You are hovering some planet in a galaxy far far away, uncertain whether it is made of matter or antimatter and hence whether or not it will be safe to land. The planet is inhabited by friendly aliens with whom you have made radio contact. They are very intelligent and understand you, and being advanced, know all about matter and antimatter.

     Naturally, they insist that they are made of matter; after all, it would be surprising if anyone chose to define their own stuff as ‘anti.’ How can we decide if their dictionary and ours coincide? What questions will unambiguously reveal whether they are made of the same stuff as us, or are anti-aliens?

     If matter and antimatter were always perfectly symmetrically counterpoised, there would be no way to settle the issue, other than gambling with a close approach of firing a tiny unmanned probe and seeing what  happens when it hits the atmosphere or anti-atmosphere. However, we know that there is an asymmetry, small but measurable, and that is what the electrically neutral variety of K mesons can reveal. They do so when they decay, producing a pion that is either positively or negatively charged accompanied by an electron or positron respectively. If matter and antimatter were perfect opposites, these two decays would also be precisely matched, the chance of each being the same. In reality, they are slightly different.

     The neutral K and anti-K are welded together in nature in such a way that they sometimes die quickly, but at other times live longer. The two possibilities are quite distinct and are known as the short- and long-lived versions. Each of these shows an asymmetry between matter and antimatter, but it is the long-lived one where the effect is biggest, they decay that leads to a positron being slightly more likely to happen than giving an electron: out of every two-thousand examples, on the average, 1,003 will give a positron and 997 give an electron. Now at last we have something to discuss with the alien.

     First, identify the K. It is no use giving its name, since the alien will certainly call it something else, but we can identify it by something we will agree about: its mass. It weighs in at slightly more than half the mass of a proton or antiproton and there are no other particles than can be confused with it. So tell the alien that we are interested in a particle whose mass is slightly more than half that of the massive particle that exists in the ‘nucleus’ at the center of the alien’s simplest atom, the proton in the hydrogen atom (or antiproton in an atom of antihydrogen.) That identifies the K.

     In addition to the neutral K, with no electric charge, there are also a K-plus and K-minus with positive or negative charge. So we much make sure that the alien and we are talking about the electrically neutral version. We must say that the property that holds the atom together is what we call ‘charge’ and that we are interested in the K that has no charge. The alien will be aware that this neutral K has two forms: one with a short life and one with a relatively long one. It is the latter that we will focus on.

     Now we come to the critical bit. In our world of matter, when the long-lived K decays into a pion and an electron or positron, it is the positron mode that is the most likely. So we ask the alien: ‘Is the lightweight particle that is produced most often in these decays the same as you find in your atoms, or is it the opposite?’ If the alien answers that it is the same, it is a positron, the alien is made of antimatter and we should look but not touch. If the alien replies that it is the opposite, an electron, then we are all made of matter and it is safe to land.

Antimatter, Frank Close

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   such a good fucking book. Frank Close is a bad ass.

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