The Development and Validation of Quantum Field Theory

FROM THE LECTURE SERIES: Redefining Reality: The Intellectual Implications of Modern Science

By Steven Gimbel, Ph.D., Gettysburg College

Science requires not just theorizing but observing. With the subatomic particles, though, we needed a different way to see: it came with the particle accelerators. How did the particle accelerators help establish the standard model and validate the quantum field theory?

Graphic illustration of high energy Hadron collisions taking place.
Hadron collisions taking place inside a particle accelerator, helping us form a model and validate the quantum field theory. (Image: Giroscience/Shutterstock)

New Discoveries Using Particle Accelerators

In case of large objects like a pen, we see it when light bounces off it and our eyes observe that light. What’s absorbed and what’s reflected tells us about the thing. For subatomic particles, though, the light not only doesn’t bounce back in the same way, but the amount of energy the light brings overwhelms the system, changing it, and scattering the particle and the light. We needed a different way of observing that didn’t involve directly seeing.

We found this with particle accelerators. To see subatomic particles, we need to first isolate them, free them from the nucleus, and then have them react with something else in such a way that the reaction has detectable properties. The particle accelerators do exactly what they say––they accelerate particles. By giving the particles more energy and then banging them against hard things or against each other, we can cause them to break apart.

This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on The Great Courses Plus.

By accelerating the particles, resulting collisions revealed a wealth of particle forms––many of which we don’t generally see around us. Throughout the 1950s and 1960s, new particle discoveries were regular occurrences, and it left physicists somewhat concerned about the proliferation.

They did what physicists do best and figured out how to categorize all these particles in such a way that clear classes emerged and underlying structures revealed themselves. Ultimately, this led to what we now call the standard model, which was put in its mature form by the Americans Sheldon Glashow and Steven Weinberg, and the Pakistani Abdus Salam.

Learn more about the quantum field theory.

Standard Model: Fermions and Bosons

According to the standard model, there are two groups of particles––fermions and bosons. Fermions make up the particles we see in the atoms, as well as others formed by banging those parts together. Fermions come in two classes––leptons and hadrons.

Leptons are particles that don’t break down any further, they’re elementary. These include the electron, the muon, and the tau. Each lepton has an antiparticle which is identical but for the charge. The electron’s antiparticle, for example, is the positron which looks just like an electron, except that it is positively charged. Additionally, each lepton has an associated neutrino and anti-neutrino.

Standard model with all its elementary particles.
Quantum field theory’s standard model of elementary particles displaying leptons, quarks, and bosons. (Image: Cush/Public domain)

The remaining fermions are the hadrons, and these are made up of quarks. Remember that leptons don’t break down any further, but hadrons do. Quarks come in six varieties or three pairs––up and down, top and bottom, and charm and strange.

Quarks and their associated anti-quark can join together to form a meson––we get the pion, psi, and upsilon mesons––but usually they come together in triplets, what are termed baryons. There are 14 of these baryons, the most famous being the proton and the neutron.

Breaking things down to their most elementary bits, from the six quarks and the three leptons, we’re now able to build up all of the particles we see. But those particles interact and the theory accounts for the interaction of particle on particle in terms of the exchange of other particles called bosons.

Bosons are named after the Indian physicist Satyendra Bose who, along with Einstein, first worked out the mathematical rules they obey. There are four gauge bosons, which carry the various forces between fermions. The most famous is the photon or light particle, which carries electromagnetic force between particles. Light is thought of as a thing, as a particle: a quantum of electromagnetic energy, which is emitted by and absorbed by fermions or collections of them. This is how forces allow particles to act on each other.

Notice how radical a shift this is. We used to think of force as some magical relationship between particles acting on each other over a distance. If you had two positively charged objects, they would repel each other. But what’s doing the pushing? You could wave your hand in between them and not feel a material thing connecting them and your hand wouldn’t disturb the force. But now, we see the four fundamental forces as the result of an exchange of gauge bosons between the particles.

Learn more about quantum mechanics.

Validation of Quantum Field Theory

In this whole scheme, there was one element left out. One remaining boson that was trouble for the model––the Higgs boson. It was named after the British physicist Peter Higgs, who was among those who first proposed it. The Higgs boson was required to give mass to those particles that possess it. It was a latecomer to the standard model and was only proposed as a fix for situations where the model gave deeply problematic anomalous results. But if this boson was added into the mix, everything worked out smoothly.

Portrait of British physicist Peter Higgs.
Nobel laureate Peter Higgs, after whom the Higgs boson particle was named. The discovery of Higgs boson validated the quantum field theory. (Image: Bengt Nyman/CC BY (https://creativecommons.org/licenses/by/2.0)

The addition of it allowed physicists to predict other particles that had not yet been seen, and on this basis they set off to find it. It took years, but in 2012, 48 years after it had been predicted, physicists at CERN, the massive accelerator based in Switzerland announced they had found it.

With the confirmation of the Higgs boson, the standard model had been accepted and nearly a complete account of the physical world.

This language and the accompanying mental picture are problematic. The usual way of talking about the standard model makes us think of the subatomic world of being composed of uncuttable particles––the classical Greek atoms. But recall that this is coming out of quantum mechanics, we need to see not just light, but matter as both wave and particle. Waves require a medium. All of the elements in the standard model are understood by physicists as excited states of the required fields, which can overlap and exchange energy. Change is the result of fluctuations in the field.

Quantum field theory and the standard model force an incredibly radical revision of our notion of reality. We move from a world of things to a world of fields. A world in which there sits an underneath reality, which undulates and condenses in a way that it creates what we think to be things. The study of atoms has thus brought us to a very non-atomic conclusion.

The uncuttable elements are indeed ultimately uncuttable in being quantized. However, they are not fundamental substances bouncing around in the void pictured by Democritus, rather they arise from space which is itself the substance of reality.

Common Questions About the Development and Validation of Quantum Field Theory

Q: What is the difference between quantum mechanics and quantum field theory?

The difference between quantum mechanics and quantum field theory is that quantum mechanics is the study of very small particles, whereas quantum field theory is the study of the interaction of these particles.

Q: Is quantum field theory accepted?

Yes, quantum field theory has been accepted, especially after the discovery of the Higgs boson particle. Its application in other physical systems has also been successful.

Q: What does quantum field mean?

The quantum field is considered as the main substratum, the underneath reality, while the particles are considered as mere excitations of the field.

Q: Is string theory a quantum field theory?

String theory is a unified theory of gravity or general relativity and quantum mechanics. So, string theory in many ways is the theory of quantum gravity.

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