String theory isn’t dead yet

String theory is a mathematical description of nature that requires space to have several additional dimensions beyond the ordinary three dimensions. These extra dimensions, too small to be noticeable in ordinary life, can take on many possible shapes or geometries (artistically depicted here) that can affect the properties of the universe and subatomic particles. Contributors: O. Knill and E. Slavkovsky

Scientists searching for the secrets of the universe want to make a model that shows how all the forces and particles in nature fit together. It would be nice to do it with Legos. But maybe connecting everything with strings would be a better option.

Not real strings, of course, but little loops or vibrating particles of energy. And “fitting together” needs to be mathematical, not neatly shaped plastic pieces. For decades, many physicists have pursued the hope that equations, especially those involving a tiny “string,” might provide the theory to unravel the ultimate subatomic mysteries of nature.

String theory, as it is called, has gained some vague cultural acclaim and has appeared on popular TV shows such as: Big Bang TheoryAnd NCIS. Reactions to the theory among physicists have been mixed. After several promising explosions of discovery in the 1980s and ’90s, Strings fell out of favor because it failed to deliver on its promises. Among these was providing a convenient way to incorporate gravity into the quantum theory of subatomic particles. Another was to uncover the mathematics that showed that the multiple fundamental forces of nature were different products of a single unified force. Promises are still being made that are not kept.

But in the time since string theory faded from the limelight, a significant cadre of string enthusiasts have struggled to tie up all the loose ends. Success remains elusive, but real progress has been made. Questions that preoccupy physicists about the properties of not only the smallest particles of matter but also the entire universe may yield to the efforts of string theorists.

“Many of the unsolved problems in particle physics and cosmology are deeply intertwined,” the physicists write. Fernando Marchesano, Gary Shiu And Timo Weigand in 2024 Annual Review of Nuclear and Particle Science. String theory may provide a way to solve these problems.

Equations of reality

One important approach in this quest is to find out whether string theory can explain what is known as the Standard Model of particle physics. The Standard Model, developed in the last half of the 20th century, provides a kind of list of all the fundamental particles in nature. Some provide the building blocks of matter; Others transfer forces between matter particles, determining how they behave.

It is quite simple to draw a graph showing these particles. For matter particles you need 12 points; six quarks and six leptons. You need four points for force particles (collectively known as bosons) and one point for the Higgs boson, which is needed to explain why some particles have mass. But the math underlying the graph is mind-bogglingly complex; It’s a combination of equations that makes hieroglyphs seem self-explanatory.

These equations work perfectly to describe the consequences of almost any particle physics behavior. But the Standard Model cannot be the whole story of the universe. “Despite the Standard Model’s incredible success in describing observed particle physics down to currently accessible energy scales, there are compelling arguments as to why it is incomplete,” Marchesano and collaborators write.

First of all, their equations do not include gravity, which has no place in the Standard Model table. And the mathematics of the Standard Model leaves many questions unanswered, such as why some particles have such precise masses. Standard Model mathematics also does not include the mysterious dark matter lurking in and between galaxies, nor does it explain why empty space is full of it. a type of energy that causes the universe to expand at an increasing rate.

Some physicists who study these problems believe that string theory could help, since the string version of the Standard Model would include additional mathematics that could account for its shortcomings. In other words, if string theory is correct, the Standard Model would be just one part of string theory’s full mathematical description of reality. The problem is that string theory describes many different versions of reality. This is because strings exist in a realm with multiple dimensions of space beyond the ordinary three. It’s kind of like the Twilight Zone on steroids.

String theorists agree that daily life proceeds just fine in a three-dimensional world. Therefore, the extra dimensions of the string world must be imperceptibly small: They must shrink or “squeeze” down to microscopic sizes. This is like an ant living on a large piece of paper perceiving a two-dimensional surface without ever realizing that the paper has a third, very small dimension.

String theory’s extra dimensions not only shrink, but can also shrink into countless different configurations, or geometries, of space. One of these possible geometries may be the correct form of reduced dimensions to describe the properties of the Standard Model.

“The Standard Model… properties, questions, and puzzles can be reformulated according to the geometry of the extra dimensions,” Marchesano and his colleagues write.

Because the mathematics of string theory can be expressed in several different ways, theorists must explore multiple possible paths to find the most efficient formulation. So far, series approximations have been found that describe many features of the Standard Model. However, different compaction geometries of the vacuum are needed to explain each feature. The challenge, Marchesano and his colleagues point out, is to find a geometry for the vacuum that simultaneously combines all these properties and also includes the properties that define the known universe.

For example, successful compression of extra dimensions would create a vacuum in space containing the right amount of “dark energy”, the source of the accelerating expansion of the universe. And cosmic dark matter candidates should also appear in string mathematics. In fact, a whole additional set of force and matter particles emerge from string equations, which involve a mathematical property called supersymmetry. “Almost all string theory models similar to the Standard Model exhibit supersymmetry at the compression scale,” Marchesano and his co-authors write.

Versions of string theory involving supersymmetric particles are called “superstring theory”. Such “super particles” have long been suspected to contain the universe’s dark matter. But attempts to detect them in space or create them in particle accelerators have so far failed.

When it comes to gravity, particles that carry the gravitational force appear naturally in the mathematics of string theory; Initially one of the most interesting points of the theory. But the fact that many formulations of string theory include gravity does not tell you which formulation provides the correct description of the real world.

Tests are possible

If string theory is correct, fundamental particles of nature Standard theory has no zero-dimensional point-like objects. Instead, the different particles would arise from different modes of vibration of a one-dimensional string, either a loop or a particle whose ends are attached to multidimensional spatial objects called branes. Such strings would be smaller than an atom, roughly to the extent that an atom is smaller than the solar system. They are so small that there is no possible way to detect them directly. The amount of energy required to probe such small scales is beyond the reach of any practical technology.

But if string theory can explain the Standard Model, it will also include other features of reality that are accessible through experiments, such as types of particles that are not included in the Standard Model table. “Array structures that realize the Standard Model always include additional energy-scale sectors that can be tested in the near future,” Marchesano and colleagues write.

Ultimately, string theory remains a promising candidate for putting all the pieces of the cosmic puzzle together. If it works, scientists will finally be able to unravel mysteries about how quantum physics’ relationship to gravity and the properties of particles and forces in nature are deeply interconnected. “String theory,” Marchesano and colleagues write, “has all the ingredients to help us understand this deep connection.”

10.1146/knowable-112124-2

Tom Siegfried He is a science journalist in Avon, Ohio. his book Number of HeavensThe book about the history of the multiverse was published by Harvard University Press in 2019.

This article was first published on: Knowable MagazineIt is a work of journalism independent of the Annual Reviews. Sign up newsletter.