTR quantitative prediction of the amount of de Sitter precession over the quantitative preditions of the other theories", this phraseology seems to miss one of the points I was trying to emphasize. See the review paper by Will on the webpage I cited and look for the diagrams showing regions of "allowed theories" parameterized by the PPN formalism. Observe that in the PPN picture, gtr sits smack dab in the center of a very thin "acceptable sliver" sitting inside a large (but finite dimensional) space of alternative gravitation theories. This sliver represents the theories which are in agreement with current observation/experiment. (Modulo inevitable quibbles over what observations to accept at face value.) However, there are still points in this sliver which are arbitrarily close to the point representing gtr.
The "acceptable sliver" has just been reduced in one dimension, if you like, because GP-B has measured the de Sitter precession to greater accuracy than ever before. Now, gtr still sits in the center of the newly reduced "sliver", but given the fact that limits of accuracy will always be with us, you can never hope to reduce this sliver to a single point! By Occam's razor, gtr is preferred theoretically because it happens to be the simplest of all these theories, which is one reason for paying so much more attention to it than to the other theories in the "acceptable sliver".
As for "hedging", the whole point of my post was to explain why there is no "hedging" from the Stanford team! Rather, they said that they are not yet ready to say anything about the more interesting Lense-Thirring precession. While this is certainly disappointing, they went on to carefully explain why that is the case. They carefully explained why they think they know what they have to do in order to "safely remove" the unanticipated errors in order to finally obtain the desired clean test, and they said they hope to complete this work by December. Science is hard, and sometimes you just have to be patient while people work out some wrinkles.