Space Travel: Pioneer Anomaly Solved?
    By News Staff | October 10th 2012 10:00 AM | 4 comments | Print | E-mail | Track Comments
    The 100 Year Spaceship Symposium, an international event advocating human expansion into other star systems, has some crucial hurdles to overcome. Basically, interstellar travel will depend upon extremely precise measurements of every factor involved in the mission, which isn't possible yet.  But a University of Missouri researcher thinks he has found the solution to a puzzle that has stumped astrophysicists for decades.

    The Pioneer spacecraft, two probes launched into space in the 1970s, seemed to violate the Newtonian law of gravity by decelerating anomalously as they traveled, but there was nothing in physics to explain why this happened. Sergei Kopeikin, professor of physics and astronomy, says he has a theoretical explanation: "My study suggests that this so-called Pioneer anomaly was not anything strange. The confusion can be explained by the effect of the expansion of the universe on the movement of photons that make up light and radio waves."

    Beams of radio waves were sent to and bounced off the Pioneer spacecraft to measure the probes' movement. The time it took for the photons to complete a round trip was used to calculate the spacecrafts' distance and speed. Kopeikin suggests that the photons move faster than expected from the Newtonian theory thus causing the appearance of deceleration, though the craft were actually traveling at the correct speed predicted by the theory. The universe is constantly expanding and this alters the Earth-based observations of the photons bouncing off the spacecraft, causing the Pioneer probes to appear to slow down.

    "Previous research has focused on mechanical explanations for the Pioneer anomaly, such as the recoil of heat from the craft's electrical generators pushing the craft backwards," Kopeikin said. "However that only explains 15 to 20 percent of the observed deceleration, whereas it is the equation for photons that explains the remaining 80-85 percent."

    Physicists must be careful when dealing with propagation of light in the presence of the expansion of space, noted Kopeikin, since it is affected by forces that are irrelevant in other equations. For example, the expansion of the universe affects photons, but doesn't influence the motion of planets and electrons in atoms.

    "Having accurate measurements of the physical parameters of the universe help us form a basis to make plans for interstellar exploration," Kopeikin said. "Discerning the effect of the expansion of the universe on light is important to the fundamental understanding of space and time. The present study is part of a larger on-going research project that may influence the future of physics."

    “Celestial ephemerides in an expanding universe” was published in the journal Physical Review D.


    Assuming this is correct, should the evidence for accelerating universe be reinterpreted or refitted?

    Say what?

    The paper that really finished up the pioneer anomaly was several months ago; by Slava Turyshev and others. Turyshev has long been involved in the careful investigation of this issue, from its very beginning. His final solution -- recoil of heat -- has long been the most plausible solution right from when the anomaly was first detected. The difficulty has been that the effect is tiny, and careful accounting of radiation from the craft has has been difficult. That was what was needed, in order to go from a plausible guess to an explanation that can be quantified sufficiently well to be taken as the solution.

    So what's this new paper?

    One of the most entertaining things about the pioneer anomaly over the years has been the more unusual or quirky ideas. Unfortunately for those hoping for some new revolution in physics, the explanation has been more mundane, and involves no new physics. Some of the more amusing attempts to deal with the problem are not so much credible speculations about new physics; but out and out cranks who hit on anything unusual as a handle to vindicate their ideas.

    It may not be quite that bad... but it doesn't look good. Expansion of the universe has long been looked at in the context of this and other problems. There are two things that recur in credible study of this.. expansion of the universe doesn't occur uniformly across space. Local concentrations of matter (like a galaxy, or a cluster of galaxies) do not expand in the same way. The simplest models for the expanding universe treat space as fairly uniform fluid of matter and energy, and this works well for the large scales of the universe. Not for motions in a galaxy let alone the solar system. And the other thing is that the effects are tiny; much MUCH smaller than what was detected with pioneer.

    I expect that this paper is going to drop into obscurity, or else get a brief bit of exposure when its errors are identified. I am pretty sure that expansion of the universe has nothing at all to do with the pioneer anomaly, and that this possibility had already been considered and checked years ago.

    I expect that this paper is going to drop into obscurity, or else get a brief bit of exposure when its errors are identified.
    Why not be part of the solution and identify the errors in an article and we'll put it here.
    It will be fun to see how this pans out, but my own knowledge of GR is not going to be enough for me to be confident of identifying specific errors. It's more a couple of red flags showing up for me; and guessing that this is not going to fly.

    I could be wrong; and it would be really exciting if I am wrong. The explanation published by Turyshev is more mundane. The papers are all on arxiv.

    Support for the thermal origin of the Pioneer anomaly
    by Slava G. Turyshev, Viktor T. Toth, Gary Kinsella, Siu-Chun Lee, Shing M. Lok, Jordan Ellis
    published in Phys. Rev. Lett. 108, 241101 (2012)
    DOI: 10.1103/PhysRevLett.108.241101


    Celestial Ephemerides in an Expanding Universe
    by Sergei Kopeikin
    published in Phys. Rev. D 86, 064004 (2012)
    DOI: 10.1103/PhysRevD.86.064004

    I'm having a further look, but don't expect to resolve the dispute on my own behalf. If I get anything new or interesting I'll add it.

    Cheers -- Chris