http://www.arxiv.org/abs/gr-qc/0411020
It seems that it is a rather attractive topic for many people - and there have even been conferences dedicated to this problem. See, for example,
http://www.zarm.uni-bremen.de/Pioneer/
What's the problem?
Two spacecrafts (Pioneer 10 and Pioneer 11) were launched in 1972 and 1973. They have done a good job and informed us about the distant planets in our solar system. Eventually NASA lost contact with them.
Nevertheless, the radio measurements of the position of the spacecrafts during the last years of their existence seem to show a puzzle: it seems that there is a universal unexpected contribution to the acceleration towards the Sun whose magnitude is roughly
a = (8 +- 1) x 10^{-10} meters/second^2
A shocking feature of this value of a is that it seems to be the same acceleration as the critical acceleration relevant for the MOND theories that replace dark matter, see another article on this blog
/2004/10/mond-and-holography.html
More precisely, the value of this acceleration is again a=Hc - it is the Hubble constant multiplied by the speed of light, plus minus roughly ten percent. If I calculated correctly, the usual Newtonian acceleration at the distance of 70 AU from the Sun (where the Pioneer anomaly was measured) is roughly 1.2 times 10^{-6} meters per second squared which is approximately 1500 times bigger than the anomalous acceleration a.
In other words, the integrated deviation from the expected position of both spacecrafts is roughly 400,000 kilometers in both cases - that's like 10 thousand of marathones!
Most particle physicists and string theorists would of course be skeptical about the existence of this new effect: we know from the observation of planets that such a large effect does not seem to exist for planets. Moreover, we have tested the equivalence principle pretty well, and a special "universal force acting on the spacecrafts" violates the equivalence principle. Furthermore, we have already learned that the people in NASA are sometimes capable to make as stupid errors as confusing inches and centimeters.
Nevertheless the numerical value of the acceleration and the possible relations with MOND (and holography) makes this topic sufficiently interesting for me so that I decided to write this short article, and encourage you to write your knowledge and opinions about this topic. Don't you think that there can be some new effect, associated with the breakdown of GR at very small accelerations, at least for small enough objects, associated with the breakdown of locality in holographic theories?
Obviously, Europe finds this topic more attractive, and some proposals to launch a spacecraft (well, NASA with LANL with the University of Bremen) whose only purpose is to measure its own motion have been suggested:
http://physicsweb.org/articles/world/17/9/3/1
These new probes could increase the precision by a factor of one thousand, and they should almost definitely settle the question whether the anomaly is a result of
- a stupid systematic error,
- the influence of new dark matter at the boundary of the Solar system
- or a proof of new physics.
