Particle Physics Issues - Essay Example

The universe as a whole is a very complex system the dynamics of which can be represented and explained by the matter and the forces acting on it (Wit & Smith, 1986; Hooft Gerard’t, 2005). Our endeavours in exploring, understanding and quantifying these dynamics saw the emergence of some path-breaking theories, concepts and models like the Quantum Field Theory (QFT), Relativistic Field Theory, Standard Model, Supersymmetric Models and other concepts of particle physics and quantum mechanics (See, Aitchison & Hey, 1989; Ryder, 1985; Cheng & Li, 1984; Hoddeson et.al., 1997).

No single theory or model can provide a complete explanation of the universe and its dynamics on an individual basis. The complete picture of the universe can be given only by such models that are capable of accounting for all the forces of nature-the three common forces and the gravitational force as well and this is exactly what the theorists and physicists have been searching for long (Hooft Gerard’t, 2005). The search for a complete particle physics model is still on and experts are exploring the options of either producing a completely new innovative theory or model or constructing an efficient model merging the relative gravity and the Standard Models. This search saw the emergence of a variety of new-age models beyond the Standard Model. However, merging these theories is not as simple as it sounds and some difficulties need to be tackled (Green et. al., 1987).

These models are built upon different basic assumptions and concepts and this is why one finds it difficult to merge such models. QFT depends on particle fields embedded in the flat space-time of special relativity whereas, General Relativity accounts for gravity as a curvature within space-time that changes as mass moves (Zee A., 2003). The simple option of merging these two theories considering gravity as another particle field seems to end up creating the so-called renormalization problem. As per the traditional understanding, gravity particles would attract each other and add up all the interactions resulting in many infinite values which cannot be easily cancelled out by mathematical interventions (Linde, 1990), thereby ruling out the possibility of getting any sensible or finite results or values. This outcome is in contrast with quantum electrodynamics where, the addition of the interactions results in comparatively lesser infinite values which can be removed or cancelled out via renormalization (Banks, 1985).

The long-range forces like the electromagnetic and gravity forces are believed to be meditated by massless particles with spin j ≥ 1 and as such, proper description of such massless particles in quantum field theory (QFT) is very important (Aitchison & Hey, 1989). This directionally brings in the fascinating approach of emergent gravity indicating that gravity may not be a component of fundamental physics. However, this concept of emergent gravity theory has been branded to be a misguided one by Steven Weinberg and Edward Witten (See, Jackson & Okun, 2001). Gravity was speculated to be an emergent phenomenon during the 1980s and supporting this, many theories like the ‘Preon theories', Technicolor theories’, etc. went on to regard that gluons might be composite. Noticing this, Weinberg & Witten came out with a ‘no-go theorem’ called the ‘Weinberg-Witten Theorem’ that excludes the hypothetical composite and emergent theories (Wienberg & Witten, HUTP-80). This theorem indicates that an interacting graviton cannot emerge from an ordinary Quantum Field Theory (QFT) in the same space-time. The Weinberg-Witten theorem forbids the existence of any massless particles with helicity j>1 in any theory with a Lorentz covariant energy-momentum tensor and also restricts charged massless particles of helicity>1/2 in any theory with a Lorentz covariant conserved current. This prohibits the existence of both graviton and the gluon but this can be avoided due to gauge symmetries.

Taking on from here, this paper proceeds to decode and present the proof of the Wienberg-Witten theorem based on the theorem as published originally. This has been conceived to be exploratory research predominantly based on secondary sources of information. The paper starts with a brief outline of the concepts of massless particles with spin 1 and spin 2 as mediators of long-range forces in relativistic quantum field theory. Particular attention has been given to highlighting how a description of such Lorentz covariance-compatible particles leads to redundancy in the mathematical description of the physics. As purposefully planned, this paper shall run through the following topics and objectives:
- Concepts & Theoretical Ideas (massless particles, representations of the Poincare Group, symmetry, spins, etc.).
- Presenting the proof of the Wienberg-Witten theorem.
- Evading the theorem in the case of graviton and gluon.
- Emergent gauge symmetries and relevance of the theorem.
- Further implications of the theorem.