Is a Theory of Everything Possible in Physics? | Zara Walia

The simple fact that we do not wholly understand the mechanics of the universe we exist within is one of simultaneous beauty and tragedy; hence the relentless chase of this elusive, singular framework of physics to describe both the large scale and the small, the high mass and the low. A theory that encompasses all aspects of the universe, uniting the physics of galaxies and black holes with that of subatomic particles. A theory that explains all known forces, particles and dimensions. A theory of everything.

The search for such a theory has seen several attempts spanning centuries beginning in the 17th with Newtonian Mechanics – revolutionary for its time, describing forces including gravity and the behaviour of objects in response to such forces.

However, the theory inevitably ran into limitations that challenged its validity; one such limitation was the unusual orbit of Mercury. Its perihelion was observed to shift slowly over time by a discrepancy of 43 arcseconds (Info. Ex. NASA.gov) that could not be accounted for by the Newtonian predictions, a precession that could only be explained by Einstein’s general relativity (1915). It explains that the missing value can be explained by gravitation being mediated by the curvature of space, since mercury follows a geodesic which is not a perfect ellipse, causing a shift with every revolution. Another such attempt rose with Planck’s Quantum Hypothesis (1900) aiming to solve the infamous ultraviolet catastrophe which demonstrated that classical physics could not explain how energy is absorbed or emitted. He birthed the idea that energy is carried in discreet packets known as quanta as opposed to continuously, represented by E = hf (Planck, 1900, pg.237), which further revolutionised conceptual understanding of physics however contradicted the successful theories of Einstein. This history suggests the success yet simultaneous failure that the hunt for a theory of everything has yielded. It can be argued that although one has not yet been confirmed, the pursuit has resulted in some of the most integral discoveries in all of physics, eliminating its redundancy.

It is obvious that past attempts to achieve the theory of everything, although not complete, have been successful in immensely deepening our understanding of the universe, but how does the present research match up in its utility? Several approaches to achieve the unification of quantum mechanics and Einstein’s general relativity are currently being investigated by physicists yet their validity is debatable within the field. String theory, a framework that suggests fundamental particles are not point-like structures but rather one-dimensional strings, whose vibrations correspond to different particles (G. Veneziano, 1968), is particularly controversial. The extra spatial dimensions required that are theorised to be curled up or compacted at unobservable scales make empirical evidence of the theory extremely difficult. Furthermore, string theory predicts supersymmetry (SUSY) stating that each particle has a corresponding ‘superpartner’ with alternative spin properties (Golfand, Likhtman, 1971), aiming to solve the hierarchy problem (the question of why gravity is much weaker than other forces). However, due to this lack of empirical evidence, as no superpartners have been detected experimentally, this cannot be proven. The energy scales at which the theory’s predictions like supersymmetry become apparent are near the Planck scale (1019 GeV). Achieving such levels is far beyond the capabilities of current particle accelerators, that operate at levels many orders below the Planck scale. This disparity between the necessary conditions to test string theory and the limitations of our equipment suggest that perhaps a theory of everything is only possible once we as a civilisation evolve and develop new technology. Perhaps a theory of everything is indeed possible, just not yet available.

Other contributions to the currently debated theories include Loop Quantum Gravity (Rovelli, Smolin, 1994)– an attempt to quantise gravity without the use of strings. This theory, in the same way that Planck quantised energy, treats spacetime as composed of discreet loops or chunks of quantum fields that form a ‘spin network’. Although these loops would describe both gravitational force and quantum mechanics, a fully developed and peer reviewed theory has yet to be published and therefore its validity is questioned frequently – a recurring theme with the majority of these unification attempts. Namely, Causal Dynamical Triangulation theory which attempts to formulate a quantum field theory which is non-perturbative. It describes spacetime as formed from building blocks called ‘simplices’- triangles in higher dimensions (J.Ambjorn, J. Jurkiewicz, R. Loll, 2010).

However, there are also significant issues with the current form of this proposition. Firstly, it does not fully include the matter fields in relation to LQG or string theory – this lack of a full quantisation of matter means the framework is not exactly complete, so is it truly a theory of everything? Additionally, the incorporation of non-trivial spacetimes such as wormholes prove to be difficult for CDT in a manner consistent with both general relativity and quantum mechanics; this limits its ability to model a full illustration of the universe, especially the most enigmatic problems including singularities at the centre of black holes. Finally, consistent with a trend in approaches to quantum gravity, CDT does not make definitive experimental predictions that can be confirmed with existing experiments. The various limitations of the current researched theories weaken the hope of achieving a theory of everything in the near future, yet researchers and physicists remain relentless in their pursuit.

To conclude, the question of whether a theory of everything is possible in physics is one of great complexity and depends on specific criteria – the ability to observe the predictions of these theories just as the predictions of general relativity are still being observed currently, and the limitations of the technologies of our civilisation which dictate the boundaries of our potential observations. A concrete theory of everything requires not just logic but empirical evidence. This is the underlying problem surrounding current research suggesting a change in perspective on our vision of the mechanics of our world is required before any further progress can be made. Once this barrier is broken the proverbial floodgates guarding the understanding of the universe should open. Whether these floodgates will even be dented in this lifetime, or in any lifetime, remains to be seen.

Bibliography

1. Causal Dynamical Triangulations and the Quest for Quantum Gravity, J.Ambjorn, J. Jurkiewicz, R. Loll (2010) https://arxiv.org/pdf/1004.0352

2. Construction of a crossing-symmetric, Regge behaved amplitude for linearly rising trajectories, Veneziano (1968)

3. Information Exchange on Newtonian gravity and Einstein's Theory of General Relativity, NASA.gov

4. https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/educators/workshops/docs/InquiringIntoNatureUniverse/Newton_Einstein_FactCards.pdf

5. Loop Quantum Gravity, Edge, Lee Smolin (2003) https://www.edge.org/conversation/lee_smolin-loop-quantum-gravity-lee-smolin

6. Loop Quantum Gravity Information Page, Wikipedia (2019) https://en.wikipedia.org/wiki/Loop_quantum_gravity

7. On the law of Energy Distribution in the Normal Spectrum, Planck (1900)

8. Supergravity and Supersymmetry, Yuri Golfand & Evgeny Likhtman (1971, 1974)

9. Supersymmetry Information Page, CERN https://home.cern/science/physics/supersymmetry