Tricky place, the Universe, and not at all keen to give up its secrets. Do you remember school physics, specifically when you did acceleration? If so, you probably recall that the Earth’s gravity tugs on everything at sea level with a pull of about 9.81 metres per second per second. In other words, if you drop something, it will pick up speed at a rate of 9.81 m/s every second until it lands on your toe. Or with a bit of extra maths, that’s the way you work out how deep a well is by dropping a stone down it.
Now, imagine an acceleration of 0.9 nanometres per second per second. That is one ten billionth of the acceleration exerted by the Earth’s gravitational field. Hard to measure? Yes, all the more so when you are doing so not at the Earth’s surface, but from a distance of many billion kilometres.
So I for one have always doubted that the Pioneer Anomaly was real. Launched in 1972 and 1973, the Pioneer 10 and 11 spacecraft are objects that give you and me bragging rights as members of the human race. Only one species we know of has the pizazz to find out about the solar system by sending automated probes to the outer planets. Not only that, but the Pioneers were tracked as they entered previously unknown areas of space beyond the planets.
And this is where the fun began. Both Pioneers started slowing up in a way that had never been anticipated, at the stately rate of 0.9 nm per second per second.
Just think what making this measurement involves. First, you have to detect the spacecraft’s minute radio signal – from an 8W transmitter about 10 billion kilometres away. NASA Deep Space Network did just this as the spacecraft crossed and left the solar system. Then you have to measure the exact wavelength of the incoming signal so the Doppler effect can be used to gauge the speed of the vessel. Then a host of other corrections kick in, starting with the need to allow for the Earth’s own movement round the Sun.
Even after this exquisite feat of engineering and science had been accomplished, and the anomaly had been shown to be real, things were not simple. There have long been a range of alternative theories of gravitation, such as MOND, Modified Newtonian Dynamics. Maybe the Pioneers really had flown into a region where the old rules of mechanics don’t apply. But a big complex spacecraft is not a rock flung through the air. It is a complex creature with pipes, wires, booms, antennas, nuclear heat sources (held at a safe distance from the sensitive instruments) and other bits. It has its own internal dynamics which are well capable of influencing its speed though space. And so it proved. We now know that the anomaly is caused by minute amounts of heat, from electrical circuits and the Pioneers’ plutonium heat sources, pushing the spacecraft backwards. It’s the opposite to the way those toy machines rotate when the sunlight strikes their vanes.
By the way, a good account of this by my mate Paul Sutherland is at the SEN website. The project to solve the anomaly, he says, involved 43GB of data – tiny now, but massive in the 1970s era of card and tape.
So what have we learned from this? First, the standard model of physics lives to fight another day. But the fuss also reminds me of a shorter-lived but more recent and higher-profile one, the superluminal neutrinos that turned out not to be flying from the CERN accelerator to the Gran Sasso lab in Italy at lightspeeed-plus.
Here the problem was to time a journey through solid rock – how to measure both the distance and the time, to exceptional accuracy and while beyond the reach of GPS. Timing technology (even with one end in the home of the expensive watch) is never easy. Try setting your clock by two different digital radios: you’ll find they each produce the time signal at a different time because they build in different delays. Same thing here.
By contrast and remaining at CERN, there are times when it all goes right, for example the discovery of the Higgs Bison. This too called for massive equipment, sure enough, but also for two incredibly delicate experiments to reach comparable findings after painstaking observation.
And there are plenty of other fun cases. Is the expansion of the universe slowing up, speeding up, or neither? You can only find out by astoundingly exact measurements, backed by daring theory, of faint supernovae billions of light-years away.
So although our big picture of the universe is a very satisfying and complete one, it depends upon technology and method at the far extremes of our ability as a race. After dodging the Pioneer Anomaly and the Superfast Neutrino scare, we still have to face the possibility that one day, one of these anomalous measurements may just fail to go away.