by Frank Close
Electric forces operate on the familiar maxim ‘like charges repel; unlike charges attract’. Thus, negatively charged electrons are held in their paths in atoms by the electrical attraction to the positively charged central nucleus.
Charges in motion give rise to magnetic effects. The north and south poles of a bar magnet are an effect of the electrical motions of atoms acting in concert.
The electromagnetic force is intrinsically much more powerful than gravity; however, the competition between attractions and repulsions neuter its effects over large distances, leaving gravity as the dominant effect at large. However, the effects of swirling electric charges in the molten core of the Earth cause magnetic fields to leak into space. A compass needle will point to the North Pole, which may be thousands of miles distant, due to such effects.
It is the electromagnetic force that holds atoms and molecules together, making bulk matter. You and I and everything are held together by the electromagnetic force. When the apple fell from a tree in front of Isaac Newton, it was gravity that guided it; but it was the electromagnetic force – responsible for making the solid ground – that stopped it continuing down to the centre of the Earth. An apple may fall for many seconds from a great height, accelerated by the force of gravity. But when it hits the floor, it is stopped and turned to pulp in an instant: that is the electromagnetic force at work.
Here is an idea of the relative strengths of the two forces. In a hydrogen atom are a negatively charged electron and a positively charged proton. They mutually attract by their gravity; they also feel the attraction of opposite electrical charges. The latter is 1040 times stronger than their mutual gravity. To give an idea of how huge this is, consider the radius of the visible universe: it has been expanding at a fraction of the speed of light, about 1016 metres per year, for some 1010 years since the Big Bang, so the whole universe is at most 1025 metres in extent. The diameter of a single proton is about 10–15 metres. So 1040 is even bigger than the size of the universe compared to the size of a single proton. Clearly we can safely ignore gravity for individual particles at present energies.
The attraction of opposites holds the electrons in atomic paths around the positively charged nucleus, but the repulsion of like charges creates a paradox for the existence of the nucleus itself. The nucleus is compact, its positive electrical charge due to the many positively charged protons within it. How can these protons, suffering such intense electrical repulsion, manage to survive?
The fact that they do gives an immediate clue to the existence of a ‘strong’ attractive force, felt by protons and neutrons, which is powerful enough to hold them in place and resist the electrical disruption. This strong force is one of a pair that act in and around the atomic nucleus. Known as the strong and weak, their names referring to their respective strengths relative to that of the electromagnetic force on the nuclear scale, they are short range forces, not immediately familiar to our gross senses, but essential for our existence.
The stability of the nuclei of atomic elements can be a delicate balance between the competing strong attraction and electrical repulsion. You cannot put too many protons together or the electrical disruption will make the nucleus unstable. This can be the source of certain radioactive decays, where a nucleus will split into smaller fragments. Neutrons and protons feel the strong force equally; only the protons feel the electrical repulsion. This is why the nuclei of all elements other than hydrogen contain not just protons, but have neutrons to add to the strong attractive stability of the whole. For example, uranium 235 is so called because it has 92 protons (which define it as uranium due to the 92 electrons that will neutralize the atom) and 143 neutrons, making a total of 235 protons and neutrons in all.
At this point you might wonder why nuclei favour any protons at all, as an excess of neutrons doesn’t seem to lead to instability. The answer depends in detail on quantum mechanical effects that are beyond the scope of this book, but a major part is due to the extra mass of a neutron relative to a proton. As we saw earlier, this underlies an intrinsic instability of neutrons, whereby they can decay, turning into protons and ejecting an electron – the so-called ‘beta’ particle of ‘beta radioactivity’.
The force that destroys a neutron is the weak force, so called because it appears weak by comparison to the electromagnetic and strong at room temperatures. The weak force disrupts neutrons and protons, causing the nucleus of one atomic element to transmute into another through beta radioactivity. It plays an important role in helping convert the protons – the seeds of the hydrogen fuel of the Sun – into helium (the process by which energy is released, eventually emerging as sunshine).
The gravitational attractions among the multitudinous protons in the Sun pull them inwards until they are nearly touching. Occasionally two move fast enough to overcome their electrical repulsion momentarily, and they bump into one another. The weak force transmutes a proton into a neutron, the strong force then clumps these neutrons with protons, after which they build up a nuclei of helium. Energy is released and radiated courtesy of the electromagnetic force. It is the presence of these four forces and their different characters and strengths that keep the Sun burning at just the right rate for us to be here.
In ordinary matter, the strong force acts only in the nucleus and fundamentally it is due to the presence of the quarks, the ultimate basic particles from which protons and neutrons are formed. As the electric and magnetic forces are effects arising from electric charges, so is the strong force ultimately due to a new variety of charge, which is carried by quarks but not by leptons. Hence leptons, such as the electron, are blind to the strong force; conversely, particles such as protons and neutrons that are made of quarks do feel the strong force.
The laws governing this are fundamentally similar to those for the electromagnetic force. Quarks carry the new charge in what we can define to be the positive form, and so antiquarks will carry the same amount but with negative charge. The attraction of opposites then brings a quark and an antiquark together: hence the bound states that we call mesons. But how are baryons, which are made of three quarks, formed?
It turns out that there are three distinct varieties of the strong charge and to distinguish among them we call them red (R), blue (B), and green (G). As such they have become known as colour charges, though this has nothing to do with colour in its familiar sense – it is just a name. As unlike colours attract, and like repel, so would two quarks each carrying a red colour charge, say, mutually repel. However, a red and a green would attract, as would three different colours, RBG. Bring a fourth quark near such a trio and it will be attracted to two and repelled by the third which carries the same colour charge. The repulsion turns out to balance the net attraction such that the fourth quark is in some sort of limbo; however, should it find two other quarks, carrying each of the two other colour charges, then this trio can also tightly bind together. Thus we begin to see the attractions of trios, as when forming protons and neutrons, is due to the threefold nature of colour charges. As the presence of electric charges within atoms leads to them clustering together to make molecules, so do the colour charges within protons and neutrons lead to the clusters that we know as nuclei.
The underlying similarity in the rules of attraction and repulsion give similar behaviour to the electromagnetic and strong forces at distances much less than the size of an individual proton or neutron; however, the threefold richness that positive or negative colour charges have in comparison with their singleton electric counterparts leads to a different behaviour in these forces at larger distances. The colour-generated forces saturate at distances of around 10–15 metres, the typical size of a proton or neutron, and are very powerful, but only so long as the two particles encroach to within this distance – figuratively ‘touch’ one another – hence the colour-induced forces act only over nuclear dimensions. The electromagnetic force, in contrast, acts over atomic dimensions of some 10–10 metres when building stable atoms, and can even be felt ove
r macroscopic distances, as in the magnetic fields surrounding the Earth.
This brings us naturally to the question of how forces spread their effects across space.
22. Attraction and repulsion rules for colour charges. Like colours repel; unlike colours can attract. Three quarks each carrying a different colour attract to form a baryon. A quark and an antiquark carry opposite colours and can also attract to form a meson.
Force carriers
How do forces, such as the electromagnetic force, manage to spread their effects across space? How does a single proton manage to ensnare an electron that is 10–10 metres away, thereby forming an atom of hydrogen? Quantum theory implies that it is by the action of intermediate agents – the exchange of particles; in the case of the electromagnetic force these are photons, quantum bundles of electromagnetic radiation, such as light.
Electric charges can emit or absorb electromagnetic radiation, and its agents, photons; analogously the colour charges also can emit and absorb a radiation, whose agents are known as gluons. It is these gluons that ‘glue’ the quarks to one another to make protons, neutrons, and atomic nuclei. The weak force analogously involves force carriers known as W or Z bosons.
The W boson differs from the photon in two important ways: it has electric charge and a large mass. Its electric charge causes its emission to leak charge away from the source – thus a neutral neutron turns into a positively charged proton when a W– is emitted; this is the source of neutron beta decay, the W– turning into an electron and neutrino. The W mass is some 80 times greater than that of a proton or neutron. If you were in a car weighing one tonne and suddenly 80 tonnes were ejected, you would complain that something was wrong! But in the quantum world this kind of thing can happen. However, this violation of energy balance is ephemeral, limited in time such that the product of the imbalance, Delta-E (ΔE), and the time it can last, Delta-t, (Δt) cannot exceed Planck’s quantum h, or numerically ΔE × Δt 6 × 10–25 GeV – sec. This restriction is one form of the ‘Heisenberg Uncertainty Principle’.
This means that for one second you could overdraw the energy account, or ‘borrow’, the trifling amount of 10–25 GeV. ‘Borrowing’ 80 GeV (the minimum energy to make a single W) can occur for some 10–24 s, during which time not even light could travel more than about one-tenth of the distance across a proton. Hence the distance over which the W can transmit the force is considerably less than the size of a single proton. So the short-range nature of the weak force is due to the excessively large mass of its carrier particle. Now, this is not a statement that the force exists up to a certain point and then turns off suddenly; instead it dies away and its strength has fallen away radically by distances of the order of a proton size. It is at such distances where beta decay is manifested, and it is thus that the force became known as ‘weak’.
In 1864 James Clerk Maxwell had successfully unified the disparate phenomena of electricity and magnetism into what we today call electromagnetism. A century later, Glashow, Salam, and Weinberg united the electromagnetic force and the weak force into what has become known as electroweak theory. This explained the apparent weakness of the ‘weak’ component of this unified force as due to the large mass of the W, whereas the photon of the electromagnetic force is massless. Their theory would only work if in addition to the electrically charged W+ and W–, there was a heavy neutral partner, the Zo, with a mass around 90 GeV. One implication of their work was that if one could provide enough energy, of the order of 100 GeV or more, whereby the W or Z could be produced directly in the laboratory, one would see that the force has a strength akin to that of the electromagnetic, and is not excessively weak after all. Such experiments have been done and confirmed this phenomenon.
23. Beta decay via W: a neutron converts to a proton by emitting a W, which then turns into an electron and neutrino.
24. The relative strengths of the various forces when acting between fundamental particles at low energies typical of room temperature. At energies above 100 GeV, the strengths of the weak and electromagnetic forces become similar. The carriers of the forces are shown: the gluons, photon, and graviton are all massless; the W+, W–, Zo are massive. Examples of entities that have special affinity for the various forces are also shown.
The W and Z were discovered at CERN in 1983–4, where they were fleetingly produced among the debris arising from the high-energy, head-on collisions between protons and antiprotons. Such collisions produce large numbers of pions and only rarely is a single W or Z produced. This led to a dedicated accelerator, LEP, where counter-rotating beams of electrons and positrons are mutually annihilated, tuned to a total energy of 90 GeV. This energy matches that of a Z at rest, and so LEP was able to produce Z particles cleanly. During a decade of experiments, over 10 million examples of Z were made and studied. These experiments proved that the concept of the merging of electromagnetic and weak forces into a single electroweak force is a correct one. It is the large masses of the W and Z that gave the apparent weakness when they were involved in historical experiments, at energies far below 100 GeV, such as in beta radioactivity.
Finally we have the strong force, whose origins are the colour charges carried by quarks or antiquarks. In this case the force is transmitted by ‘gluons’. As a quark can have any of three colours, labelled R, B, or G, the gluon radiated can itself carry colour charge. For example, a quark with charge R can end up carrying colour B if the gluon carries a charge that is like ‘positive R, negative B’. The relativistic quantum theory, known as quantum chromodynamics, or QCD, allows for a total of eight different colours of gluons.
As gluons carry colour charge, they can mutually attract and repel as they travel across space. This is unlike the case for photons when transmitting the electromagnetic force. Photons do not themselves carry (electric) charge, and so do not mutually suffer electromagnetic forces. Photons can voyage across space independently, filling all the volume, the intensity of the resulting force dying out as the square of the distance – the famous ‘inverse square law’ of electrostatics. Gluons, carrying colour charges, do not fill space in the same way as photons do. Their mutual interactions cause the ensuing force to be concentrated in a line, along the axis connecting the two coloured quarks.
So while photons fill space and travel independently, the gluons cluster. One consequence of this clustering is the possibility that gluons mutually attract to form short-lived composite states known as glueballs. It is this mutual affinity amongst gluons while they are transmitting the force that causes the long-range behaviours of the electromagnetic and colour (strong) force to differ radically. The electromagnetic force dies with the inverse square of the distance; the colour forces do not. The energy required to pull two colour sources, such as quarks, apart grows with distance. At a separation of around 10–15 metres this energy tends to become infinite. Thus individual quarks cannot be separated from their siblings; they remain clustered in the trios, such as baryons, or quark and antiquark, as in mesons. It is thus that the effects of colour charges become ‘strong’ at large distances.
At short distances, as are probed in experiments at high energy, the electroweak and colour forces appear to act as if exhibiting a grand unity. It is at lower energies, such as were the norm until the latter part of the 20th century, that they exhibit their different characters: the massive W and Z causing an apparent weakness; the mutual interactions among the gluons causing the colour force by contrast to take on great strength.
That much we know. Extrapolate the effects of the colour forces, and of the weak and electromagnetic to extreme energies, far beyond what we can measure in the lab, and it appears that all three become alike. The behaviour of atomic particles at very high energies, akin to those that were abundant just after the Big Bang, suggests that the colour forces are enfeebled, and similar in strength to the familiar electromagnetic force. A tantalizing hint of unity has emerged. This is known as grand unification of the forces. It suggests that there is an underlying simp
licity, unity, to Nature and that we have only glimpsed a cold asymmetric remnant of it so far. Whether this is really true is for future experiments to test.
Chapter 8
Exotic matter (and antimatter)
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Nature has a three-party system – ‘generations’. We look at antimatter, and the mystery of why there is so little of it; symmetry between generations which appear effectively identical but for their different masses; ideas that the multiple generations might have something to do with the disappearing antimatter; experiments that are trying to find out if this is so; and strange matter.
* * *
Strangeness
We have met the basic particles from which matter on Earth is ultimately made. However, in Nature’s scheme, there is more than this. Cosmic rays from outer space are continuously hitting us. These consist of the nuclei of elements produced in stars and catastrophic events elsewhere in the cosmos; they hurl through space and some, trapped by the magnetic fields of the Earth, hit the upper atmosphere and produce showers of secondary particles. In the 1940s and 1950s, cosmic rays provided an active source of discovery of forms of matter that had not hitherto been known on Earth. Some of these had unusual properties and became known as ‘strange’ particles. Today we know what distinguishes them from the familiar protons, neutrons, and pions: they contain a new variety of quark, which has become known as the strange quark.
