- #1
- 6,724
- 429
An analysis of the orbits of Neptune's satellites seems to show that the Pioneer anomaly can't be gravitational.
http://www.technologyreview.com/blog/arxiv/24541/
http://www.technologyreview.com/blog/arxiv/24541/
IMO, there were really never any reason to expect that the PA should involve anybcrowell said:An analysis of the orbits of Neptune's satellites seems to show that the Pioneer anomaly can't be gravitational.
http://www.technologyreview.com/blog/arxiv/24541/
Old Smuggler said:But the fact that the anomalous acceleration is very close to cH, would suggest that
the PA has something to do with mismodeling of null curves in an expanding Universe.
This does not work in GR, but alternative models exist. See, e.g., gr-qc/9910054.
edpell said:what is cH please?
The observations might be consistent with a variable speed of propagation of light through space that is polarized by gravitation of embedded masses. Such an interpretation of observations would be a game-changer and thus would be anathema to BB cosmology. If it's true, prepare to wait for a century or so before it can become "accepted".Garth said:The unexplained residual is consistent with cH.
What actually was being detected was a residual blue shift in the radio signals, which is interpreted as an extra Sun-wards acceleration.
It could just as well be a time drift between atomic and ephemeris clocks.
Garth
We have plenty of experimental data, and not just from the Pioneers, but from other probes that are at least partially spin-stabilized. If the apparent "acceleration" is not real, and is not caused by gravitational forces on the probes themselves, we must be prepared to consider that the radio signals could be propagating at something other than c. Einstein's view of space as a polarizable ether was not well-received by his contemporaries nor by modern-day physicists, but it was a view that he held from 1920 onward.lalbatros said:"The observations might be consistent with a variable speed of propagation of light through space that is polarized by gravitation of embedded masses. Such an interpretation of observations would be a game-changer and thus would be anathema to BB cosmology. "
The use of the conditional tense is appropriate indeed, as always when pure speculation without enough experimental data is involved.
Einstein said:In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its result hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light).
Einstein "On the Ether" 1924 said:The ether of the general theory of relativity differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in it locally variable properties by ponderable matter.
http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.0399v1.pdfInitial efforts to explain the Pioneer anomaly focused on the possibility of on-board systematic forces.While these cannot be conclusively excluded (Anderson et al. 2002; Turyshev et al. 2005), the evidence to date did not support these mechanisms: it was found that the magnitude of the anomaly exceeds the acceleration that these mechanisms would likely produce, and the temporal evolution of the anomaly differs from that which one would expect, for instance, if the anomaly were due to thermal radiation of a decaying nuclear power source.
Do all the different probes show the same magnitude of anomalous acceleration as a function of distance from the Sun, or is it different for different probes?turbo-1 said:While analysis to that level is pretty impressive, there is another more compelling reason to discount systematics. The anomaly is not confined to the Pioneer craft. Other probes also show evidence of anomalous Sunward acceleration, and when probes have different masses, cross-sectional areas and thermal signatures (due to the various ways RTGs are mounted) it is difficult to come up with scenarios in which the anomaly can be produced for each of them.
If there's no clear evidence that the magnitude of the anomalies for different probes are too similar to be explained by quirks of engineering, would you then disagree with turbo-1's statement that "when probes have different masses, cross-sectional areas and thermal signatures (due to the various ways RTGs are mounted) it is difficult to come up with scenarios in which the anomaly can be produced for each of them"?Garth said:The Pioneer Anomaly acceleration is constant from the orbit of Saturn outwards and equal on both the Pioneer probes.
Inside the orbit of Saturn other effects would have masked the effect, as indeed they have on other deep space probes, although signs of the effect have been detected on the Voyager probes as well. The Voyager spacecraft 's three-axis stabilization maskes the effect.
The Flyby anomalies are different, though they may be connected.
Part of my skepticism of design-dependent systematics arises from the fact that even though the anomaly can be masked in some probes that are not fully spin-stabilized, the anomaly still results in some blue-shifting. If off-gassing, leaking thruster fuel, or thermal radiative pressures can somehow be responsible for the anomalies, why is the thrust always directed away from the Sun resulting in braking? Why no acceleration out of the system resulting in red-shifting?JesseM said:If there's no clear evidence that the magnitude of the anomalies for different probes are too similar to be explained by quirks of engineering, would you then disagree with turbo-1's statement that "when probes have different masses, cross-sectional areas and thermal signatures (due to the various ways RTGs are mounted) it is difficult to come up with scenarios in which the anomaly can be produced for each of them"?
edpell said:Sound like a worthwhile experiment send out 10 probes in different directions. All the probe needs to do is ping once per ten seconds and point towards Earth (that is point the antenna part towards Earth). Say 20Kg per probe 200Kg total. But at so respectable velocity (we don't have all century). Anyone have a feel for the cost?
While analysis to that level is pretty impressive, there is another more compelling reason to discount systematics. The anomaly is not confined to the Pioneer craft. Other probes also show evidence of anomalous Sunward acceleration,
. If off-gassing, leaking thruster fuel, or thermal radiative pressures can somehow be responsible for the anomalies, why is the thrust always directed away from the Sun resulting in braking?
Well, that would depend on your definition of "interesting physics". Some people apparentlybcrowell said:If I were peer-reviewing a proposal for a new dedicated probe, the question in my mind would be this: if it's not a gravitational effect, then what are the chances that it's even interesting physics at all?
That's not exactly the mainstream position. The mainstream position is that within star systems and galaxies and clusters, gravity is just strong enough to dominate "dark energy", while the gravitational attraction at larger distances is too weak to counteract dark energy.Old Smuggler said:This confronts the
"well-known" mainstream position that the cosmic expansion does not apply to
gravitationally bound systems.
Even with no dark energy the expansion would still happen, just at a different rate, right? But either way I guess GR predicts that the local gravity in a bound system dominates over cosmic expansion (I wonder if any numerical simulations have been done to check exactly how small the effects of expansion should be within the solar system according to GR)Al68 said:That's not exactly the mainstream position. The mainstream position is that within star systems and galaxies and clusters, gravity is just strong enough to dominate "dark energy", while the gravitational attraction at larger distances is too weak to counteract dark energy.
I know what the mainstream position is. Since I was talking about observable consequencesAl68 said:That's not exactly the mainstream position. The mainstream position is that within star systems and galaxies and clusters, gravity is just strong enough to dominate "dark energy", while the gravitational attraction at larger distances is too weak to counteract dark energy.
Several studies have been done on the effects of the cosmic expansion within the solarJesseM said:But either way I guess GR predicts that the local gravity in a bound system dominates over cosmic expansion (I wonder if any numerical simulations have been done to check exactly how small the effects of expansion should be within the solar system according to GR)
Sure, but without dark energy, the rate of expansion would be dominated even by the weak gravity over large distances, ie the rate would be slowing.JesseM said:Even with no dark energy the expansion would still happen, just at a different rate, right?
I wonder about that, too, but I would think that it would be negligible, since compared to intergalactic space, the gravitational forces within the solar system are huge, as well as within galaxies and clusters.But either way I guess GR predicts that the local gravity in a bound system dominates over cosmic expansion (I wonder if any numerical simulations have been done to check exactly how small the effects of expansion should be within the solar system according to GR)
It seems that you have the cosmic redshift in mind. But no sane person would identifyGarth said:The PA is not cosmological expansion detected within the solar system, the anomalous acceleration is in the opposite direction!
Garth
You have to be very careful in your language to avoid confusion.Old Smuggler said:It seems that you have the cosmic redshift in mind. But no sane person would identify
the apparent anomalous blueshift inferred from the PA with the cosmic redshift.
However, the cosmic expansion may have another, more subtle effect, namely an
anomalous delay of radio signals used in the telemetry. Since the PA essentially appears
from the calculation of trajectories of the probes, such an anomalous delay could be
interpreted as if the probe's coordinate velocity were slowing down anomalously, thus
indicating an anomalous acceleration towards the observer.
This explanation does not work in GR, but it has been shown to work in another theory.
The bottom line is thus that the interpretation of the PA is crucially theory-dependent,
and that the PA really might be the cosmological expansion detected within the solar system.
Expansion per se (I mean unaccelerated, a~t) can be removed by a coordinate transformation. Therefore it can't have any physical effect on the solar system.JesseM said:Even with no dark energy the expansion would still happen, just at a different rate, right?
Ich said:Expansion per se (I mean unaccelerated, a~t) can be removed by a coordinate transformation. Therefore it can't have any physical effect on the solar system.
(didn't check your math, but it seems wrong at first glance)First, if I take a metric [itex]
ds^2=dt^2-a^2(dr^2+r^2 d\theta^2+r^2\sin^2\theta d\phi^2 )
[/itex], with a=ut, and calculate the Einstein tensor, I get [itex]
G^t_t=3t^{-2}
[/itex] , along with other diagonal elements that are nonzero. Doesn't this mean that the a=ut is observable?
Expansion is not unobservable, but it has no physical effect. If all galaxies are tied to constant FRW coordinates, they are moving away from each other in the standard minkowski coordinates. That is surely observable, but it doesn't mean that physics is different. It just means that matter in the universe is arranged such that it is moving apart. No reason for e.g. a solar system to follow that trend, it's the same as if other galaxies were not moving apart. It simply doesn't matter what other things do.Also, there seems to be something missing from your argument, since it makes it sound as though expansion would be unobservable not just at the scale of the solar system but at the cosmological scale as well.
FYI, Ich has a serious misunderstanding of differential geometry; he believes that bybcrowell said:First, if I take a metric [itex]ds^2=dt^2-a^2(dr^2+r^2 d\theta^2+r^2\sin^2\theta d\phi^2 )[/itex], with a=ut, and calculate the Einstein tensor, I get [itex]G^t_t=3t^{-2}[/itex], along with other diagonal elements that are nonzero. Doesn't this mean that the a=ut is observable?
bcrowell said:if I take a metric [itex]ds^2=dt^2-a^2(dr^2+r^2 d\theta^2+r^2\sin^2\theta d\phi^2 )[/itex], with a=ut, and calculate the Einstein tensor, I get [itex]G^t_t=3t^{-2}[/itex], along with other diagonal elements that are nonzero.
Ich said:(didn't check your math, but it seems wrong at first glance)
load(ctensor);
ct_coords:[t,r,theta,phi];
a : u*t;
lg:matrix([1,0,0,0],
[0,-a^2,0,0],
[0,0,-a^2*r^2,0],
[0,0,0,-a^2*r^2*sin(theta)^2]);
cmetric();
einstein(true);
3
(%t7) ein = --
1, 1 2
t
1
(%t8) ein = --
2, 2 2
t
1
(%t9) ein = --
3, 3 2
t
1
(%t10) ein = --
4, 4 2
t
Ich said:It is the minkowski metric if you use https://www.physicsforums.com/showthread.php?p=2283519#post2283519"
Ich said:Hi and a happy new year,
sorry, I've been sloppy: What I've been referring to is the empty universe, with negatively curved space and a~t. Try your calculations (or the transformation) with this metric:
[tex]
ds^2=dt^2-u^2t^2(dr^2+r^2 sinh(u r)^2 (d\theta^2+r^2\sin^2\theta d\phi^2) )
[/tex]
(I hope it's correct)
Ich said:Expansion per se (I mean unaccelerated, a~t) can be removed by a coordinate transformation. Therefore it can't have any physical effect on the solar system. What is relevant for solar system physics are gravitational sources that are unaccounted for in the standard calculations. There are two such sources of "cosmological origin", Dark Matter and Dark Energy. We should expect that Dark Matter dominates around the sun, producing indeed an anomalous blueshift - but several orders of magnitude below the observed effect. So cosmology is not the solution, at least not in GR or other metric theories consistent with observation.