The Standard Model: Theory and Problems

By Robert Hazen, Ph.D.George Mason University

In particle physics, there are six quarks, six leptons, six antiquarks, and six antileptons. Then there are four different forces in the universe: gravity, electrostatic or electromagnetic, strong, and weak forces. According to the Standard Model theory, all the 24 different fundamental particles and the four forces are one and the same thing.

An illustration of the Standard Model
The Standard Model theory states that the different fundamental particles and the different forces are one and the same thing. (Image: Spectral-Design/Shutterstock)

The Standard Model Theory

In particle physics, there are 24 particles. Then there are forces that act on the matter.

There is gravity; the universal force between any two masses. Then there is the electrostatic force, or the electromagnetic force; the force between charged particles. The third force is called the strong force; it is what holds the nucleus together against the electrostatic repulsion of two protons, two positive charges. Finally, there’s the weak force. We know the weak force exists because electrons and neutrinos go flying out of a nucleus, so there has to be some force that causes them to fly away.

An illustration of the Big Bang explosion.
After the Big Bang, the temperature was so unimaginably high that everything was in a single, uniform state. (Image: Ismagilova/Shutterstock)

According to the modern theory of particle forces and different kinds of fundamental leptons and quarks and so forth, this is called the Standard Model. According to this theory, all the 24 different fundamental particles, all the four forces are, in some deep sense, one and the same thing. 

Imagine the moment of the Big Bang, the very origin of the universe, when all the matter, all the energy in the universe, was concentrated down to a point. The temperatures were so unimaginably high that everything was in a single, uniform state. All the matter, all the energy, all the forces, all the motions were just a single entity. As the universe expanded and cooled were these different forces and the different particles condensed out in a sense.

Learn more about the Big Bang theory.

Sequence of Freezing

Theorists tell us the sequence of freezings that took place. The first of these, according to calculations, was at 10-43 seconds; that’s when the force of gravity split off from everything else. At 10-35 seconds, you had freezing of the strong force. For the first time, quarks could condense and form nuclei, at least nuclear particles like protons and neutrons. At 10-10 seconds, the weak force and the electromagnetic forces separated, so at last, all four forces were present; but this was well into the history of the universe, at 10-10 seconds, one ten-billionth of a second.

Before 10-5 seconds, all the particles were just quarks and leptons, apparently with slightly more matter than antimatter; that’s the asymmetry we are talking about. At 10-5 seconds, the quarks combine to form hadrons. Then at three minutes, you get the first nuclei, with protons and neutrons fusing together to form deuterium. 

The present ratio of deuterium to hydrogen resulted from that time, at about three minutes into the origin of the universe. Finally, at 500,000 years—so, half a million years later—the universe cooled sufficiently that the first atoms formed. 

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

The Standard Model’s Problems 

The Standard Model of particle physics—and by extension the universe itself—seems like a neat little package, but there are nagging problems with this model. The most significant one is that the mathematical description of the Standard Model requires more than a dozen separate, fundamental constants. 

An illustration of trails of light, showing the concept of the speed of light.
Apparently, there is no mathematical relationship between the mass of electrons and the speed of light. (Image: jaras72/Shutterstock)

These are not things that you can predict from theory; you just have to measure them and accept them as they are; things like the mass of the electron, the speed of light, the gravitational constant, and so forth and so on. Each of these constants has to be determined separately, and there’s no mathematical relationship, seemingly, between them. 

Many physicists feel that this aspect of the Standard Model lacks elegance, in a sense. It lacks the kind of beauty and simplicity that you might expect from a universe. Another problem with the Standard Model is the force of gravity has not yet been fully integrated into the model. We really don’t know how gravity relates to the other forces in any fundamental way.

Learn more about the Bohr atom.

Not Enough Explanation about Dark Matter

Also, the Standard Model doesn’t fully explain dark matter. Might there be other classes of particles? Might there be WIMPs, these Weakly Interacting Massive Particles that are not part of our 24-particle inventory right now? 

In fact, there are some modifications of the Standard Model that have been proposed that double the number of particles. Some of these models are called supersymmetry or string theory; and if they double the number of particles, some of those particles may correspond to WIMPs that we have not yet detected or seen in any way.

Common Questions about the Standard Model

Q: What’s the Standard Model Theory? 

At the time of the Big Bang, due to the very high temperatures, all particles and forces were in the same state. Only after cooling occurred did the particles and forces begin to form. This is the crux of the Standard Model theory.

Q: According to theorists, what was the sequence of freezing after the Big Bang? 

After the Big Bang, the first freezing occurred at 10-43 seconds; that’s when the force of gravity split off from everything else. At 10-35 seconds, there was freezing of the strong force. At 10-10 seconds, the weak force and the electromagnetic forces separated.

Q: What are the drawbacks of the Standard Model? 

One of the most important drawbacks of the Standard Model is that the mathematical descriptions of this model need more than a dozen separate, fundamental constants. Another problem is that the force of gravity has not yet been fully integrated into the model.

Keep Reading
The Discovery of Magnet and Magnetic Compass
Magnetism, Static Electricity, and Gravity: The Three Hidden Forces
Benjamin Franklin and the Study of Electrostatic Phenomenon

The Standard Model: Theory and Problems