Seeking a comprehensive theory – delineating all the forces and elements of the cosmos – is arguably the ultimate quest in physics. Even though each of its principal theories operates remarkably effectively, they also conflict with each other – prompting physicists to hunt for a more foundational, underlying theory. Yet, is a comprehensive theory truly essential? And how close are we to realizing one?

Our two paramount theories about the natural world are quantum mechanics and general relativity, which characterize the minutest and vastest scales of the cosmos, in that order. Each has shown tremendous efficacy and has been validated through repeated experiments. The issue, however, is their pronounced discrepancies, mathematical included. “General relativity revolves around spatial configurations. It pertains to the bending of space and the curvature of space-time – a singular entity comprising three spatial dimensions and one temporal dimension,” conveys Vlatko Vedral, a physics scholar at Oxford University in the UK. “Conversely, quantum mechanics centers on algebraic principles.”

Scientists have successfully combined quantum theory with Einstein’s secondary major theory: special relativity (detailing how velocity impacts mass, time, and dimensions). In unison, these create a structure termed “quantum field theory”, which underpins the Standard Model of Particle Physics – our most refined system for outlining the fundamental components of the cosmos. The standard model accounts for three of the four core forces in the cosmos – electromagnetism, along with the “strong” and “weak” interactions that rule the atomic core – omitting gravity.

While the standard model elucidates much of what we observe in particle physics studies, certain discrepancies remain. To address these, an augmentation named “supersymmetry”, implying particles share a profound linkage, has been introduced. Supersymmetry posits that every particle possesses a “super counterpart” with identical weight, but a reversed spin. Regrettably, instruments like the Large Hadron Collider (LHC) at Cern in Switzerland have not provided proof of supersymmetry, even though they were specifically crafted for that purpose.

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Conversely, there are fresh indications from both the LHC and Fermilab in the US alluding to a potential fifth natural force. If these findings are duplicated and verified as genuine breakthroughs, it would affect the unification of quantum mechanics and gravity. “I believe [the uncovering of a novel force] would be astounding,” states Vedral. “It would question the accepted notion that’s been in place for over fifty years that only four core forces exist.” Vedral contends that the primary action upon discovering a fifth force would be to determine if it aligns with quantum mechanics. Should it align, it would imply that quantum theory might be more intrinsic than general relativity, encompassing four of the five forces – hinting that general relativity might require adjustments. If it doesn’t, it would rattle the world of physics – insinuating that amendments to quantum mechanics might be in order.


How about other enigmatic characteristics?

Yet, what must a comprehensive theory encompass? Is it sufficient to merge gravity with quantum mechanics? And concerning other enigmatic characteristics like dark energy, which propels the universe’s expansion at an increasing speed, or dark matter, a concealed element constituting the majority of the universe’s matter?

As articulated by Chanda Prescod-Weinstein, a junior professor of physics and astronomy at the University of New Hampshire in the US, experts in physics tend to opt for the phrase “quantum gravity theory” rather than “theory of everything”.

“Dark substance and shadowy force constitute the bulk of the material and energy presence in the cosmos. Thus, it isn’t truly a comprehensive doctrine if it doesn’t encompass the majority of the universe’s material and energy makeup,” she contends. “That’s the reason I’m pleased we don’t commonly refer to it as the ‘universal theory’ in our research.” Even if they don’t encompass everything, there are numerous hypothesized theories of quantum gravity. String theory is one such, positing that the cosmos is fundamentally composed of minuscule oscillating filaments. Yet another is loop quantum gravity, which postulates that Einstein’s concept of space-time originates from quantum interactions.

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“A significant advantage many highlight about string theory is its foundation on quantum field principles,” elaborates Prescod-Weinstein. “It integrates the entire standard framework, something that loop quantum gravity doesn’t achieve in an identical manner.” However, string theory isn’t without its pitfalls, she maintains, like the assumption of additional dimensions for which we’ve found no tangible proof. Testing these theories experimentally is a challenge – demanding energy levels beyond what our labs can generate. Vedral postulates that although direct examination of these minuscule scales for evidence of quantum gravity theories might be elusive, there’s potential in magnifying such phenomena so they could be indirectly witnessed on grander scales through bench-top experiments.

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