Scientists aiming to uncover the mysteries of the universe wish to create a model illustrating how all of nature’s forces and particles interconnect. While using Lego pieces might seem appealing for this task, employing string theory could offer a more promising approach to link these elements together.
Certainly — these aren’t meant to be literal sequences, but rather minuscule loops or fragments of oscillating energy. The idea of fitting them together should rely on mathematics instead of well-shaped components made from plastic. Over several decades, numerous physicists have been chasing the possibility that specific equations describing an incredibly small “string” might offer the theoretical framework needed to unravel some of nature’s deepest subatomic enigmas.
String theory, known for its nature, has gained a somewhat hazy cultural recognition, making appearances in popular television series such as The Big Bang Theory and NCIS Among physicists, opinions about the theory have varied. Following several encouraging periods of discovery during the 1980s and '90s, string theory lost some popularity as it failed to fulfill expectations. One such expectation was offering an appropriate method for incorporating gravity into the quantum framework governing subatomic particles. Additionally, it had yet to demonstrate through mathematical means how all of nature’s various fundamental forces could originate from a single unified force. The promised breakthroughs remain unrealized.
Even though string theory stepped back from the spotlight over recent years, many dedicated followers of this theory have continued working diligently to connect all the remaining pieces. While achieving complete success still proves challenging, genuine advancements have indeed been made. Issues that continue to perplex scientists regarding both the tiniest particles and the characteristics of our whole cosmos might ultimately be unraveled through the endeavors of these string theorists.
As physicists point out, many unresolved issues in particle physics and cosmology are closely interwoven. Fernando Marchesano , Gary Shiu and Timo Weigand in the 2024 Annual Review of Nuclear and Particle Physics String theory could potentially offer the solution to those issues.
Reality’s equations
A key strategy in this pursuit involves determining whether string theory can elucidate what is referred to as the Standard Model of particle physics. Formulated during the latter half of the 20th century, the Standard Model offers an inventory of sorts for all of nature’s fundamental particles. Certain particles make up the structure of matter, while others mediate interactions among these matter particles, dictating their behavior.
Drawing a chart for these particles is quite straightforward. There should be twelve positions allocated for matter particles—six quarks and six leptons. Additionally, you'll require four spaces for force particles, collectively referred to as bosons, along with one spot designated for the Higgs boson—a key component explaining how certain particles acquire mass. However, the mathematical framework behind this chart is incredibly intricate; combining such equations makes even hieroglyphs appear easy to understand by comparison.
These equations excel at elucidating the outcomes of nearly all particle physics phenomena. However, the Standard Model likely doesn’t encompass the entire narrative of our universe. As Marchesano and colleagues point out, “Even with the remarkable achievements of the Standard Model in accounting for known particle physics within current observable energies, strong evidence suggests that this framework falls short.”
Firstly, its equations fail to account for gravity, which doesn’t find a place on the Standard Model diagram. Additionally, the mathematics of the Standard Model cannot answer numerous questions; for instance, why certain particles possess specific masses. Furthermore, this mathematical framework does not incorporate the enigmatic dark matter found both inside and outside galaxies, nor does it elucidate why empty space is permeated with energy. a type of energy driving the expansion of the cosmos at an accelerating rate.
A number of physicists exploring these issues think that string theory might offer assistance, as a string-based rendition of the Standard Model may include extra mathematics capable of addressing its limitations. To put it another way, should string theory prove accurate, the Standard Model would merely represent a fragment within the broader mathematical framework provided by string theory’s depiction of reality. However, the challenge lies in the fact that string theory outlines numerous potential realities. This complexity arises from the existence of strings in a multidimensional spatial domain extending far beyond our familiar three dimensions—think of it as an enhanced and more intricate version of "The Twilight Zone."
String theorists agree that everyday experiences work perfectly well within our familiar three-dimensional universe. Consequently, these additional dimensions proposed by string theory must be so minute they remain undetectable; hence, they need to curl up or become compactified at scales smaller than what we can observe microscopically. This concept is akin to how an ant navigating across a large piece of paper would only experience a flat, two-dimensional space without being aware that the paper has a third dimension which is extremely tiny.
String theory’s additional dimensions not only have to contract, but they can do so in countless distinct arrangements or spatial geometries within the vacuum state of space. Among these potential configurations could lie the precise arrangement needed for the shrunken dimensions to account for the characteristics of the Standard Model.
Marchesano and colleagues state that "the characteristics, inquiries, and enigmas of the Standard Model can be restated in terms of the geometry associated with additional dimensions."
Due to the fact that string theory mathematics can be articulated in various ways, researchers must investigate numerous potential paths to identify the most promising version. Up until now, string theories have successfully depicted many aspects of the Standard Model. However, distinct geometric configurations of the vacuum are required to elucidate each aspect. As Marchesano and his team highlight, the difficulty lies in discovering a single vacuum configuration capable of simultaneously explaining all these facets while also accounting for characteristics that align with our current understanding of the universe.
For example, an effective compactification of the extra dimensions could result in a vacuum containing the appropriate quantity of “dark energy,” which drives the universe’s accelerated expansion. Additionally, potential candidates for cosmic dark matter ought to manifest within the mathematics of strings. Actually, a new series of force and matter particles arises from string equations through a mathematical principle known as supersymmetry. The authors state: “Virtually all string theory models that mirror the Standard Model exhibit supersymmetry at the level of compactification.”
Theoretical frameworks within string theory that incorporate supersymmetric particles are referred to as "superstring theory." These hypothetical "superparticles" were once thought to make up the elusive dark matter pervading our cosmos. However, efforts to observe these superparticles either in outer space or through experiments with particle colliders have thus far yielded negative results.
When discussing gravity, particles responsible for transmitting the gravitational force emerge inherently within string theory mathematics—one of the key reasons this theory was initially so appealing. However, just because numerous versions of string theory incorporate gravity doesn’t mean they accurately depict our actual universe.
Tests are possible
If string theory holds true, fundamental particles of nature Would not be the point-like entities found in conventional theories. Rather, distinct particles would emerge from various vibrational patterns of a single-dimensional string, which could form as loops or segments anchored at points within multi-dimensional spaces known as branes. These strings would essentially be minuscule compared to how much an atom is smaller than our solar system—extremely minute, without any realistic method for direct detection. The quantity of energy required to investigate such infinitesimally small structures vastly exceeds what current technological capabilities can achieve.
However, if string theory manages to explain the Standard Model, it would encompass various aspects of reality that experimental methods might access, including particle types absent from the Standard Model diagram. As Marchesano and his team point out, "Constructions within string theory that align with the Standard Model invariably include extra sections… at an energy level that may soon be examined through upcoming experiments."
In the end, string theory still stands as a promising contender for solving the entire cosmic jigsaw puzzle. Should it prove successful, researchers might at last decipher the enigma surrounding the relationship between quantum mechanics and gravity, along with understanding how the characteristics of nature’s fundamental particles and forces interconnect profoundly. As Marchesano and his team point out, "string theory possesses every element necessary to aid our comprehension of this deep linkage."
10.1146/knowable-112124-2
Tom Siegfried is a science journalist based in Avon, Ohio. His work includes a book The Count of the Skies The book discussing the history of the multiverse, released in 2019 by Harvard University Press.
The piece initially surfaced in Knowable Magazine , an independent journalistic endeavor from Annual Reviews. Sign up for the newsletter .
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