End of the String for String Theory: How Physics’ Most Coveted Theory Fell From Grace

Image credit: Mae Mu/Unsplash

Science has been, seemingly eternally, a group of disparate theories with only the weak unifying theme of the search for knowledge about the universe. Until the late 20th century when a ragtag group of scientists embarked on a seemingly absurd quest - to unify every branch of science, and by extension the entirety of our knowledge about the universe, into a united and coherent theory. They started a frenzy of research where a multitude of papers were released and increasingly anomalous discoveries became a decadal occurrence. The math started to check out and with an otherworldly momentum string theory charged forth. Even a leading physicist, Micho Kaku spoke about string theory as if it has already been proven. If only.

Background

Our entire understanding of reality is built on two major theories: quantum mechanics and general relativity. Each is unique and built on its own fundamental philosophical, mathematical, and logical foundations. General relativity is an elegant theory claiming that gravity appears from the curvature of a fabric woven from space and time. Its most appealing feature is the fundamental notion that gravity is a unique force created from curving space and time. It is this intuitive elegance that has allowed general relativity to captivate the mind.

Quantum mechanics is almost entirely the antecedent of general relativity. Unlike general relativity’s minimalist intuitive aesthetics, quantum mechanics is a mass of unintuitive, seemingly illogical theories that go against common sense. But quantum mechanics challenges reality in such a way that it is a potent drug. It is tempting to flirt with such concepts as superposition, entanglement, and quantum spin replacing simple logical reality.

Together quantum mechanics and general relativity comprise modern physics and together they are enough to explain the basic principles of every branch of science from chemistry to biology. So it should come as a surprise that rather than complementing each other they explicitly rule out each other. The notion that our two fundamental theories of reality are incompatible should come as a shock, and indeed it has been a major cause of insomnia for scientists for decades.

After decades of sleepless nights, napkin equations, and fruitless work that can only be described as tedious, a group of scientists found a theory appearing to end the dispute between the two most important theories in human history. String theory was born.

String Theory: A Mathematical Eden

String theory had an odd, somewhat off-putting start. It was originally a failed theory attempting to explain the inner workings of protons which we later discovered was explained by quantum chromodynamics. Despite a handful of early bumps, string theory became incredibly competitive for a wide variety of reasons. For example:

1. Mathematical elegance: when a theory is mathematically elegant it has a substantially higher chance of being accurate. String theory happens to be an example of near perfection mathematically speaking.

2. Intuitive appeal: in the same manner as general relativity string theory is exceedingly easy to communicate to the general public. Unlike theories like the Higgs Mechanism, string theory (despite its complex math) is built on a comparably simple concept; replace points with strings.

But beyond any of those advantages, physicists have a deep physiological reason for proving string theory. 

The fundamental notion of science is to systematically derive information through a mix of experimentation and conjecture. But to have the entirety of the natural world explained through one theory would essentially be the completion of science. There would still be unexplored frontiers but for the first time science would have a solid foundation with which to work from (akin to set theory in mathematics). And for that reason I believe that the vast majority of scientists have a deep underlying hope for a proof of string theory. With its characteristic charm, elegance, and abstraction string theory turned thousands of physicists into string puppets in a global game.

A String Runs Out of Yarn

String theory became the most common way to unify general relativity and quantum mechanics. Initially it had a steady stream of discoveries, for example it was discovered that multiple branches of string theory could be turned into a single coherent theory. But every well runs dry.

By the 2010s string theory was under a perfect storm. The first and perhaps most concerning attack on string theory was that it wasn't one theory. In fact it had an upper estimate of being 10^500 different theories, which is one followed by five hundred zeros. In order to explain why, it is important to understand the math for the leading branch of string theory–M-theory–requires an eleven dimensional (10 spatial, 1 temporal)universe. As one has hopefully ascertained, our universe is four dimensional (3 spatial, 1 temporal). String theory reconciles these two worldviews by having the extra dimensions compactified in Calabi-Yau manifolds. They look fairly recognizable in their 2d form, one is shown on the cover of this article.

But there is no one way to compactify space. There are an astronomical number of ways to compactify space. Each compacification scheme yields a new version of string theory, each with its own predictions. It is hard to trust a theory that predicts more than a google different configurations of the universe.

Compactification was only the beginning of string theory's woes. Competitive theories such as loop quantum gravity have gained ground in the quest to unify general relativity and quantum mechanics. String theory hasn't seen a major discovery in decades. In general, science theories shouldn’t take such an obscenely long period of time to prove. Nor should they require such a large piece of the resources of the science community with no visible ROI in decades. 

Can String Theory Still Explain The Universe?

Despite the excruciating decadal wait, string theory is still enticing. The fact that it can’t be proven might just mean we do not have mental vision to see the complete theory. A recent study provided evidence for string theory, while it was weak and far from a complete proof, it did help to quench string theory’s researcher’s deep thirst for progress. The sheer scale of the impact on science has led numerous researchers to continue developing string theory. Over the coming decades there is still a significant chance string theory will be proven through some novel experiment. Despite everything it has gone through, thousands of physicists (and myself) still find string theory the best explanation for the incompatibility between general relativity and quantum mechanics, and for generating a coherent basis for our understanding of the universe.

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