What are the Strings in String Theory?

SPEAKER_1 (0:00)

College football is back, so Hilton called on me, the superstition concierge, to make your fan rituals a reality. Need a room to match your lucky number? We got you. Wanna make sure our team doesn't wash your lucky jersey? Ooh, that smells lucky.

Hilton's unmatched hospitality can keep up with any superstition. Even a marching band wake up call at 555 in 55 seconds. Hit it! When you need a team that will do whatever it takes on game day, it matters where you stay. Hilton, for the stay.

SPEAKER_2 (0:30)

When you need to build up your team to handle the growing chaos at work, use Indeed Sponsored Jobs. It gives your job post the boost it needs to be seen, and helps reach people with the right skills, certifications, and more. Spend less time searching and more time actually interviewing candidates who check all your boxes. Listeners of this show will get a $75 sponsored job credit at indeed.com/podcast. That's indeed.com/podcast. Terms and conditions apply. Need a hiring hero? This is a job for Indeed Sponsored Jobs.

Matt O'Dowd (1:03)

Thank you to 23andMe for supporting PBS Digital Studios. You may have heard the usual pop-side description of string theory. There are these tiny vibrating strings. And that's where all the forces particles, including gravity and the entire universe come from.

This raises more questions than it answers. Like, why strings? What are they made of? And what's all this nonsense of extra dimensions?

In physics, we like to reduce our description of the mechanics of reality down to the simplest possible form. We expect the most fundamental machinery to have the fewest possible moving parts, or free parameters. This is why the standard model of particle physics is considered incomplete. Its equations predict many things with stunning accuracy, but they first require us to tune many mathematical knobs and dials. We need to use physical measurement to fix 19 free parameters, like the masses of particles. And then there's gravity, which doesn't fit into the standard model at all. So surely there exists a deeper set of cogs and wheels, a theory that brings all observable phenomena into the same mechanical framework. That would be a theory of everything, and this is the great hope of String Theory. In the following episodes, we're going to explore the gory details of String Theory. But today, it's String Theory 101

Where did this crazy idea come from? I mean, why tiny vibrating strings versus literally any other tiny vibrating anything? What exactly are the strings of String Theory? First, let's do a quick primer on the origins of String Theory. The idea started in the 60s with efforts to understand the behavior of hadrons, collections of quarks bound by the gluons of the strong nuclear force. That includes protons and neutrons as well as mesons, which are a combination of a quark and an anti-quark.

Peculiarities in the interactions between pairs of mesons, as well as an odd relationship between their angular momenta and masses, suggested that the quarks in mesons are connected by, you guessed it, strings.

In this case, the strings are stretched out tubes of strong nuclear force, vibrating elastic bands made of gluons.

A lot of work went into figuring out a quantum theory for the strong interaction based on the physics of strings. The theory had some success, but kind of got stuck and was ultimately replaced by quantum chromodynamics. One of the reasons this strong force version of string theory got stuck is that it predicted the existence of unexpected and unwanted vibrational modes in the gluon field of these strings. What's a vibrational mode in a quantum field? It's a particle. And one of those modes appeared to be a massless spin-2 particle.

But the only hypothetical massless spin-2 particle is the graviton, the conjectured quantum particle of the gravitational field. If the gravitational field is made of quantum particles, which it might be, we really don't know, but if it is, then the quanta of gravity should have an uncanny resemblance to the type of particle produced by this little investigation into hadronic strings.

Except that there's no way anything like the graviton should appear in that sort of string. This realization came in the early 70s. A bold new proposal emerged. Forget mesons. What if the math of this theory could be used in the theory of quantum gravity? In fact, what if all force carrying particles result from oscillations in tiny strings? All we needed was to...

SPEAKER_1 (5:00)

College football is back. So Hilton called on me, the superstition concierge, to make your fan rituals a reality. Need a room to match your lucky number? We got you. Want to make sure our team doesn't wash your lucky jersey? Oh, that smells lucky.