Elementary Particles

4. FUNDAMENTAL FORCES OF NATURE

So far we have talked about particles. We know some of them are fundamental, while others are composite, made up of quarks. Our universe exists because the fundamental particles interact with each other and these interactions include attractive and repulsive forces, decay, and annihilation. There are four fundamental interactions between particles, and all forces in the universe can be attributed to these four interactions! Our next topic of discussion will therefore be the fundamental forces of nature and their place in the Standard Model. The four fundamental forces are: (a) Gravitation (b) Electromagnetism (c) Strong and (d) Weak.

Gravity is a property of matter and space. It binds matter in planets and stars, and holds stars together in galaxies. Isaac Newton formulated gravity as a force between two objects as directly proportional to their masses and inversely proportional to the square of the distance between them. In his general theory of relativity, Albert Einstein envisioned gravity as the distortion of space caused by mass. Einstein showed that Newton’s ideas were a special case for objects moving at low speeds in a weak gravitational field.
The electromagnetic interaction is responsible for the repulsion of like and the attraction of unlike electric charges. It also explains the properties of light, holds electrons to nuclei in atoms, binds atoms into molecules, and is responsible for the properties of solids, liquids and gases. The formulation of the electromagnetic theory was provided by James Clerk Maxwell in the 19th century
The strong interaction holds protons and neutrons together in the atomic nucleus. In the 1970s scientists formulated a theory for the strong force known as quantum chromodynamics.
The weak interaction is responsible for nuclear beta decay and other similar decay processes. It is also essential for the nuclear reactions in the centers of stars like the Sun, where hydrogen is converted into helium.

The four forces are often described according to their relative strengths.

Table 2: FUNDAMENTAL FORCES
table2

The strong force is the most powerful force in nature followed by the electromagnetic, weak, and gravitational forces in descending order of relative strength. Despite its strength, the effect of strong force cannot be felt in the macroscopic universe because of its extremely limited range, being confined to an operating distance of about 10^{-15} meter (1 fm)—about the diameter of a proton. The range of the weak force is even shorter, about 10^{-17} meter. By contrast, the ranges for gravitational and electromagnetic forces are infinite. That has the implication that gravity acts between all objects of the universe, no matter how far apart they are, and an electromagnetic wave, such as the light from a distant star, travels undiminished through space until it encounters some particle capable of absorbing it.

4.1 FORCE FIELD

This brings us to the important concept of force fields. Field is a region in space where each point is affected by a force. The electromagnetic field is a familiar example whose affect can be seen by scattering iron filings around a magnet. The gravitational field is another example whose effect can be felt by the gravitational pull towards the center of the earth. Then there are the strong field active inside a nucleus and the weak field responsible for radioactivity.

4.2 FORCE CARRIERS

basketball

The manifestation of all four fundamental forces is thought to be through the exchange of one or more particles called charged intermediary particles (the gauge bosons). The exchange particle concept is a central element of modern particle physics and needs to be discussed further. If two basketball players represent matter particles, then the intermediary particles are like basketballs tossed between matter particles (basketball players). Forces are actually the effects of force carrier particles on matter particles.

Table 3: THE STANDARD MODEL TABLE OF PARTICLES AND THE FORCE CARRIERS. ANTIPARTICLES ARE NOT SHOWN HERE IMAGE CREDIT:
Table1

Each fundamental force has its own intermediary particle (or quantum) shown in Table 3.

553px-Standard_Model_of_Elementary_Particles.svg_
Figure 4-1: THE STANDARD MODEL TABLE OF PARTICLES AND THE FORCE CARRIERS. ANTIPARTICLES ARE NOT SHOWN HERE. CREDIT:

The notion of charged intermediary particles originates from quantum field theory (QFT), according to which particles can be held together by a “charge-exchange” force, which is carried by the charged intermediary particles. QFT extends quantum mechanics from single localized particles to fields that exist everywhere. Heisenberg was the first to try this idea in the context of nuclear force, but it was Yukawa who invented a new particle as the carrier of the nuclear binding force. The binding force is short-ranged and constrained by the size of a nucleus. Yukawa correctly calculated the mass of the new intermediary, which is about 200 times the electron’s mass, or 100 MeV. In the process, he provided the mathematical insight that the mass of an exchanged particle was inversely linked to the range of the nuclear force. And it was Yukawa who established that the nuclear or the strong force is a new fundamental force of nature along with gravity and electromagnetism rather than being a secondary effect explained in terms of established physics.

A few years later the pion was discovered which nicely fitted Yukawa’s prescription of the charged intermediary particle for the strong force between the nucleons (protons and neutrons).

With the introduction of the force carriers, the particle table looks like Figure 4-1 (the antiparticles are not shown here). Notice that the graviton has not found its place in this table. To know why, one needs to understand the properties of the force carriers first. So we start our discussion with the photons, the quanta of electromagnetic radiation.

4.3 PHOTONS

Albert Einstein was the first to propose that light can behave like particles made up of photons. In the classical Maxwellian theory, light was regarded as a wave. In 1905, Albert Einstein published a paper explaining the photoelectric effect where a photon acts like a particle. He was the first to propose that energy quantization was a property of electromagnetic radiation itself. Although he accepted the validity of Maxwell’s theory, Einstein pointed out that many anomalous experiments could be explained if the energy of a light wave were localized into point-like quanta that move independently of one another, even if the wave itself is spread continuously over space. Furthermore, in light of Planck’s law of black-body radiation, the energy quanta must also carry momentum, making them full-fledged particles.

4.4 GLUONS

In the 1960s Quantum Chromodynamics (QCD) emerged as the new theory of strong interactions for describing the interactions between quarks inside the nucleons with gluons as the intermediary particles. While Yukawa had shown that the strong force between nucleons could be explained as the exchange of pions, it eventually became clear that the forces between quarks had a deeper fundamental origin. In QCD the strong force came to be described in terms of the exchange of gluons between quarks rather than pions between nucleons. Like photons, gluons are massless and travel at the speed of light. But they differ from photons in one important respect: they carry what is called “color” charge, a property analogous to electric charge. Gluons are able to interact together because of color charge, which at the same time limits their effective range. We will talk about color charge in section .

The gluon was discovered in 1979 at the TASSO experiment at the Deutsches Elektronen-Synchrotron (DESY) electron-positron (anti-electron) collider in Germany. A quark—antiquark pair is created in a typical collisions between electrons and positrons, resulting in two distinct particle jets in a detector such as the cloud chamber. But if the energies of the incoming electron and positron are sufficiently high, a third jet appears —which represents gluons escaping the nucleus. The appearance of 3-ject events provided experimental proof for the existence of gluons, whose presence had been suspected for a while.

4.5 THE W AND Z BOSONS

The Standard Model of particle physics includes the electromagnetic, strong and weak forces and all their carrier particles. However, attempts to incorporate gravitons into the Standard Model have run into serious theoretical difficulties at high energies because of infinities arising due to quantum effects (in technical terms, gravitation is not renormalizable). Classical general relativity and quantum mechanics are incompatible at such energies. Some proposed models of quantum gravity attempt to address these issues, but these are speculative theories.

In spite of the problem of incorporating gravity into the Standard Model, physicists have long sought to show that the four basic forces are simply different manifestations of the same fundamental force. The most successful attempt at such a unification is the electroweak theory, proposed during the late 1960s by Steven Weinberg, Abdus Salam, and Sheldon Lee Glashow. Their theory treats the electromagnetic and weak forces as two aspects of a more-basic electroweak force that is transmitted by four carrier particles. Not surprisingly, one of these carrier particles is the photon (\gamma) of electromagnetism, while the other three—the electrically charged W^+ and W^- particles and the neutral Z^0 particle—are associated with the weak force. Unlike the photon, the W^\pm and Z^0 bosons are massive, and it is the mass of these carrier particles that severely limits the effective range of the weak force. The W^\pm and Z^0 bosons were successfully detected at the CERN Super proton-antiproton Synchrotron (SPS) in the 1980s by an international team of scientists, an account of which can be found later in this article.

4.6 GRAVITONS

One of the central predictions of general relativity is the existence of gravitational waves: ripples in space-time that propagate outwards from its source as waves. Direct detection of gravitational waves was announced by LIGO with striking aplomb in February 2016. Earlier, a strong indirect evidence of the existence of gravitational waves was reported in the timing of the orbital decay rate of the binary pulsar PSR1913+16.

Gravitons are the proposed quanta of the gravitational interactions, the carriers of the gravitational force at the quantum level, just as the electromagnetic force is carried by the photon. Gravitons are expected to appear naturally in a future theory of quantum gravity that is yet to be formulated although general relativity, which is a classical theory, gives us some insight into the nature of gravitons where the distribution of mass and energy in the universe is described by a four-by-four matrix that mathematicians call a tensor of rank two. If this tensor is the source of gravitation, it can be shown that the graviton must be a particle possessing a spin of 2 (the graviton is the only massless, spin-two particle proposed so far).

Quantum Field Theories of the three fundamental forces—electromagnetism, weak and strong (or the Standard Model) are well established—their predictions have been repeatedly confirmed, sometimes with surprising accuracy, although the Standard Model is not particularly elegant as it requires 19 finely tuned input parameters, and nobody knows why. Sadly, as we know, the techniques of QFT fail for gravity. In technical terms, the force of gravity is said to be non-renormalizable. QFT can be used to provide a partial sketch for gravitation at low energy, but extending this is impossible—the theory makes useless predictions at high energies because it needs an infinite number of constants2.

graviton_E_image
Credit: CERN BULLETIN.

Furthermore, the weakness of gravitational interactions makes the detection of a single graviton a remote proposition. In fact, Freeman Dyson and others have estimated that it may be impossible to physically construct a detector sensitive to individual gravitons since the detector will have to be so massive that it would collapse into a black hole in the process. In that case, is it meaningful to talk about gravitons as mediators of gravitation?

In a paper (Krauss & Wilczek, 2014) published in February 2014, Lawrence Krauss, from Arizona State University, Frank Wilczek, from MIT and ASU, proposed a possible way out of this. They argued that the only direct way of verifying the existence of gravitons can come from the measurement of minute changes in the Cosmic Microwave Background radiation of the Universe. The detector, in their case, is the expanding Universe, the largest entity one can think of. Then in 2015, researchers of the BICEP2 experiment thought they had indeed detected gravitational waves from the imprint of their polarization on the cosmic microwave background, which they interpreted as the footprint of gravitons. The implications would have been truly profound if confirmed—the first direct evidence, not just of cosmic inflation, but of the quantum nature of space and time. Unfortunately, the researchers soon realized that they could not isolate the signal from scatterings off space dust. A new experiment with higher resolution, BICEP3, is currently under way, but we have to wait for the results to come out.

You can probably guess by now why the graviton is conspicuously absent from the elementary particle table. While it is firmly predicted by theory, the prospect of a direct observation is exceedingly difficult, although scientists have not given up in their attempts to detect them in direct experiments.


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