Forces of Nature
Introduction
An apple fell from a tree and hit him on the head.
Everyone is familiar with the story of Isaac Newton and how he came up with the theory of gravity, though Newton himself merely said he saw an apple falling. Gravity is perhaps the best known of the major forces, since we come across it every day of our lives. But what is a force? Simply it is an interaction between particles. There are many different forces but most of these can be explained as special cases of the four main forces, and even these four are thought to be different aspects of the same force.
Elementary Particles
Before we can look at the forces we must first consider the particles they act upon. Protons and neutrons consist of smaller particles known as quarks. There are six different types of quarks, known as flavours, and they are: up, down, top, bottom, strange and charmed. Each of these also comes in three colours: red, blue and green. These "colours" just distinguish the quarks; they are not actually visible as the particles are smaller than the wavelength of light. Protons consist of two up and one down quark, and a neutron consists of two down and one up quark. The three quarks in a proton or neutron are all different colours. The other four flavours of quarks can be artificially combined to produce unstable particles that decay to protons and neutrons. All these sub-atomic particles have a property known as spin. This shows how a particle looks from different directions. Particles can have spin of 0, 1, 2 or ½. Spin 0 means the particle looks the same from all directions, like a full stop. Spin 1 looks the same when you rotate it a full 360 o , much like an arrow. A particle with spin 2 looks the same when rotated through 180 o , like a double headed arrow. Spin ½ is more unusual in that it needs two complete revolutions before it looks the same. All matter particles (quarks, electrons, etc) are spin ½, and all particles of spin 0, 1 or 2 are force carrying particles. Spin ½ particles must obey Pauli's exclusion principle, that is to say no two particles can exist in the same quantum state - particles with equal velocities must have different positions, and those with very nearly the same position must have different velocities. However, integer spin particles do not obey this principle and therefore the forces they carry can add up to be very strong.
The Forces
The first force is the electromagnetic force. Again, this is one that most people are somewhat familiar with; everyone has seen magnets repelling and attracting each other. In the same way, electric charges also attract or repel depending on the relative charges. Just as two north magnetic poles will repel, so will two positive charges. In fact, there are many similarities between electricity and magnetism, which lead to the unification of the two theories by Maxwell in 1865. The electromagnetic force is responsible for holding atoms together - the force of attraction between the positive central nucleus, and the orbiting electrons. As such, on an atomic scale, it is a powerful force (though not the strongest), but on a more day to day level, it is not so apparent. Most large bodies contain nearly equal numbers of positive and negative charge, and since the force is sometimes repulsive and sometimes attractive, it tends to cancel out.
The force carrying particle here is the photon, the particle form of electromagnetic radiation. Any charged particle in classical (non-quantum) physics is surrounded by an electric field. A moving charge is surrounded by a magnetic field. However, thanks to wave-particle duality, what this means is that the charge is continually emitting and absorbing photons. An electron emits a photon which then collides with another electron and its energy is absorbed, changing the velocity of the second electron in the same way as if there was a force between the two. The photon is a virtual particle, as it cannot be directly detected in this exchange. We know it exists because we can measure its effect - the electromagnetic force! Sometimes a real photon may be emitted, such as when an electron moves to a lower energy shell or orbit in the atom. This real photon is directly detected as light (provided the wavelength is correct), or by a photon detector.
It can be wrong to think that the electromagnetic force only acts on charged particles such as protons and electrons. Anything that consists of charged particles, even if it is itself uncharged, can undergo electromagnetic interactions. A neutral atom, with equal numbers of protons and electrons, experiences this force. On a smaller scale, the electrically neutral neutron consists of charged quarks, and so can undergo electromagnetic interactions. This is referred to as residual electromagnetism, and is very weak.
The strength of the electromagnetic force falls off according to the square of the distance between the two charged particles. That is to say, if you double the distance between them, the force decreases by a factor of four. However, it will never actually reach zero. Therefore the range of the force is infinite. This is because of the nature of the photon, in particular its mass - which is very close to zero, if not exactly. The theory of relativity says that E=mc 2 , but for a virtual particle, conservation of energy prevents this equation from holding. This results in a discrepancy between the energy and mass of the particle, and the higher the mass, the higher this discrepancy is. This discrepancy is allowed by the Heisenberg Uncertainty Principle provided the time is short. The larger the discrepancy, the shorter the time allowed. However, the discrepancy is proportional to the mass of the virtual particle, and the photon has no mass. Therefore the discrepancy can be infinitesimally small, and the time period can be effectively infinite. Given this limitless amount of time, the photon can wander a large distance from the charged particle that emitted it before being absorbed by another, giving the electromagnetic force an infinite range. If the virtual particle has a mass, it will only have a short lifetime and therefore a short range. This is the case for other forces.
A powerful consequence of this force occurs during nuclear detonations - the electromagnetic pulse, EMP. When a nuclear explosion occurs, many gamma rays are produced. These gamma ray photons interact with the air molecules and produce electrons by the Compton Effect. The photon collides with an electron and is absorbed to form an intermediary particle. This particle then emits a lower energy photon travelling at some angle to the incident photon, and the electron is also diverted as it has a higher kinetic energy. An electric field is created between the atom and the Compton electron. The lower-energy electrons produced by collisions with the Compton electrons are attracted to the positive ions. These ions produce a conduction current. This current is directly related to the strength of the Compton Effect. Also, this conduction current flows in a direction opposite to the electrical field produced by the Compton Effect. Because of this, the conduction current limits the electrical field and stops it from increasing. Though short-lived, the strength of the electromagnetic field can be anything up to about 50kV/m, and can induce large voltages in conductors such as power lines, communication cables, large antennas and metal fences. In essence, a single nuclear blast at an altitude of 300km can produce an EMP that covers all of North America instantly, and seriously damages all electronic equipment exposed to it.
As we have seen, the electromagnetic force is responsible for binding electrons to the positive nucleus. This makes sense in the case of the simplest atom, hydrogen, which consists of one proton and one electron. However, in all other atoms, there is more to consider. The neutrons are bound inside the nucleus, but they have no charge and are unaffected by the electromagnetic force. Furthermore, if there is more than one proton, they will repel each other. This means there must be another force that keeps the nucleus together, that doesn't depend on electric charge, and is stronger than the electromagnetic force. This brings us to the aptly named strong nuclear force.
Just as the electromagnetic force depended on charge, the strong force depends on colour. Only coloured particles, quarks and the particles they combine to form, are affected by it. The strong force is carried by the gluon. There are many differences between the photon and the gluon. Although the photon carries the electromagnetic force, it has no charge itself. The gluon, however, itself has colour. Related to this, there are different types of gluons of different colours, while all photons are identical.
The strong force has a property known as confinement; it combines particles into combinations that are neutral in colour. Neutrons and protons must therefore be made up from a red, blue and green quark, held together by a string of gluons. Quarks can also combine with anti-quarks, such as red and anti-red, to form the mesons (pion and kaon). These mesons are unstable since the quark and anti-quark could easily annihilate each other. Also, since the gluon has colour, it is only found in combinations that are neutral in colour. This collection of gluons is known as a glueball and is also unstable. Due to this confinement property, the quarks bound up inside protons and neutrons have no way of escaping - very useful, otherwise all matter would be unstable and decay into the constituent quarks. However this makes it difficult to detect the existence of quarks and gluons.
Just as neutral particles had residual electromagnetism, so colourless particles experience a residual strong force due to their coloured constituents. In this interaction between protons and neutrons, it is the pion that carries the strong force. The pion consists of a quark and an anti-quark, and comes in three different types, depending on whether up or down quarks are used. An up quark (charge +2/3) and a down anti-quark (charge +1/3) make a pion that is positively charged. A down quark (charge -1/3) and an up anti-quark (charge -2/3) make a pion that is negatively charged. An up quark with an up anti-quark, or a down quark and anti-quark, make a pion that is electrically neutral . Protons (consisting of up up down quarks) and neutrons (consisting of up down down quarks) interact with each other by the exchange of virtual pions . The proton emits a pion, transforming into a neutron. Another neutron absorbs the emitted pion, and transforms into a proton. Because of this, any stable nucleus that has more than one proton must also contain neutrons.
The pion has a mass of about one seventh of the mass of a proton, and therefore unlike the photon, it does not have an infinite range. The lifetime is just 26 nanoseconds, which results to a range of 10 -15 m - which is the same size as an average atomic nucleus. Nuclei larger than this size are unstable as the nucleons on one side are not held to the nucleons on the other side as strongly, and so the nucleus breaks up. The consequence of this short range is that atomic interactions are electromagnetic rather than strong.
The next force is known as the weak nuclear force. Macroscopically we think of strength of forces as being measured in Newtons or equivalent units. On a sub-atomic scale, however, strength is governed by the likeliness of the interaction taking place. If a pion and a proton approach each other, they are very likely to interact due to the residual strong force, provided they are in range. However, a proton and an electron are less likely to interact due to the electromagnetic force. Therefore the electromagnetic force is not as strong as the strong nuclear force. The weak force is far, far, far less likely to take place between the particles that can experience it.
Weak interactions can occur for all particles except gluons and photons. The exchange particles involved are known as the W + , W - or Z 0 bosons. These particles are extremely massive, about eighty times the mass of a proton. This results to an extremely short range, and is the reason the force is so weak. Particles must pass very close for there to be a chance of an interaction. A good demonstration of this is to imagine a tube of water about 1,000 light years long. If we fire a stream of neutrinos, which are electrically neutral and colourless and so are only affected by the weak force, down the centre of the tube, roughly half of them will emerge at the other side without interacting with anything on the way through.
Weak interactions are responsible for the decay of particles into more stable particles. It is this force that leaves us with matter particles containing up and down quarks, rather than other quarks. If there were no weak interactions, many more types of matter would be stable. The theory behind the weak force was first formulated to explain beta decay. When radioactivity was discovered, it was noted that there three types of radiation. The first, alpha radiation, can be explained using the strong nuclear force, since it consists of two protons and two neutrons (a helium nucleus). Gamma radiation is understood by the electromagnetic force. Beta decay, the emission of electrons, needed something else to explain it. What the weak force does is to change the flavour (type) of quarks. An electrically uncharged electron type neutrino interacts with a negatively charged down quark by emitting a W + boson. The neutrino, having lost positive charge, becomes an electron. The W + is then absorbed by the down quark, which changes it to a positively charged up quark. If this quark was part of a neutron (up down down), the neutron is transformed into a proton (up up down), and appears to emit an electron.
This property of changing flavours is unique to the weak force and makes it extremely important. The nuclear fusion that occurs in the sun and provides the heat that is emitted, involves four protons fusing into a helium nucleus of two protons and two neutrons. The weak force is responsible for transforming the up quarks in two of the protons into down quarks.
As was stated previously, many unstable particles would be stable were it not for the weak force. For example, a muon (or heavy electron), has a mass about 200 times that of an electron. It decays due to the weak force, emitting a W - boson and becoming a muon neutrino. The W - boson then decays into an electron and an anti-electron type neutrino. A similar process happens with the tau particle and the other flavours of quark (top, bottom, strange and charmed).
The fourth and final force is the one that opened the essay. Gravity appears to be the most powerful force out of them all; it makes the moon orbit the earth, the earth orbit the sun, it even causes neutron stars and black holes. However, on a microscopic scale, it is by far the weakest of all forces. Gravitational attraction between individual particles is to all intents and purposes negligible. However, it has two special properties that make it unique. The exchange particle is thought to be the graviton, which acts on mass in the same way the strong force acts on colour and the electromagnetic force on charge. The graviton has never been detected since the force is tiny on the atomic level. The graviton is massless and so gravity is long range, and it is always attractive. In a large body such as the earth, the weak and strong forces are insignificant since they are only short range. The electromagnetic force, though long range, is sometimes attractive and sometimes repulsive, depending on the charge of the particle. Overall the earth is uncharged as there are roughly equal numbers of negative and positive particles. However, the always-attractive effects of gravity do not cancel out, and so the effect is greater the greater the mass is. Large bodies such as the sun have a lot of mass and therefore a strong gravitational field.
Like electromagnetism, gravity acts as a field, continually emitting and absorbing gravitons - or at least, this is what is assumed. The strength of the field falls off proportional to the square of the distance between the two objects. When a real graviton is produced, it is presumably given off as gravity waves in the same way real photons are electromagnetic waves. However, these waves are so weak they have yet to be detected.
Bringing It All Together
A major goal of physics today is to unify all the separate theories into one theory of everything. Currently, there are two major theories that are widely accepted - quantum mechanics, which governs the microscopic world and ignores gravity, and general relativity, which describes the macroscopic world and ignores quantum effects. But how do the forces fit into these?
The electromagnetic and the weak nuclear force were the first to be unified. Until 1967, the weak force and the cause of it were not well understood. This changed with the theories proposed by Abdus Salam and Steven Weinberg, suggesting that the weak force was carried by three massive bosons, the W + , W - and Z 0 . Their theory also suggested that although these particles are massive, about 80 times the mass of the proton, at sufficiently high energies they would be massless and indistinguishable from the photon. At lower energies, spontaneous symmetry breaking occurs, and the W + , W - and Z 0 bosons acquire mass, giving their force short range. Meanwhile, the photon remains massless and continues to have infinite range. The critical energy was predicted to be about 100 GeV (giga electron Volts), and at the time the theory was proposed, particle accelerators could not reach these energies to produce real W + , W - and Z 0 bosons to confirm the theory. However, the theory also made predictions about the forces at lower energies which were confirmed by experiment over the next ten years, leading to Salam and Weinberg, together with Glashow who had produced a similar theory, winning the Nobel Prize in 1979. Finally, in 1983, the CERN laboratories in Geneva discovered the W + , W - and Z 0 particles with the predicted masses and properties. The electromagnetic and weak forces were now unified as the electroweak force.
The next step was to unify the strong force with the electroweak. There is a property of the strong force known as asymptotic freedom which means that at high energies, the strong force becomes weaker and gluons and quarks can become free. However, the electroweak force is not asymptotically free, and the strength of the force increases at high energies. There is a point at which the electroweak and the strong force have the same strength, and it is therefore likely that these forces are aspects of the same unified force. This is known as the Grand Unified Theory or GUT. There are still some problems with the GUT; it doesn't include gravity and so isn't totally unified, and it contains parameters that are not predicted from theory but are included based on experimental values. Also, the energy at which the two forces are unified is unknown but thought to be of the order of a thousand million million GeV - a lot higher than any particle accelerator can manage. As with the electroweak theory, there are lower energy predictions made by the GUT that can be tested, and are currently being worked on.
Unification of the final force is proving to be a lot more problematic. For a start, there is no real quantum theory of gravity. We do know some of the properties that a quantum theory would have, based on the discoveries of the other forces. If there is a particle that carries the gravitational force, it would have to be massless due to the long range, and of spin 2 - the graviton, as mentioned earlier. This is about as far as we can get currently with this theory. However, there is a relatively new theory that may solve the problem of quantum gravity - string theory.
String theory works on a different principle conceptually to the usual idea of particles. A single guitar string can produce many different notes depending on where it was plucked, and the tension it is under. String theory relates this to unifying particles; one string can be any particle (or 'note') depending on its tension. The string is thought to be of size somewhere near the Planck length, which is the smallest length allowed by quantum mechanics, 10 -34 m. How does all this help us with understanding gravity though? We can predict the existence of the graviton through the usual measures, but the mathematics of this breaks down if we try. Particle theory says that interactions occur at a single point, the point of collision, with no distances between them. String theory replaces this by saying the strings collide over a small finite distance, as if the collision was 'smeared out'. The mathematics for this scenario produces sensible answers. Now we have our quantum theory of gravity.
Currently there are many different string theories, depending on whether fermions (matter particles) are included, leading to superstring theory, and whether the string is closed or open loop. As work continues in this field, it is hoped that these different theories will be compacted into a single theory that describes all the forces and particles that make up the universe; the theory of everything.
