Understanding It Doesn’t Make it Less Freaky

Dennis Overbye has a nice article this week in the NYTimes on the recently published explanation of the Pioneer Anomaly. As he explains,

The story starts with the Pioneer 10 and 11 space probes, which went past Jupiter and Saturn in the late 1970s and now are on their way out of the solar system. In the 1980s it became apparent that a mysterious force was slowing them down a little more than should have been expected from gravity of the Sun and planets.

Was there an unknown planet or asteroid out there tugging on the spacecraft? Was it drag from interplanetary gas or dust? Something weird about the spacecraft? Or was something wrong in our calculation of gravity out there in the dark?

 The last question is really one about whether Einstein's account of gravity in General Relativity is showing any cracks. As Overbye notes, that would be big news indeed, given that the GPS system requires corrections rooted in General Relativity to achieve the precision it does (see Relativity and the Global Positioning System for some details if you're interested).

The solution determined by Slava Turyshev and his team at the Jet Propulsion Laboratory in California is that the spacecraft are slowing down due to asymmetric thermal emissions from the electronics and fuel source on board. Thermal means heat which means light. Light carries momentum and any light source experiences a recoil force opposite to the direction the light is emitted just like shotgun recoils against the bullet into your shoulder when you fire it. Now, it's not just that the Pioneer probes are thermally radiating (cooling off, effectively), but that they are doing so asymmetrically -- emitting more light from the front than the back. This leads to a net drag, slowing down the spacecraft. The same thing happens when you turn your headlights on in your car -- it slows you down. (Though, I wouldn't rely on it for braking at stoplights.)

You can read  article for yourself in Physical Review Letters or get the preprint version here. You can also read another account of the paper at arsTechnica.com.

I like this particular case for a number of reasons. First, the anomaly arises in the difference between what we expect to be the case and what we find to the case. Understanding is increased by trying to bring those expectation into line with those findings. Many options are entertained including throwing out the best theory we have so far to describe planetary scale physics. Just like the recent bruhaha over the hints that neutrinos travel faster than the speed of light. (It turned out to be a loose cable in the experimental setup.) It also reminds me of the early days of sub-atomic physics and the discovery of the neutrino. In the beginning all that was observed was missing energy. Our best, most honest explanations couldn't account for distribution of the energy of the electron in beta-decay. Niels Bohr openly considered whether or not we won't need to just throw out Conservation of Energy as a principle in physics. (Just as we had thrown out Conservation of Mass as a fundamental principle of physics not much earlier.) However, in the case of the neutrino there really was something new that hadn't been seen -- it really was new physics. In the case of the Pioneer anomaly, it's as Overbye says "You may dream of freaky new physics, but sometimes freaky old physics is all you need."

Second, it makes me reflect on the effect of scale, both in size/distance and in (for lack of a better term) detail, on what we consider to be true about the world and what is a good practical point of view. And, for that matter, what the real differences are between those two claims. I joked earlier that turning on your headlights slows you down, but the practical fact is that it doesn't matter one iota at my everyday physical scale. In order to worry about such things I need to be traveling gargantuan distances through deep space in order for the effect to show up with any significance. Does this mean that my "everyday" understanding about the effect of my headlights on the speed of my car is false? As an approximation, is it not true? This might be right for something like the effects of special relativity because there is one fairly straightforward parameter -- velocity -- that can be turned up and down. So, the world is truly relativistic and space is not Euclidean. I leave myself content with working with approximations most of the time simply because they are easier to work with. But even with this pragmatic disposition of the perpetual revisability of scientific understanding, it doesn't seem quite right to always consider the understanding of the beings and interactions at one scale simply false when one uncovers more structure at other (generally smaller) scales. Indeed, it's not at all clear that all of the large-scale properties of materials are embodied in the presumptively "more fundamental"  smaller-scale phenomena.



  1. Evan says

    There’s a lot here to agree with, but I’ve heard you guys mention in an an off hand way the possibility that not all of the large scale phenomena we are familiar with are embodied in the smaller scales. This just strikes me as supremely doubtful. I’m finishing my PhD in molecular biology, a field that is entirely devoted to attaching observed large scale phenomena to their small scale roots. There just aren’t any cases at all, that I can think of, where I’ve ever heard it postulated that this project might be impossible. There are soooo many very solid cases of larger scale phenomena being well explained by their small scale features (think DNA base pairing). I’ve watched many many unsolved problems yield to this approach, and I’ve solved a few myself. At this level of understanding (understanding things between 1nm and 1mm, roughly), we really seem to be in a Kuhnian “normal science” paradigm.

    What are your sources for asserting that this reductionist paradigm may eventually run up against discontinuities,may eventually find a case where some large scale phenomenon just *isn’t explained at all* by its smaller scale components? Or do you mean something else entirely? For example, I agree that we can’t understand cells in terms of Schrodinger’s equation; but we *can* understand them in terms of genes and proteins. Do you mean that we can’t conclusively rule out the possibilty of reductionism failing? I agree, but we also can’t conclusively rule out solipsism, and so what?

    Love the podcast, and I’m particularly glad to have your perspective present on it Dylan. Keep the good stuff coming-

    • Michael says

      I don’t think Dylan is saying that there are large scale phenomena which can’t be explained in terms of their lower scale makeup- he’s saying that some larger scale phenomena occur which do not occur at lower sizes. There are certainly cases where larger scale phenomena arise which do not manifest at lower size scales, for instance, consciousness appears to be a physical phenomena which only arises in the way that it does from a large scale coordination of smaller phenomena which are not themselves conscious. Consciousness can be explained in terms of these smaller scale phenomena, as you say, but consciousness is not present in these individual components (at least not in the same way as the global effect of them).

    • Profile photo of Dylan Casey says

      Thanks, Evan.

      Indeed, I did not mean that reductionist techniques aren’t extraordinarily successful. Like you, I’ve had first-hand experience using reductionist techniques to explain larger scale phenomena. (In my case it is particle physics.) In some part, my comment is about emergence, which, admittedly, is not a settled matter in philosophy or science. I would point to Paul Anderson’s classic article P.W. Anderson. “More is Different”. Science, 177, 1972 (http://www.andersonlocalization.com/pdf/more_is_different.pdf) for a very nice, physics-related discussion of the issue of scale what is and is not explained at different scales. I’d also point to a nice piece of work in the the history of science on the Periodic Table by E. Scerri. Here, the question of kinds comes up, especially regarding whether the organizing principles of the periodic table (the kinds) represent something authentically different than what would be described by lower-scale phenomena (i.e., sub-atomic physics).

      • says

        Hey Dylan, the link didn’t seem to work for Paul Anderson’s article. It’s interesting to hear what a particle physicist and a molecular biologist have to say about this. It seemed to me from Dylan’s post that he was getting at the idea of emergence and I am definitely far from the technical understanding of this you all have, but there are a few recent books on the topic that introduce emergence, both from a philosophical and scientific perspective. The recent kerfuffle over Deacon’s book “Incomplete Nature,” which erupted over the supposed similarity between his perspective on emergence and Alicia Juarrero and Evan Thompson’s more philosophical perspective made me wonder about what both scientists and philosophers think the questions of emergence are. It seems Deacon is trying to take it into an entirely new direction by attempting to scientifically ground such notions as end-directedness (telos) and absence (propagation of constraint form lower to higher levels). So what I take Deacon’s point to be is that it’s not what is embodied at lower levels, but rather what is not embodied at lower levels (constraint) that’s important. But the success of reductionist techniques in science makes me curious about what you all might think about these ideas?

        Here are links to some of these ideas:


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