Today the space shuttle Endeavor was launched on its final mission to deploy the Alpha Magnetic Spectrometer: a 2 billion dollar orbiting observatory for cosmic ray particles. The launch itself is an amazing technical achievement – and the 8 tonne (!) detector is astonishing in itself. But the discoveries yet to be made with the AMS may prove to be more spectacular still.
Cosmic Rays is a term that was initially used for the highest frequency electromagnetic waves – higher frequency even than gamma rays. But in fact it is still not clear whether or not the rays are indeed waves, or particles. This might seem an amazing degree of ignorance, but in fact, we never see the cosmic ray particles themselves, we only see the debris of their collisions with atoms in our atmosphere. So we have to deduce their properties by examining the debris – like trying to determine the properties of a missile after it has exploded! This is the rationale for putting the detector into space – that short 450 km hop should make a big difference.
Historically, studies of cosmic ray debris has resulted in breathtaking discoveries. In 1912 Victor Hess launched balloon borne experiments and found that levels of ionising radiation increased with height. He deduced the existence of radiation from space and for this he shared the 1936 Nobel Prize. In 1938 Auger and Maze showed that cosmic rays detected 20 metres apart were synchronised, and they deduced that they were seeing secondary ‘showers’ from an initial source. Later they showed that the showers extended over hundreds of metres at ground level, and calculated that the energy of the initial particles sometimes exceeded 1015 electron volts. This was a billion times more energetic than the most energetic particle then available for study and is still a thousand times more energetic than the particles created at CERN today. WOW! And amongst the debris from the collisions, scientists first discovered positrons, muons, pions, and anti-protons. More recent studies have detected particles with an energy exceeding 1020 electron volts which is so energetic that absorbing just a single such particle would warm a cubic centimetre of water by more than 1 °C.
Mysteries. The flux of the low energy cosmic rays is enormous – one or two probably go through your body every second (Yes really!). But the flux of the highest energy cosmic ray particles is very low, below 1 per square kilometre per year, and as I mentioned earlier, the identity of the particles is as yet unknown. The AMS will have to sift through billions of low energy particles (probably protons) before it encounters a single ultra-high energy particle. It will then have to answer the question “What was that?” : electromagnetic wave? proton? iron nucleus? or maybe something new? And hopefully identifying the particle will give us a clue as to where it came from.
Calculations indicate that the Universe as a whole should be ‘opaque’ to particles travelling with energy greater than 1019.7 electron volts. So particles with energy less than this could have come from great distances – even from outside the galaxy. But particles with higher energies, must have come from somewhere much closer. However we have no idea how any combination of electric, magnetic, gravitational and strong fields could combine to make particles travel with quite this much energy.
I am filled with admiration for the scientists and engineers – the thousands of them – that have brought this experiment to life and I eagerly await their results. My hope is history will repeat itself, and that ‘Cosmic Rays’ will surprise us again with something we just hadn’t thought of.
My source for most of this article was a recent review Ultra High Energy Cosmic Rays by Danilo Zavrtanik. (Contemporary Physics 51, No. 6 2010, pp 513-529) which is sadly not available on line.