Why neutrinos "go through anything and everything"
The ties that bind
To understand the bizarre behaviour of the neutrinos, it is best to start by examining the different ways that matter "sticks together".
Yet gravity is actually the weakest of the forces. It dominates everything on a large scale because it always attracts: everything attracts everything else and there is no way to "shield" oneself from this effect.
The other forces which hold matter together are different: they form small clumps which are very tightly bound, but have only a weak effect on the world outside. The most famous of these clumps of matter is the atom:
Atoms, and the electromagnetic interaction
An atom is held together by electromagnetic interactions. The tiny nucleus, containing 99.98% of the mass of the atom, defines its centre, and the attraction between this positively-charged centre and the negatively-charged electrons binds the electrons within the atom. (The cartoon of a carbon atom shown here is purely schematic - electrons do not travel in neat orbits within an atom and can't really be said to travel along a path at all - one just has to draw them somehow.) Objects with a positive charge would be repelled, rather than attracted, by the nucleus.
The nucleus, and the "strong force"
Electrons and other leptons (such as muons and neutrinos) do not participate in the strong nuclear interaction. This accounts for the vast difference of scale within the atom:
- the electrons, bound to the atom by the electromagnetic interaction only, are confined within a distance of a ten-billionth of a metre or so;
- the protons and neutrons, bound together by the strong nuclear interaction, are confined within a distance of about a hundred-trillionth of a metre: ten-thousand times smaller.
This extra interaction also means that a "free" particle which is strongly interacting - a proton, say, or a pion - will not travel very far in matter, no matter how much energy it possesses. Sooner or later the particle will interact with a nucleus, losing most of its energy either in a violent scattering, or in producing a spray of lower-energy particles: this happens about every 70 metres in air, and within only 17 cm in iron. A neutron, which has no electromagnetic interaction with its surroundings, will also be stopped in this way.
What we previously called the "strong nuclear force" is a residual force, the left-over "clingy-ness" between the colour-charged constituents of the protons and neutrons. Between protons in a nucleus, this clingy-ness is about twenty times as strong as the electromagnetic interaction; at distances much larger than that, a proton or neutron looks completely "colourless" or "colour-neutral" and there is no strong interaction at all.
(As usual, the cartoon of the proton shouldn't be taken too seriously. While there are certainly three quarks confined within the proton, the proton is much more complicated than that. For a start, the strong interaction is mediated by quanta [units or "particles"] called gluons - "pieces of force" if you like - which are themselves colour-charged, and which carry much of the momentum, and most of the spin, of the proton. The strong interactions within a proton, or any other hadron, are lively and complicated.)
Neutrinos and the "weak nuclear force"
Neutrinos do not participate in any of this:
- As leptons, they do not feel the strong force, and so they are not bound into the nucleus.
- Electrically neutral, they are not bound in the atom by the electromagnetic interaction.
They thus have no place in the small-scale, stable structures of matter. Neither will they interact with matter when they pass through it: since they have no charge, they don't lose energy by displacing electrons or scattering from nuclei, and since they don't feel the strong force, they don't ever "run into" a nucleus the way a neutron eventually does.
The only way a neutrino can interact is through the so-called weak nuclear force, the last of the four types of interaction. The more colloquial term "force" is inappropriate: the weak nuclear interaction does not bind anything together, or bend the path of particles in flight. It is the way in which one of the fundamental particles changes into another.
On the rare occasions that a neutrino (or an anti-neutrino) interacts with matter, it does so through a similar process:
Suppose that the neutrino is of the electron-type. It turns into an electron, emitting a W-plus, which strikes a down quark and turns it into an up quark: a neutron (two down plus one up) has been turned into a proton (one down plus two up quarks). The process is something like a forced beta-decay, induced by the neutrino.
This inverse-beta process is almost unbelievably rare. A neutrino will pass through air, water, solid rock, even metal, as if it were not there: if the whole universe were filled with water, for example, neutrinos would travel 100 light years (on average) before interacting in this way. Therefore most of the neutrinos produced by the sun travel straight through the earth, which is a mere twelve-thousand kilometres thick. Most, but not all: a tiny fraction of the neutrinos interact with the rock (or air, or whatever).
There is no "magic" to this, but rather the relative strength of the interactions: of the three types of interaction, strong, electromagnetic and weak, the weak interaction is by far the most feeble - in this context, this means "the least likely to actually occur" - and this is the only type of interaction of which neutrinos are capable.
Once again gravity
Well, not quite. We've been ignoring gravity. Everything is subject to the gravitational interaction - absolutely everything, including neutrinos. One would never suppose this to be relevant, except that neutrinos are extremely common, as common as electromagnetic radiation: light, radio waves, etc. We expect there to be between 300 and 600 neutrinos in every cubic centimetre of space, "left over" from the Big Bang, and long since thinned-out and cooled-down as the universe has expanded.
If neutrinos are massless, then only the miniscule kinetic energy of these relic neutrinos would act on their surroundings by gravity. But if neutrinos have a mass - even a hundred-thousandth of the mass of the electron, otherwise the lightest of the particles - their gravitational influence may be helping to hold the universe together.
There seems to be more matter in the universe than the matter we can see: the "missing" material goes by the somewhat dramatic name of dark matter. For example, in a spiral galaxy like our own Milky Way, which rotates about its centre, the speed of rotation provides a measure of how much matter the galaxy contains - the faster the rotation, the more mass which must be present to hold the galaxy together. (By way of comparison: if the moon took two weeks to complete its motion around the earth rather than one month, the earth would have to be four times as massive as it actually is, in order to hold the moon in its present orbit.)
When we do measure the rate at which our galaxy rotates, and compare this with the rate we expect by counting up all the stars, gas and dust, we find that the galaxy is rotating too fast. There is matter there that we can't see - probably in a large, spherical cloud extending out past the visible "rim" of the galaxy - and the same is true of other galaxies, clusters of galaxies and so on.
The diagram above shows the "rotation curve" for our galaxy. The vertical axis shows the speed of rotation (in kilometres per second), the horizontal axis shows the distance from the centre of the galaxy (in kiloparsecs, a unit equal to 3262 light-years, or 31 thousand million million kilometres).
The dotted line shows the speed of rotation we'd expect if the visible matter was all there is. The solid line shows the actual speed of rotation.
There are other more subtle reasons to suppose that there is dark matter out there - and there may be more than one "type". In any case, neutrinos-with-mass are one of the important candidates, and the dark matter problem is one of the driving forces behind the search for neutrino oscillations - which can only take place if neutrinos have mass. This is especially true of an experiment like NOMAD, which is most sensitive to neutrinos with masses which are "cosmologically interesting", as they say.
Whether neutrinos make a significant contribution to the dark matter remains to be seen. If true, it would be remarkable that these elusive particles should play so vital a role on so large a stage.
About this document ...
This material was originally prepared by Bruce Yabsley for a presentation at the MLC School for Girls, Burwood, in July 1996.
The galaxy image is a Hubble Space Telescope picture of the spiral galaxy M100, provided by the Space Telescope Science Institute (operated by the Association of Universities for Research in Astronomy, Inc., from NASA contract NAS5-26555), and is reproduced with their permission. The full-size image and others are freely available (with acknowledgement) from their Web site.