Do Forces Holding Matter Together?

All the known matter from proton to atoms of liquids, solids and huge conglomerations of matter (in the stars and galaxies) is simply made of three quarks u, d and s. To form all these structures, the underlying particles must work together and interact in some way. They do this through only a few basic interactions, which we can think of as forces. The action of the forces often holds matter together and gives it shape, but sometimes the forces can blow matter apart.

Familiar Forces

The most familiar basic force is gravity. It keeps our feet on the ground and the planets in motion around the Sun. All particles of matter feel the influence of gravity, but on the scale of individual particles, the effects are extremely small. Only when we have matter in bulk as in ourselves, or on larger scales in planets, stars and galaxies then gravity dominate. A much stronger fundamental force manifests itself in the effects of electricity and magnetism. Comb your hair and electric charge builds up on the comb, especially in dry weather. The comb can then attract small pieces of paper, so that you can pick them up, against the force of gravity. On a larger scale, a magnet will lift up pins and nails, while a large industrial magnet can lift scrap metal even cars. These effects are all due to the same underlying electromagnetic force. Unlike gravity this can give rise to both attractive and repulsive effects. Opposite electric charges (positive and negative) and opposite magnetic poles (north and south) attract, but charges or poles of the same type repel each other. The electromagnetic force binds negative electrons to the positive nuclei in atoms, and underlies the interactions between atoms that give rise to molecules and to solids and liquids. It is also ultimately responsible for larger-scale effects, such as surface tensions and friction, which depend on the forces acting on the atoms near the surface of materials.

Unfamiliar Forces

At scales of the size of atomic nuclei and smaller, two unfamiliar forces come into play in the interactions between the basic particles of matter. These are called the weak force and the strong force. The weak force leads to the decay of neutrons, and many other particles, including the pions and muons common in cosmic rays. In ordinary matter in the world about us, the decay of the neutron underlies many natural occurrences of radioactivity. The weak force is also responsible for hydrogen burning in the centre of stars, where hydrogen the lightest element is converted into the second lightest element, helium. In the very hot dense centre of a star, ordinary hydrogen nuclei (protons) can come close enough to form a heavier nucleus of hydrogen, “heavy hydrogen” or deuterium, which consists of a proton and a neutron. This first step in the conversion of hydrogen to helium involves the weak force, as a proton changes into a neutron.

The strong force is responsible for holding quarks together within protons, neutrons and other particles, including pions. It also prevents the protons in the nucleus from flying apart under the influence of the repulsive electrical force between them. (Protons should repel each other as they all have the same positive charge.) This is because within the proton the strong force is about 100 times stronger than the electromagnetic force. The strong force keeps quarks bound within larger particles, so that quarks never appear alone. This is because as you try to pull two quarks apart, the force becomes stronger. This is unlike the more familiar effects of gravity and electromagnetism, where the forces become weaker with distance.

How forces work?

The basic forces, or interactions, between the particles of matter all act through a “force carrier”, which is exchanged between the interacting particles. In this way, interactions between particles are like a game of “catch”, which can either bring the particles together (an attractive force) or push them apart (a repulsive force). There is a different type of carrier for each of the basic forces Photons, the “particles” of light, carry the electromagnetic force. The photons have no mass and no electric charge, and can be exchanged over large distances so that the electromagnetic force is infinite in range. The carriers of the weak force are called W+, W- and Z. The W is electrically charged (W+ and W-), while the Z is neutral (Z0). These carriers are massive, each weighing about 100 times as much as a proton. This makes them difficult to exchange at low energies, so the weak force appears weak. The carriers of the strong force are called gluons. They have no electric charge and no mass, but they carry a special charge called colour that gives them their power to hold quarks together so strongly that the quarks are never seen as individual particles. The force carrier of gravitation called graviton yet not discovered.

How the invisible are studied?

The basic building blocks of which we and everything in the world about us are made are extremely tiny. Even if you enlarged one of these tiny particles a million million (1012) times, it would still be smaller than a full stop. Like high-flying aeroplanes they are invisible, but just like aeroplanes, in the right conditions we can see the trails they make.

First we need to knock the particles out of the atoms where they normally hide. For this we use machines, called accelerators. Once we have done this we can follow their trails in special detectors, and then the excitement begins!

The mass mystery

The various matter and force-carrying particles weigh in with a wide range of masses. The photon, carrier of the electromagnetic force, and the gluons that carry the strong force, are completely massless, while the conveyors of the weak force, the W and Z particles, each weigh as much as 80 to 90 protons or as much as a reasonably sized nucleus. The most massive fundamental particle found so far, the super heavyweight, is the top quark. It is twice as heavy as the W and Z particles, and weighs about the same as a nucleus of gold. The electron, on the other hand, is approximately 350,000 times lighter than the top quark, and the neutrinos may even have no mass at all.

Why there is such a range of masses is one of the remaining puzzles of particle physics. Indeed, how particles get masses at all is not yet properly understood. In the simplest theories, all particles are massless, which is clearly wrong, so something has to be introduced to give them their various weights. In the Standard Model, the particles acquire their masses through a mechanism named after theorist Peter Higgs. According to the theory, all the matter particles and force carriers interact with another particle, known as the Higgs boson. It is the strength of this interaction that gives rise to what we call mass: the stronger the interaction, the greater the mass.

Experiments have yet to show whether this theory is correct. The search for the Higgs boson (or bosons) has already begun at the colliders and will continue into this century with new machines like the Large Hadron Collider.

Particles and the Universe

Looking into outer space means looking back in time. When you look at a galaxy a million light years away, you are looking at it as it was a million years ago. Looking at the sky at night is like reading the history of the Universe. Looking into inner space – into the structure of matter – also provides a view back in time. Experiments today collide together particles at the highest possible energies in order to penetrate into the deepest layers of matter. The enormous concentration of energy leads to the creation of new matter just as when matter was first created in the initial instants after the Big Bang with which the Universe began. Studies of the smallest structures in the Universe, in high-energy particle physics are therefore intimately linked with observations in astronomy of the largest structures. This meeting point between particle physics and cosmology is one of the most fascinating aspects of modern physics. Indeed, through the scenario of the Big Bang, observations in astronomy have testable consequences in particle physics and vice versa.

Into Outer Space

Astronomical evidence shows that the Universe is expanding, as if from a great explosion, the Big Bang, nearly 15 thousand million years ago. In the beginning, the Universe was unimaginably hot and dense, its whole size smaller than a single atom! Since then it has expanded, and cooled to only 3 degrees above the absolute zero of temperature. When we look at distant stars and galaxies we are also looking back in time. Some of the mysterious objects called quasars are so far away that when we look at them we are seeing light that was emitted more than 10 thousand million years ago. (The quasars are more than 10 thousand million light years distant.) It might seem that by looking at objects even further away we could come close to observing the origin of the Universe in the Big Bang. However, the conditions in the early Universe prevent us from doing this.

Before the Universe was about 300,000 years old it was too hot for neutral atoms to exist. Instead the Universe would have been hot plasma of freely moving charged particles – electrons and nuclei in the later stages, or more exotic mixes at earlier times. Light would then have been absorbed and re-emitted by the charged particles – in other words, the Universe would have been opaque. Only at an age of 300,000 years would the Universe have cooled sufficiently for electrons to bind to nuclei to form atoms. Only then would light begin to travel freely and the Universe become transparent. The early opaqueness prevents astronomers from seeing directly back to the Big Bang, before the formation of atoms. To study the creation of matter itself requires high-energy particle physics and experiments.

Into Inner Space

Particle physicists study the basic blocks of matter and the forces between them by colliding particles of matter together at very high energies. The collisions create the fundamental constituents that no longer exist in ordinary matter, but which were common in the energetic early Universe.

By concentrating a large amount of energy into the smallest possible volume, equal numbers of particles of matter and antimatter are created from pure energy according to the equation E = mc2. The energy concentrations created correspond to the conditions prevailing in the early Universe, less than a tenth of a thousandth of a millionth of a second (10-10 s) after the Big Bang. In this way, particle physics probes the conditions of the very early Universe minute fractions of a second after it began. The highest energy concentrations achieved at different experiments so far have been with nuclei of lead atoms accelerated. In these collisions, the energy concentration may be sufficient for the nucleons (protons and neutrons) within the nuclei to “merge” for an instant, so that the quarks and gluons normally locked within the nucleons form”quark-gluon plasma”. This state of matter should have existed in the very early Universe, before it had cooled sufficiently for the quarks and gluons to condense out to form the protons and neutrons of familiar matter. In experiments like these, particle physics probes forms of matter unseen since the very early Universe.

Cosmic Messengers

Among the commonest particles in the Universe are neutrinos. Like photons, they outnumber the protons and neutrons of bulk matter by around one thousand millions to one. Each cubic centimeter of space contains a hundred or so neutrinos left over from the Big Bang at the beginning of the Universe. Every second, 60 thousand million neutrinos from the Sun pass through each square centimeter of the Earth surface and through you! When supernova 1987a blazed forth, the Earth was soaked with a tidal wave of neutrinos. Most passed through the Earth unheeded, but a few were caught in underground particle detectors. Paradoxically, neutrinos are probably the least understood of particles. They have no electric charge, and interact with other matter only through the weak force. There are three types of neutrino, which are like electrically neutral versions of the electron, and its two heavier relatives, the muon and the tau. These neutrinos are all much lighter than their charged counterparts, but we have no idea how much lighter or whether indeed they have any mass at all. Neutrinos are so common in the Universe that even if they have only a small mass their total effect could dominate the Universe! The masses of the neutrinos are certainly smaller than we can measure by standard methods, so experiments have to be more ingenious. One technique is to look for neutrinos changing or “oscillating” from one type to another. This would be possible if, but only if, one or more of the neutrino types have some mass. This exciting possibility could explain why fewer neutrinos reach Earth from the Sun than expected, as the present detectors pick up only one type of neutrino, the electron neutrino. Experiments with beams of neutrinos produced from particle collisions are also testing whether neutrinos oscillate, at higher energies than the Sun produces.

Grand Unification And Super-Symmetry

One of the major breakthroughs in particle physics in the 1970s was the successful development of “electroweak” theory. This brings together electricity and magnetism, light and radioactivity, in a unified description of the electromagnetic and weak forces that underlie these very different phenomena. Now theorists are attempting a broader “grand unification”, which will also include the strong force that holds the bulk of matter together at the nuclear level.

Experiments show that the strong force becomes weaker in its effects as energies increase. This suggests that at very high energies, the strengths of the electromagnetic, weak and strong force are the same, and the forces are basically indistinguishable. The energies involved are thousand millions of times greater than particle accelerators can reach, but they would have existed in the very early Universe, almost immediately after the Big Bang, when the Universe was a mere 10-34 seconds old.

Fortunately for present-day experiments, grand unified theories do have consequences at lower energies. In particular, for the theories to be sensible they generally require that Natures has a deep symmetry, known as “super-symmetry”, which so far has Super-symmetry links the matter particles (the quarks and leptons) with the force-carrying particles, and implies that there are additional “super particles” necessary to complete the symmetry. These super particles must be much heavier than their ordinary relations, and so have not been seen. But the lightest super particles should be only around ten times heavier than the heaviest particles studied so far. Which requires machines for accelerate much higher than before.

Antimatter And The Six Quarks

Normal matter in the world around us is built from two types of quark, called “up” and “down”, which form neutrons and protons. It also requires two types of lepton: the electron and the electron-neutrino (which emerges for example in radioactive decays). This pattern repeats itself in two heavier “generations”, each with two quarks and two leptons.

Recent results from high the accelerator experiments and from astrophysics show that there can be no more generations of this type. We do not know why there are only three generations, or why the one that forms the world about us was not enough. However, this puzzle may be linked with one of the mysteries of the Universe: Where did the antimatter go? Experiments in particle physics show that matter and antimatter are always created in equal quantities, indicating that this should also have been so during the extremely energetic conditions of the early Universe. But if that were so, why did the antimatter not completely annihilate the matter, leaving only energy (photons) in the Universe? It seems instead that there was some small but significant asymmetry between matter and antimatter. The only clue we have to the origin of this asymmetry comes from a breakdown of the symmetry between particles and antiparticles known as CP-violation. Present understanding of this effect is inextricably tied up with the existence of three generations. So far it has been seen only in the neutral forms of particles called kaons, which contain “strange “quarks of the second-generation, and are currently made with protons beams. The new machines for accelerating particles like LHC at CERN’s is expected produce particles containing the heavier “bottom” quarks of the third generation. If the theory is right, particles containing bottom quarks should also reveal the symmetry breaking effect of CP violation.

But is this All?

The present description of matter – the Standard Model of particles and forces is being tested at high-energy experiments at different laboratories to great precision and proving remarkably successful. This is all the more surprising because the origin of one of the most fundamental properties of matter – mass – remains unknown. We do not know yet whether particles really do gain their different masses through the Higgs mechanism. In addition there are important questions that the Standard Model cannot answer. Can the electroweak and the strong forces be unified? Why are there six kinds of quark? Do neutrinos have mass? Such questions also relate to current mysteries about the Universe. Is there more to the Universe than meets the eye? Why does matter dominate antimatter? With all these questions in mind, Physicists are preparing for new machines, which will reach higher energies than ever before.

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