"When you have eliminated the (im)possible...

... whatever remains, however improbable, must be the truth" - A slight paraphrasing of Sherlock Holmes, but very apt for science. I've been back in Sydney for almost a week, and so have post-holiday blues, so time for a rant.

The subject of today's post, the Pioneer anomaly.

First, a little background. The Pioneer probes where launched in the 1970s to undertake grand tours of the Solar System. They have been trundling for almost as long as I have been on the planet, and are now well beyond the orbit of Pluto.
 Out there, gas is extremely tenuous, and we would expect the Pioneers to be simply test particles, whose paths are completely due to the influence of gravity, mainly the Sun, but also the planets.

Now, we can predict the locations of the planets extremely accurately, so you'd think with the mathematics of gravity in hand, we should be able to do the same with the Pioneers, and we can.

But there is a problem. The Pioneer probes are not behaving exactly as predicted. There is a tiny, and I mean tiny, residual acceleration that is not inline with the gravitational expectations. The fact that we can measure this 0.9 nanometers per second squared is testament to the accuracy of the observations of the two space craft.

But what's the source of the residual acceleration? I have been following this from the 1990s, and the web has been full of radical claims that the Pioneer anomaly is due to new physics, due to Newton and Einstein being wrong, that modifications to general relativity are needed. Heck, it's even been used to support creationist cosmologies.

And of course, the web loved it. Stupid scientists, to blind to see that their universe is built on a house of cards. but in the background, there continued to be story that the acceleration is potentially due to a much more mundane source, such as a little leaky fuel, or non-uniform heating. However, that was not as exciting as physics being wrong.

Studying the heating and cooling of spacecraft is not easy. They ain't spherical cows! And so it takes a lot of work to get it right. But, surprise, surprise, after almost a decades worth of work, NASA has shown that Study Finds Heat is Source of ‘Pioneer Anomaly’ (taken from Bryan's twitter).

How dull! All those fantastic ideas struck down by simple facts! (well, actually complex observations and modelling etc). I am sure there will be claims that this is a NASA conspiracy to keep the colourful crayon brigade suppressed.

But wake up shashdotters - this is the way science goes. Seeing something weird in your data can be the start of a road to a revolution in science. But you must convince yourself that you have not fooled yourself, that the "thing" you see in your data really is something new, and is not the result of something boring and mundane that you have forgotten. You must check, and check again. And this is what NASA has done. And what OPERA did, and many other "Oh wow!!" science results

Yes, it is great (and important) to play "what ifs" over in your head, but before you claim your Nobel prize (or just publish), then remember, the motto of the scientist should be a rewriting of Sherlock Holmes:
"Before you publish "the truth", however, improbable it may seem, be sure that you have fully eliminated all of the possibles, no matter how dull and mundane they may be"


  1. What a refreshing approach Geraint!!..I have seen too many "other" scientists completely ignore this type of advice.


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