Two Times String Theory Taught Us Stunning New Physics

Surprising physics results that were first discovered via string theory

String theory has a PR problem. Driven by narratives from prominent critics, many have been convinced that string theory is a massive cult dedicated to an unproductive pursuit.

I have experienced the pull of this critique myself. When I was entering grad school, Smolin’s “Trouble With Physics” was one of the key reasons I ditched string theory and choose loop quantum gravity instead. Yet here I am today, won over by string theory and defending it.

Critics often point to the fact that string theory does not produce predictions that we know how to test. There's undeniable validity to this concern, and I will deal with string theory and its predictions in another post. For this post, I want to talk about something that these narratives consistently leave out: the remarkable success of string theory in guiding us to profound new physics.

Gravity=Gauge

My first example is gauge¹/gravity duality(aka holography aka AdS/CFT). Some of you will have heard of the duality in the context of a holographic universe. In this post, I will not get into its usefulness for quantum gravity, but highlight a simple yet under-emphasized fact.

First, some background. Gauge theories (actually all field theories) come with a parameter -- a "coupling constant" which we will call g which tells us how strongly the fields are interacting. When g is small (aka “weak coupling”), we know how to solve the theory. The physicist’s usual trick of expanding everything in powers of g works perfectly.

When g is large(“strong coupling”), gauge theories turn into a dark forest. We are pretty much lost.

Enter gauge-gravity duality. Derived from string theory by Juan Maldacena in 1998, gauge-gravity duality says that certain gauge theories are secretly equivalent to a gravitational theory in one higher dimension. Moreover, strong coupling in gauge theory maps to weak coupling in gravity.

The duality turns the dark forest into a neat, geometric path.

Example: the KSS viscosity bound popped out of the gravity side, later independently derived in field theory.

Here is what I wanted to highlight: field theorists never figured this path on their own before string theory showed us. Even now, 27 years after the discovery of gauge/gravity duality, we still don't have a first principles proof from inside gauge theory. And we have been studying gauge theories for a very long time now.

It's not just the usefulness of gauge-gravity duality that is impressive, it's the fact that gauge theory secretly contains geometry and we can’t yet carve it out with gauge‑theory tools alone.

Whether you label that math or physics, the takeaway is that well-studied quantum theories still hide deep structure from us. But string theory sees these hidden structures. Without string theory, we would never have discovered gauge-gravity duality.

Gravity=Gauge*Gauge

The strings that string theory describes can be closed or open. In 1986, Kawai, Lewellen & Tye noticed that any closed‑string "scattering amplitude"² can be written as a product of two open‑string amplitudes. This is now known as the KLT relations.

The KLT formula is nothing more mysterious than the fact that you can have vibrations moving both left and right on a closed string, and each of those behave like vibrations of an open string. From the string POV, it is intuitive.

But if you zoom out i.e look at energies far below the string scale where everyday physics lives, you discover something surprising. In that field‑theory limit, the same trick says that the product of two gauge theory scattering amplitudes, after some reformatting involving swapping color factors, gives a gravitational amplitude. In other words: Gravity ≈ (Gauge Field) × (Gauge Field).

These are the double copy relations, so named in the seminal 2008 paper of Bern, Carrasco and Johansson(BCJ).

The double copy relations are totally wild. It says that you can take results from one theory (gauge) and do some stuff with it and end up with results from a completely different theory (gravity).

Since the BCJ paper, double‑copy has become a highly active field. Many results have been uncovered, going beyond its stringy origins. Maps between classical gravity solutions(like Kerr-Schild) and classical gauge field configurations have been found. As it is easier to compute scattering amplitudes in gauge theories than gravity, double copy even helps crunch numbers for gravitational‑wave signals.

While one can derive the double copy relations within gauge theories no one fully understands why the prescription works so broadly. Why does nature organize itself so that gravity = gauge^2? String theory still gives the only intuitive picture of why it must be true.

Bottom Line

No other candidate for quantum gravity or TOE has led us to previously unknown physics results, let alone results as powerful as gauge/gravity or double copy relations.

This isn’t to claim that the gauge/gravity duality or the double‑copy are smoking guns, but that they are powerful circumstantial evidences for string theory. What they demonstrate is string theory’s ability to see the deep structures hidden wihin the theories that describe real world physics.

Understanding how it achieves this seems a pursuit worth our effort.

1. Gauge theory is a type of quantum field theory. Strong, weak and electromagnetic interactions are all described by gauge theories.

2. Scattering amplitudes are quantities that tell you that if you smash a bunch of particles(or strings) together, what is the chance you’ll see X-particle fly out.