The Unexpected Muon

On the edge of space, a naked proton that has wandered the universe for hundreds of millions of years manages by a statistical fluke to pass through the relatively tiny target that is the Inner Solar System.  The average distance of the Earth from the Sun is 8.32 light-minutes: gaps between stars are typically three or four light-years in our region of the galaxy.  The Inner Solar System is 0.0000000174% of the stellar neighbourhood, or about than 1 in 5,760 million.

It is rare for an individual wandering proton to pass through the Inner Solar System.  Even more flukish for it to happen to hit our planet.  The Earth viewed a circular target is just 0.000000000659% of the Inner Solar System, or less than 1 in 157 billion.   And yet free protons manage to hit us all the time.  They do this because they exist in enormous numbers, despite space being a better vacuum that the one you’d find in the vacuum flasks that we use to keep drinks warm.  Atoms are small.  Vast numbers of them exist in what is a vacuum from a human viewpoint.

Our everyday world contains far more atoms than most people realise.  A litre of water contains 130,000,000,000,000,000,000,000,000 atoms.  A litre of air at the Earth’s surface contains 26,520,000,000,000,000,000,000.  Outer space, though relatively empty, still contains enough free particles for some of them to hit us.

This proton – identical to the atomic nucleus of an atom of ordinary hydrogen, but in this context called a Cosmic Ray – slams into the atmosphere at very high velocity.  It hits the nucleus of another atom that is part of our atmosphere.  Which atom it hits barely matters: proton meets proton with enough energy to disrupt or abolish the unobservable quarks within each proton.

The velocity of cosmic rays vary a lot, but can come close to the speed of light.  Let’s say that this one comes in at about 200,000 kilometres per second.  That’s a speed way outside of human experience: the famous supersonic Concorde aircraft flew at less than one kilometre per second.  Humans returning from the moon broke all previous speed records by moving at just over 11 kps.  Cosmic rays – which are various things, but most commonly protons – go a lot faster than that.

The result of the proton-proton collision is an infinitesimal moment of pure undefined energy, similar to the very first moments after the Origin Event (Big Bang).  Then it transforms into something more regular: specifically, a bizarre particle called a pion.  The pion rapidly decays into something stranger but less unstable; a muon, along with a neutrino.

The neutrino will almost certainly pass undisturbed through the entire solid Earth and out into interstellar space, most likely never again encountering normal matter.  The muon will pass through the atmosphere; pass through you if you happen to be in the way.  Solid rock or metal will occasionally stop it, but mostly not.  It will normally perish deep underground, not because it hits anything but because it is unstable.  Its proper lifetime is 2.2 microseconds, but its speed is close enough to light-speed that time slows for it: from our viewpoint it lasts much longer.  The muon that derived from a pion that derived from an impacting cosmic ray will end its brief existence by becoming an electron and two more neutrinos.

How often does this happen?  Continuously.  Some ten thousand muons reach every square meter of the earth’s surface every minute.  Each has an energy of more than a hundred million electronvolts.  But don’t let that worry you: the kinetic energy of a single flying mosquito is much larger, a full trillion electronvolts.  It would take the entire energy of a million million muons to light a modern bicycle lamp for a second.  Muons and other subatomic particles are amazingly tiny.  Muons from cosmic rays are part of the background radioactivity that we evolved with.  Radiation that sometimes damaged the DNA of living creatures and caused random mutations, a few of which were useful for the eventual emergence of complex life and then ourselves.

It is possibly that without muons or something similar, we would not be here.  There are of course other sources of radiation: one is ordinary potassium, which makes bananas measurably but harmlessly radioactive.  It might be that the extra radiation from muons was necessary to tip the balance in favour of complex life.  More probably not, but no one truly knows.

Muons must have existed ever since the universe became cool enough to allow them.  (They can be created by other processes besides the one described above.)  But they made no sense before the Standard Model of subatomic particles was put together in the 1960s and 1970s.  Until then they were an oddity.  An anomaly that produced the famously comic comment ‘Who ordered that?’ from a noted subatomic theorist.

No one ‘ordered’ the muon: it was unexpected and unwanted.  As surprising as if a dish of fried rabbits-ears arrived at the table of a group of British or US friends having a celebration in a Chinese restaurant.  But restaurant dishes only appear if the menu offers them and then someone orders them (unless the restaurant is pulling a spoof).  That’s human culture: muons come from the other side of reality, the things that existed long before we did.  Things which are also utterly independent of our observations, and would be exactly the same if we had never existed.

Some aspects of subatomic physics have been interpreted as meaning that they require a human observer to be real: I’ll go into this in detail later on.  For now, I want to emphasise that many things in subatomic physics were not looked-for or expected.  Yet they were real.  They forced themselves into our vision of reality by their surprising and mostly unwelcome existence.


The famous New Zealand physicist Ernest Rutherford had worked out that the atomic nucleus was a collection of protons, positively charged particles.  You might have wondered what would hold them together.  If you know basic physics, you’d know that identical charges repel.  North repels North for magnets.  Positive repels positive for electricity.  So theorists correctly deduced that some other unknown force held the nucleus together.  And that it was probably very short-range.  It had to be something that would not be seen in the normal world, so we would not previously have had any hint of its existence.

This was in fact the Strong Nuclear Force.  (Confusingly, there was another unrelated force also involved in atomic nuclei, inelegantly known as the Weak Nuclear Force.)  Its nature remained obscure until the 1970s, when it was discovered to be just one aspect of something called the Strong Interaction or Colour Force that operates between quarks.  But it was possible to work out quite a lot about the Strong Nuclear Force without fully understanding it.  Specifically; in 1935 a Japanese physicist called Hideki Yukawa worked out that this force would be carried by a particle with a mass intermediate between the electron and the proton.  Electrons have a mass that is about 1/1836 that of the proton, a difference that remains unexplained.  But on the basis of the radius of the atomic nucleus, Yukawa worked out that this particle would have a mass somewhere between the two.  For this reason the name mesotron was suggested, ‘middle particle’.  But meson was preferred.

When the oddities now known as muons turned up in cosmic rays, they seemed to be this predicted particle.  The mass was about right, so they were called mesons.  But further work showed that these ‘mesons’ were not behaving as expected.  As I said earlier, it was as if they had predicted the existence of dogs but then encountered cats.  These ‘mesons’ were not what had been expected.  They ignored the Strong Nuclear Force, meaning that they could not be the ‘Yukawa particle’.

Then in 1947, careful studies of cosmic rays turned up something else: very short-lived mesons that gave rise to the known mesons.  When examined, this new type of meson did fit Yukawa’s predictions.  The original ‘meson’ was renamed the mu-meson.  The new one became the pi-meson.

The names were later changed again.  The ‘pi-meson’ turned out to be one a large class of particles composed of pairs of quarks.  (Protons and neutrons are made of different combinations of three quarks.)  The ‘pi-meson’ is now known as the pion, one of many mesons made of different paired quarks.  The muon is no longer called a meson, because it was something quite different; a heavyweight relative of the electron.

The muon was the first human discovery of a portion of something unexpected and unwanted: the Second Generation of Elementary Particles.  These are weird overweight alternatives to the ordinary particles that make up the familiar world.  And there is also a Third Generation, even heavier and much rarer.

As far as anyone can tell, our universe could work fine without these extra ‘generations’.  It’s a puzzle that they exist.  To be exact, there is no confirmed theory of particle physics that requires the existence of Second Generation or Third Generation particles in order for a universe like ours to exist.  They do influence the behaviour of First Generation particles.  It might be that without them, the early universe would have produced exactly equal amounts of matter and anti-matter and would have ended in mutual annihilation with nothing left to form stars or planets.  Or some other vital relationship might depend on the extra particles being there.  But no one currently knows.

It seems to me that the unexpected appearance of the muon and the entire Second and Third Generations of particles discredits the popular notion that subatomic particles are only there because humans observe them or were expecting them.  Things happen in the subatomic realm that contradict our common sense: but that’s probably because ‘common sense’ was developed by people dealing just with solids, liquids and gases on a human scale.  The basic notion that things exist whether or not you notice them is fundamental to our understanding of the material world.  And it should not be confused with the social world, where everything is more complex and where some things genuinely don’t exist unless humans accept them as existing.  The pattern of stars seen from Earth in independent of human will: but they could have been 68 constellations or 128 constellations rather than the 88 officially recognised by astronomers.


I said earlier what a muon is not.  To say more about what it is requires some wider explanations.  Starting with more about quarks, the extraordinary objects that are believed to lie behind the normal world of matter, yet never appear directly.

The idea of quarks came from two sets of data.  Firstly, experiments known as ‘deep inelastic scattering‘ (and modelled on Rutherford’s original idea of probing the atom with alpha particles).  These produced subtle indications that there were smaller and denser objects within each proton.  And secondly from the discovery in cosmic rays and in particle colliders of short-lived particles similar to but distinct from the protons and neutrons that made up the atomic nuclei of normal matter.  All of these strange particles and their properties could be neatly explained on the basis of three ‘flavours’ of quark, called Up, Down and Strange.  Protons and neutrons and various similar particles were composed of various combinations of three quarks, with protons being two Up quarks and a Down, while a neutron was one Down and two Ups.  The other six known ‘baryons’ had some mix including a Strange quark.  Various combinations of quarks could also pair up as mesons, with the Pion being one Up quark and one Down Quark.  All very satisfactory and logical, yet baffling because it was unclear why it existed.  The man responsible for this system even called it the Eightfold Way, alluding to the Noble Eightfold Path of Buddhism.

Then they found a fourth flavour of quark.

The existence of a fourth quark, the Charmed Quark, was proposed in relation to something called the GIM mechanism.  It was then proved to exist when they found something that came to be known as the J/psi meson.  This new meson only made sense if you assumed it was composed of a pair of Charmed quarks, one of them an anti-particle.  This happened in 1974, and during the 1970s the Standard Model of particles was put together from various bits of evidence and some deeper theoretical exploration.  The Standard Model tied everything together even more neatly than the three-quark model.  But in an unwanted complication that experimental results imposed upon theorists, it was found it had to have three generations of particles.  I mentioned these earlier: here I need to say more about them.

The First Generation consisted of four components: the Up and Down quarks, the electron and the neutrino.  The Second Generation had another four: the Strange and Charmed quarks, along with the muon and its own distinctive variety of neutrino.  And beyond this, there was already evidence for a fifth quark, for which the name Beauty was proposed.  Scientists typically dislike using soft and romantic terms for their ideas, so instead of Beauty the name Bottom quark became the standard.  It had a heavier partner, the Top quark, and also a Tau electron and Tau neutrino, making up the Third Generation.

The muon finally made sense: it was a ‘heavy electron’ that belonged in the Second Generation, along with the Strange and Charmed quarks.

It is not known why these generations exist ,or why there are exactly three of them.  It is pretty definite that three is the limit: a Fourth Generation would produce visible effects on various interactions if it existed, just as the existence of the Second Generation and Third Generation were deduced before direct evidence was found.  Since these effects are not observed, it is assumed that there is no Fourth Generation.

(Assumed rather than solidly proven, because it is remotely possible that a fourth generation exists, but manages somehow to avoid being observed.  Just as free quarks can not be observed under conditions that humans can create.)

Note also that no one thinks that the Standard Model is the complete answer.  It gives accurate answers if one starts from various known facts, but gives no indication as to why these are facts.  The current position is perhaps similar to 19th century chemistry, when it was known what the chemical elements would do, though not why.  Where unknown elements could be predicted using the Periodic Table.  In the 19th century, the rules seemed arbitrary.  Only when it was known that atoms were composed of electrons, protons and neutrons did they start to make sense.

(Which is not to say that the subatomic particles are necessarily made up of something still smaller.  There are a large collection of theories that try to do this, mostly using the term ‘preon’ for the supposed subcomponents.  {Other suggeted names include prequarks, subquarks, maons, alphons, quinks, rishons, tweedles, helons, haplons, Y-particles and primons.}  Since quarks have exactly one-third or two-thirds of the charge of an electron, the obvious approach is to believe in ‘preons’ that have this one-third charge.  Logically, three of them would make an electron, or else would make a muon in combination with something else that would explain the muon being more massive.  And that’s the sort of complication that burdens all attempts at a successful ‘Preon Model’.  The true answer may be something so radical that no one has yet thought of it.)

Despite the incompleteness of the Standard Model, we do at least know the difference between a pion and a muon.  A pion is a meson composed of a quark and an anti-quark.  In more detail, a pion is any of four rather similar things: an Up quark paired to a Down anti-quark, an Up anti-quark paired to a Down quark, an Up quark paired to a Up anti-quark, or a or a Down quark paired to a Down anti-quark.  Only the first two, Charged Pions, give rise to muons.  An Up or Down quark paired with its own anti-particle is a Neutral Pion.  A Neutral Pion is much harder to detect, and most likely to decay into a pair of gamma-ray photons, or possibly a photon and two electrons.

(You may be wondering why the quark and the anti-quark do not annihilate each other, since anti-matter annihilates ordinary matter.  The answer is that they do, but it takes time.  Something else may happen first.  Also a quark can engage in mutual annihilation with an antiquark only if they are of the same ‘flavor’.  An Up quark cannot annihilate an anti-Down quark, for instance.)

Pions decay into muons, but the muon is nothing like a pion.  Muons and electrons are leptons, as are the tau electron and the three known types of neutrino.  Leptons are quite unlike quarks, but quarks and leptons form part of a larger class of particles called fermions.  The other known category is bosons, which include photons and the recently discovered Higgs Boson.

Remarkably, subatomic particles readily transmute into other very different particles, if the total energy is similar and values like charge and ‘strangeness’ can be conserved.  A proton (two Up quarks and a Down quark) hits another proton, and the result is a Charged Pion, either an Up quark and a Down anti-quark or else the other way round.  This then decays into something utterly different, the muon, the inexplicable heavy cousin of the electron.  Or the original collision may produce a Neutron, which mostly decays into two gamma-ray photons (bosons) or else a photon and two electrons (leptons).  These transformations are possible because the product and result both have much the same energy, while conserving qualities like electrical charge.

Why does it work?  As far as I know, there is no real explanation: just a knowledge that these exchanges do happen and have a consistent and predictable pattern.  It’s as if you purchased something from a department store and then they let you exchange it for something quite different that happened to be at the same price.  A saucepan for a scarf, say, and then exchanged again for three teaspoons.  (Or a brazier for a pair of pillow cases, as in the old joke.)

None of this was at all in line with human expectations.  Looking into the depths below the atom, we have found a wholly baffling world.  A world much stranger than any of the imagined worlds of myth or magic or legend.  Yet this is the basis of our real existence.

This is a from a much longer and more diverse article, Physics and the Nature of Reality,  which has relevant references.  You will also find the sections of this article separated by other observations.
You can read more about constellations here.