A feature common to all of the quantum field theories is that the exchange particles are all bosons, not fermions. This is actually required by the same properties of spin that led us to the Pauli exclusion principle in the first place. All of the known exchange bosons have spin quantum number equal to one. The mass of the force-carrying particle has an impact on the range of the force. The fact that the photon is massless allows the electromagnetic force to have infinite range, and still not violate the conservation of energy and momentum, while the weak and strong nuclear forces have severely limited ranges. If a quantum field theory is ever developed for gravity, it will demand the existence of a massless boson we might call the “graviton.”
QUANTUM QUOTE
Young man, if I could remember the names of these particles, I would have been a botanist.
—Nobel laureate Enrico Fermi, to future Nobel laureate Leon Lederman
All these observations about the known quantum field theories can be neatly packaged into something we call the standard model of particle physics. Though imperfect, it goes a long way to help us understand what can happen at the quantum scale and what cannot. As we have seen, the standard model still struggles to incorporate gravity. But for the rest of the fundamental interactions, it does a great job of guiding calculations and computer models that correctly predict the outcomes of a wide variety of experiments.
The accuracy of standard model predictions for the electromagnetic interaction and the weak nuclear interaction is truly astounding. For the strong interaction it has correctly predicted the existence and even masses of new particles before they were observed. The exact way that the interaction between quarks leaks out to hold protons and neutrons into nuclei is still a bit mysterious, but we are now confident that we understand the basic structure of protons, neutrons, and nuclei.
The standard model, which pieces together all of the known entities of quantum field theory, can be packed into one concise figure. Only the matter side is shown here, since the antimatter side would look almost exactly the same.
The qualitative features of the standard model can be summarized in one compact figure. It displays the fundamental fermions on the left, and the force carrying bosons on the right. The fermions are separated into the quarks and the leptons, with each group coming in pairs multiplied by three generations.
Gluons, as the exchange particles carrying the strong force, only interact with quarks and with themselves. Photons only interact with particles that have nonzero electric charge, which leaves out the gluons, all the neutrinos, and the neutral Z boson. Even though it is weaker than the strong force or electromagnetism, the weak force holds sway over more of the fundamental particles. All of the fundamental fermions (quarks and leptons) participate in the weak interaction, mediated by the W and Z bosons. The newest piece of the standard model is the Higgs boson and its accompanying, all-pervasive field, from which some of the particles derive their masses.
The standard model has been a great success, and remains a valid representation of a wide swath of very basic physics, but it is not complete. It still has a few rough edges and missing pieces. The biggest shortcoming is the lack of a quantum field theory for gravity itself. This glaring omission has become the holy grail of quite a few physicists, and it is to this significant effort that we will now turn.
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