Pioneer Anomaly

Farsight

A couple of matters have caused me some curiosity of late. One is the "Pioneer Anomaly", wherein exploratory spacecraft are slightly off course:

http://en.wikipedia.org/wiki/Pioneer_anomaly

The other is something I read about recently in New Scientist, about a revised theory of gravity making Dark Matter unnecessary.

http://www.newscientistspace.com/article/dn8631

Can anybody tell me more and/or give me a lowdown on whether these items are mere journalistic sensation?

Farsight

Here's an excerpt from the New Scientist link:

"It's as if gravity is stronger near the centres of galaxies", Brownstein told New Scientist. "Then, at a certain distance, the stars become sparse, and the gravitons don't contribute that much." So at larger distances, gravity returns to the behaviour described by Newton......

Furthermore, the team tested the theory against observations of NASA's 34-year-old Pioneer 10 spacecraft, which appears about 400,000 kilometres away from its expected location in the outer solar system.

Farsight

Anybody? Gravity and dark matter?

ukmicky

http://www.newscientistspace.com/art...ar-system.html


the-kink-at-the-edge-of-the-solar-system

Farsight

Thanks micky. I'm in the UK too.

matt.o

A couple of things about the cluster paper;

-They model clusters as isothermal spheres. There are clear temperature gradients in clusters, particularly within ~200Kpc.

-They make no mention of substructure within the clusters. The assumption of hydrostatic equilibrium is violated in the presence of a major merger. A quick look at the clusters in their study tells me there are quite a few there that are known mergers. A lot of these seem to fit their metric-skew-tensor model, whilst some of the more dynamically relaxed clusters seem to not fit well. I would like to see a study done on relaxed systems alone. I have a feeling that their model may not fit this revised sample so well.

As for the Pioneer anomaly, I think a test specifically designed to measure the anomaly will solve the puzzle.

Farsight

Hello, here's a paper Gravitational Solution to the Pioneer 10/11 Anomaly by Brownstein and Moffat:

http://arxiv.org/PS_cache/gr-qc/pdf/0511/0511026.pdf

Can anybody tell me in a nutshell what metric skew tensor gravity is?

selfAdjoint

From the abstract of reference 4 in that paper, http://arxiv.org/abs/astro-ph/0412195,

Gravitational Theory, Galaxy Rotation Curves and Cosmology without Dark Matter
Authors: J. W. Moffat

So this is general relativity with an additional force, carried by a massive particle which is described by a skew tensor field, [tex]F_{\mu\nu\lambda}[/tex]. Skew means the same thing as anti-symmetric; if you interchange any two of the indexes the result is the same as multiplying the tensor by -1 [tex]F_{\nu\mu\lambda} = - F_{\mu\nu\lambda}[/tex] and so on. The Faraday tensor of electromagetism is a skew tensor, but that's not the same is the one describing this new force because Faraday is a rank two tensor (two subscripts) while the new Moffat tensor is defined to be rank 3 (three subscripts).

Forces in GR are described by tensor fields because tensor equations are true in every diffeomorphism class, the coefficient of the tensors change but the equations remain true.

An important point here is that Moffatt originally made up his theory to explain galactic rotation curves and now is asserting that it can account for the Pioneer anomaly too.

Farsight

Thanks selfAdjoint. I was hoping for a geometrical one liner. I found this powerpoint presentation which looked pretty good, but I don't understand the "phion field". Sorry, I can't link direct, so search google on Edinburgh Moffat Talk Gravity and it's at the top.

http://www.google.co.uk/search?hl=en...G=Search&meta=

I didn't know this sort of thing had been kicking around for so long. Where have I been?

MOND is a modification of the usual Newtonian force law, hypothesized in 1983 by Moti Milgrom of the Weizmann Institute, as an alternative to Dark Matter.

PRodQuanta

MOND really just says that the measurements for gravitational acceleration that were taken during the time of Newton were too crude and that the crude measurement lead to an improper assumption that objects under such sparse gravitational conditions will abide by the same gravitational correlation.

For the Pioneer Anomaly, MOND is used to try and say that there is no blueshift taking place; rather, our calculations of where the probes should be are wrong because of our incorrect assumption of classical Newtonian Dynamics.

Paden Roder

Garth

It is important to understand what is being measured as the Pioneer Anomaly.

It is not the actual acceleration of the space craft, nor its actual position, but rather the frequency of the carrier wave repeated back from the space craft to a ground station.

See: The Pioneer Anomaly: Seeking an explanation in newly recovered data
The acceleration and position anomalies are deductions obtained by comparing this frequency count with expectations from the classical Newtonian model.

There are two interpretations of a real effect: The Study of the Pioneer Anomaly: New Data and Objectives for New Investigation(emphasis mine)

The PA is a deviation of the measurement of frequency with prediction, it is a comparison of two time rates.

Thus the primary measurement is one of clock drift. It is only by applying doppler does this become an anomalous acceleration and hence a deviation of velocity and position from that predicted by Newton.

Garth

Farsight

Many thanks for that Garth. I've printed the papers and will enjoy some bedtime reading.

PRodQuanta

So what you are saying is that the PA is an (de-)acceleration of clocks? I think I read something about that, Ephemeris time time vs. atomic time.

Paden Roder

Garth

It could be.
The primary measurement is the comparison of two clock rates: that received from the space-craft compared with that predicted by Newtonian physics.

If the space-craft are actually behaving as Newton predicts then the anomaly is one of clock acceleration, as you say Ephemeris v atomic time. (Incidentally as predicted by SCC)

If the two clock rates are identical then the space-craft are not behaving as Newton predicts.

In this case either there is an extra force decelerating the space-craft such as radiation imbalance, gas emission, dust drag, or Planet X (this last option being the reason the data was examined in the first place!)
or
gravity does not behave as Newton predicts at large ranges or small accelerations (MOND).

If there is an extra unaccounted force it is strange that the sun-wards acceleration is constant and the same for both space-craft.

If there is some MOND-type effect it is surprising it has not been detected elsewhere in planetary orbits.

Garth

Garth

Read all about it! Today's New Scientist:Have we got gravity all wrong?

Garth

marcus

Moffat has a new paper this week where he suggests a way to falsify his theory by measuring some time-delays from probes in the outer.

http://arxiv.org/abs/gr-qc/0605141
Time Delay Predictions in a Modified Gravity Theory
J. W. Moffat
5 pages
"The time delay effect for planets and spacecraft is obtained from a fully relativistic modified gravity theory including a fifth force skew symmetric field by fitting to the Pioneer 10/11 anomalous acceleration data. A possible detection of the predicted time delay corrections to general relativity for the outer planets and future spacecraft missions is considered. The time delay correction to GR predicted by the modified gravity is consistent with the observational limit of the Doppler tracking measurement reported by the Cassini spacecraft on its way to Saturn, and the correction increases to a value that could be measured for a spacecraft approaching Neptune and Pluto."

selfAdjoint

Sounds good. From his previous paper and the original rotation curve one it looked liked he was tuning his theory to accomodate whatever "appearance" he was looking at, a la epicycles.